This paper discusses the types and characteristics of cutting tools materials and cutting fluids together with their main advantages and disadvantages. The paper is organised into two parts; the first part is dedicated to cutting tools materials and the second part to cutting fluids.
CUTTING TOOLS MATERIALS
The selection of cutting tool material and grade is an important factor to consider when planning a successful metal cutting operation. A basic knowledge of each cutting tool material and its performance is therefore important so that the correct selection for each application can be made. Considerations include the work piece material to be machined, the component type and shape, machining conditions and the level of surface quality required for each operation.
Cutting tool materials have different combinations of hardness, toughness and wear resistance, and are divided into numerous grades with specific properties. To produce quality parts a cutting tool must possess following typical characteristics:
-Hardness - hardness and strength at high temperatures, to resist flank wear and deformation -Toughness - to resist bulk breakage, so that tools don’t chip or fracture -Wear resistance - having acceptable tool life before needing to be replaced •Non-reactive with the work piece material
•Chemical stability - to resist oxidation and diffusion
•Resistance - to sudden thermal changes.
Cutting tool materials can be divided into two main categories: stable and unstable. Unstable materials are “substances that start at a relatively low hardness point and are then heat treated to promote the growth of hard particles inside the original matrix, which increases the overall hardness of the material at the expense of some its original toughness. Since heat is the mechanism to alter the structure of the substance, and at the same time the cutting action produces a lot of heat, such substances are inherently unstable under machining conditions”. Stable materials on the other hand are “substances that remain relatively stable under the heat produced by most machining conditions, as they don't attain their hardness through heat. They wear down due to abrasion, but generally don't change their properties much during use” . Most stable materials tend to be hard and very fragile and are generally used for large, heavy and stiff machinery. Unstable materials tend to be softer and thus tougher and are generally used in hand tools and light machinery. The main materials used for cutting tools are:
-High Speed Steel (HSS)
-Cast Cobalt Alloy
-Cubic Boron Nitride (CBN)
This material is one of the earliest cutting materials used in machining. It is however now virtually superseded by other materials used in engineering because it is extremely sensitive to heat – it starts to temper at about 220oC. This softening process continues as the temperature rises. As a result cutting using this material for tools is limited to speeds up to 0.15 m/s for machining mild steel with lots of coolant. Carbon steel is generally unstable and very inexpensive. Although it is considered obsolete today it is still found in non-intensive applications such as hand operated tools (e.g. reamers and taps). It has a hardness of up to about HRC 65 and sharp cutting edges are achievable with it. High Speed Steel (HSS)
This range of metals contains about 7% carbon, 4% chromium plus additions of tungsten, vanadium, molybdenum and cobalt. These metals maintain a hardness of up to about HRC 67 at temperature up to about 600o, but soften rapidly at higher temperatures. These materials are suitable for cutting mild steel at speeds up maximum rates of 0.8 m/s to 1.8 m/s and sharp cutting edges possible. HSS is unstable but inexpensive. In fact it is the most common cutting tool material used today...