CIRP Encyclopedia of Production Engineering

Living Edition
| Editors: The International Academy for Production Engineering, Sami Chatti, Tullio Tolio

Hard Material Cutting

  • Hans Kurt ToenshoffEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-35950-7_6406-4

Synonyms

Definition

Cutting hard materials like hardened steel with hardness values exceeding 47 HRC is called hard material cutting. All cutting processes like turning, drilling, milling, broaching, and other processes with geometrically defined cutting edges (as distinct from abrasive processes) comprise this category of production method.

Theory and Application

History

In the past, hard materials were – unless in need of repair – exclusively processed and brought to the final shape by abrasive processes. The development of appropriate cutting tools, machine tools, and machining strategies through the 1980s and 1990s has enabled the majority of hard materials to be cut, and hard cutting is now established in industry. In particular, tool materials of high hardness and sufficient strength at elevated temperatures were the primary technical breakthrough, most notably polycrystalline cubic boron nitride (PCBN), but also mixed oxide (Al2O3/TiCN) ceramic with enhanced toughness and ultrafine-grain carbide. As early as the mid-1960s, steels exceeding 60HRC were successfully machined with alumina ceramics with highly negative effective rake angles (King and Wheildon 1966); however, it was not until the development of CBN/TiCN composites in the 1980s that sufficient reliable tool performance levels were realized. Over the last decade or so, PVD coatings have been successfully applied to PCBN tools, enabling higher cutting speeds and in certain applications, more than a doubling in tool lives. In industry, hardened steels represent a considerable percentage of the hard materials cut, of which the majority are case-hardened steels within the automotive industry (Schreiber 1976; Toenshoff et al. 1986, 1997, 1998; Denkena and Toenshoff 2012; Barry 2012).

Process Design

In many industrial segments including machine- and automotive-building, parts are designed to bear increasingly higher forces, stresses, and power densities. Such parts, therefore, require higher strength, hardness, and wear resistance. In the past, materials – most commonly steels – of limited strength were sufficient, whereas now highly hardened and tempered materials must be applied. The spectrum of preshaping and finishing processes can be enlarged by hard cutting in addition to abrasive processes. In addition to the strength and hardness of load-bearing components, the quality criteria of these parts are more stringent than in the past. These quality demands have to be fulfilled by hard cutting – an example of a typical component is shown in Fig. 1 (Klocke et al. 2005). In addition, hard cutting provides several capabilities which either complement or supersede the capabilities of abrasive processing techniques. With hard turning processes, for example, it is possible to machine several diameters, faces, and or tapers with one tool in one clamping. Furthermore, the majority of hard turning and all hard milling processes are undertaken dry.
Fig. 1

Typical quality requirements of highly loaded components (Source: Kanfir)

Hard Turning

For turning processes, the cutting edge is engaged with the workpiece mostly without interruption. Due to the high specific cutting pressure of hardened steels, the density of heat energy generated within the chip forming zone is high – of the order of 5 J/mm3. This leads to high thermal loads of the cutting wedge (Brandt 1995; Koch 1996; Schmidt 1999; Toenshoff et al. 2000; Barry and Byrne 2002). Therefore, the cutting speeds have to be limited with respect to tool wear so as to realize acceptable tool lives. Table 1 shows typical process conditions.
Table 1

Typical process conditions for hard cutting processes (Source: Toenshoff)

Process

Tool material

Cutting parameters

Surface and dimensional quality

Hard turning

PCB, ceramics, fine grain carbide

vc = 100–220 m/min

Rz = 1–3 μm

It 6–it 7

f = 0.05–0.2 mm

Hard drilling

Carbide, TiN-coated

vc = 40–60 m/min

Rz = 2–4 μm

f = 0.02–0.04 mm

IT 7–IT 9

Hard milling

PCB, fine grain carbide

vc = 200–350 m/min

Rz = 2–5 μm

fz = 0.1–0.2 mm

IT 7–IT 10

The tool life is largely dependent on the hardness of the material being cut and the cutting speed as shown in Fig. 2 by trend. The composition of the steel plays a significant role also – in particular, the carbide content. Steels with higher contents of carbide-forming elements result in more aggressive abrasive wear, in addition to the thermochemical wear mechanisms operative with low alloy steels. The size of the carbide grains is also influential; for example, the finer carbides found in PM-produced steels result in significantly lower wear rates than the coarse carbides in conventional steels (Chou and Evans 1997).
Fig. 2

Hardness and tool life, with appropriate cutting speeds indicated (Source: Toenshoff)

The most appropriate tool materials are polycrystalline cubic boron nitride (PCBN), mixed oxide ceramics (SC), and fine-grain tungsten carbides (TC), the latter only under certain conditions of use. PCBN is the material of choice for large volume, hard rough- and finish-turning (H05–H30) processes, whereas ceramics are used for low volume processes with minimal interrupts in the workpiece. Cemented carbides are primarily limited to milling processes where they cater for geometrically more complex tools; however, very high volume processes such as constant-velocity joint ball-track milling are now the domain of PCBN. ∼In addition to case-hardened steels, hot- and cold-work tools steels, sintered steels, high-chrome, white cast irons, and even cemented carbides are routinely cut with defined edge tools (Barry 2012).

Figure 3 shows the relationship between PCBN tool life and cutting speed in a Taylor diagram. The atypical curves demonstrate – compared to the straight lines when cutting soft steel – that there is generally a limited optimal range of speed for hard turning. The hardness and composition of the machined steels were varied in the tests – reflected partly in the hardness of the workpiece. Note that the tool life decreases with increasing Cr content. For practical purposes, it must be considered that a small decrease in hardness brings a remarkable gain in tool life. It should also be noted that as with unhardened steels, hard steels are often subject to relatively widely specifications in terms of the elements known to impact their machinability. As such, batch to batch variation in the machinability of the steel is sometimes observed, often due to differences of several hundreds of ppm in (usually) oxide-forming elements such as Ca and Al (Barry 2000).
Fig. 3

Taylor curves for PCBN in finish hard turning (Source: Koch)

It is useful to appreciate the actual geometrical conditions of tool engagement in hard material cutting. For finish hard turning, small feeds are mostly employed with relatively large corner radii. As such, even where negatively chamfered tools are employed, a highly negative cutting geometry results due to the undeformed chip thickness being comparable and often less than the dimension of the edge hone. This is to decrease the specific load (load per length) on the cutting edge.

Figure 4 shows the tool engagement conditions in real geometric proportions, as determined by the parameters f, ap, rβ, and rε. The view on the rake face (left) shows that the local engagement angle κ(ϕ) is very small and that the nominal engagement angle of the straight cutting edge is not relevant for the effective (or actual) cutting geometry.
Fig. 4

Engagement conditions in hard turning (both vertical and horizontal dimensions are shown to the same scale so as to preserve the actual aspect ratios present in typical hard cutting processes) (Source: Toenshoff)

Therefore, an effective engagement angle κeff is defined as (all formulas can be found in Denkena and Toenshoff 2012):
$$ {\upkappa}_{\mathrm{eff}}=\frac{1}{2}\mathrm{arc}\;\cos \left(\frac{{\mathrm{r}}_{\upvarepsilon}-{\mathrm{a}}_{\mathrm{p}}}{{\mathrm{r}}_{\upvarepsilon}}\right) $$
Also normal to the rake face, there are characteristic geometric conditions in hard material machining, which differ from conventional turning – namely, the very small undeformed chip thickness. This varies from h = 0 to the maximum thickness of hmax = f × sin κeff. The length of the contact arc – the part of the cutting edge, which is engaged – follows from:
$$ {\upiota}_{\upkappa}=2\cdotp {\upkappa}_{\mathrm{eff}}\cdotp {\mathrm{r}}_{\upvarepsilon} $$
Figure 4 (right) shows that the real rake angle ϒeff is normally negative because of the cutting edge radius rβ. For the cutting edge with h < rβ, it is:
$$ {\upgamma}_{\mathrm{eff}\left(\upvarphi \right)}=\mathrm{arc}\;\sin \frac{{\mathrm{r}}_{\upbeta}-{\mathrm{h}}_{\left(\upvarphi \right)}}{{\mathrm{r}}_{\upbeta}} $$

Where optimizing hard cutting processes, it is instructive to determine the above quantities as these are likely to relate in a more fundamental manner to the process output variables, than do the typical process conditions of feed, depth of cut, and nose radius.

Applying hard turning as a finishing process is widely used to substitute grinding operations. Figure 5 gives a practical example where hard turning is obviously more favorable. A friction disk made of bearing steel DIN100Cr6 (corresponding to SAE 52100) has three surfaces finished – the outer cylinder (A), a face (B), and a shorter inner cylinder (C).
Fig. 5

Economic manufacture by hard turning (Source: Toenshoff)

The economic and ecological advantages indicated in the above figure can only be taken as typical, and do not apply to every component/process – the actual benefits of turning over grinding depend very much on the component tolerance and surface integrity criteria, stock removal allowances, and component geometrical complexity. Additional criteria include manufacturing cost, flexibility, capital investment requirements, and ecological sustainability.

It is also important to note that a minimal undeformed chip thickness exists in hard turning and hard cutting processes in general (Toenshoff et al. 1997). As such, very small reductions in part diameter – of microns or several tens of microns – are difficult or impossible. However, grinding can be controlled by “spark-out” until the normal force is diminished and diameter reduction ceases. As a consequence, hard turning operations cannot be used, if long, slender, and very compliable workpieces have to be machined. In many cases, the decisive advantage of hard turning is the high flexibility of the shape. The workpiece is machined by a controlled movement of the tool, whereas grinding often involves plunge operations copying the shape of a profiled grinding wheel. This flexibility of the shape may lead to a shortening of the process chains in the sense of complete machining, resulting in less complexity cost and less investment.

Hard Drilling

It is frequently required to make holes in cylindrical parts, which are mostly manufactured by turning or grinding. Such holes need to be drilled. With case-hardened steels, a given drill has to work consistently in cutting through hard and soft material regions of the component. With through hardened materials, there is also a limited process window in hard drilling as shown in Fig. 6 – most notably, small feed values and relatively low cutting speeds (Spintig 1995).
Fig. 6

Tool life for hard drilling (Source: Spintig)

In most cases, there are used short solid tungsten carbide drills. Tungsten carbide with ultrafine grain size is the most suitable cutting material. For larger diameters (>12 mm), there are applied boring tools with indexable tips, mostly coated with titanium nitride (TiN), TiAlN, or similar coatings. In addition to being more wear resistant, such coatings minimize the friction between tool and hole surface as well as between tool and chips. Under finishing conditions with solid tungsten carbide drills, it is possible to achieve diameter qualities in the range of IT 7–IT 9 and a surface roughness of Rz = 2–4 μm in special cases, Rz = 1 μm.

For case-hardened parts, the manufacturing chain can be reduced considerably as with the example shown in Fig. 7. The key point is that it is possible to avoid a necessary split thermal treatment or an additional coating for the carburization.
Fig. 7

Minimizing of working steps by hard drilling (Source Spintig)

The possibility to remove heat is especially difficult in hard drilling, because the power density in front of the cutting edges is high and the energy transportation out of the hole is very limited. Consequently, tools may be subject to high temperatures and thermal expansion. The reduced clearance between the thermally expanded drill body and the wall causes further increased friction and exacerbation of the problem, such that eventually, the drill may jam in the hole. To avoid such behavior, drills with diameter reduction along the length of the shaft were developed (Spintig 1995).

Chip Formation and Forces

If materials which are hard and exhibit limited ductility are plastically deformed, they will normally develop cracks due to components of shear or tensile stress which evolve. Therefore, it is only logical to question why the cut surface of a hard workpiece does not contain numerous cracks. The small undeformed chip thickness and the highly negative effective rake angle (as shown in Fig. 4) result in a high hydrostatic compressive stress in the work material immediately adjacent to the cutting edge. It is known, however, that even brittle materials are highly deformable under high hydrostatic pressure. Thus, the material in front of the cutting edge is deformed without cracking. In addition, the heat of plastic deformation acts to soften the work material such that it exhibits some level of ductility.

The deformation behavior of hard work materials with limited strain hardening capacity also dictates the chip formation mechanisms. The chips produced in hard cutting exhibit a distinct “sawtooth” morphology, which results from period catastrophic shear (or fracture) as material passes through the primary shear zone. In cross section, these localized shear bands appear white under an optical microscope – a feature arising from their “apparent” resistance to etching. These bands are characterized by an extremely fine grain size, typically of the order of 50 nm, which are formed due to intense plastic shear with rapid cooling (following the cessation of deformation). The white layers observed in cross sections of hard cut surfaces exhibit a similar ultrafine, equiaxed grain structure, similarly resulting from intense shear in the tertiary shear zone. The presence of flank wear on the cutting tool accentuates the depth of white layer formation (Barry 2012).

Due to the very negative effective rake angle, the passive (or normal) force is the dominant force component in hard cutting (Sölter 2010). It was noted that the cutting force components are strongly dependent on the tool wear and that they increase near linearly with flank wear (Fig. 8).
Fig. 8

Tool wear and force components (Source: Toenshoff)

The force and pressure distribution between tool and workpiece in the contact area determine the resulting surface integrity (Borbe 2001). The thrust force on the cutting edge Fd is the vectorial sum of the feed and passive force Ff and Fp. Assuming a friction factor μ (Coulomb’s law), the normal and tangential forces on the flank F and F can be estimated at:
$$ {\mathrm{F}}_{\mathrm{N}\alpha }=\frac{1}{1-{\upmu}^2}\left({\mathrm{F}}_{\mathrm{d}}-\upmu \cdotp {\mathrm{F}}_{\mathrm{c}}\right) $$
And
$$ {\mathrm{F}}_{\mathrm{T}\alpha }=\frac{\upmu}{1-{\upmu}^2}\left({\mathrm{F}}_{\mathrm{d}}-\upmu \cdotp {\mathrm{F}}_{\mathrm{c}}\right) $$
At this point, the contact-related friction power may be determined as:
$$ {\mathrm{P}}_{\upalpha}^{\prime }=\frac{{\mathrm{P}}_{\upalpha}}{\upiota_{\upkappa}}=\frac{\upmu \cdotp \, {\mathrm{v}}_{\mathrm{c}}\cdotp \sqrt{{\mathrm{F}}_{\mathrm{p}}^2+{\mathrm{F}}_{\mathrm{f}}^2}}{\upiota_{\upkappa}} $$

It was found out that a friction factor of μ = 0.25 till 0.28 is suitable in most cases (Wobker 1996).

For practical applications of hard cutting (e.g., for antifriction bearings or other dynamically, highly loaded parts), the question of surface integrity is of the utmost interest. As noted above, the wear of the cutting edge has a dominant influence on the contact load and depth and magnitude of structural modifications to the final workpiece surface. The corresponding effect may be seen in residual stresses and in microstructural changes near the surface. Figure 9 shows the residual stresses, the passive force, and the microstructure for a hard-turned case-hardened steel DIN16MnCr5 (corresponding SAE 5115). With increasing tool wear, the thermo-mechanical loading of the workpiece surface is increased and results in a distinct newly hardened layer which appears white under an optical microscope (Brinksmeier and Reckling-Wilkening 1992). As a consequence of the thermo-mechanical loading of the work material surface, tensile residual stresses occurred which increased in magnitude and depth with the flank wear. In general, however, with adequate control of tool wear and process conditions, the integrity of hard-turned surfaces is rarely a concern and under certain conditions, very favorable compressive stresses can be generated.
Fig. 9

Physical surface state is dependent on tool wear (Source: Toenshoff).

Cross-References

References

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© CIRP 2018

Authors and Affiliations

  1. 1.Institute of Production Engineering and Machine ToolsLeibniz University HannoverGarbsenGermany

Section editors and affiliations

  • Garret O'Donnell
    • 1
  1. 1.Trinity College DublinDublinIreland