How To Choose The Cutting Amount Reasonably?

Selecting a reasonable cutting amount is a very important part of the cutting process.

Cutting amount are not only important parameters that must be determined before machine tool adjustment, but also have a very important impact on machining quality, machining efficiency and production cost.

When selecting reasonable cutting amount, the machining properties must be considered.

Reasonable cutting amount refer to the cutting amount that give full play to the cutting performance of tools and machine tools on the premise of ensuring machining quality, so as to obtain high productivity and low machining cost.

The common principle is the higher productivity that can be obtained at the lowest cost.

(1) Basic principles for selecting cutting amount in rough machining

High production efficiency is the basic goal of rough machining.

This goal is usually expressed by the minimum number of man hours per piece or the maximum volume of metal removed per unit time.

Take the outer circle longitudinal car as an example.

Working hours of single piece of motor vehicle:

Where

  • L- feed length, unit: mm;
  • dw — workpiece blank diameter, unit: mm;
  • nw — workpiece speed, unit: R / min;
  • h — machining allowance, unit: mm.

Let A0 = πdwLh / 1000, then

Increasing vc, f and ap can improve productivity.

(2) Basic principles for selecting cutting amount in finish machining

The choice of cutting amount in finish machining should first ensure the machining accuracy and surface quality, while taking into account the necessary service life and production efficiency.

Smaller ap and f should be used in finish machining to reduce cutting force and elastic deformation of process system and improve the quality of machined surface.

ap is usually determined according to the machining allowance, and the improvement of f is mainly restricted by the surface roughness.

After the determination of ap and f, the reasonable cutting speed vc is determined on the premise of ensuring the reasonable tool service life.

(3) Ways of selecting cutting amount

There are many factors affecting the cutting amount, including workpiece material, tool material, tool geometric parameters, process category, machining process and specific service conditions, such as machine tool, clamping, cutting fluid and many other factors, coupled with the uncertainty and difficult to quantify characteristics of these factors, there is no method to accurately calculate the cutting amount.

Generally, there are three ways to obtain cutting amount:

1. Check the manual

Generally, cutting speed, feed rate and tool life can be found.

The data in the manual is to establish the mathematical model for calculating the cutting amount through the cutting test under specific conditions.

Due to the difference between the specific use conditions and the test conditions, it is necessary to make some correction when using, and make appropriate adjustment in the future use.

2. Check the sample of the tool manufacturer or the CD provided by them

While introducing the products, these samples or CDs also recommend the corresponding cutting speed and feed rate.

These data not only have reference value for the company’s cutting tools, but also can be used as a reference for the same type of cutting tools to process the same kind of workpiece materials. They should also be adjusted in the application to make them more in line with the use conditions.

3. Cutting test

The test conditions are in line with the actual service conditions as much as possible, and the results obtained are the most reliable.

In order to make the cutting test results comparable, the international organization for standardization has made strict provisions on the test conditions, procedures and data processing of turning tools, end mills and drill bits.

(4) Selection method of reasonable cutting amount

1. Selection of back cutting amount ap

Among the three elements of cutting amount, the back cutting amount is usually selected first, which is generally determined according to the machining nature and machining allowance.

We should also consider the specific process factors such as workpiece materials, tool materials, machine tools and machining processes, and make a reasonable allocation of the allowance.

From the productivity calculation formula of turning, it can be seen that the productivity

A0 is constant.

The increase of ap is directly proportional to the increase of productivity, because ap has the least impact on tool life.

When the tool life is fixed, increasing ap ratio and increasing f is beneficial to improve productivity.

Therefore, during rough machining, the ap value should be as large as possible to reduce the times of feeding and improve the production efficiency.

However, increasing ap will increase the cutting force, especially when the allowance is large.

The increase of ap should be limited by the machine tool power and system rigidity.

During semi finishing or finishing, the machining allowance shall be taken as a smaller value to reduce the cutting force and deformation, increase the tool life and ensure the machining quality. However, for materials with severe work hardening, the minimum back cutting amount shall exceed the depth of the hardened layer produced in the previous process.

ap is generally determined according to machining properties and machining allowance.

Cutting is generally divided into rough machining (surface roughness Ra = 50 ~ 12.5 μ m) , semi finishing (Ra= 6.3 ~ 3.2 μ m) and finishing (Ra = 1.6 ~ 0.8μ m).

For example, when turning the outer circle with cemented carbide tools on medium power machine tools, ap= 2 ~ 6mm for coarse turning, ap = 0.3 ~ 2mm for semi fine turning and ap = 0.1 ~ 0.3mm for fine turning.

In the following cases, the rough turning shall be fed several times:

(1) The machining allowance is too large, and the cutting force of one feed is too large, so that the power of the machine tool is insufficient or the strength of the tool is insufficient.

(2) When the rigidity of the process system is low, or the machining allowance is very uneven, causing great vibration, such as machining slender shafts and thin-walled workpieces.

(3) When cutting intermittently, the tool will be greatly impacted, resulting in cutting.

Even in the above cases, the ap of the first feed or previous feeds should be as large as possible. In case of two feeds, the ap of the first feed is generally 2 / 3 ~ 3 / 4 of the machining allowance.

Should try to make the back of the knife more than the thickness of the hard skin or cold hard layer, in order to prevent premature wear of the knife tip.

2. Selection of feed rate f

The influence of feed rate on tool life is larger than back feed rate, but smaller than cutting speed.

The feed rate is mainly selected according to the strength, hardness, tool material category, processing procedure and other factors of workpiece materials, and according to experience or samples, manuals and other data.

(1) During rough machining, the requirements for the machined surface quality are not high.

At this time, the cutting force is large, and the selection of feed rate f is mainly limited by the cutting force.

When the rigidity of cutter bar, workpiece and the strength of blade and machine tool feed mechanism allow, select a larger feed rate.

(2) During semi finishing and finishing, due to the small back draft ap, the cutting force is small, and the selection of feed rate is mainly limited by the machining surface quality.

When the cutter has reasonable transition edge and polishing edge and adopts high cutting speed, the feed rate f can be appropriately larger to improve production efficiency.

In production, the feed rate is often selected by looking up the table according to experience.

During rough machining, the feed rate can be selected according to the workpiece material, turning tool bar size, workpiece diameter and the determined back feed ap.

The feed rate of rough turning outer circle of cemented carbide tool is given in table 1-9.

Table 1-9 reference feed rate of rough turning outer circle of cemented carbide tool

Workpiece material

Section dimension of turning tool bar

Workpiece diameter D / mm

Back cutting amount a / mm

≤3

>3-5

>5-8

>8-12

>12

Feed rate f (mm / R)

Carbon structural steel, alloy structural steel and superalloy

sixteen × twenty-five

20

0.3~0.4

40

0.4-0.5

0.3~0.4

60

0.5-0.7

0.4-0.6

0.3-0.5

100

0.6~0.9

0.5~0.7

0.5~0.6

0.4~0.5

400

0.8-1.2

0.7-1.0

0.6-0.8

0.5-0.6

twenty × three thousand and twenty-five × twenty-five

20

0.3-0.4

40

0.4~0.5

0.3~0.4

60

0.6-0.7

0.5~0.7

0.4-0.6

100

0.8~1.0

0.7~0.9

0.5-0.7

0.4~0.7

400

1.2~1.4

1.0~1.2

0.8-1.0

0.6~0.9

0.4~0.6

Cast iron and copper alloy

sixteen × twenty-five

40

0.4~0.5

60

0.6~0.8

0.5~0.8

0.4-0.6

400

1.0~1.4

1.0~1.2

0.8-10

0.6~0.8

twenty × three thousand and twenty-five × twenty-five

40

0.4~0.5

60

0.6~0.9

0.5~0.8

0.4-0.7

100

0.9~1.3

0.8~1.2

0.7-1.0

0.5~0.8

400

1.2-1.8

1.2-1.6

1.0-1.3

0.9-1.1

0.7-0.9

Note:

① When machining intermittent surfaces and workpieces with impact, the feed rate in the table shall be multiplied by the coefficient K = 0.75 ~ 0.85.

② When there is no outer skin processing, the feed rate in the table shall be multiplied by the coefficient K= 1.1.

③ When processing superalloy, the feed rate shall not be greater than 1mm / r.

④ When machining quenched steel, the feed rate should be reduced.

When the hardness of steel is 44 ~ 56HRC, the multiplication coefficient is 0.8;

When the hardness of steel is 57 ~ 62HRC, the multiplication factor is 0.5.

During semi finishing and finishing, f shall be selected according to the value of machined surface roughness, workpiece material, pre estimated cutting speed vc and tool tip arc radius rε, as shown in table 1-10.

Table 1-10 reference feed rate of semi finishing outer circle of cemented carbide turning tool

Workpiece material

Surface roughness valueRa/um

Cutting speed Vc(m/ min)

Tool tip arc radius r / mm

0.5

1

2

Feed rate f / (mm / r)

Cast iron, bronze, aluminum alloy

10-15

unlimited

0.25-0.40

0.40-0.50

0.50-0.60

5-2.5

0.15-0.25

0.25-0.40

0.40-0.60

2.5-1.25

0.10-0.15

0.15-0.20

0.20-0.35

Carbon steel and alloy steel

10-5

<50

0.30-0.50

0.45-0.60

0.55-0.70

>50

0.40-0.55

0.55-0.65

0.65-0.70

5-2.5

<50

0.18-0.25

0.25-0.30

0.30-0.40

>50

0.25-0.30

0.30-0.35

0.35-0.50

2.5-1.25

<50

0.10

0.11-0.15

0.15-0.22

50-100

0.11-0.16

0.16-0.25

0.25-0.35

>100

0.16-0.20

0.20-0.25

0.25-0.35

According to the rough turning feed rate f determined by experience, in some special cases, such as large cutting force, large length diameter ratio of workpiece and large extension length of cutter bar (inner hole), it is necessary to verify the strength and stiffness of cutter bar, blade strength, feed mechanism strength of machine tool and workpiece stiffness.

Finally, the feed rate should be determined according to the machine tool manual.

3. Selection of cutting speed VC

The cutting speed VC can be calculated when the tool service life T, back draft ap and feed f are determined.

Where Kvc- correction coefficient of cutting speed, which is related to tool material, geometric parameters, workpiece material, etc.

The values of Cv, xv, yv, m and Kvc can be found in table 1-11.

Table 1-11 coefficients and indexes in cutting speed formula during turning

Material Science

Processing form

Tool material

Feed rate /(mm/r)

Coefficient and index

C

X

Y

m

Carbon structural steel

Rm=0.637GPa

Outer Park longitudinal car

P10 (no cutting liquid)

≤o.3

291

0.15

0.2

0.2

≤0.7

242

0.35

>0.7

235

0.45

W18Cr4V (with cutting fluid)

≤0.25

67.2

0.25

0.33

0.125

>0.25

43

0.66

Cutting and grooving

P10 (without cutting fluid)

38

0.8

0.2

W18Cr4V (with cutting fluid)

21

0.66

0.25

Stainless steel 1Cr18Ni9Ti

Outer Park longitudinal car

K30 (without cutting fluid)

110

0.2

0.45

0.15

W18Cr4V (with cutting fluid)

31

0.55

Grey cast iron

190HBW

Cylindrical longitudinal car

K20 (without cutting fluid)

≤0.4

189.8

0.15

0.2

0.2

>0.4

158

0.4

W18Cr4V (no cutting liquid)

≤0.25

24

0.15

0.3

0.1

>0.25

22.7

0.4

Cutting and grooving

K20 (without cutting fluid)

68.5

0.4

0.2

W18Cr4V (without cutting fluid)

18

0.15

malleable iron

150HBW

Cylindrical longitudinal car

K20 (without cutting fluid)

≤0.4

317

0.15

0.2

0.2

>0.4

215

0.45

W18Cr4V (with cutting fluid)

≤0.25

68.9

0.2

0.25

0.125

>0.25

48.8

0.5

Cutting and grooving

K30 (without cutting fluid)

86

0.4

0.2

W18Cr4V (with cutting fluid)

37.6

0.5

0.25

Copper alloy (medium hardness heterogeneous)

100-140HBW

Cylindrical longitudinal car

W18Cr4V (without cutting fluid)

≤0,20

216

0.12

0.25

0.23

>0.20

145.6

0.5

Aluminum silicon alloy and cast aluminum alloy

R=0.098-0.196GPa, ≤65HBW

Duralumin r = 0.294 ~ 0.392gpa, ≤ 100HBW

Cylindrical longitudinal car

W18Cr4V (without cutting fluid)

≤0,20

485

0.12

0.25

0.28

>0.20

328

0.5

Note: the following processing shall multiply the calculated vc by the dressing factor:

① Boring: Kvc = 0.9.

② When machining stainless steel and cast steel with high-speed steel turning tool without cutting fluid, Kvc = 0.8.

After the cutting speed is determined, the machine speed n (r / min) can be calculated.

The selected speed shall be determined according to the machine tool manual (take the similar lower speed n), and finally calculate the actual cutting speed according to the selected speed.

The general principles for selecting cutting speed in production are:

(1) In rough turning, ap and f are larger, so choose lower vc;

On the contrary, select higher vc when finishing.

(2) When the strength and hardness of workpiece material are high, lower vc should be selected.

(3) The cutting speed of cutting alloy steel is 20% ~ 30% lower than that of cutting medium carbon steel;

The cutting speed of steel in the state of cutting quenching and tempering is 20% ~ 30% lower than that in the state of cutting normalizing and annealing;

The cutting speed of non-ferrous metals can be increased by 100% ~ 300% compared with that of medium carbon steel, as shown in table 1-11.

(4) The better the cutting performance of tool materials, the higher the cutting speed.

For example, the cutting speed of cemented carbide is several times higher than that of high-speed steel tools, the cutting speed of coated tools is higher than that of uncoated tools, and higher cutting speeds can be adopted for ceramic, diamond and CBN tools.

When determining the cutting speed, the following points should also be considered:

(1) During finish machining, the area where chips and scale thorns are generated shall be avoided as far as possible.

(2) During intermittent cutting, in order to reduce impact and thermal stress, the cutting speed should be appropriately reduced.

(3) When vibration is easy to occur, the cutting speed should avoid the critical speed of self-excited vibration.

(4) When machining large workpieces, slender workpieces, thin-walled workpieces or workpieces with skin, the cutting speed shall be appropriately reduced.

Verify the machine power, cutting power:

PE can be queried from the machine tool manual.

If it cannot meet the above formula, it is necessary to analyze and appropriately reduce the selected cutting amount.

The three factors of cutting amount have different influence on tool life, and the order of influence from large to small is vc, f and ap.

In order to maintain the determined tool life, if one element is increased, the other two elements must be reduced accordingly (see table 1-12 and table 1-13).

It can be seen that the influence of the three factors of cutting amount on productivity is different.

Table 1-12 recommended cutting amount of cemented carbide tools

Workpiece material

Rough machining

Finish machining

Cutting speed / (M / min)

Feed rate / (mm / R)

Back cutting amount / mm

Cutting speed / (M / min)

Feed rate / (mm / R)

Back cutting amount / mm

carbon steel

220

0.2

3

260

0.1

0.4

low alloy steel

180

0.2

3

220

0.1

0.4

High alloy steel

120

0.2

3

160

0.1

0.4

cast iron

80

0.2

3

140

0.1

0.4

stainless steel

80

0.2

2

120

0.1

0.4

titanium alloy

40

0.3

1.5

60

0.1

0.4

Grey cast iron

120

0.3

2

150

0.15

0.5

Ductile iron

100

0.2

2

120

0.15

0.5

aluminium alloy

1600

0.2

1.5

1600

0.1

0.5

Table 1-13 recommended common cutting amount

Workpiece material

Processing content

Back cutting amount / mm

Cutting speed / (M / min)

Feed rate / (mm / R)

Tool material

Carbon steel r > 600MPa

Rough machining

5~7

60~80

0.2~0.4

Class P

Rough machining

2~3

80~120

0.2~0.4

finish machining

2~6

120~150

0.1~0.2

Drilling center hole

500~800r/min

W18Cr4V

drill hole

25~30

0.1~0.2

Cut off (width < 5mm)

70~110

0.1~0.2

Class P

Cast iron < 200hbw

Rough machining

50-70

0.2~0.4

Class k

finish machining

70~100

0.1~0.2

Cut off (width < 5mm)

50~70

0.1~0.2

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