Davim J.P.Machining of Hard Materials

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1 Machining of Hard Materials – Definitions and Industrial Applications 17

normally achieved in grinding so it is logical to use hard boring and/or reaming as

the finishing operation after the work material has been hardened.

If distortion and size variation as a result of heat treatment places unreasonable

constraints on the “soft stage” machining, increasing its tooling cost, hard boring

will provide a cost-effective, scrap-reducing alternative. The greater the distortion

(due to part asymmetry, for example) and the length-to-diameter ratio, the greater

advantages of hard boring.

Moreover, when properly used, hard boring and reaming is much more produc-

tive than internal grinding. According an example presented by a leading tool

manufacturer, when internal grinding was replaced by hard boring in machining of

hardened steel of hardness 63 HRC, the cycle time reduced from 26

min to 2

min

20 s. As such, the direct tooling cost and manufacturing costs related to the opera-

tion were reduced by 50

% and 100

%, respectively.

Typical boring and reaming tools for hard machining are shown in Figure 1.9.

PSBN, cermet, or ceramic inserts are clamped into cartridges or directly to the tool

body. To achieve high productivity, multi-insert tools are used. However, when

high precision of the machined hole is required, single-blade tools with supporting

pads are used. For bore sizes smaller than 12

mm in diameter, clamping of small

inserts into small boring bars becomes progressively more difficult. This problem

is solved by using brazed tools. The PCBN tip is brazed directly to a tungsten

carbide shank, providing a rigid boring tool capable of producing good surface

finishes and providing a highly productive alternative to internal grinding.

1.4.3 Hard Milling

Throughout the last few years, hard milling has captured the attention of manufac-

turers around the world. These manufacturers are typically focused on the mold

Figure 1.9 (a) Common design of boring tool for hard machining, and (b) family of hole-

finishing tools

18 V.P. Astakhov

and die industry where materials such as P20, H13, W5, S7, and others are com-

monly cut. Traditionally, core and cavities from these materials are manufactured

in the hardened state using electrical-discharge machining. Through the years, new

technologies have been developed where these materials can be, in most cases,

machined directly into hardened material using new toolpath processing tech-

niques to form hard milling. These materials can range from 45 HRC to as hard as

64 HRC. Advanced moldmakers have realized that adopting new technology can

be one of their keys to survival against global competition.

According to Zurek [35], successful hard milling is the result of implementing a

system including the machine, cutting tools and toolholders, and the computer-

aided design/manufacturing system. The machine tool is the key component of the

system. The machine must be designed for hard milling, along with having some of

the same characteristics found in a high-speed machining center. The base construc-

tion and the individual components of the machine, such as the drive train, spindle,

and CNC system, must be able to handle the demands of hard milling. A rigid base

with good vibration damping characteristics is of prime importance. Polymer con-

crete bases are a good choice for high-speed and hard milling applications because

they typically have vibration damping 6–10 times greater that of cast iron.

Digital drives that can handle fast acceleration/deceleration provide good con-

touring accuracy while helping to minimize cutting-tool wear. Spindles should

provide flexibility, offering high torque at low speeds and high power over a large

speed range.

Mold shops use three general types of hard milling tools: solid carbide endmills,

indexable carbide inserts, and, most recently, ceramic indexable inserts. Each of

these tools has its strengths and weaknesses depending upon the application. Solid

carbide endmills are usually precision ground, coated, and quite expensive. The

second type of hard milling tool is a cutter with indexable carbide inserts. In most

cases the carbide grades and geometry of these inserts are not designed well for hard

milling, and they do not offer optimal tool life or productivity in hardened materials.

The third type is ceramic indexable inserts, more specifically, whisker-reinforced

ceramic inserts

. The benefits of using a system of cutters with indexable ceramic

inserts include faster cycle times and a reduced number of operations per part. A full

line of cutters for hard milling with whisker-reinforced ceramics enables a shop to

rough out a part from a solid hardened block – including face milling, pocketing and

profiling with indexable inserts – and finish it in one setup. Cutters engineered to

mill with ceramics are capable of secure, high-speed milling from large face mills

down to small diameter endmills – all using indexable ceramic inserts. It is impor-

tant to use cutters designed for hard milling with ceramics at high velocity for se-

cure insert clamping. Modern whisker-reinforced ceramics have a melting point of

more than 2000

°C, which means that ceramic inserts can operate at speeds well

beyond the point where carbide tools fail. In fact, whisker-reinforced ceramics work

better above the melting temperature of carbide inserts. Coolant is not recom-

mended for hard milling applications with ceramic inserts, but air blast is suggested

especially when pocket milling to keep from recutting chips. Reduced coolant usage

and disposal cost is an added benefit when using ceramic inserts for hard milling.

1 Machining of Hard Materials – Definitions and Industrial Applications 19

Toolholders play an important role in hard milling. Because hard milling re-

quires a large range of rotational speeds (from low for a roughing application to

high for finishing), the hollow-shank tooling interface between the toolholder and

spindle interface should be used. It provides rigid and balanced tooling setup over

the ISO taper interface. Although collet chucks and hydraulic expansion toolhold-

ers are an excellent choice for roughing and semi-finishing operations, for finish-

ing hardened cavities and cores with a high degree of accuracy and quality, power

and heat-shrink toolholders provide excellent characteristics. Today, all systems

are commonly available from most tooling suppliers.

1.4.4 Hard Broaching

In the field of metal cutting for mass production of parts with complex profiles,

the broaching operation is the most economical method if high production rates

combined with great consistency of machined parts are required. The advantages

of broaching are based on its technical principle which includes a multi-toothed

tool with cutting edges one after the other and graduated in depth of chip thick-

ness. The profile of a part can be broached in single stroke. Internal broaching is

started from a pre-machined hole, while external broaching is to machine a surface

profile. Broaching is possible in both directions horizontal and vertical. Cutting

motion can be linear or helical.

Two different methods of hard broaching are feasible with this tool configuration:

• hard broaching without defined stock removal: the parts are finish-broached

before hardening and hard broaching means only clearing the heat distortion;

• hard broaching with defined stock removal (0.1–0.2

mm diametrical), which

requires a corresponding finish of the pre-broaching tool considering the ex-

pected heat distortion.

It is known that the cutting speed in common broaching operations is in the

range of 0.5–6

m/min. This speed is 10–30 times less than that in turning and mill-

ing of the same work material. Among many reasons for these low speeds, the

common tool material, high-speed steel, is the root cause. The advent of hard

broaching gave rise to the use of more advanced tool materials such as carbides

and PCBN. Since then, the concerns about productivity, quality of the machined

surface, and the efficiency and reliability of hard broaching came to the attention

of researchers and practitioners.

The following summarize the results obtained so far:

• Coated carbide broaches designed as a flank cutting tool show the best results in

terms of quality and reliability of broaching operations. Profile quality IT 7–8

can be achieved. Surface finish is normally in the range of

0.3–0.5

μm and

is dependent on the combination of the carbide grade and coating as well as the

adhesion properties of the work material.

20 V.P. Astakhov

• An effective cutting speed of 60–63

m/min is determined as an important hard-

broaching process parameter when carbide broaches are used. By this speed,

the broaching stroke lasts 1–2

s, implicating shorter cycle times in comparison

to green (soft) broaching. In some operations, as for example, internal gear

manufacturing or spline broaching of sun gears for automotive transmissions,

hard broaching is the only feasible and economical way to improve quality of

ring and sun gears after their heat treatment.

• Smaller uncut chip thickness t

(the rise per tooth) compared to conventional

broaching is the case. As such, the tool life characterized by the intensity of tool

wear (the linear wear rate

) [36] is proportional to t

, as reported by Makarov

et al. [37] in machining of high-nickel (35

%) alloy, when t

0.02

mm,

μm/m, t

0.06

mm, v

μm/m, t

0.10

mm, v

3.5

μm/m.

• PCBN can also be used as the tool material in hard broaching, allowing higher

cutting speeds (up to 100

m/min) compared to carbide broaches [38]. However,

the process is unstable in terms of cutting-edge chipping after only few meters of

machining, which was attributed to the quality of PCBN with no reason given.

• Hard broaching is only possible on specially designed machines as ordinary

broaching machine cannot deliver the high cutting speed and cannot withstand

excessive broaching axial force.

1.4.5 Hard-gear-manufacturing Operations

The function of gears remains an important element in modern machinery. Gear

manufacturers face numerous challenges to improve their production capabilities

in today’s competitive markets. They must improve efficiency to meet customer

demands for gear sets with lower vibration, lower noise in wide frequency range,

and increased load ability. Customers also require increased gear hardness and

improved tooth surface quality and accuracy. While addressing these demands,

gear manufacturers also remain concerned with the environmental impact of using

oil products or other pollutants. Reducing the use of cutting fluids, or completely

removing the coolant by dry cutting, must be considered in the production process.

To meet all these challenges, gear manufacturers must increase their production

capacity and reduce costs through the use of precision cutting techniques.

The mentioned requirements for gears result in the implementation of special

alloys for gears and their heat treatment to achieve the required strength and

hardness which are requirements for gear accuracy, low noise, and durability. To

achieve accuracy requirements in machining of hard gears, additional operations

for gear finishing are required. Normally, it is necessary to have/purchase expen-

sive specialized equipment for gear-finishing operations such as grinding and

honing, applied after gear heat treatment to achieve the required quality. These

operations increase the cost of gears significantly. As an alternative, hard-gear-

cutting operations such as hard hobbing, shaving, and broaching can be used.

Hard hobbing is able to do both rough machining before heat treatment and fin-

ishing work after heat treatment. It reduces the cost of gears, increases the produc-

1 Machining of Hard Materials – Definitions and Industrial Applications 21

tion rate, and allows centralization of equipment. Hard hobbing, however, sets some

special requirements on the tool, the machine, and the operation as a whole. Under-

standing these requirements results in successful implementation of this process.

The most common hob used today in industry is made of high-speed steel. Be-

cause this tool material does not possess sufficient wear resistance and red hard-

ness,

its use has been limited to the machining of relatively soft work materials

having Brinell hardness of no more than 250. Further, a hob made of high-speed

steel cannot finish up the surface of even such relatively low hardness materials

but can machine it only to an extent as to require further finishing work by other

gear manufacturing tools, for example, shavers. The major reason is an insuffi-

cient flank angle along the teeth profile.

Generally in manufacturing, it is customary to form the cutting wedges of

form-cutting tools with a flank angle of 3–4° to improve the properties of a fin-

ished surface [19]. In hobbing tools, the flank angles used are much smaller be-

cause it is necessary to keep the tooth profile within a narrow tolerance on hob

multiple resharpenings. In machining of hard materials, the springback of the work

materials is great due to a high yield strength, so excessive rubbing on the flank

surface is the case. High temperatures occurring due to this rubbing cause adhe-

sion between the flange and the work material that ruins the surface finish and

reduces tool life. Therefore, tool materials that can withstand much higher contact

temperatures and prevent high-temperature adhesion with the work materials are

used for hard hobbing.

Sintered carbides are common tool materials used in hard hobbing. Figure 1.10

shows a solid carbide hob used for small modules and an indexable carbide insert

gear hob. The latter shows the maximum advantage in hard hobbing although it is

an expensive tool that requires a long pre-setting time and proper handling. Al-

though the advantages and potentially efficiency gains in using of cermet and

PCBN hobs have been revealed, the practical use of these tool materials in hard

hobbing requires additional studies, and new machines and tooling designs.

The property of being hard enough to cut metals even when heated to a dull-red color.

Figure 1.10 (a) Solid carbide hob, and (b) double-start indexable carbide insert gear hob

22 V.P. Astakhov

1.4.6 Skiving

1.4.6.1 Hard “Skiving” Turning

Traditionally when cutting tools with polycrystalline superhard materials (PSHM)

are used, the cutting feed does not exceed 0.2

mm/rev due to a common requirement

of obtaining surfaces with roughness of R

and R

below 1.25 and 5

μm, respectively

(Figure 1.11) [39]. Increased turning efficiency is possible by using cutting tools

with wiper geometry [40], tools with non-planar (sculptural) rake faces [41], and

“skiving” cutting [39].

The working principles and advantages of cutting tools with wiper geometry

and with sculptural rake faces in terms of improving allowable feed are well known

[42] while there is very little information about “skiving” machining with tools

equipped with CBN-based PSHM. The results of a study on hard turning of high-

chromium–nickel alloy (hardness of 60–62 HRC) were discussed by Klimenko and

Manokhin [39]. As pointed out, surface roughness of R

0.07–

0.14

μm was

achived at feed of 0.05

mm/rev. In machining the SCM415 steel with a hardness of

61 HRC using Sumitomo PCBN-based tool material, the surface roughness R

were

1.25 and 1.75

μm at feeds 0.1–0.3

mm/rev, respectively [13].

As discussed by Astakhov [19], the proper determination of the uncut (unde-

formed) chip thickness is of prime importance in the optimization of any machin-

ing operation and in the determining the energy balance. Due to “skiving” turning

special features, the determination of the cross-section parameters of the uncut

chip thickness in “skiving” turning is of particular interest as it is the prime pa-

rameter to calculate the chip compression ratio and thus the energy spent in plastic

deformation of the work material as discussed above.

The model of “skiving” turning proposed by Klimenko and Manokhin [39] is

shown in Figure 1.13. In this model, region 1–2 of the chip is the line of inter-

section of the plane, which coincides with the tool rake face having a zero rake

angle with the surface (machined at the previous turn) that represents a hyper-

Figure 1.11 Minimum and maxi-

mum feeds used in various hard-

turning methods: standard tools

0.05–0.12

mm/rev) (1), turning

with tools having a cylindrical rake

face (f

0.1–0.2

mm/rev) (2), turning

with tools having a wiper geometry

0.1–0.4

mm/rev) (3), traditional

“skiving” turning (f

0.3

mm/rev) (4),

and advanced “skiving” turning (f up

to 1.0

mm/rev) (5)

1 Machining of Hard Materials – Definitions and Industrial Applications 23

boloid of rotation. Region 2–3 is the line of intersection of surface to be ma-

chined on the workpiece (a cylinder) with the rake face. The shape of the chip

cross-section is bounded by the tool cutting edge and curve 1–2–3.

The uncut chip thickness t

in each point of the cutting edge is understood as

a distance between the cutting edge on the transient surface and line 1–2–3 meas-

ured along the normal to the tool cutting edge.

()

222

sin cos tan for

sin for

rX fX XX X

RX X X X

λλ λ

λλ

λλλ

⎧

−++ ≤<

⎪

⎨

⎪

−≤≤

⎩

(1.4)

where

22 22

12 3

2cos sin cos sin

Rr f Rr

XX X

λλλ

−− −

==−= (1.5)

Figure 1.12 Hard “skiving” turning

Figure 1.13 Model for the determi-

nation of the uncut chip thickness in

“skiving” turning

24 V.P. Astakhov

In these equations, r is the radius of the machined surface, R is the radius of

the workpiece, f is the cutting feed, and

is the cutting-edge inclinational angle

[19, 43].

Figure 1.14 shows dependences of the average uncut chip thickness on vari-

ous machining parameters. As seen, the average chip thickness depends on the

diameter of the workpiece, cutting-edge inclination angle,

, depth of cut, d

and cutting feed, f. In the range of machining parameters f

0.5–2.0

mm/rev,

0.05–0.15

mm, and λ

15–60°, the average chip thickness is in the range of

10–60

μm. The results obtained show that feed and cutting-edge inclination have

the strongest effect on the average uncut chip thickness (Figure 1.14 (a)) and

with an increase of one of these characteristics the degree of the influence of the

other increases. In comparison with the above characteristics, an increase in the

cutting depth increases the average uncut chip thickness to a lesser degree (Fig-

ure 1.14 (b)) and as the diameter of the part machined increases, the chip thick-

ness decreases.

Summarizing the experimental results [39], the following important features of

“skiving” hard turning (compared to usual hard turning with PCBN tools) can be

represented as:

• High productivity and efficiency: the cutting speed in “skiving” turning is the

same while the cutting feed is up to five times greater.

− Long tool life: the average uncut chip thickness is relatively small even

when the cutting feed (known in industry as the chip load) reaches 1

mm/rev

(Figure 1.14). This assures lower machining temperatures and uniform wear

of the rake and flank tool contact areas as shown in Figure 1.15.

Figure 1.14 Effect of the machining parameters on the average uncut chip thickness: (a)

mm, t

0.1

mm, and (b) f

mm/rev, λ

45°

1 Machining of Hard Materials – Definitions and Industrial Applications 25

• The most distinguishing feature of “skiving” turning is that the roughness of

the machined surface does not deteriorate proportionally to the tool wear as

in the conventional hard machining (Figure 1.2). Figure 1.16 shows an ex-

ample. As seen, if the tool life criteria is accepted to be VB

0.25

mm then

practically no surface finish deterioration occurs over the entire period of

tool life.

• High quality of the machined surface: the depth of the “white” layer is in the

range from 5 to 16

μm. No solid phase transformation due to high machining

temperature was found.

• Surface roughness is much smaller compared to usual hard turning, while the feed

is up to five-fold higher. Figure 1.17 shows an example of the experimental data

[39]. At cutting speeds in the range of 0.9–1.2

m/s, the roughness of the machined

surface depends on the cutting feed as: at f

0.09–0.67

mm/rev, R

0.3–0.6

μm,

1.5–6

μm; at f

0.67–1.30

mm/rev, R

0.6–0.8

μm, R

4–9

μm; at f

1.30–

2.60

mm/rev, R

0.80–1.25

μm, R

6–12

μm.

Figure 1.15 (a) Rake and (b) flank faces having uniform wear pattern

Figure 1.16 Roughness of the machined surface variation with the tool flank wear in machining

AISI E52100 (0.98–1.1

% C, 1.45

% Cr, 0.35

% Mn) of hardness 60–62 HRC with f

0.67

mm/rev,

1.32

m/s, λ

50°

26 V.P. Astakhov

Figure 1.17 Peak-to-valley

height R

vs. feed and cutting-

edge inclination angle. Work-

piece material AISI 4340, 48–

50 HRC, d

0.15

mm,

2.5

m/s

1.4.6.2 Gear Skiving

Skive hobbing is finish hobbing of hardened gears. This process removes the

inaccuracies in the gear flanks and profile which occur during heat treatment.

The result is improved gear accuracy. It is possible to hob hardened gears in the

range up to 63 HRC using tungsten carbide hobs. This gear-finishing process is

capable of achieving gear quality levels of up to DIN class 6 or AGMA class 11

in standard module ranges of 1 to 6

mm. Gear skiving has gained in recent years

a considerable reputation among gear producers and is nowadays a powerful

alternative to traditional grinding and hard hobbing [44]. This became possible

with the development of highly evolved and automated machine tools, which

make the method practical and economical [45]. Moreover, the introduction of

well-designed cemented carbide tools and the implementation of stock-dividing

systems in gear-hobbing machine tools make the skiving process attractive and

efficient.

Figure 1.18 shows Gleason’s skive hobbing of a module 16

mm hardened spur

gear using a Gleason P 600 hobbing machine. The hob tool consists of carbide

cutting inserts brazed on a high-speed steel body. The skiving process removes in

one cut 0.5

mm stock per gear flank. Besides significant reduction of tooling cost

and cycle time, advantages of this process include capabilities such as gear

flank modifications like taper and crown in the range of 100

µm and more in

only a single cut. DIN 8 quality was achieved on the profile, which is the most

sensitive quality characteristic in this process.

The special geometry of the skiving hob teeth is shown in Figure 1.19. As it

appears for many researchers, the main differences between skiving and gear hob-

bing cutting teeth are the negative tool-in-hand rake angle γ

formed due to the

tooth rake offset δ

[44]. It appears that the negative rake angle protects the car-

bide cutter from shocks and instantaneous overloading.