Hard Material Cutting
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
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).
Typical process conditions for hard cutting processes (Source: Toenshoff)
Surface and dimensional quality
PCB, ceramics, fine grain carbide
vc = 100–220 m/min
Rz = 1–3 μm
It 6–it 7
f = 0.05–0.2 mm
vc = 40–60 m/min
Rz = 2–4 μm
f = 0.02–0.04 mm
IT 7–IT 9
PCB, fine grain carbide
vc = 200–350 m/min
Rz = 2–5 μm
fz = 0.1–0.2 mm
IT 7–IT 10
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).
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.
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.
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.
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.
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).
It was found out that a friction factor of μ = 0.25 till 0.28 is suitable in most cases (Wobker 1996).
- Barry J (2000) Machining hardened steels, cutting tool wear, acoustic emission, chip formation and surface integrity. PhD thesis, University College, DublinGoogle Scholar
- Barry J (2012) Personal notice on occasion of the reviewing processGoogle Scholar
- Borbe C (2001) Bauteilverhalten hartgedrehter Funktionsflächen [Behavior of components depending on hard machining of functional surfaces]. Dr.-Ing. Thesis Univ, HannoverGoogle Scholar
- Brandt D (1995) Randzonenbeeinflussung beim Hartdrehen [Subsurface influences of hard turning]. Dr.-Ing. Thesis Univ, HannoverGoogle Scholar
- Brinksmeier E, Reckling-Wilkening K (1992) Gefügebeeinflussung durch spanende Bearbeitung [Structure influencing by cutting]. Praktische Metallogr, Sonderbd 23:47–56Google Scholar
- Chou YK, Evans CJ (1997) Finish hard turning of powder metallurgy M50 steel. Trans NAMRI/SME 25:81–86Google Scholar
- Denkena B, Toenshoff HK (2012) Cutting fundamentals. Springer, HeidelbergGoogle Scholar
- King AG, Wheildon WM (1966) Ceramics in machining processes. Academic, New YorkGoogle Scholar
- Koch K-F (1996) Technologie des Hochpräzisions-Hartdrehens [Technology of high precision hard turning]. Dr.-Ing. thesis, RWTH, AachenGoogle Scholar
- Schmidt J (1999) Mechanische und thermische Wirkungen beim Drehen gehärteter Stähle [Mechanical and thermal effects of turning hardened steels]. Dr.-Ing. thesis, Univ. Hannover/Fortschritt-Berichte VDI, Reihe 2, Nr. 519, Berichte aus dem Institut für Fertigungstechnik und Spanende Werkzeugmaschinen, Universität Hannover. VDI-Verlag, DuesseldorfGoogle Scholar
- Schreiber E (1976) Die Werkstoffbeeinflussung weicher und gehärteter Oberflächenschichten durch spanende Bearbeitung [Material influences of soft and hard surfaces by cutting]. VDI-Bericht Nr. 256, Duesseldorf, p 67–79Google Scholar
- Sölter J (2010) Ursachen und Wirkungsmechanismen der Entstehung von Verzug infolge spanender Bearbeitung [Causes and mechanisms of distortion development by machining]). Dr.-Ing. thesis, Univ. Bremen/Forschungsberichte aus der Stiftung Institut für Werkstofftechnik Bremen. Shaker, AachenGoogle Scholar
- Spintig W (1995) Werkstoffbeeinflussung und Prozessführung beim Hartbohren [Material influences and process conduct of hard drilling]. Dr.-Ing. thesis, Univ. Hannover/Fortschritt-Berichte VDI, Reihe 2, Nr. 358, Berichte aus dem Institut für Umformtechnik und Umformmaschinen, Universität Hannover. VDI-Verlag, DuesseldorfGoogle Scholar
- Toenshoff HK, Bußmann W, Stanske C (1986) Hartbearbeitung durch Drehen und Fräsen [hard cutting by turning and milling]. tz für Metallbearbeitung 80:35–40Google Scholar
- Toenshoff HK, Karpuschewski B, Borbe C (1997) Comparison of basic mechanisms in cutting and grinding of hardened steel. Prod Eng IV(2):5–8Google Scholar
- Toenshoff H K, Karpuschewsky B, Borbe C (1998) Hard machining: state of research. In: Proceedings of CIRP/VDI conference on international high performance tools, VDI Berichte, Ausgabe 1399. Duesseldorf, p 253–277Google Scholar
- Wobker H-G (1996) Hartbearbeitung [Hard machining]. Habilitation thesis, Univ. Hannover/Fortschritt-Berichte VDI, Reihe 2, Fertigungstechnik, Nr. 420, Berichte aus dem Institut für Fertigungstechnik und Spanende Werkzeugmaschinen, Universität Hannover. VDI-Verlag, DüsseldorfGoogle Scholar