Laboratory work number 8.

Purpose of work: To learn how to choose the right cutters for cutting ends and ledges

Learn how to cut the ends of workpieces with various cutters rough and clean, longitudinal and transverse feeds.

Methodical materials: this development, the poster "Basic turning work".

Equipment: screw-cutting lathe TV4 (TV6).

Tool: through straight cutter, through bent cutter, through thrust cutter, scoring cutter.

On products manufactured on lathes, the following surfaces are distinguished: 1. Flat surfaces that limit the length of the part - ends. Requirements for the end face of the workpiece. 1. It must be perpendicular to the longitudinal axis of the workpiece. 2. The end surface must be flat without bulges and concavities. 3. The cleanliness of the processing of the end surfaces must comply with the requirements of the working drawing.

2. Surfaces obtained as a result of the rotation of the forming products around the central axis of the machine - steps, the total length of which is equal to the length of the product. A body of revolution that has several different sections, with different diameters, is called a stepped body. A section of a body of revolution that has a constant diameter is called a step. Flat surfaces that limit the length of a step are called ledges. Requirements for ledges: 1. Perpendicularity to the longitudinal axis of the product. 2. Lack of convexity and concavity. 3. The finish of the ledge must comply with the requirements of the working drawing. 4. The accuracy of the location of the ledge in relation to other steps of the shaft.

Sometimes, in order to increase the resistance of the product to torsional moments, instead of a ledge, a smooth transition from one step to another is performed - a fillet.

The manufacture of a part on lathes must begin with cutting the end of the workpiece, because. the end face of the workpiece serves as a reference surface, from which the length of the product is measured. By cutting the end, a cut of the burrs is achieved, the perpendicularity of the end to the longitudinal axis of the workpiece, obtaining a reference base for the lengths of the workpiece and its individual steps.

Cutting the ends of the workpiece is performed by longitudinal and transverse feeds of the cutter. As cutters for trimming the ends of the workpiece, bent through, straight through, trimming and cutting cutters are used. Cutting the end face of the workpiece can also be done with a right-hand straight cutter, but for this it must be rotated together with the tool holder by about 15 - 20 °.

The workpiece overhang from the chuck, when turning it only in the front center, should be minimal, but not more than 5 of its diameters.

Practical part: 1. Come to the machine. Visually check the condition of the machine, the presence of all its components, the presence of enclosing devices and grounding.

2. Remove the tool and all items from the machine to the bedside table.

3. Install a workpiece in the front center with an overhang of no more than 3 of its diameters.

4. Turn on the machine.

5. Check that the workpiece rotates without runout.

6. Turn off the machine.

7. Install a bent cutter in the tool holder with a cutter overhang of no more than 1.5 h

8. Calculate the frequency of rotation of the workpiece according to the formula V= Dn/ 1000, where V is the cutting speed m/min; D is the workpiece diameter, mm; n is the frequency of rotation of the workpiece rpm. The maximum cutting speed is determined by the material of the cutting edge of the cutter. For high speed steel it is 20 m/min; for cutters with brazed carbide inserts 60 m/min, for cutters with diamond tips cutting speed over 20,000 m/min.

Trimming ends in a three-jaw chuck with a transverse cutter feed.

1. Observe the conditions for safe work on the machine. Correctly and firmly fix the workpieces in the chuck. Be careful when cutting the end close to the jaws of the chuck, so as not to allow the cutter to cut into the jaws of the chuck.

2. Trim the ends with a trimming end cutter.

2.1. Place a cylindrical workpiece in a three-jaw self-centering chuck. The workpiece is installed and fixed in the chuck with a reach from the jaws of no more than 40-50 mm.

Rice. 1. Fig.2 Fig.3.

Fig.4 Fig.5.

2.2. Install the scoring end cutter. Set the scoring face cutter with its apex at the level of the axis of the machine centers in the same way as the through thrust cutter.

2.3. Set the desired spindle speed. The spindle speed is determined by the selected cutting speed and the diameter of the workpiece being machined.

2.4. Turn on the machine.

2.5. Trim the first end of the workpiece. touch top 3 cutter heads 2 (see Fig. 1.) the end of the workpiece 1 and pull the cutter towards you. Then move the cutter to the left in the direction of the arrow BUT to the required amount of cut layer and move it in the direction of the arrow B manual cross feed on the workpiece I. reducing the amount of feed when the cutter 2 approaches the axis of centers (Fig. 2.), slightly move the cutter to the right from the end of the workpiece and move it to its original position. The initial position of the cutter is considered to be when its top is at a distance of 5-8 mm from the end of the workpiece.

2.6. Switch off the machine.

2.7. Check the straightness of the end. Face straightness a check blanks 1 after processing with a measuring ruler (Fig. 3.). Butt bulge a not allowed (Fig. 4.). it can be detected by swinging the measuring ruler or caliper rod on the center of the butt. The concavity of the end is allowed insignificant.

2.8. Determine the amount of allowance for cutting the 2nd end. Unfasten the workpiece, measure its length and determine the allowance. Fix the workpiece with the other end in the chuck.

2.9. Turn on the machine.

2.10. Trim the second end, maintaining the length of the workpiece according to the drawing. By moving the cutter 2 (Fig. 5.) from the end face of the workpiece 1 in the direction of the arrow BUT to the required allowance 3. leaving 0.1-0.2 mm for fine cutting. Counting the allowance value is carried out according to the limb of the screw top slide or along the limb of the longitudinal feed screw of the caliper.

Trim the end by moving the cutter towards the center (see Fig. 2.) with a manual cross feed.

Rice. 6. Fig.7. Fig.8.

On the limb of the screw of the upper slide of the caliper, move the cutter to the left by the amount of the remaining allowance and cut the end completely.

2.11. Switch off the machine. Pull the cutter to the right to a position that allows you to freely remove the workpiece. Unfasten and remove the workpiece.

2.12. Measure the length of the cut piece. Check the length of the workpiece with a measuring ruler or caliper. If the length of the workpiece turned out to be more than required according to the drawing, cut the 2nd end, checking first the straightness of the end surface.

2.13. Switch off the electric motor.

2.14. Loosen and remove the cutter.

3. Trim the ends with a through-thrust cutter.

3.1. Install and secure the cutter and workpiece. When removing a small layer of metal, a through thrust cutter 2 (Fig. 1.) set the main cutting edge to the surface of the end face of the workpiece 1 at an angle of 10-15°.

3.2. Trim the end with the removal of a small layer of metal. Crash with the tip of the cutter into the end near its center in the direction of the arrow BUT to the required depth. Move the cutter first to the center of the workpiece, and then from its center in the direction of the arrow B.

3.3. Trim the end with the removal of a significant layer of metal. In this case, the through-thrust cutter 2 (fig. 7.) set so that the main angle in the plan is 95 °: cut the end in several working moves, each time feeding the cutter for cutting in the direction of the arrow BUT, for plunging in the direction of the arrow B, those. stepwise, and so on to the very center of the workpiece. Then apply the cutter for a small plunge and backfeed in the direction of the arrow AT(from the center of the workpiece) cut the end face completely.

3.4. Turn off the motor, unfasten and remove the workpiece and cutter.

4. Trim the ends with a through thrust cutter using a half-center.

4.1. Install the cutter in the tool post, half-center in the tailstock quill. Cutter 2 (fig. 8.) install and fix in the tool holder so that the main angle in the plan is approximately 95-100 °; the rear semi-center 5 with its cut should be turned towards the incisor.

4.2. Install blank 1 into a three-jaw chuck, pressing it with the rear half-center.

4.3. Turn on the machine and cut the end, maintaining the specified length of the workpiece.

4.4. Switch off the machine. Unfasten and remove the part, cutter, half-center.

5. Trim the ends with a curved through-thread cutter with a multi-faceted, non-regrindable carbide insert.

5.1. Check workpiece dimensions. Check the diameter and linear dimensions of the workpiece according to the drawing for the part.

5.2. Install, align and secure the workpiece in the chuck and the cutter in the tool holder. Set the cutter exactly at the level of the axis of the centers of the machine.

5.3. Cut off the first end. When rough cutting the end, I (Fig. 9.) mix the cutter 2 from outer surface blanks to its center in the direction of the arrow BUT with manual or mechanical feed. When finishing cutting with the removal of a small layer of metal, it is recommended to move the cutter from the center of the workpiece to its outer surface in the direction of the arrow B.

5.4. Unfasten the workpiece, rearrange the other end and secure.

5.5. Cut the second end b, maintaining the size of the length of the workpiece. When cutting the second end, make sure that its surface is parallel to the surface of the first end, which is achieved by carefully aligning the workpiece. If possible, insert the workpiece into the chuck until it stops in its body.

Types of marriage in the processing of ends.

Checkout laboratory work according to the sample.

Cutting with cutters is performed with the selected feed rate at a certain depth of cut and with an allowable (optimal) cutting speed. Cutting conditions are a combination of these values. When choosing turning modes, it is advisable to use the materials of the reference book “Metal cutting conditions”, namely: “General instructions for calculating cutting conditions” (p. 7 ... 8), conventions values ​​relating to all sections of the reference book (pp. 9 ... 10), as well as the materials given in sec. 1 "Cutting conditions on lathes", references to which will be given when choosing cutting conditions. In map T-1 sec. 1 on sheets 1 ... 3 subsection. “Lathes” describes the “Method of calculating cutting conditions when processing on single-spindle lathes” (p. 11 ... 13).

The depth of cut t depends on the machining allowance and the type of machining (rough or fine turning). Processing is carried out with the smallest possible number of passes.

Consider the sequence of determining cutting conditions when turning on single-spindle machines.

1. Determining the length of the working stroke L p.x of the caliper at the working feed, mm (or each caliper, if there are several), based on the L values ​​calculated for individual caliper tools and the sequence of their work. The calculation is carried out for one cutter, i.e. L p.x \u003d L:

L \u003d L p + L p + L d,

where L p - cutting length, mm; L p - the length of the supply, cutting, overrun of the tool, mm; L d - additional stroke length, due to the features of the setup and configuration of the part, mm.

2. Appointment of the support feed per spindle revolution S o , mm / rev, based on the material being processed, the type of tool, the depth of cut t, the requirements for the quality of processing, including surface roughness (during finishing).

For example, the feed per revolution S o for rough turning with through cutters is shown in Table. 2.1.

Then, the feeds are refined according to the machine passport, if it contains feeds per revolution.

Table 2.1. Feed per revolution S o for rough turning with straight cutters, scoring cutters and boring cutters

Notes.

1. The given feed rates reflecting production experience, depend on the rigidity of the technological system: large feeds are assigned with greater rigidity.
2. SMP - replaceable polyhedral plates.
3. The following restrictions must be taken into account when assigning innings:
   • with interrupted cutting with carbide SMP S o ≤ 0.4 mm/rev;
   • feed values ​​should be no more than 0.5 radius at the tip of carbide cutters.
4. When working with a cutter with SMP made of cutting ceramics, it is advisable to reduce feeds during insertion and exit of the cutter in order to increase the reliability of the tool.

3. Determination of resistance T p tool, min (or a group of limiting tools for multi-tool processing) is made according to table. 2.2. Tool life Tr, min (limiting), for which the cutting speed is calculated, is determined by the formula

T p \u003d T m πλ,

where T m - normative resistance of tools in minutes of the main processing time; λ is the cutting time factor.

Table 2.2. Normative resistance Тm of tools

The cutting time factor λ is calculated as the ratio of the number of revolutions of the spindle during the cutting time for the tool in question to the total number of revolutions of the spindle during the working cycle.

When working with one support λ ​​= L p / L p.x. ,If it is obvious that the cutting time factor λ > 0.7, then it can be taken equal to one and not taken into account.

4. Calculation of cutting speed v, m/min, and spindle speed n, rpm.

In this example, the calculation is made for machines with a constant spindle speed during the working cycle, based on the known parameters: the angle in the approach φ, the depth of cut t, the feed per revolution S o and the accepted tool life T p.

Determination of the initial values ​​of v tools with resistance T p carried out according to the table. 2.3).

Cutting speed v 1 for steels and cast irons is determined by the formula

v \u003d v table K 1 K 2 K 3,

where v table - speed according to the table, m / min; K 1 , K 2 , K 3 , - coefficients depending, respectively, on the grade and hardness of the material being processed, the hard alloy group and the tool life T p.

Table 2.3. Steel turning
Cutting speed v table when turning through, cutting and boring cutters

The values ​​of the coefficients K 1 , K 2 , K 3 are given in the same map. The calculation of the value n corresponding to the initial value v is made by the formula

n = 1000v/(πD),

where D is the diameter of the workpiece, mm.

The value specified in the machine passport should not exceed the smaller of the calculated values ​​of n by more than 10...15%. If the machine passport specifies feed values ​​S M, mm/min, then it is necessary to determine the calculated value S M = S o n and specify it according to the machine passport.

5. Calculation of the main processing time T o, min, at a constant feed S o and rotational speed n of the spindle is made according to the formula

Тo = L p.x /(S o n),

where L p.x is the length of the working stroke of the caliper, mm.

6. Correction of cutting conditions. In the case when the main time T o calculated in step 5 is less than the main time corresponding to a given performance, one should consider the feasibility of lowering cutting conditions to improve reliability, improve technical and economic indicators while ensuring a given productivity and quality; in this case, the initial data are two values ​​of the main time T o calculated at step 5 and corresponding to the specified performance.

Table 2.4. Cutting force R.tabl

7. Performing verification calculations for cutting power consists of two stages.

7.1. The cutting force is determined by the formula

Р z = Р ztable t,

where P ztabl is the main component of the cutting force, kN (Table 2.4); t - cutting depth, mm.

7.2. Cutting power, kW, is determined by the formula

where v - cutting speed, m/min.

Engine power is checked by peak load and heating.

Threading on lathes

Consider the methods of processing threads with cutters and round dies.

Cutters cut external threads with a diameter of d H = 1 ... 1000 mm, pitch P = 0.25 ... 100 mm, 6 ... 8 degrees of accuracy. The highest machining productivity in serial production, including on CNC machines, is 5 pcs/min for threads with a minimum diameter, pitch and length of no more than 2d H .

Table 2.5. Determination of the total depth of cut t 1 and the number of passes i when turning external and internal metric threads on structural steel parts

Table 2.6. Radial feed per pass S when cutting outer metric thread on structural steel parts

Table 2.7. Cutting speed v for thread turning

The calculation of threading modes with cutters is completed by determining the main time.

When turning a thread, the main time

T o \u003d L p.x iq / (Pn),

where L p.x is the length of the working stroke of the cutter, mm; P - thread pitch, mm; n - the frequency of rotation of the workpiece, rpm, determined by the formula

n = 1000v/(πD),

based on the tabular speed v, taking into account the capabilities of the machine, determined by the passport data; i - number of passes; q is the number of thread starts.

Round dies cut threads with a diameter of d H = 0.2 ... 72 mm, pitch P = 0.08 ... 3 mm, 5 ... 8th degree of accuracy. The highest productivity - 5 pieces/min.

Cutting speed v, tool life T p, torque M kr, main time T o when threading with round dies can be determined from the RG-1 map of the reference book.

test questions

1. What are the values ​​of cutting conditions when machining parts by turning?
2. What threading methods are used on lathes?
3. How to choose cutting conditions for rough turning of parts from structural steels according to the given tables?
4. Give an example of the choice of cutting conditions for thread turning.

The concept of processing allowance. Machine parts processed on machine tools, are made from castings, forgings, pieces of rolled metal and other blanks. The part receives the required shape and dimensions after all excess material is cut off from the workpiece, or, as they say, the allowances resulting from its manufacture.

allowance(general) is a layer of metal that must be removed from the workpiece to obtain a part with a final finished form.

Some parts are processed sequentially on several machines, on each of which only a part of the total allowance is removed. So, for example, parts, the diametrical dimensions of which must be very accurate, and the surfaces must have a very small roughness, are preliminarily processed on lathes, and finally on grinding machines.

The layer of metal removed lathe, is called allowance for turning. When processing cylindrical parts, there are - side allowance and diameter allowance. The diameter allowance is equal to twice the allowance per side. It can be defined as the difference in diameters in the same section before and after processing.

The part of the metal removed (cut) from the workpiece during its processing is called shavings.

The wedge is the basis of any cutting tool. The cutting tools used in the processing of parts on machine tools, in particular lathes, are very diverse, but the essence of their work is the same. Each of these tools is a wedge, the device and operation of which are well known.

The knife with which we sharpen the pencil is wedge-shaped in cross section. The joiner's chisel is also a wedge with an acute angle between its sides.

The most commonly used tool when machining a part on a lathe is a cutter. The section of the working part of the cutter also has the shape of a wedge.

Rice. No. 1 Wedge as the basis of any cutting tool

Cutting motions in turning. Figure 2 schematically shows the turning of part 1 with cutter 2. The part rotates along the arrow υ, and the cutter moves along the arrow s and removes chips from the part. The first of these movements is main. It is characterized by cutting speed. Second movement - feed motion.

Rice. №2 Movements and cutting elements when turning

Cutting speed. Each point of the part processed on the surface (Fig. 2), for example, point A, passes in a unit of time, for example, in one minute, a certain path. The length of this path can be more or less depending on the number of revolutions per minute of the part and on its diameter and determines the cutting speed.

Cutting speed called the length of the path that passes in one minute the point of the machined surface of the part relative to the cutting edge of the cutter. Cutting speed is measured in meters per minute and is denoted by the letter υ. For brevity, instead of the words "meters per minute" take to write m / min.

The cutting speed during turning is found by the formula

υ = πDn / 1000

where υ is the desired cutting speed in m / min; π is the ratio of the circumference to its diameter, equal to 3.14; D - diameter of the processed surface of a detail in mm.; n is the number of revolutions per minute. The product πDn in the formula must be divided by 1000 so that the found cutting speed is expressed in meters. This formula reads as follows: the cutting speed is equal to the product of the circumference of the workpiece and its number of revolutions per minute, divided by 1000.
Innings. The movement of the cutter during cutting, depending on the working conditions, can be faster or slower and is characterized, as noted above, by the feed.
Submission is the amount of movement of the cutter for one revolution of the workpiece. Feed is measured in millimeters per revolution of the part and is denoted by the letter s (mm/rev).
Submission is called longitudinal, if the movement of the cutter is parallel to the axis of the workpiece, and transverse when the cutter moves perpendicular to this axis.
Cutting depth. When moving, the cutter removes a layer of material from the part, the thickness of which is characterized by the depth of cut.
Depth of cut called the thickness of the removed layer of material, measured perpendicular to the machined surface of the part. The depth of cut is measured in millimeters and is denoted by the letter t. The depth of cut for external turning is half the difference between the diameters of the workpiece before and after the cutter passes. Thus, if the diameter of the part before turning was 100mm, and after one pass of the cutter it became 90mm, then this means that the depth of cut was 5mm.
Slice, its thickness, width and area. As a result of the residual deformation of the chips, which occurs in the process of its formation, it is wide and especially its thickness is larger than b and a in fig. 2. The length of the chips is less than the corresponding size of the machined area of ​​the workpiece surface. Therefore, the area ƒ shaded in Fig. 2 and is called a cut, does not reflect the cross section of the chips removed in this case.
cut called the cross section of the metal layer removed at a given depth of cut and feed. The dimensions of the cut are characterized by its thickness and width.
slice thickness called the distance between extreme points working part of the cutting edge of the cutter. The cutting width is measured in millimeters (mm) and is indicated by the letter b. The quadrangle shaded in Fig. 2 shows the cut area.
The cut area is equal to the product of the feed and the depth of cut. The cut area is measured in mm², denoted by the letter ƒ and is determined by the formula ƒ= s t, where ƒ is the depth of cut in mm.
Surfaces and planes in the process of cutting. On the workpiece, when removing chips from it with a cutter, surfaces are distinguished: machined, machined and cutting surface (Fig. 3).

Rice. 3. Surface and plane during cutting

processed surface called the surface from which the chips are removed.
Surface treated called the surface of the part obtained after chip removal.

cutting surface called the surface formed on the workpiece directly by the cutting edge of the cutter.

To determine the angles of the cutter, the concepts are established: the cutting plane and the main plane.

cutting plane called the plane tangent to the cutting surface and passing through the cutting edge of the cutter.

Main surface called a plane parallel to the longitudinal and transverse feeds. It coincides with the supporting surface of the cutter.

Parts of the cutter and elements of its head. The cutter (Fig. 4) consists of a head, i.e. working part, and the body that serves to secure the cutter.

Rice. 4. Parts of the cutter and elements of its head.

The surfaces and other elements of the cutter head have been given the following names.
Front surface of the cutter called the surface on which the chips come off.
Back surfaces of the cutter are called surfaces facing the workpiece, and one of them is called main, and the other auxiliary.
cutting edges incisor are called lines formed by the intersection of its anterior and posterior surfaces. The cutting edge that performs the main work of cutting is called main. The other cutting edge of the cutter is called auxiliary.
From fig. 4 it can be seen that the main rear surface of the cutter is the surface adjacent to its main cutting edge, and the auxiliary one is adjacent to the auxiliary cutting edge.
The tip of the incisor the place of conjugation is called the main and auxiliary edge. The top of the incisor can be sharp, flat-cut or rounded.
Cutter angles. The main angles of the cutter are the main relief angle, the rake angle, the taper angle and the cutting angle. These angles are measured in the main cutting plane (Fig. 5).
Principal cutting plane there is a plane perpendicular to the main cutting edge and the main plane.
The main clearance angle is the angle between the main back surface of the cutter and the cutting plane. This angle is denoted by the Greek letter α (alpha). Tapering angle called the angle between the front and main back surfaces of the incisor. This angle is denoted by the Greek letter β (beta).
front angle called the angle between the front surface of the cutter and the plane drawn through the main cutting edge perpendicular to the cutting plane. This angle is denoted by the letter γ (gamma).
Injection cutting is called between the front surface of the cutter and the cutting plane. This angle is denoted by the Greek letter δ (delta)>

.

Rice. 5. Corners turning tool.

In addition to those listed, the following cutter angles are distinguished: auxiliary clearance angle, main angle in the plan, auxiliary angle in the plan, angle at the top of the cutter and the angle of inclination of the main cutting edge.
Auxiliary rear corner called the angle between the secondary back surface and the plane passing through the secondary cutting edge perpendicular to the main plane. This angle is measured in the minor secant plane perpendicular to the minor cutting edge and the major plane and is denoted α¹.
Leading angle is the angle between the main cutting edge and the feed direction. This angle is denoted by the letter φ (phi).
Auxiliary angle in plan is called the angle between the secondary cutting edge and the feed direction. This angle is denoted φ ¹ .
apex angle called the angle formed by the intersection of the main and auxiliary cutting edges. This angle is denoted by the Greek letter ε (upsilon).
The simplified image of the corners of the cutter, adopted in practice, is shown in fig. 6, a and b (line AA - cutting plane). On fig. 6c shows the angles of the cutter in plan.
The main cutting edge of the cutter can be of various angles of inclination with a line drawn through the top of the cutter parallel to the main plane (Fig. 7).

Rice. 6. Simplified image of the corners of a turning tool.

Tilt angle measured in a plane passing through the main cutting edge perpendicular to the main plane, and denoted by the Greek letter λ (lambda). This angle is considered positive (Fig. 7, a) when the tip of the cutter is the lowest point of the cutting edge; equal to zero (Fig. 7, b) - with the main cutting edge parallel to the main plane, and negative (Fig. 7, c) - when the tip of the cutter is the highest point of the cutting edge.

Rice. 7. The angles of inclination of the main cutting edge: positive (a), equal to zero (b) and negative (c)

The value of the angles of the cutter and general considerations when choosing them. All of the above angles are importance for the cutting process and the choice of size they should be approached very carefully.
The larger the rake angle γ of the cutter, the easier the chip removal. But with an increase in this angle (Fig. 6, a), the angle of sharpening of the cutter decreases, and therefore its strength.
The rake angle of the cutter can therefore be relatively large when machining soft materials and, conversely, must be reduced if the material being machined is hard. The rake angle can also be negative (Fig. 6, b), which helps to increase the strength of the cutter.
From fig. 6a, it is clear that with a decrease in the rake angle of the cutter, the cutting angle increases. Comparing this with what was said above about the dependence of the rake angle on the hardness of the material being machined, we can say that the harder the material being machined, the greater the cutting angle should be, and vice versa.
To determine the value of the cutting angle δ, when the rake angle of the cutter is known, it is sufficient, as can be seen from Fig. 6, a, subtract from 90º the given value of the front angle. For example, if the rake angle of the cutter is 25º, then its cutting angle is 90º - 25º = 65º; if the rake angle is -5º, then the cutting angle will be 90º - (-5º) = 95º.
The clearance angle of the cutter α is necessary so that there is no friction between the rear surface of the cutter and the cutting surface of the workpiece. If the back angle is too small, this friction is so significant that the cutter becomes very hot and becomes unusable for further work. With too large a back angle, the angle of sharpening is so small that the cutter becomes fragile.
The taper angle value β is determined by itself after the back and front corners of the cutter are selected. Indeed, from Fig. 6, but it is obvious that to determine the angle of sharpening of a given cutter, it is enough to subtract from 90º the sum of its rear and front angles. So, for example, if the cutter has a rear angle equal to 8º, and a front 25º, then its sharpening angle is 90º - (8º +25º) \u003d 90º -33º \u003d 57º. This rule should be remembered, as it is sometimes necessary to use it when measuring the angles of the cutter.
The value of the main angle in the plan φ follows from the comparison of Fig. 8, a and b, which schematically show the operating conditions of the cutters at the same feeds s and depth of cut t, but at different values ​​of the main angle in the plan.

Rice. 8. Influence of the main angle in the plan on the cutting process.

With an entering angle of 60º, the force P that occurs during cutting causes less deflection of the workpiece than the same force Q at an entering angle of 30º. Therefore, a cutter with an angle φ=60º is more suitable for processing non-rigid parts (relatively small diameter with a large length) in comparison with a cutter with an angle φ=30º. On the other hand, at an angle φ=30º, the length l² the cutting edge of the cutter, directly involved in its work, is greater than the corresponding length l¹ at φ=60º. Therefore, the cutter shown in Fig. 8, b, better absorbs the heat generated during the formation of chips and works longer from one sharpening to another.
The meaning of the slope λ is that by choosing a positive or negative value of it, we can direct the outgoing chips in one direction or another, which in some cases is very useful. If the angle of inclination of the main cutting edge of the cutter is positive, then the curling chip moves to the right (Fig. 9, a); at an angle of inclination equal to zero, the chips move away in the direction perpendicular to the main cutting edge (Fig. 9, b); at a negative angle of inclination, the chips move to the left (Fig. 9, c).

Rice. Fig. 9. The direction of chip flow at positive (a), equal to zero (b) and negative (c) angle of inclination of the main cutting edge.

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section three

Fundamentals of the theory of metal cutting.
Choice of cutting data

Chapter VI

Fundamentals of the theory of metal cutting

The founders of the theory of cutting metals were the outstanding Russian scientists I. A. Time (1838-1920), K. A. Zvorykin (1861-1928), Ya. G. Usachev (1873-1941) and others. The works of these scientists, which received world recognition have not yet lost their value. However, in the conditions of backward tsarist Russia, all these works did not find practical application because the industry was underdeveloped.

The science of metal cutting gained wide scope only after the Great October Socialist Revolution, especially during the Soviet five-year plans, when science was placed at the service of socialist industry.

Soviet scientists V.D. Kuznetsov, V.A. Krivoukhov, I.M. metals, distinctive feature which is the close collaboration of science with production, scientists with innovators of production.

An important role in the development of the science of cutting metals was played by the movement of innovators in production. In an effort to increase labor productivity, production leaders began to look for new ways to improve cutting conditions: they created a new cutting tool geometry, changed cutting conditions, mastered new cutting materials. Each workplace Turner-innovator has become like a small laboratory for the study of the cutting process.

A broad exchange of experience, possible only under the conditions of a socialist economy, and close cooperation between leading workers in production and science ensured the rapid development of the science of cutting metals.

1. Work of the cutter

Wedge and his work. Working part any cutting tool is wedge(Fig. 44). Under the action of the applied force, the tip of the wedge cuts into the metal.

The sharper the wedge, that is, the smaller the angle formed by its sides, the less force is required to cut it into the metal. The angle formed by the sides of the wedge is called taper angle and is denoted by the Greek letter β ( beta). Therefore, the smaller the taper angle β, the easier the wedge penetrates into the metal, and, conversely, the larger the taper angle β, the greater the force that must be applied to cut the metal. When assigning the taper angle, it is necessary to take into account the mechanical properties of the metal being processed. If you cut hard metal with a cutter having a small sharpening angle β, then the thin blade will not withstand and will crumble or break. Therefore, depending on the hardness of the metal being processed, an appropriate wedge sharpening angle is assigned.

The layer of metal being processed, located directly in front of the cutter, is continuously compressed by its front surface. When the force of the cutter exceeds the forces of adhesion of metal particles, the compressed element is sheared and shifted by the front surface of the wedge upwards. The cutter, moving forward under the action of the applied force, will continue to compress, chip and shift the individual elements from which the chips are formed.

Basic movements in turning. When machining on lathes, the workpiece rotates, and the cutter receives movement in the longitudinal or transverse direction. The rotation of the workpiece is called main movement, and the movement of the cutter relative to the part - feed motion(Fig. 45).

2. The main parts and elements of the turning tool

The cutter consists of two main parts: the head and the body (rod) (Fig. 46). Head is the working (cutting) part of the cutter; body serves to secure the cutter in the tool holder.

The head consists of the following elements: front surface, along which the chips come off, and rear surfaces facing the workpiece. One of the rear surfaces facing the cutting surface is called main; the other, facing the treated surface, - auxiliary.

Cutting edges are obtained from the intersection of the front and back surfaces. Distinguish home and auxiliary cutting edge. Most of the cutting work is done by the main cutting edge.

The intersection of the main and secondary cutting edges is called incisor tip.

3. Surface treatment

Three types of surface are distinguished on the workpiece (Fig. 47): machined, machined and cutting surface.

processed surface is the surface of the workpiece from which chips are removed.

Surface treated called the surface of the part obtained after chip removal.

cutting surface called the surface formed on the workpiece by the main cutting edge of the cutter.

It is also necessary to distinguish between the cutting plane and the base plane.

cutting plane called the plane tangent to the cutting surface and passing through the cutting edge of the cutter.

Main plane called a plane parallel to the longitudinal and transverse feeds of the cutter. For lathes, it coincides with the horizontal support surface of the tool holder.

4. Cutter angles and their purpose

The angles of the working part of the cutter greatly affect the flow of the cutting process.

By choosing the right angles of the cutter, you can significantly increase the duration of its continuous operation until blunting (durability) and process per unit of time (per minute or hour) large quantity details.

The cutting force acting on the cutter, the required power, the quality of the machined surface, etc. also depend on the choice of the angles of the cutter. That is why every turner must study well the purpose of each of the sharpening angles of the cutter and be able to correctly select their most advantageous value.

The angles of the cutter (Fig. 48) can be divided into the main angles, the angles of the cutter in the plan and the angle of inclination of the main cutting edge.

The main angles include: back angle, front angle and taper angle; the angles of the cutter in the plan include the main and auxiliary.

The main angles of the cutter should be measured in the main cutting plane, which is perpendicular to the cutting plane and the main plane.

The working part of the cutter is a wedge (shaded in Fig. 48), the shape of which is characterized by the angle between the front and main rear surfaces of the cutter. This corner is called taper angle and is denoted by the Greek letter β (beta).

back angle α ( alpha) is the angle between the main flank and the cutting plane.

Clearance angle α serves to reduce friction between the back surface of the cutter and the workpiece. By reducing friction, we thereby reduce the heating of the cutter, which, due to this, wears out less. However, if the relief angle is greatly increased, the incisor is weakened and quickly destroyed.

front angle γ ( gamma) is the angle between the front surface of the cutter and the plane perpendicular to the cutting plane, drawn through the main cutting edge.

The rake angle γ plays an important role in the chip formation process. With an increase in the rake angle, it is easier to cut the cutter into the metal, the deformation of the cut layer is reduced, the chip flow is improved, the cutting force and power consumption are reduced, and the quality of the machined surface is improved. On the other hand, an excessive increase in the rake angle leads to a weakening of the cutting edge and a decrease in its strength, to an increase in wear of the cutter due to chipping of the cutting edge, and to a deterioration in heat removal. Therefore, when processing hard and brittle metals, to increase the strength of the tool, as well as its durability, cutters with a smaller rake angle should be used; when machining soft and ductile metals, cutters with a large rake angle should be used to facilitate chip removal. In practice, the choice of the rake angle depends, in addition to the mechanical properties of the material being machined, on the material of the cutter and the shape of the rake surface. Recommended rake angles for carbide cutters are given in Table. one.

Plan angles. Leading angle φ ( fi) is called the angle between the main cutting edge and the feed direction.

The angle φ is usually chosen in the range of 30-90° depending on the type of processing, the type of cutter, the rigidity of the workpiece and the cutter and the method of their attachment. When processing the majority of metals with pass-through peeling cutters, it is possible to take the angle φ = 45°; when processing thin long parts in the centers, it is necessary to use cutters with an approach angle of 60, 75 or even 90 ° so that the parts do not bend or tremble.

Auxiliary angle in planφ 1 is the angle between the secondary cutting edge and the feed direction.

Angle λ ( lambda) inclination of the main cutting edge(Fig. 49) is the angle between the main cutting edge and the line drawn through the top of the cutter parallel to the main plane.

Table 1

Recommended rake and clearance angles for carbide tools
Note. The mechanical properties of metals are determined on special machines and instruments, and each property is given its own designation. The designation σ b given in this and subsequent tables expresses the tensile strength of the metal; the value of this limit is measured in kg/mm ​​2 . The letters HB denote the hardness of the metal, which is determined on the Brinell device by pressing a hardened steel ball into the surface of the metal. The value of hardness is measured in kg / mm 2.

Cutters whose apex is the lowest point of the cutting edge, i.e. angle λ positive(Fig. 49, c), are more durable and resistant; with such cutters it is good to process hard metals, as well as intermittent surfaces that create an impact load. When processing such surfaces with carbide cutters, the angle of inclination of the main cutting edge is adjusted to 20-30°. Cutters whose apex is the highest point of the cutting edge, i.e. angle λ negative(Fig. 49, a), it is recommended to use for processing parts made of soft metals.

5. Materials used for the manufacture of incisors

When working on the cutting edges of the cutter, high pressure occurs, as well as high temperature (600-800 ° and above). The friction of the rear surface of the cutter on the cutting surface and chips on the front surface of the cutter causes more or less rapid wear of its working surfaces. Due to wear, the shape of the cutting part changes and the cutter after some time becomes unusable for further work; such a cutter must be removed from the machine and resharpened. To increase the tool life without regrinding, it is necessary that its material resists wear at high temperatures well. In addition, the material of the cutter must be strong enough to withstand the high pressures generated during cutting without breaking. Therefore, the following basic requirements are imposed on the material of the cutters - hardness at high temperature, good wear resistance and strength.

Currently, there are many tool steels and alloys that meet these requirements. These include: carbon tool steels, high speed steels, hard alloys and ceramic materials.

Carbon tool steel. For the manufacture of cutting tools, steel with a carbon content of 0.9 to 1.4% is used. After quenching and tempering, the cutting tool made of this steel acquires high hardness. However, if during the cutting process the temperature of the cutting edge reaches 200-250 °, the hardness of the steel drops sharply.

For this reason, carbon tool steel is currently of limited use: it is used to make cutting tools that operate at a relatively low cutting speed when the temperature in the cutting zone reaches a small value. Such tools include: dies, reamers, taps, files, scrapers, etc. Cutters are not currently manufactured from carbon tool steel.

High speed steels. High speed steels contain a large number of special, so-called alloying elements - tungsten, chromium, vanadium and cobalt, which give steel high cutting properties - the ability to maintain hardness and wear resistance when heated during cutting to 600-700 °. HSS cutters allow 2-3 times higher cutting speeds than carbon cutters.

At present, the following grades of high-speed steel (GOST 9373-60) are produced in the USSR: R18, R9, R9F5, R14F14, R18F2, R9K5, R9KYU, R10K5F5 and R18K5F2.

Cutters made entirely of high-speed steel are expensive, therefore, in order to save high-speed steel, cutters with welded plates are mainly used.

Hard alloys . Carbide alloys are characterized by very high hardness and good wear resistance.

Hard alloys are made in the form of plates from tungsten and titanium powders combined with carbon. The combination of carbon and tungsten is called tungsten carbide, and with titanium, titanium carbide. Cobalt is added as a binder. This powdery mixture is pressed under high pressure to obtain small plates, which are then sintered at a temperature of about 1500°. The finished plates do not require any heat treatment. The plate is soldered with copper to the cutter holder made of carbon steel or attached to it with the help of adjustments and screws (mechanical fastening of the plates).

Main advantage hard alloys lies in the fact that they resist well to abrasion by descending chips and the workpiece and do not lose their cutting properties even when heated to 900-1000 °. Thanks to these properties, cutters equipped with carbide inserts are suitable for machining the most solid metals(hard steels, including hardened ones) and non-metallic materials (glass, porcelain, plastics) at cutting speeds that are 4-6 times or more higher than the cutting speeds allowed by high-speed cutters.

The disadvantage of hard alloys is increased brittleness.

At present, two groups of hard alloys are produced in the USSR. The main ones are - tungsten(VK2, VKZ, VK4, VK6M, VK6, VK8 and VK8M) and titanium-tungsten(T30K4, T15K6, T14K8, T5K10). Each of these groups has a specific scope (Table 2).

All tungsten alloys are intended for processing cast iron, non-ferrous metals and their alloys, hardened steels, stainless steels and non-metallic materials (ebonite, porcelain, glass, etc.). For the processing of steels, hard alloys of the titanium-tungsten group are used.

Ceramic materials. AT recent times Soviet metallurgists have created cheap materials with high cutting properties, which in many cases replace hard alloys. These are ceramic materials thermocorundum), produced in the form of plates white color, reminiscent of marble, which, like hard alloys, are either soldered to the cutter holders or mechanically attached to them. These plates do not contain such expensive and scarce elements as tungsten, titanium, etc. At the same time, ceramic plates have a higher hardness than hard alloys and retain their hardness when heated up to 1200 °, which makes it possible to cut metals with them with high speeds cutting.

The disadvantage of ceramic plates is their insufficient viscosity. Cutters equipped with ceramic inserts can be used for finishing or semi-finishing of cast iron, bronze, aluminum alloys and mild steels.

6. Sharpening and finishing of cutters

In factories, the sharpening of cutters is usually carried out centrally on sharpening machines by special workers. But the turner himself must be able to sharpen and finish the cutters.

table 2

Properties and purpose of some grades of hard alloy

Sharpening and finishing of high-speed cutters is carried out in compliance with the following rules:
1. The grinding wheel should not hit, its surface should be even; if the working surface of the circle has developed, it should be corrected.
2. During sharpening, you need to use a handpiece, and not hold the cutter on weight. The handpiece should be installed as close as possible to the grinding wheel, at the required angle and give reliable support to the cutter (Fig. 50, a-d).
3. The cutter to be sharpened must be moved along the working surface of the circle, otherwise it will wear unevenly.
4. In order not to overheat the cutter and thereby avoid the appearance of cracks in it, do not strongly press the cutter to the circle.
5. Sharpening must be carried out with continuous and abundant cooling of the cutter with water. Drip cooling, as well as periodic immersion of a highly heated cutter in water, is not allowed. If continuous cooling is not possible, it is better to switch to dry sharpening.
6. Sharpening of cutters made of high-speed steel should be done using electrocorundum wheels of medium hardness and grain size 25-16.
The order of sharpening cutters is set as follows. First, the main back surface is sharpened (Fig. 50, a). Then the auxiliary back surface (Fig. 50, b), then the front surface (Fig. 50, c) and, finally, the radius of the top (Fig. 50, d).
7. It is strictly forbidden to sharpen cutters on machines with the protective cover removed.
8. Be sure to wear safety goggles when sharpening.

After sharpening the cutter, small notches, burrs and risks remain on its cutting edges. They are eliminated by finishing on special finishing machines. Finishing is also carried out manually using a fine-grained whetstone moistened with mineral oil. First, with light movements of the touchstone, the rear surfaces are adjusted, and then the front and the radius of the top.

Sharpening and finishing of cutters equipped with carbide inserts. Sharpening of cutters with plates of hard alloys is carried out on grinding machines with circles of green silicon carbide. Sharpening is carried out both manually (Fig. 50, a-d), and with the fixing of the incisors in the tool holders. The order of sharpening these cutters is the same as for cutters made of high-speed steel, i.e., first the cutter is sharpened along the main back (Fig. 50, a), then along the auxiliary back surfaces (Fig. 50, b), then along the front surface (Fig. 50, c) and, finally, round off the top of the incisor (Fig. 50, d).

Preliminary sharpening is carried out with green silicon carbide wheels with a grit of 50-40, and final sharpening with a grit of 25-16.

The cutter should not be strongly pressed against the working surface of the circle in order to avoid overheating and cracking of the hard alloy plate. In addition, it must be constantly moved relative to the circle; this is necessary for uniform wear of the circle.

Sharpening can be carried out both dry and with abundant cooling of the cutter with water.

After sharpening a carbide cutter, it is imperative to finish its surface. Finishing is done manually or on a finishing machine. Manual finishing is carried out using a cast-iron or copper lap, the working surface of which is rubbed with a special paste or boron carbide powder mixed with machine oil or kerosene is applied to the surface in an even layer. Finishing is carried out to a width of 2-4 mm from the cutting edge.

More productive finishing on a special finishing machine using a cast-iron disk with a diameter of 250-300 mm, rotating at a speed of 1.5-2 m / s; a paste or powder of boron carbide mixed with machine oil or kerosene is applied to the surface of this disc.

7. Chip formation

Types of shavings. The detached chip under the action of the pressure of the cutter greatly changes its shape or, as they say, is deformed: it shortens in length and increases in thickness. This phenomenon was first discovered by Prof. I. A. Time and named chip shrinkage.

The appearance of the chip depends on the mechanical properties of the metal and the conditions under which cutting occurs. If viscous metals are processed (lead, tin, copper, mild steel, aluminum, etc.), then the individual elements of the chips, tightly adhering to each other, form a continuous chip that curls into a tape (Fig. 51, a). Such a strand is called drain. When processing less viscous metals, such as hard steel, chips are formed from individual elements (Fig. 51, b), weakly connected to each other. Such a strand is called chipping chips.

If the metal being machined is brittle, such as cast iron or bronze, then the individual elements of the chips break and separate from the workpiece and from each other (Fig. 51, c). Such a chip, consisting of individual irregularly shaped flakes, is called broken chips.

The considered types of chips do not remain constant, they can change with changing cutting conditions. The softer the metal being processed and the smaller the chip thickness and cutting angle, the more the chip shape approaches the drain. The same will be observed when cutting speed is increased and cooling is applied. With a decrease in cutting speed, instead of a drain chip, chipping chips are obtained.

Outgrowth. If you examine the front surface of the cutter that was used for cutting, then at the cutting edge you can sometimes find a small lump of metal welded to the cutter under high temperature and pressure. This is the so-called outgrowth(Fig. 52). It appears under certain cutting conditions of ductile metals, but is not observed when processing brittle metals. The hardness of the build-up is 2.5-3 times higher than the hardness of the metal being processed; thanks to this, the growth itself has the ability to cut the metal from which it was formed.

The positive role of the build-up is that it covers the cutting blade, protecting it from wear by descending chips and the action of heat, and this somewhat increases the durability of the cutter. The presence of a build-up is useful when peeling, since the cutting blade heats up less and its wear is reduced. However, with the formation of build-up, the accuracy and cleanliness of the machined surface deteriorate, since the build-up distorts the shape of the blade. Therefore, the formation of build-up is unfavorable for finishing work.

8. The concept of the elements of the cutting mode

In order to perform processing more efficiently in each individual case, the turner must know the basic elements of the cutting mode; these elements are depth of cut, feed and cutting speed.

Depth of cut called the distance between the machined and machined surfaces, measured perpendicular to the latter. The depth of cut is indicated by the letter t and is measured in millimeters (Fig. 53).

When turning a workpiece on a lathe, the machining allowance is cut off in one or more passes.

To determine the depth of cut t, it is necessary to measure the diameter of the workpiece before and after the cutter passes, half the difference in diameters will give the depth of cut, in other words,

where D is the diameter of the part in mm before the cutter passes; d is the diameter of the part in mm after the cutter has passed. The movement of the cutter in one revolution of the workpiece (Fig. 53) is called filing. The feed is denoted by the letter s and is measured in millimeters per revolution of the part; for brevity, it is customary to write mm / rev. Depending on the direction in which the cutter moves relative to the frame guides, there are:
a) longitudinal feed- along the bed guides;
b) cross feed- perpendicular to the bed guides;
in) oblique feed- at an angle to the guides of the bed (for example, when turning a conical surface).

Sectional area of ​​cut denoted by the letter f (eff) and is defined as the product of the depth of cut by the feed (see Fig. 53):

In addition to the depth of cut and feed, they also distinguish the width and thickness of the cut layer (Fig. 53).

Cutting layer width, or chip width, - the distance between the machined and machined surfaces, measured along the cutting surface. It is measured in millimeters and is denoted by the letter b (be).

Cut thickness, or chip thickness, is the distance between two successive positions of the cutting edge in one revolution of the part, measured perpendicular to the chip width. Chip thickness is measured in millimeters and is denoted by the letter a.

With the same feed and depth of cut, as the main angle φ decreases, the chip thickness decreases, and its width increases. This improves heat dissipation from the cutting edge and increases the tool life, which in turn allows you to significantly increase the cutting speed and process more parts per unit time. However, a decrease in the main angle in the plan φ leads to an increase in the radial (repulsive) force, which, when processing insufficiently rigid parts, can cause them to bend, loss of accuracy, and also strong vibrations. The appearance of vibrations, in turn, leads to a deterioration in the purity of the machined surface and often causes chipping of the cutting edge of the cutter.

Cutting speed. When machining on a lathe, point A, located on a circle of diameter D (Fig. 54), in one revolution of the part travels a path equal to the length of this circle.

The length of any circle is approximately 3.14 times its diameter, therefore it is equal to 3.14 D.
The number 3.14, showing how many times the length of a circle is greater than its diameter, is usually denoted by the Greek letter π (pi).

Point A in one revolution will make a path equal to πD. The diameter D of the part, as well as its circumference πD, is measured in millimeters.

Assume that the workpiece will make several revolutions per minute. Let us denote their number by the letter n revolutions per minute, or abbreviated as rpm. The path that point A will take in this case will be equal to the product of the circumference and the number of revolutions per minute, i.e. πDn millimeters per minute or abbreviated mm / min, and is called circumferential speed.

The path traveled by the point of the workpiece surface during turning relative to the cutting edge of the cutter in one minute is called cutting speed.

Since the part diameter is usually expressed in millimeters, to find the cutting speed in meters per minute, divide πDn by 1000. This can be written as the following formula:

where v is the cutting speed in m/min;
D is the diameter of the workpiece in mm;
n is the number of revolutions per minute.

Example 3 Processed roller diameter D = 100 = 150 rpm. Determine the cutting speed.
Decision: Spindle speed count. When turning a part of a known diameter, it may be necessary for a turner to adjust the machine to such a number of spindle revolutions in order to obtain the required cutting speed. For this, the following formula is used: where D is the diameter of the workpiece in mm;

Example 4 What number of revolutions per minute should a roller with a diameter of D \u003d 50 mm have at a cutting speed of v \u003d 25 m / min?
Decision:

9. Basic information about the forces acting on the cutter and cutting power

Forces acting on the cutter. When removing chips from the workpiece, the cutter must overcome the force of adhesion of metal particles to each other. When the cutting edge of the cutter cuts into the material being processed and the chip is separated, the cutter experiences pressure from the metal being separated (Fig. 55).

From top to bottom, the force P z presses on the cutter, which tends to press the cutter down and bend the part up. This force is called cutting force.

In the horizontal plane in the direction opposite to the feed movement, the cutter is pressed by the force P x, called axial force, or feed force. This force during longitudinal turning tends to press the cutter towards the tailstock.

In the horizontal plane, perpendicular to the feed direction, the cutter is pressed by the force P y, which is called the radial force. This force tends to push the cutter away from the workpiece and bend it in a horizontal direction.

All listed forces are measured in kilograms.

The largest of the three forces is the vertical cutting force: it is about 4 times the feed force and 2.5 times the radial force. The cutting force loads the parts of the headstock mechanism; it also loads the cutter, the part, often causing large stresses in them.

Experiments have established that the cutting force depends on the properties of the material being processed, the size and shape of the section of the chip being removed, the shape of the cutter, cutting speed and cooling.

To characterize the resistance of various materials to cutting, the concept of cutting coefficient has been established. The cutting factor K is the cutting pressure in kilograms per square millimeter of the cut section, measured under certain cutting conditions:

Depth of cut t......................5 mm
Feed s......................1 mm/rev
Rake angle γ......................15°
Leading angle φ.......45°
The cutting edge of the cutter - rectilinear, horizontal
The tip of the cutter is rounded with a radius r = 1 mm
Work is done without cooling

In table. 3 shows the average values ​​of the cutting factor for some metals.

Table 3

Average values ​​of the cutting factor K when turning

If the cutting factor K is known, then by multiplying it by the cross-sectional area of ​​​​the cut f in mm 2, you can find the approximate value of the cutting force using the formula

P z \u003d Kf kg. (eight)

Example 5 A shaft made of machine-made steel with σ b = 60 kg / mm 2 is turned on a lathe. Determine the cutting force if the depth of cut t = 5 mm and the feed s = 0.5 mm/rev.
Decision. According to formula (8), cutting force P z \u003d Kf kg. (eight) We determine the value of f: f \u003d ts \u003d 5x0.5 \u003d 2.5 mm 2. According to the table 3 we find the value of K for machine-made steel with σ b \u003d 60 kg / mm 2: K \u003d 160 kg / mm 2. Therefore, z = Kf = 160x2.5 = 400 kg. cutting power. Knowing the cutting force and cutting speed, you can find out how much power is required to cut chips of a given section.
Cutting power is determined by the formula (9) where N res - cutting power in hp;
P z - cutting force in kg;
v - cutting speed in m/min.

The power of the electric motor of the machine tool should be somewhat greater than the cutting power, since part of the power of the electric motor is spent on overcoming friction in the mechanisms that transmit movement from the electric motor to the machine spindle.

Example 6 Determine the cutting power for turning the shaft, considered in the previous example, if the processing is carried out at a cutting speed, υ = 60 m/min. Decision . According to formula (9), cutting power

Cutting power is usually expressed not in horsepower, but in kilowatts (kW). A kilowatt is 1.36 times horsepower, so in order to express power in kilowatts, you need to divide horsepower by 1.36:

and vice versa,

10. Heat of cut and tool life

With an increase in the cutting force, the friction force increases, as a result of which the amount of heat released during the cutting process increases. The heat of cutting increases even more as the cutting speed increases, as this speeds up the entire process of chip formation.

The generated heat of cutting with insufficient removal of it softens the cutter, as a result of which the wear of its cutting part occurs more intensively. This makes it necessary to change the cutter or sharpen it and reinstall it.

The time of continuous work of the cutter before blunting is called the tool life (measured in minutes). Frequent change of the cutter (short tool life) causes additional costs for sharpening and installing the cutter, as well as for replenishing worn cutters.

Therefore, tool life is an important factor when choosing cutting data, especially when choosing cutting speed.

The durability of the cutter depends primarily on the qualities of the material from which it is made. The most resistant will be the cutter, which is made of a material that allows the highest heating temperature without significant loss of hardness. The cutters equipped with hard alloy plates, mineral-ceramic plates have the greatest resistance; significantly less resistance - cutters made of high-speed steel, the smallest - cutters made of carbon tool steel.

The resistance of the cutter also depends on the properties of the material being processed, the cut section, the sharpening angles of the cutter, and the cutting speed. Increasing the hardness of the material being machined reduces the tool life.

By changing the sharpening angles and the shape of the front surface, it is possible to achieve a significant increase in the durability of the cutters and their productivity.

Cutting speed has a particularly strong effect on tool life. Sometimes even the slightest increase in speed leads to rapid blunting of the cutter. For example, if, when processing steel with a high-speed cutter, the cutting speed is increased by only 10%, i.e., 1.1 times, the cutter will become dull twice as fast and vice versa.

With an increase in the cross-sectional area of ​​the cut, the tool life decreases, but not as much as with the same increase in cutting speed.

The tool life also depends on the size of the tool, the shape of the cut section and cooling. The more massive the cutter, the better it removes heat from the cutting edge and, consequently, the greater its durability.

Experiments show that with the same section of cut, a large depth of cut and a smaller feed provide greater tool life than a smaller depth of cut with a correspondingly larger feed. This is explained by the fact that with a greater depth of cut, the chips come into contact with a greater length of the cutting edge, so cutting heat is better removed. That is why, with the same cut section, it is more profitable to work with a greater depth than with a greater feed.

The durability of the cutter increases significantly when it is cooled.

Coolant must be supplied plentifully (emulsion 10-12 l/min, oil and sulfofresol 3-4 l/min); a small amount of liquid not only does not benefit, but even spoils the cutter, causing small cracks to appear on its surface, leading to chipping.

11. Choice of cutting speed

The productivity of labor depends on the choice of cutting speed: the higher the cutting speed, the less time spent on processing. However, as the cutting speed increases, the tool life decreases, so the choice of cutting speed is influenced by the tool life and all the factors that affect the tool life. Of these, the most important are the properties of the material being machined, the quality of the material of the cutter, the depth of cut, the feed, the dimensions of the cutter and sharpening angles, and cooling.

1. The longer the tool life should be, the lower the cutting speed should be selected and vice versa.

2. The harder the material being processed, the less tool life, therefore, to ensure the necessary resistance when machining hard materials, the cutting speed has to be reduced. When machining cast and forged workpieces, on the surface of which there is a hard crust, shells or scale, it is necessary to reduce the cutting speed against that which is possible when machining materials without crust.

3. The material properties of the cutter determine its durability, therefore, the choice of cutting speed also depends on these properties. Other equal conditions high speed steel cutters allow significantly higher cutting speeds than carbon steel cutters; even higher cutting speeds allow cutters equipped with hard alloys.

4. In order to increase the resistance of the cutter when processing viscous metals, it is advantageous to use cooling of the cutters. In this case, with the same tool life, it is possible to increase the cutting speed by 15-25% compared to machining without cooling.

5. The dimensions of the cutter and the angles of its sharpening also affect the allowable cutting speed: the more massive the cutter, especially its head, the better it removes the heat generated during cutting. Incorrectly selected cutter angles that do not correspond to the material being processed increase the cutting force and contribute to faster wear of the cutter.

6. With an increase in the cut section, the tool life decreases, therefore, with a larger section, it is necessary to choose a cutting speed that is lower than with a smaller section.

Since small chips are removed during finishing, the cutting speed during finishing can be much higher than during roughing.

Since an increase in the cut section has less effect on the tool life than an increase in the cutting speed, it is advantageous to increase the cut section due to a slight decrease in the cutting speed. The processing method of the innovator turner of the Kuibyshev Machine Tool Plant V. Kolesov is based on this principle. Working at a cutting speed of 150 m/min, T. Kolesov finishes steel parts with a feed rate of up to 3 mm/rev instead of 0.3 mm/rev, and this leads to a reduction in machine time by 8-10 times.

The question arises: why do advanced turners often increase labor productivity by increasing cutting speed? Doesn't this contradict the basic laws of cutting? No, it doesn't contradict. They increase the cutting speed only in cases where the opportunities to increase the section of the cut are fully used.

When semi-finishing or finishing is performed, where the depth of cut is limited by a small allowance for machining, and the feed is limited by the requirements of high purity of machining, an increase in the cutting mode is possible by increasing the cutting speed. This is what advanced turners do, working on semi-finishing and finishing. If it is possible to work with large sections of the cut (with large allowances), then first of all, it is necessary to choose the largest possible depth of cut, then the largest possible technologically permissible feed, and, finally, the corresponding cutting speed.

In cases where the machining allowance is small and there are no special requirements for surface finish, the cutting mode should be increased by using the largest possible feed.

12. Cleanliness of the machined surface

When machining with a cutter, irregularities in the form of depressions and scallops always remain on the machined surface of the part, even with the most careful finishing. The height of the roughness depends on the processing method.

Practice has established that the cleaner the surface of the part is treated, the less it is subject to wear and corrosion, and the part is stronger.

Careful surface finishing when machining a part is always more expensive than a rough surface finish. Therefore, the cleanliness of the machined surface should be assigned depending on the operating conditions of the part.

Designation of surface cleanliness in drawings. According to GOST 2789-59, 14 classes of surface cleanliness are provided. To designate all purity classes, one sign is established - an equilateral triangle, next to which the class number is indicated (for example, 7; 8; 14). The cleanest surfaces are graded 14 and the roughest grade 1.

The surface roughness according to GOST 2789-59 is determined by one of two parameters: a) the arithmetic mean deviation of the profile R a and b) the height of the irregularities R z .

To measure the roughness and assign the treated surface to a particular class, special measuring instruments are used, based on the method of feeling the surface profile with a thin diamond needle. Such devices are called profilometers and profilographs.

To determine the roughness and classify the treated surface to one or another class of cleanliness in workshop conditions, tested samples of various classes of cleanliness are used - the so-called purity standards, with which the machined surface of the part is compared.

Factors Affecting Surface Finish. Practice has established that the cleanliness of the machined surface depends on a number of reasons: the material being machined, the material of the cutter, the sharpening angles and the condition of the cutting edges of the cutter, the feed and cutting speed, the lubricating and cooling properties of the liquid, the rigidity of the system machine - cutter - part, etc.

Particularly important for obtaining a surface High Quality when turning, it has a cutting speed, feed, lead angles and a radius of curvature of the tool tip. The smaller the feed and entering angle and the larger the corner radius, the cleaner the machined surface. Cutting speed greatly affects surface finish. When turning steel at a cutting speed of more than 100 m/min, the machined surface is cleaner than at a speed of 25-30 m/min.

To obtain a cleaner machined surface, attention should be paid to careful sharpening and finishing of the cutting edges.

test questions 1. What shape is the chip formed when machining viscous metals? When processing brittle metals?
2. Name the main elements of the incisor head.
3. Show the front and back surfaces on the incisor; front and rear corners; sharpening angle.
4. What is the purpose of the front and rear corners of the incisor?
5. Show lead angles and lead angle.
6. What materials are cutters made of?
7. What grades of hard alloys are used in steel processing? When processing cast iron?
8. List the cutting mode elements.
9. What forces act on the cutter?
10. What factors and how do they affect the magnitude of the cutting force?
11. What determines the durability of the cutter?
12. What factors influence the choice of cutting speed?

According to the type of processing, turning tools are divided into through, scoring, boring, cutting, slotting, grooving, fillet, threaded and shaped (Fig. 11.10).

Rice. 11.10.

a- boring a blind hole with a boring cutter; b- grooving and cutting off with a cut-off scoring tool; in– longitudinal turning with a through cutter; G– turning grooves with a grooving cutter; d– cutting of conical grooves; in– fine turning with a rounded cutter; well– fine longitudinal turning with a wide cutter; h– longitudinal turning with a bent cutter; and - threading with a threaded cutter; to– longitudinal turning with a thrust cutter; l– shaped turning with a prismatic shaped cutter

A boring cutter is used for boring pre-drilled axial holes, both through and blind (Fig. 11.10, a).

Trimming (Fig. 11.10, b) of the end surfaces of cylindrical and the processing of planes of body parts is performed with a transverse feed of the caliper with scoring cutters.

Cutting parts and cutting grooves (Fig. 11.10, b, d) also carried out with a transverse feed of the caliper. However, in this case, cutting and grooving cutters are used, respectively.

The outer cylindrical surfaces are turned with straight or persistent through cutters (Fig. 11.10, c, f, g, h). The blanks of smooth shafts are turned by setting them in the centers, stepped shafts - according to the schemes for dividing the allowance or length of the blank into parts. Cylindrical surfaces are obtained by turning with a longitudinal feed of the caliper.

External and internal threads are cut with threaded cutters (Fig. 11.10, i), which allow you to get all types of threads: metric, inch, modular and pitch with any profile - triangular, rectangular, trapezoidal, semicircular, etc. The productivity of the process is low.

Longitudinal turning to a ledge is carried out with a thrust cutter (Fig. 11.10, to).

Various types of shaped surfaces of revolution are formed mainly by the same methods as when turning. Prismatic and disk shaped cutters are used (Fig. 11.10, l) or mechanical, electrical or hydraulic copiers.

For turning rounded grooves and transitional surfaces, fillet cutters are used.

Cutting conditions

The main technological parameters for controlling the cutting process are: cutting speed V, tool feed S, processing depth t, tool material and parameters of its geometry, composition, methods and intensity of supply of the lubricating-cooling medium.

Approximately with rough turning, the depth of processing can reach 12 mm, with finishing - no more than a few tenths of a millimeter. The feed, depending on the depth of cut and the material, is -0.3-2.0 mm / rev, the cutting speed is 1.5-7.5 m / s. For machine tools without CNC, cutting conditions, depending on specific conditions, are selected from the tables of general machine building standards. Modern machines with CNC control systems have in memory extensive databases for materials, standard designs, tools, etc. This allows the operator to automatically receive information about the processing route, types of tools when entering the initial and final profiles of the workpiece, dimensions and accuracy of the part, material properties, etc. and start making it.

hard turning called turning of workpieces with a hardness above 47 HRC and special cutting conditions. This is a new, growing type of machining of revolutions, which is often a more cost-effective alternative to grinding. Modern tool materials, technologies and designs of machine tools make it possible to introduce this process into production more and more widely.

There are rough, precise and extra-precise hard turning. Roughing is implemented at processing depths of 0.5–3 mm, cutting speeds of 50–150 m/min and feeds of 0.1–0.3 mm/rev and requires maximum rigidity and drive power from the machine. With precision hard turning, the depth of cut does not exceed 0.1–0.5 mm at a cutting speed of 100–200 m/min and a feed rate of 0.05–0.15 mm/rev. Machining accuracy corresponds to the 5th or 6th grade with surface roughness after machining R z 2.4–4 µm. Particularly precise hard turning ensures machining accuracy within the 3rd or 4th grade with roughness up to R z 1 µm. The depth of cut is in the range of 0.02–0.3 mm at a cutting speed of 150–220 m/min and a feed rate of 0.01–1 mm/rev.

Functionally, the principle of hard turning is to heat the workpiece material 1 in the zone of contact with the cutting edge 4 up to the glow temperature (Fig. 11.11,11.12). No cutting fluids are used in the process. Specially selected tool geometry and processing modes heat up the material, which results in the cut zone 2 tempered to a hardness of about 25 HRC. After chip separation 3 material is rapidly cooled.

Rice. 11.11.

1 – workpiece (62 HRC); 2 – cut zone (HRC 25); 3 - chips (HRC 45); 4 - cutting edge

As a result, the hardness of the part is reduced by no more than 2 units, and the resulting chip has a hardness of about 45 units. The part in its bulk practically does not heat up. An example of hard turning is shown in fig. 11.12.

Rice. 11.12.

For the implementation of hard turning, it is necessary to use machine tools with high accuracy, static and dynamic rigidity, temperature stability and providing free chip flow.

The tool material of the working part of cutters for hard turning is cutting ceramics and cubic boron nitride.