Cutting Speed and Chip Formation

 

In machining metals the cutting speed is of great importance. It is defined as the speed of the relative motion between the cutting tool and work. In planning and shaping, for example, the tool moves through a straight path over the work, detaching chip. The cutting speed V can be calculated by the following formula:

                                  (6.1)

 

Cutting speed is normally expressed in meters per minute; for high-speed tools, e.g. grinding wheel, the cutting speed is expressed in meters per second.

In the case of rotating tools or workpieses the formula of the cutting speed is as follows:

,                              (6.2)

where D is diameter of workpieses (turning) or tools (milling, grinding) in mm;

n is revolution speed of workpiece or tool, revolution per minute, or (r.p.m).

 

When the cutting tool is pressed into the material, chip is produced. The production of chips and their shape are determined by the properties of the material to be tooled, the shape of the tool edge, the cutting edge geometry and the cutting speed. The different types of the chips produced are known as flowing (Fig. 6.3a) shearing (Fig. 6.3b) and tearing chips (Fig. 6.3c).

In the case of flowing chip, which is cut from tough materials (steel 10, 15, 25, 30, Cu, Al, Mg) at high cutting speed, a very smooth surface of the workpiece is produced. When tough materials are tooled at low cutting speeds, or when materials with low toughness are tooled, the shearing chip is produced (steel 50X, 65Г, 60XTC).

Fig. 6.3. The basic types of chip: a – flowing; b – shearing; c – tearing

 

In the case of brittle materials (grey cast-iron, hard steels, hard brass) the advancing crack is relatively large. The brittle materials fracture and discontinuous chip is formed. This type is called tearing chip. At low cutting speeds the work surface will be very rough.

 

Cutting Materials

 

The cutting edge of tool is especially stressed by the cutting forces and the heat evolved in cutting because of friction and plastic deformation of machined materials. Thus, the tool is exposed to wear and tempering (softness at high temperature). Frictional heat and wear may be reduced by the use of special coolants or cutting fluids. But at high cutting speed the rise in temperature at the cutting edge may exceed the specified limit so that the tempering and loss of hardness may take place.

So the cutting materials must have a high hardness at room and high temperatures. The last is named red hardness. The higher red hardness the higher cutting speed may be taken.

The table 6.1 illustrates the composition and properties of the cutting materials.

 

 

Table 6.1. Composition and properties of cutting materials

N	Material	Composition	HRC	Red hardness temperature, °C	Max.cutting speed V, m/min	Notes
1	Carbon tool steels У7,У8...У13	0.7...1.3%C	60...62	200	15	For chisels, axes, saws, files
2	Alloying tool steels 6XC 9XBГ, 8X4B3M3Ф2	0.6...0.9%C+Cr, Si, W, Mo,V	61...65	250...400	20...25	Cutting, surgical, measuring tool
3	High-speed steels P9, P12, P18, P6M5, P9Ф5, P9K10...	~1%C, 4%Cr, 2%V +6…18% W (first figure) + Mo, V, Co	62...64	600...650	up to 100	Cutting tool (unbroken solid or composite)
4 4.1 4.2   4.3	Hard alloys BK2...BK25 T5K10,T14K18 TT7K12	WC+2...25%Co WC+5(14)% TiC+18%Co WC+7 %(TiC+ TaC)+12 %Co	74...86	800...1000	up to 800	Monocarbide alloys, two carbide, three carbide alloys
5	Cermets	A12O3 as base + ZnO2, MgO and etc as a binders	HRA 90	1200	1500 (250 mps)	Cheap, but brittle, is used as insert for finish cutting (without shocks)
6	Super-hard tool materials	Diamonds, elbor BN, silica carbide SiC	HRA 94...96	700...1800	1200 (200 mps)	As cutting tip of tool and indentors for hardness measurement
7	Abrasive grains	A1203, SiC, BN, diamond+ +binder	-	1800...2000	900 to 6000 (15...100 mps)	As a component of grinding tools

The selection of the materials to be used for the production of tools depends on the materials, from which works to be machined are made. Another point of view for option is the cutting speed and hardness of tool at elevated temperatures.

Unalloyed or carbon tool steel, also known as plain tool steel, having a carbon content from 0.7 to 1.3 % can be used in the lower range of cutting speeds. It has a low elevated temperature (red temperature) and hardness (only about 200°C). At higher temperature the tempering process causes hardness decrease.

In addition to 0.6...0.9% of carbon, a lloy tool steel contains chromium, silicon and other alloying elements and have a red temperature from 250 to 400°C and maximum cutting speed about 25 m/min. Cr, W, Mo, and V form carbides at elevated temperatures. But to further increase of red temperature special carbides should be formed, such as M₂₃C₆, M₆C, MC (M means metal). Therefore, high-speed steels have about 1%C, 4 % Cr, 2 % V, and, in addition to that, 6...18 % W, 0...5 % Mo, 0...10 % Co, up to 5 % V. High-speed steels loose their cutting capability only at 600...650°C. This means that they can be used at high cutting speeds (up to 100 m/min). That is the reason why they are called "high-speed" steels.

Extremely high cutting speeds and a long service life can be achieved with hard metals also known as cutting metals. Their elevated temperature hardness or red temperature is about 800°C. Cutting metals are made by sintering the carbides of tungsten, titanium, tantalum and, sometimes, other elements with metallic cobalt as a binder in this case. That is why these cutting materials are also known as cemented carbides. Carbides are chemical compounds of the mentioned metals and carbon. The melting temperature of these carbides is very high (about 5000°C), but melting point of cobalt is 1494°C. Cemented-carbide products for tools are used in the form of small plates (inserts) to be attached to the tip of tools. Hard metals have a high price.

Newcomers to the cutting-tool field are cermets, or ceramics, or cemented oxides. The principal ingredient of this tool is cheap aluminum oxide (melting point 2050°C), with varying percentages of the other oxides. The material is sintered at very high temperatures (about 2000°C). The material is ceramic, and as a consequence, has high brittleness. The present application seems to be for finishing purposes without shocks. Long service life and the ability to cut the newer materials of high hardness are important ceiling points of this material.

Synthetic diamonds, cubic borous nitride BN, named elbor or belbor and silica carbide SiC are produced at high temperature (1700...2500°C) and pressure (10⁴...10⁵ MPa). In the form of tip of cutting tool diamond and BN may be used for cutting either very hard materials, or very tough (viscous) ones (hard rubber, bakelite, plastics, aluminum, brass, etc.) Like most hard materials they are quite brittle and cannot stand shocks.

Abrasive grains of aluminium oxide, borous nitride, silica carbide and synthetic diamonds in various forms (loose, bonded into wheels and stones, and embedded in papers and coating) find wide usage in industry. They are used for grinding and sharpening of hard materials such as tools, carbides in all forms, and alloys, which have been previously hardened. They are also used when a superior finish is desired on hardened or unhardened materials.