How To Select PCBN and PCD Cutting Tools?

At present, PCBN tools are mostly used for cutting hardened steel, chilled cast iron and surface thermal spraying materials.

PCD has high hardness, excellent wear resistance and thermal conductivity, and low friction coefficient.

It is suitable for cutting non-ferrous metals and their alloys (Cu, Al, Mg), non-metallic materials and composite materials.

(1) Selection of PCBN cutters

PCBN tools often fail to show good cutting performance when processing materials with medium hardness.

For example, when processing workpieces with medium and low hardness, the service life of PCBN tools is not as long as that of cemented carbide tools.

When processing workpieces with hardness above 50HRC, the tools begin to exert their superhard performance.

For example, when processing workpieces with hardness of about 60HRC, the service life of PCBN tools is more than 10 times longer than that of cemented carbide tools.

Therefore, only when PCBN tool is used to process high hardness materials can it give full play to its superior performance.

The effect of CBN grain size on the cutting performance of PCBN tool material.

The wear resistance of PCBN tool is related to the size of CBN grain in the sintered body.

The size of CBN grain affects the strength of the tool.

Fine grain can increase the grain boundary area of the grain, improve the sintering strength and the ability to resist crack propagation, so as to increase the wear resistance of PCBN tool.

When the particle size increases by one time, the tool life will be reduced by 30% ~ 50%.

Fig. 2-28 shows the tool wear curves of three grain sizes when cutting hardened steel.

It can be seen from fig. 2-39 that the smaller the grain, the stronger the wear resistance.

This is because the smaller the grain, the higher the strength of the composite sheet and the longer the tool life.

Table 2-40 shows the selection of PCBN tools with different CBN particle sizes, and table 2-41 shows the selection of PCBN tools with different CBN contents.

effect of grain size on wear

(Cutting parameters:vc=100m/min;f=1mm/r;ap=0.2mm)

Fig. 2-39 effect of grain size on wear

Table 2-40 selection of PCBN tools with different CBN sizes

CBN particle size /umPurpose
4~5Machining of superhard and difficult cutting materials / finishing of cast iron to semi finishing general rough machining of steel and cast iron / intermittent cutting of cast iron
3~4Continuous cutting of high toughness cast iron, high hardness materials, iron series sintered metals, etc
2~3High speed cutting of general cast iron, cutting of difficult to machine materials such as Inconel superalloy and high hardness materials such as rolls

Table 2-41 selection of PCBN tools with different CBN contents

CBN contentPurpose
CBN (wt.50%)Continuous cutting hardened steel (45~65hrc)
CBN (wt.65%)Semi intermittent cutting hardened steel
CBN(wt.80%)Machining Ni Cr cast iron, heavy intermittent cutting hardened steel
CBN(wt.90%)High speed cutting of cast iron, intermittent cutting of hardened steel, machining of cemented carbide, sintered metal, heavy alloy
CBN(wt.80%~90%)High speed cutting cast iron (vc=500~1300m/min), rough machining and semi finishing hardened steel

1. Determination of PCBN geometric parameters

Whether the geometric parameters of the cutting part are properly selected or not has a great impact on the service life of PCBN tools, sometimes the difference can be several times to more than ten times.

Because the toughness of PCBN composite is higher than that of ceramic but lower than that of cemented carbide, the strength of cutting edge and tip should be considered when designing and selecting the geometric parameters of PCBN tool.

(1) Front corner:

Generally, the recommended range is 0 ° ~ -10 °. Generally, 0 ° front angle is mostly used, and negative front angle or positive front angle is also used.

As for the size of the negative front angle, it can be determined according to the specific conditions of the machined parts.

(2) Rear corner:

Generally, it is selected within the range of 6 ° ~ 10 ° to ensure that the wedge angle of the cutting part of the tool is large enough.

(3) Blade inclination:

If there are no special requirements, it is usually taken as 0 °, sometimes it can also be taken as a negative value to ensure that the cutting edge has a better stress state when cutting.

(4) Tip angle:

Except for special requirements, it shall not be less than 90 ° generally, and the tool tip shall be ground into a circular arc with rε=0.4 ~ 1.2mm to ensure that the tool tip has sufficient strength.

(5) Cutting edge:

Generally, the strength of cutting edge is increased by grinding negative chamfer.

The negative chamfer width is related to the cutting parameters and the properties of the processed material, which is generally taken as 0.2mm × (-15°~-30°)。

The recommended negative chamfer parameters are shown in table 2-42.

The selection of main deflection angle and auxiliary deflection angle is mainly based on the specific processing conditions.

Table 2-42 recommended negative chamfering parameters

Workpiece material

BZN6000/BZN7 000S

BZN8 200

Rough machining

Finish machining

Finish machining

Hardened steel

-20° × 0.20mm

 

-25° × 0.10mm

Spraying materials

-20° × 0.20mm

-20° × 0.20mm

-25° × 0.10mm

Soft cast iron

-20° × 0.20mm

-20° × 0.20mm(0.25mm)

 

Heat-resisting alloy

-20° × 0.20mm

0.025~0.05mm or no chamfer

 

(6) Main deflection angle and auxiliary deflection angle:

The selection of these two parameters is mainly based on the specific processing conditions.

2. Use of cutting fluid

Although the PCBN composite sheet can withstand the cutting temperature of 1200 ~ 1300 ℃, the welding part between the PCBN composite sheet and the tool body can only withstand the temperature of about 800 ℃ (in case of brazing).

Therefore, it is best to use cutting fluid for cooling in this regard.

However, at about 1000 ℃, CBN and water produce hydrolysis, resulting in serious wear of PCBN tools.

Therefore, water or other oxidizing cutting fluids should not be used to avoid aggravating oxidation wear.

The suitable cutting fluid is the cutting oil with extreme pressure.

3. Particularity of influence of workpiece hardness on PCBN tool life

When cutting hardened steels with different hardness, with the increase of the hardness of the processed materials, the service life of PCBN tools does not decrease monotonically, but is the lowest at 40HRC.

When the hardness of the workpiece material is higher than this hardness, the service life of PCBN tools increases instead.

When the hardness is lower than this hardness, the service life of PCBN tools is lower than that of cemented carbide tools.

This is mainly due to the small hardness loss of PCBN material at high temperature and the large hardness loss of the processed material.

This characteristic of PCBN is very suitable for machining high hardness materials.

The most representative is that the turning instead of grinding technology in hard cutting can obtain the machined surface quality that can only be obtained by grinding in the past.

At present, this cutting technology called “Red Crescent” is being studied and popularized.

The tool life of PCBN tool when cutting Cr12Mn with different hardness is shown in Fig. 2-40.

tool life of PCBN tool when cutting Cr12Mn with different hardness

Fig. 2-40 tool life of PCBN tool when cutting Cr12Mn with different hardness

4. High speed PCBN tool life

When the cutting speed exceeds a certain limit, the higher the cutting speed, the higher the cutting temperature, the lower the hardness of the machined material, and the lower the wear speed of the PCBN tool, that is, the service life of the PCBN tool at high speed is higher than that at low speed.

Therefore, PCBN tools are most suitable for high-speed machining of high hardness materials.

At present, high-speed and ultra-high-speed cutting technology has shown its advantages.

As an advanced manufacturing technology, it has been widely valued. PCBN is the most promising tool material for high-speed cutting technology.

5. Key points for reference and selection of PCBN

(1) Select appropriate cutting parameters

1) Optimization of cutting speed.

PCBN tool material has high hardness at room temperature and high temperature, and good thermal stability.

The mechanism of cutting hard materials is to soften the metal in the small area in the cutting area, so as to increase the hardness difference between the tool and the workpiece, making cutting easier.

The cutting speed is too low to generate enough cutting heat to soften the metal in the cutting area, so the cutting performance of PCBN tool can not be brought into play.

Therefore, the selection value of cutting speed is much higher than that of cemented carbide tools.

In addition, PCBN shows a special rule when it is used for cutting at higher speeds.

Many experimental studies on cutting hardened steel, high-speed steel, bearing steel and other high-hardness materials and cast iron with PCBN tools show that when the cutting speed is higher than a certain value, the relationship between the cutting speed and the tool life will no longer conform to the Taylor formula, that is, when the cutting speed is higher than a certain value, the tool life will increase.

Therefore, PCBN tools are very suitable for high-speed Ultra high speed machining.

Generally speaking, the cutting speed should be selected according to the materials to be machined.

For example, when cutting various kinds of hardened steel (50 ~ 64hrc), the turning speed can be selected within the range of 70 ~ 200m/min, while when cutting Ni based spraying (welding) parts (50 ~ 62HRC), the cutting speed can be selected within the range of 50 ~ 80m/min.

If the cutting tool is used within the optimal cutting speed range, a higher service life can be obtained.

The cutting speed of cubic boron nitride compared with that of cemented carbide tools is shown in table 2-43.

Table 2-43 cutting speed comparison between cubic boron nitride and cemented carbide tools

Workpiece material

Turning speed Vc/ (m/min)

Milling speed Vc/ (m/min)

cubic boron nitride

Cemented carbide

cubic boron nitride

Cemented carbide

Steel 150~250HBW

100-200

130-300

400~900

100~300

Steel 45~55HRC

80-160

25~45

200-500

30-70

Steel 60~70HRC

60-120

10~15

80~200

 

Grey cast iron 120~240HBW

600~1000

100~200

800~3000

70~200

High strength cast iron 160~300HBW

400~800

50~100

500~2000

50~80

Quenched cast iron 400~600HBW (40~60HRC)

50~150

10~20

200-800

10~20

2) Optimization of feed rate

Because PCBN tools generally have negative chamfering, when possible, the feed rate should be greater than the chamfering width.

In addition, PCBN tools are mainly used for finishing, and the back feed is generally small.

In order to produce metal softening effect, the feed should be relatively large.

In order to make PCBN tool produce metal softening effect during cutting, it is necessary to make the cutting area large enough.

However, the feed rate of PCBN tool should not be greater than 0.2mm/r.

3) Optimization of back cutting amount

PCBN tools are mainly used for finishing and semi finishing, so the back cutting amount is mostly less than 1mm.

When the PCBN tool is used to cut hard materials with hardness greater than 50HRC, cutting with small back feed is easy to cause rapid wear of the tool.

When the PCBN tool is used with large back feed, the wear speed of the PCBN tool is reduced because there is enough heat to soften the metal in the cutting area (the hardness is reduced).

Therefore, the softening effect of metal should be considered in the selection of the back cutting amount, which should not be too small.

Generally, the back cutting amount shall not be less than 0.3mm.

The selection of cutting parameters should pay attention to the comprehensive influence of the three.

Although the PCBN composite sheet is resistant to high temperature, the welding strength between the composite sheet and other parts often restricts the improvement of cutting efficiency.

For example, the welding between the composite sheet and the tool body is often desoldered due to high cutting temperature, resulting in falling off or cutting, damaging the quality of the machined surface.

The recommended cutting parameters are shown in table 2-44.

Table 2-44 recommended cutting parameters

Workpiece materialCutting speed Vc/ (m/min)Feed rate f /(mm/r)Back feeding amount ap/mm
Hard cast iron75~1500.13~0.650.13~2.5
Grey cast iron610~12200.13~0.650.13~2.5
Hardened steel65~1050.13~0.500.75~2.5
Spraying materials90~2150.13~0.250.13~13
powdered metal90~1800.10-0.250.10~1.3
Heat-resisting alloy180~2500.10~0.300.10~25

(2) Requirements for processing equipment.

Because the fragility of PCBN tool is greater than that of cemented carbide tool, there must be certain requirements for the process system of the machine tool when it is used.

First, the spindle deflection of the machine tool should be small, the rigidity of the tool holder and the whole processing system should be good, and the vibration of the machine tool should be small to prevent cutting.

As PCBN cutters are mostly used for cutting hard to machine materials such as hardened steel and wear-resistant cast iron, the cutters have negative chamfering.

Therefore, the radial force is large, which requires good rigidity and precision of the machine tool and small system vibration.

During the installation of PCBN tools, the tool tip should be aligned, the cantilever should be small, and the extension length of the tool head should be short, which can generally be less than 25 ~ 30mm.

During the tool alignment, it should be carefully operated to avoid collision;

It is not suitable for rough surface processing, and the workpiece flash is best chamfered;

The workpiece shall be clamped well, and the factors causing vibration during cutting shall be eliminated as far as possible, such as adding a center frame when machining a slender shaft.

The recommended PCBN brands are cb7105, cb7025, cb7035 and cb7525, as shown in figure 2-41.

Heavy duty intermittent cutting PCBN tool cb7525

Fig. 2-41 Heavy duty intermittent cutting PCBN tool cb7525

CB7015 is used for continuous to light intermittent cutting;

CB7025 is used for light to medium intermittent cutting;

CB7035 is used for medium to heavy load intermittent cutting;

CB7525 is used for heavy-duty intermittent cutting.

(2) Selection of PCD tools

The selection of PCD tools is shown in table 2-45.

The thermal stability of diamond is relatively poor.

When the cutting temperature reaches 800 ℃, its hardness will be lost.

Diamond cutting tools are not suitable for processing iron and steel materials, because diamond has a strong chemical affinity with iron.

At high temperature, iron atoms are easy to interact with carbon atoms to convert them into graphite structure, and the cutting tools are very easy to be damaged.

PCD tools are limited to processing non-ferrous materials, such as high silicon aluminum, metal matrix composites (MMC) and carbon fiber reinforced plastics (CFRP).

PCD with a large amount of cutting fluid can also be used for ultra precision machining of titanium materials.

Using single crystal diamond tool, mirror machining can be realized on ultra precision lathe.

Single crystal diamond tools are the most important tools in the field of ultra precision machining at present.

Their cutting edges can be ground very sharp.

The blunt circle radius of the cutting edge can reach 20~30nm.

The surface roughness of the machined workpiece is small, and the service life of the tools is very long.

They can be used for several hours at a time.

The suitable processing materials for diamond tools are shown in table 2-46.

At present, single crystal diamond tools are widely used to process computer disk substrates, video recorder drums, laser mirrors, various astronomical telescopes, microscopes and optical instruments.

Table 2-45 PCD Tool Selection

PCD granularity/ μ mPurpose
>20Cutting of cemented carbide and high silicon aluminum alloy
5-15Cutting of aluminum alloy, copper alloy, nonferrous metal and nonmetal (plastic)
1~3Precision machining of electronic components, carbon, gftr and other non-ferrous and non-metallic materials
0.5-~3Finishing of aluminum alloy

Table 2-46 suitable processing materials for diamond tools

Processed material

Machining object

Non ferrous metal

Aluminium and its alloys

Aircraft, automobile and motorcycle: piston, cylinder, compressor parts, transmission case, various shell parts and other precision machinery; camera, copier, measuring instrument, gas appliance and other general machinery: various pump bodies, oil presses, mechanical parts, etc

Copper and its alloys

Internal combustion engines, ships: various shafts, bearing shells, pump bodies, gears, rotor blades electronic instruments: various instruments, motors, commutators and other general machinery: various shafts, bearing shells, valve bodies, shells, etc

other

Magnesium, titanium, zinc, pliers and other non-ferrous metals

The recommended PCD brand is shown in table 2-47.

Table 2-47 recommended PCD grades

BrandPerformancePicture
CD10(N05)CD10 is a polycrystalline diamond brand, which consists of a fine to medium fine grain matrix and a coating with an average thickness of 7μm. Recommended for finishing and semi finishing of non-ferrous and non-metallic materials. 
CD1810(N10)CD1810 is a CVD diamond coating brand based on a particularly suitable substrate. High purity diamond coating with 6~8μm extremely wear-resistant surface provides good characteristics and is used for processing non-ferrous alloys.
Scroll to Top