Introduction

THIS ARTICLE covers precision and ultraprecision finish machining techniques that make use of defined cutting edges, such as polycrystalline diamond (PCD) and cubic boron nitride (CBN) compacts. Grinding techniques are discussed in the article "Finishing Methods Using Multipoint or Random Cutting Edges" in this Volume. The turning, broaching, milling, drilling, and reaming processes discussed in this article are also covered extensively in Machining, Volume 16 of the ASM Handbook (Ref 1). The same volume provides in-depth information on machining fundamentals and cutting tool materials.

Reference

1. Machining, Vol 16, 9th ed., Metals Handbook, ASM International, 1989 Finish Turning

Much of the recent progress in turning has been made in precision and ultraprecision machining, and it is generally true that today's precision engineering will be tomorrow's general engineering. At present, the highest spindle speeds in conventional lathes are 2200 to 2400 rpm. For precision finishing cuts on steel and materials with similar properties, the highest spindle speeds should range from 3000 to 5000 rpm to give cutting speeds of 120 to 915 m/min (400 to 3000 ft/min). Fine stepless feeds are desirable to provide gradual entry of the tool into the workpiece, especially at high speeds. Inserts with chip control are essential, and it is not unusual for lathes to have slant beds and screw conveyors for continuous chip disposal. Machine tools must have high rigidity for finish turning that is carried out at high speeds (Ref 2).

The cutting tool materials widely used for finishing cuts are:

• Chemical vapor deposition ceramic (aluminum oxide) coated carbides

• Chemical vapor deposition triphase (TiC/TiCN/TiN) coated carbides

• Physical vapor deposition TiN (titanium nitride) coated carbides

• Ceramics (alumina-based and silicon nitride-based)

• Polycrystalline CBN

These tool materials need to be used with specific work materials. Table 1 gives the cutting speeds, feeds, and depths of cut that correspond to some typical work materials.

Table 1 Depth of cut, feed rate, and cutting speed for finish turning of selected materials

Work material

Depth of cut

Feed rate

Cutting speed using indicated tool materials, m/min (ft/min)

mm

in.

mm/rev

coated)

Cermets

Ceramics

PCD/CBN

Low-carbon and free-machining steel

0.52.5

0.020.1

0.150.28

0.0060.011

185-455 (600-1500)

120-245 (400800)

90-230 (300375)

135-455 (4501500)

245-915 (8003000)

Medium-and high-carbon steel

0.252.5

0.010.1

0.15-0.3

0.0060.012

150-410 (500-1350)

90-215 (300700)

75-215 (250700)

170-305 (450850)

245-455 (8001500)

Low-carbon alloy steel

0.252.5

0.010.1

0.15-0.3

0.0060.012

135-335 (450-1100)

90-230 (300750)

135-305 (4501000)

75-185 (250600)

230-425 (75-1400)

Medium-carbon alloy steel

0.42.5

0.0150.1

0.08-0.2

0.0030.008

150-275 (560-900)

60-120 (200400)

55-130 (175450)

325-750 (325750)

230-410 (7501350)

Hardened irons (400-525 HB)

0.081.5

0.0030.6

0.08-0.3

0.0030.012

(80-400)

Glasses and ceramics (200250 HB)

0.121.2

0.0050.050

0.120.25

0.0050.1

(750-3300)

For ultraprecision turning the lathes mentioned above are unsuitable. Among the tool materials indicated in Table 1 for finish machining, not even PCD or CBN compacts can be used for ultraprecision turning, only single-crystal diamonds and CBN. The machines need to have stiffness, which is provided by aerostatic bearings for the spindle and hydrostatic bearings for the bedway. The machine tool should be capable of giving feeds at the nanometer level for brittle materials, and this, combined with an equally fine depth of cut and moderately high speeds, enables the machining of brittle materials such as germanium and silicon. At such low feeds and depths of cut, brittle materials behave like ductile materials, and this mode of removal is known as ductile machining (Ref 3). Glass cannot be turned on these machines as yet, but this should be possible in the near future.

Pioneering work conducted at a number of institutions (Ref 3, 4, 5, 6, 7, 8, 9) has resulted in the development of ultraprecision lathes. Two typical machines are shown in Fig. 1. The cutting tools used are single-crystal diamonds. One mode of manufacture of these tools is chemical machining, in which the material removal rate ranges from 1 to 3000 m3/s (Ref 9). The cube face of the single-crystal diamond is selected as the cutting edge to avoid cleavage, which occurs easily on the octahedral face. The cutting edge should show no nicks or chips at magnifications of 10,000*. By way of comparison, PCDs used for finish turning are inspected at 50*.

Fig. 1 Ultraprecision lathes. (a) Lawrence Livermore PAUL lathe. Source: Ref 3. (b) Rank Pneumo lathe. Source: Ref 5

Figure 2(a) shows a sketch of a single-crystal diamond/CBN tool. The cutting edge radius is around 50 nm; after wear it is about 350 nm (Fig. 2b). Brittle materials (such as silicon and its compounds, germanium used in aspheric lenses, and ceramics) and ductile materials (such as aluminum for computer hard disks and copper for laser printer mirrors) are easily machined with these tools. When they are used to finish germanium on an ultraprecision lathe, mirror-like surfaces with an average surface roughness (Ra) of 1 nm have been achieved. Even composites such as silicon-carbide-whisker-reinforced (SiCw) aluminum have been successfully machined by single-crystal diamonds to Ra of 10 nm (Ref 9).

Figure 2(a) shows a sketch of a single-crystal diamond/CBN tool. The cutting edge radius is around 50 nm; after wear it is about 350 nm (Fig. 2b). Brittle materials (such as silicon and its compounds, germanium used in aspheric lenses, and ceramics) and ductile materials (such as aluminum for computer hard disks and copper for laser printer mirrors) are easily machined with these tools. When they are used to finish germanium on an ultraprecision lathe, mirror-like surfaces with an average surface roughness (Ra) of 1 nm have been achieved. Even composites such as silicon-carbide-whisker-reinforced (SiCw) aluminum have been successfully machined by single-crystal diamonds to Ra of 10 nm (Ref 9).

Fig. 2 Single-crystal diamond machining tools. (a) Schematic of a single-crystal diamond/cubic boron nitride tool (Ref 11). (b) Cutting edge radius on a single-crystal diamond before wear (50 nm) and after wear (350 nm). Source: Ref 8

Precision tolerances for silicon, germanium, aluminum, and copper are bilateral of the order of +20 nm for a linear dimension of 20 mm (0.8 in.). Ultraprecision tolerances are unilateral from +5 nm (lower) to +10 nm (upper) for a linear dimension of 20 mm (0.8 in.). A typical product where these tolerances are easily achieved is the polygon mirror. These tolerances are not as easily achieved with SiCw aluminum alloys because of the orientation of whiskers and heavy tool wear.

References cited in this section

2. F. Koenigsberger, Design of Metal Cutting Machine Tools, Pergamon Press, 1964, p 43-161

3. P.N. Blake and R.O. Scattergood, Ductile-Regime Machining of Germanium and Silicon, J. Am. Ceram. Soc., Vol 73, 1990, p 949-957

4. J. Franse, J.W. Roblee, and K. Modemann, Dynamic Characteristics of the Lawrence Livermore National Laboratory Precision Engineering Research Lathe, Precis. Eng., Vol 13 (No. 3), July 1991, p 196-202

5. D.E. Luttrell, Machining Non-Axisymmetric Optics, Proceedings ASPE Annual Conference, (Rochester, NY), 1990, p 31-34

6. W.J. Wills-Moren et al., Some Aspects of the Design and Development of a Large High Precision CNC Diamond Turning Machine, Ann. CIRP, Vol 31 (No. 1), 1982, p 409-414

7. M. Sawa et al., Development of an Advanced Tool-Setting Device for Diamond Turning, Ann. CIRP, Vol 42 (No. 1), 1993, p 87-90

8. K. Horia et al., A Study on Damaged Layer Remaining in Diamond Mirror Cut Surface, Ann. CIRP, Vol 41 (No. 1), 1992, p 137-140

9. C.J. Yuan, L. Geng, and S. Dong, Ultraprecision Machining of SiCw/Al Composites, Ann. CIRP, Vol 42 (No. 1), p 107-109

11. F. Mason, Atomic Machining of Diamond Tools, Am. Mach., Vol 134 (No. 4), April 1990, p 49-52 Finish Broaching

Broaching is a typical multipoint process that makes use of several transverse cutting edges, which are pushed or pulled through a hole or over a surface to remove metal by axial cutting. Because a broach has roughing, semifinishing, and finishing teeth, the holes or surfaces produced have close tolerances. The dimensional accuracies and surface finishes for various materials are given in Table 2.

Table 2 Commonly broached materials and typical results

Metal

Heat treatment'3'

Hardness HRC

Tolerance

Finish

mm

in.

^m

^in.

2618-T61 Al

G

70 HRB

0.05

0.002

0.80-1.15

32-45

2014-T6 Al

G

70 HRB

0.058

0.0023

0.80

32

Ti-6Al-4V

E

36-38

0.019

0.00075

0.61-0.80

24-32

Stellite 31

B

32

0.05

0.002

2.00

80

SAE 51410 (type 410 SS)

H

32-36

0.05

0.002

1.60

63

Greek Ascoloy

I

32-38

0.025

0.001

0.80-1.07

35-42

Inconel

A

85 HRB

0.13

0.005

2.00

80

Inconel X

H

29

0.025

0.001

0.80

32

Timken 16-25-6

F

20-28

0.025

0.001

0.80-1.60

32-63

A-286

G

28-30

0.060

0.0024

0.80

32

30-35

0.025

0.001

0.89

35

32-38

0.015

0.0006

0.80

32

S-816

G

23-30

0.025

0.001

0.80-1.00

32-40

SAE 3310

E

20

0.25

0.010

1.60

63

SAE 9310

I

36-38

0.05

0.002

1.60

63

17-22A(S)

H

29-34

0.025

0.001

1.50

60

17-22A

H

35-40

0.075

0.003

SAE 9840

I

32-36

0.025

0.001

1.25

50

SAE 4130

I

32

0.013

0.0005

1.60

63

SAE 4140

I

25-29

0.05

0.002

0.80-1.60

32-63

SAE 4340

I

38

0.05

0.002

1.14-1.60

45-63

M2 tool steel

A

24-28

0.02

0.0008

0.80

32

EMS 544

40-47

0.025

0.001

0.75

30

Inconel 901

I

32-36

0.038

0.0015

1.60

63

René 41

G

40-42

0.060

0.0024

0.80

32

WAD 7823A

28

0.0076

0.0003

1.0-1.5

40-60

D-979

I

38-40

0.013

0.0005

1.50

60

EMS 73030

32-36

0.071

0.0028

1.60

63

M-308

36-38

0.060

0.0024

0.80

32

Chromoloy

31-32

0.10

0.004

0.80

32

PWA-682 (Ti)

34-36

0.025

0.001

0.80

32

Lapelloy

J

30-37

0.20

0.008

0.80

32

Type 303 SS

A

85 HRB

0.025

0.001

1.60

63

Type 304 SS

A

80-85 HRB

0.05

0.002

1.60

63

Type 403 SS

I

37-40

0.015

0.006

1.60

63

SAE 1010

D

60

0.025

0.001

0.75

30

SAE 1020

D

3-12

0.05

0.002

1.55-2.05

60-80

SAE 1037

I

15-20

0.0076

0.0003

1.60

30

SAE 1045

I

24-31

0.013

0.0005

SAE 1063

E

12-18

0.10

0.004

0.63-1.5

25-60

SAE 1070

E

5-10

0.05

0.002

0.71-1.5

28-60

SAE 1112

87 HRB

0.025

0.001

1.0-1.15

40-45

SAE 1145

C

13-18

1.25-2.5

50-100

SAE 1340

C

15-20

0.075

0.003

SAE 4047

C

8-15

0.05

0.002

1.5-2.0

60-80

SAE 5140

C

8-15

0.05

0.002

1.5-2.0

60-80

SAE 52100

D

25

0.013

0.0005

0.75

30

Gray cast iron

B

90 HRB

0.075

0.003

2.0-2.5

80-100

KP-7 cast iron

B

0.013

0.0005

3.20

SS, stainless.

Source: Metal Cutting: Today's Techniques for Engineers and Shop Personnel, McGraw-Hill, 1979

(a) Treatment or condition. A, annealed; B, as-cast; C, as-forged; D, cold finished; E, hot finished; F, stress relieved; G, solution and precipitation treated; H, air quench, furnace temper; I, oil quench, furnace temper; J, salt quench, furnace temper.

Further accuracy can be obtained by providing burnishing teeth on the same broach, thus extending its size, or by having a separate broach with burnishing teeth to be used as a second operation only when needed (Fig. 3). A burnishing broach not only increases accuracy but also provides a smoother, more wear-resistant surface. A burnishing broach produces a glazed surface, particularly in steel, cast iron, and nonferrous materials. The total change in diameter produced by a burnishing operation may be no more than 0.013 to 0.025 mm (0.0005 to 0.001 in.). Burnishing teeth are rounded; they do not cut the surface but rather compress and cold work it, improving both the Ra and Rt (total roughness) surface finish values. Burnishing tools, used when surface finish and accuracy are critical, are relatively short and are generally designed as push broaches. Burnishing buttons are sometimes included behind the finishing tooth section of a conventional broaching tool (Fig. 3). The burnishing section can be added as a special attachment or an easily replaced shell. These shells are commonly used to reduce tooling costs when high wear or tool breakage is expected. Burnishing tolerances range from 13 nm (lower) to 25 nm (upper).

Fig. 3 Two types of broaches used for burnishing the walls of broached holes. (a) Broach for burnishing only. (b) Broach for cutting and burnishing. Source: Ref 10

Reference cited in this section

10. Broaching, Machining, Vol 16, 9th ed., Metals Handbook, ASM International, 1989, p 194-211 Finish Milling

Current practice is to use computer numerical control machining centers for milling, particularly finish milling. Milling is used primarily for machining surfaces and slots. Machining centers are divided into two types: vertical and horizontal. Both types are used for milling flat surfaces, contours, and slots. The workpiece is usually set on a portable pallet, and it is not unusual to find four to six pallets on a machining center. The spindle speed ranges from 40 to 6000 rpm with the possibility of using special spindles with speeds up to 8000 rpm. Spindle design has changed from the conventional addon type to the current built-in type (Fig. 4). Machining centers are provided with automatic tool magazines (ATMs) capable of holding 20 to 30 tools. Special ATMs capable of holding 200 to 300 tools are available.

Fig. 4 Spindle head with built-in motor for high-speed (up to 12,000 rpm) finish milling

High-speed milling, which is usually associated with precision machining, involves the use of spindle speeds between 25,000 and 50,000 rpm, and in exceptional cases even as high as 100,000 rpm. A typical application on a horizontal machine is tool and die mold manufacture (Ref 12). High rake angles are used, typically 15° for carbon steels at 300 m/min (985 ft/min) and 25° for ductile materials such as copper and aluminum alloys at 500 m/min (1640 ft/min). Disposal of chips is very important, because they can envelope the tool and cause serious damage to the tool as well as the finished surface. Many machining centers therefore use a vacuum-type chip disposal system that collects the chips as soon as they are generated. However, to use this system, cutting tools must have chip breakers. Precision dies (made of high-carbon steel or chromium-molybdenum alloy steel) should be machined in the heat-treated condition (45 HRC), and in such cases CBN inserts can be used.

As with turning, ultraprecision milling is gaining ground, and a recent development is shown in Fig. 5(a). Single-crystal diamonds are used and mirror-like surfaces are obtained. With the development of diamond pseudo-end mills it is possible to manufacture sculptured surfaces such as toroids and paraboloids (Fig. 5b, c) with a typical feed and depth of cut of 2 and 75 ^m, respectively, and with the spindle rotating at 23,000 rpm. Toroids and paraboloids can be generated by grinding (Ref 14), but small products are difficult.

Fig. 5 Ultraprecision milling. (a) Ultraprecision machine for manufacturing sculptured surfaces. (b) Paraboloid surface. (c) Toroid surface. Source: Ref 13

References cited in this section

12. H. Schulz and T. Moriwaki, High Speed Machining, Ann. CIRP, Vol 41 (No. 2), p 637-643

13. Y. Takeuchi et al., Generation of Sculptured Surfaces by Means of an Ultraprecision Milling Machine, Ann. CIRP, Vol 42 (No. 1), 1993, p 611-614

14. Z.W. Zhong and V.C. Venkatesh, Generation of Parabolic and Toroidal Surfaces on Silicon and Silicon Based Compounds Using Diamond Cup Grinding Wheels, Ann. CIRP, Vol 43 (No. 1), 1994, p 323-326

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