Cutting Tools free essay sample

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 â€Å"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 â€Å"substances that remain relatively stable under the heat produced by most machining conditions, as they dont attain their hardness through heat. They wear down due to abrasion, but generally dont 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: -Carbon Steel -High Speed Steel (HSS) -HSS Cobalt Cast Alloys -Cast Cobalt Alloy -Cemented Carbide -Coated Carbide -Ceramic -Cermet -Cubic Boron Nitride (CBN) -Diamond Carbon Steel 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 It is however now virtually superseded by other materials used in engineering because it is extremely sensitive to heat – it starts to temper at about 220oC.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 softenin g 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. 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 and is used extensively on drill bits and taps. HSS Cobalt These are high cobalt versions of HSS that are very resistant to heat and thus excellent for machining abrasive and/or work hardening materials such as titanium and stainless steel. They are unstable and moderately expensive and are used extensively on milling cutters and drill bits. They have a hardness of up to about HRC 70 and sharp cutting edges possible. Cast Alloys These metals are stable and expensive but somewhat fragile. Despite their stability they dont allow for high machining speed due to low hardness and are not used much. They have a hardness of up to about HRC 65 and sharp cutting edges possible. Cast Cobalt Alloys These materials are made of various non ferrous metals in a cobalt base. They can withstand cutting temperatures of up to 760oC and are capable of cutting speeds about 60% higher than HSS. They are made of 38 – 53% Cobalt, 30 – 33% Chromium and 10 – 20 % Tungsten. They have a hardness of 58-64 HRC and good wear resistance but little toughness. They are not suitable for intermittent cutting but are good for deep boring and continuous turning is better than HSS. Cemented Carbides These materials usually consist of tungsten carbide or a mixture of tungsten carbide, titanium, or tantalum carbide in powder form, sintered in a matrix of cobalt or nickel. As they are expensive and have low rupture strength they are normally made in the form of tips which are brazed or clamped on a steel shank. The clamped tips are generally used as throw away inserts. Cemented carbide is stable and the most common material used in the industry today. It is offered in several grades and offers a resistance to abrasion. Its main use is in turning tool bits although it is very common in milling cutters and saw blades. It supports a hardness of up to about HRC 90 and sharp edges are generally not recommended. Coated Carbides The cutting system is based on providing a thin layer of high wear-resistant titanium carbide fused to a conventional tough grade carbide insert, thus achieving a tool combining the wear resistance of one material with the wear resistance of another. These systems provide a longer wear resistance and a higher cutting speed compared to conventional carbides. Ceramics Ceramics are made by powder metallurgy from aluminium oxide with additions of titanium oxide and magnesium oxide to improve cutting properties. They are stable and moderately inexpensive. Chemically inert and extremely resistant to heat, ceramics are usually desirable in high speed applications however they are brittle and have little resistance to shock. Ceramics are considered unpredictable under unfavourable conditions. Their use is therefore limited to tips used for continuous high speed cutting on vibration-free machines. The most common ceramic materials are based on alumina (aluminium oxide), silicon nitride and silicon carbide. They are used almost exclusively on turning tool bits. They have a hardness up to about HRC 93 and sharp cutting edges and positive rake angles are to be avoided. Cermets Cermets are stable and moderately expensive. They are cemented materials based on titanium carbide (TiC). They binder is usually nickel, which provides higher abrasion resistance compared to tungsten carbide at the expense of some toughness. It is also far more chemically inert than tungsten. Cermets have an extremely high resistance to abrasion. They are used primarily on turning tool bits although research is being carried on producing other cutting tools. They have a hardness of up to about HRC 93 and sharp edges with them are generally not recommended.? Cubic Boron Nitride (CBN) CBN is the second hardest substance known but it is also the second most fragile. It is stable and expensive. CBN offers extremely high resistance to abrasion at the expense of much toughness. It is generally used in a machining process called hard machining, which involves running the tool or the part fast enough to melt it before it touches the edge, softening it considerably; it is used almost exclusively on turning tool bits. CBN has hardness higher than HRC 95 and sharp edges are generally not recommended with it. Diamonds The hardest substance known to date, diamonds have limited application due to the high cost and the small size of the stones. They are stable and are used on very hard materials to produce a fine finish and on soft materials, especially those inclined to clog other cutting materials. They offer superior resistance to abrasion but also high chemical affinity to iron which results in them being unsuitable for steel machining. Extremely fragile, they are generally used at very high cutting speed with low feed and light cuts. Due to the brittleness of the diamonds the machine has to be designed to be vibration free. The tools last for 10 (up to 400) times longer than carbide based tools. Diamonds are used almost exclusively on turning tool bits although they can be used as a coating on many kinds of tools. Sharp edges generally not recommended. ? CUTTING FLUIDS Cutting fluids are used in metal machining for a variety of reasons such as: -Improving tool life -Reducing work piece thermal deformation -Improving surface finish -Flushing away chips from the cutting zone The properties of a good cutting are: Maintaining thermal stability of the work piece †¢Maximising the life of the cutting tip by lubricating the working edge and reducing tip welding. †¢Ensuring safety for the people handling it (toxicity, bacteria, fungi) and for the environment upon disposal. †¢Preventing rust on machine parts and cutters. Cutting fluids fall into one of five categories: -Liquid -Paste or gels -Aerosols -CO2 C oolants -Air or other gases Liquid There are four main types of liquid cutting fluids: †¢Straight oils †¢Soluble oils †¢Semisynthetic fluids †¢Synthetic fluids Straight Oils Straight oils are non-emulsifiable and are used in machining operations in an undiluted form. They are composed of a base mineral or petroleum oil and often contain polar lubricants such as fats, vegetable oils and esters as well as extreme pressure additives such as Chlorine, Sulphur and Phosphorus. Straight oils provide the best lubrication and the poorest cooling characteristics among cutting fluids. Synthetic Fluids They contain no petroleum or mineral oil base and instead are formulated from alkaline inorganic and organic compounds along with additives for corrosion inhibition. They are generally used in a diluted form (usual concentration 3 to 10%). Synthetic fluids often provide the best cooling performance among all cutting fluids. Soluble Oil Fluids They form an emulsion when mixed with water. The concentrate consists of a base mineral oil and emulsifiers to help produce a stable emulsion. They are used in a diluted form (usual concentration 3 to 10%) and provide good lubrication and heat transfer performance. They are widely used in industry and are the least expensive among all cutting fluids. Semi-synthetic Fluids They are essentially a combination of synthetic and soluble oil fluids and have characteristics common to both types. The cost and heat transfer performance of semi-synthetic fluids lies between those of synthetic and soluble oil fluids. Pastes or Gels They are used for some applications, in particular hand operations such as drilling and tapping. In sawing metal with a bandsaw, it is common to periodically run a stick of paste against the blade. This product is similar in form factor to lipstick or beeswax. It comes in a cardboard tube, which gets slowly consumed with each application. Aerosols Some cutting fluids are used in aerosol (mist) form. They were traditionally regarded as a health hazard to workers and notoriously difficult to use with precision. However, a newer form of aerosol delivery, MQL (minimum quantity of lubricant), avoids both of those problems. The delivery of the aerosol is directly through the flutes of the tool, i. e. through or around the insert itself. MQL is delivered in such a precisely targeted way that it can be regarded as almost like dry machining. The chips generally seem like dry-machined chips, requiring no draining, and the air is very clean. MQL doesnt provide much cooling in the sense of heat transfer, but its well-targeted lubricating action prevents some of the heat from being generated in the first place, which helps to explain its success. CO2 Coolant Carbon Dioxide can also be used as a coolant. In this application pressurized liquid CO2 is allowed to expand and following the ideal gas law, this is accompanied by a drop in temperature, enough to cause a change of phase into a solid. These solid crystals are redirected into the cut zone by either external nozzles or through-the-spindle delivery, to provide temperature controlled cooling of the cutting tool and work piece. Existing CNC machines can be retrofitted with this safe and environmentally coolant approach. In applications such as turning, milling or drilling tool life and throughput have been improved substantially, especially in High Temperature Alloys such as titanium, 4140, steels and plastics. Air or other gases Compressed air, supplied through pipes and hoses from an air compressor and discharged from a nozzle aimed at the tool, is sometimes used as a coolant. The force of the decompressing air stream blows chips away, and the decompression itself has a slight degree of cooling action. The net result is that the heat of the machining cut is carried away a bit better than by ambient air alone. Liquid nitrogen, supplied in pressurized steel bottles, is sometimes used in similar fashion. Cutting Fluid Selection Criteria The principal criteria for selection of a cutting fluid for a given machining operation are: †¢Process performance : oHeat transfer performance oLubrication performance oChip flushing oFluid mist generation oFluid carry-off in chips oCorrosion inhibition oFluid stability (for emulsions) †¢Cost Performance Environmental Performance †¢Health Hazard Performance Cutting Fluid Maintenance and Disposal Cutting fluid maintenance involves checking the following variables: -Concentration of soluble oil emulsions (using refractometers) -PH (using a pH meter) -Quantity of tramp oil (hydraulic oil leaking into the cutting fluid system) -Quantity of particulates in the fluid. Action taken to maintain the fluid includes -Adding make-up concentrate or water -Skimming of tramp oil -Adding biocides to prevent bacterial growth -Filtering the particulates by centrifuging