Mastering Metal Machining

Machining is a term covering all the various methods by which metals are shaped with powered ribbon cutting tools. The cutting operation consists essentially in removing layers of metal by the wedging action of the cutting tool. This principle applies to all cutting operations, whether carried out by hand tools or by the many types of power driven machine tools. Nowadays, a large number of these power driven tools are used in industry, all of which adapt the wedge principle to a particular purpose. For example, planers are used for smoothing large flat surfaces and milling machines for facing or slotting small components. Large holes are normally produced in boring mills, whilst for smaller sizes drilling machines are used. Then there are the more specialized types of tool used mainly in large scale production. The broach has a series of cutting edges, each of which carries the cut a stage further than the one before. The hob is a modified milling cutter designed to cut gear teeth. The grinding wheel on the other hand is one of the commoner tools. Its rough surface is really made up of thousands of minute cutting edges. Grinding is used mainly for finishing operations. The most familiar and most versatile of all machine tools is the lay. The object of any machining operation is to remove a definite amount of metal as easily and as quickly as possible, and at the same time to achieve accuracy and a satisfactory surface finish. The efficiency with which this can be done depends basically on three factors. The physical characteristics of the metal to be cut, that is, the workpiece, the design and nature of the tool, and finally during the operation, the relationship between the tool and the workpiece. This of course includes factors such as the cutting fluid, the nature of the operation, and the design of the machine tool. Each of these factors may vary considerably, and in order to get the best out of a machine tool, something must be known of the background common to all machining operations. The first thing to be considered is the nature of metal itself. If a polished specimen of pure metal is examined under the microscope, it is seen to consist of a patchwork of irregular and closely interlocked grains. Each of these grains though irregular externally is made up of atoms arranged in a regular pattern called a lattice. If pressure is applied to the metal, rows of atoms slide over each other along what are called slip planes. Pressure is transmitted from one grain to another, and so causes deformation and a rearrangement of the grain structure. If the pressure is continued beyond a certain point, however, the metal fractures by shearing along favorable slip planes. This is a general picture of what happens when any metal is machined. But different metals show varying resistance to the pressure of the cutting tool. In other words, the machine ability varies according to that particular properties. Some of these properties can be assessed by means of a few simple mechanical tests. In the eyes odd test for example, a brittle metal fractures without appreciable deformation. Cast iron is a common example. On the other hand, tough metals such as stainless steel absorb a great deal of energy before fracture. Metals that stretch considerably before breaking in the tensile test are said to be ductile. Another property that affects the machine ability of metal is hardness. One way of measuring this is to bring a hardened steel ball into contact with the test piece, and then apply a standard load to it. As in the Brunel test. Hardness may be defined as the resistance to indentation. When a metal is being machined. The tool must overcome the resistance of the workpiece. For this reason, tools in general are considerably harder than the metal that they have to cut. They must also be tough enough to withstand the heavy stresses, which are a feature of most machining operations. Although the design of cutting tools varies greatly according to the particular job they have to do, they can be broadly divided into groups having certain features in common. For example, drills, remmers, and taps are rather similar in general shape and are all used to produce internal diameters. Milling cutters, saws and broaches, on the other hand, look very different from each other, but all have one thing in common they belong to the group of cutting tools, having a number of cutting edges, in contrast to the single pointed group as used for shaping, planing and turning. But in all tools the most important feature is the cutting edge. This must be the right shape, have adequate support and must be suitably sharp. Tools are given their final form by grinding. This is a skilled job and the tools should be checked before starting on any machining operation to make sure that the angles of the various faces are appropriate. For example, the rake angles are very important. These are the angles which the working face makes with the workpiece. Then there are the clearance angles. These must be large enough to allow the tool to clear the workpiece, but not so great as to weaken the cutting edge. Following the preparation of the tool comes the setting up for a particular job. Here, the skill and experience of the setter account for a great deal, but there are some general rules for each type of machine On a lathe for example, the setter will ensure that the cutting edge is exactly the same height as the center of the workpiece, and that the overhang from the tool support is as small as possible, so that the tool has maximum rigidity. We have said that the prime object of any machining operation is to remove a certain quantity of metal as easily and as quickly as possible. Taking this shaper as an example. Let's see how this object can be achieved. The first thing to consider is the speed of cut. Obviously, the greater the speed at which the tool cuts through the metal, the greater the amount of metal removed in a given time. Most machine tools can be run at various speeds, and in theory, the cutting speed can be increased up to the limit of a particular machine. In practice, of course, the person in charge of an operation will decide on the most suitable cutting speed for the particular job. The choice of speed will depend on the sort of metal being cut and the type of tool being used. But speed of cut is not the only factor to consider when machining metal. The depth of cut can also be varied. Instead of taking a shallow cut at every stroke, the two can be set so as to cut more deeply into the workpiece. Then there is another factor the feed. This might be called the sideways amount of metal removed at each cut. As in the case of the speed and depth of cut. This can be varied according to the job. If these three factors speed of cut, depth of cut, and feed are multiplied together, we get the volume of metal removed in a given time. Each or all of them can be varied, but the final choice will depend on the limitations of the machine tool. The properties of the metal being cut. The rate of tool wear. And the surface finish and degree of accuracy required. Let us first see how the properties of the workpiece can affect a machining operation. Roughly speaking, a ductile metal is one that will stretch considerably before breaking. When a ductile metal is being cut. The grains tend to deform rather than fracture. This causes the severed portion of metal to flow over the tool as an unbroken ribbon, forming what is called a continuous chip. This picture has been slowed down and considerably magnified, and the operation performed without cutting fluid so as to show the actual process of tip formation with metal of this type. The cutting action is relatively smooth, and in such cases a high cutting speed with a moderate feed and depth of cut is normally used. In contrast to ductile metals. Brittle metals behave quite differently. The chip fractures readily under the shear stresses caused by the pressure of the tool. As a result, the metal comes away in short lengths or segments, giving what is called a discontinuous chip. Brittle metals such as cast iron are best cut at moderate speeds with a fairly high rate of feed and depth of cut. Apart from the characteristics of the metal, the conditions of machining are also affected by whether the cut is a roughing or a finishing one. During a roughing cut, metal is normally removed at high speeds as finish and accuracy are not important. But with finishing cuts, the speed, feed and depth of cut are chosen so as to give the desired accuracy and surface finish. In neither case, should the rate of metal removal be so high as to cause excessive tool wear. When a metal is machined, heat is produced at the tool tip as the result of friction. Some of this is internal friction caused by the deformation involved in producing the chip. Some is external friction caused by the chip rubbing against the tool face as the cutting rate or the pressure is increased. The temperature rises until it reaches a point where the tool may be damaged. During the 19th century, metals were normally machine dry. This meant that cutting speeds had to be kept down to avoid damage through overheating. But towards the end of the century, a discovery was made which had a great effect on the efficiency of machining operations. It was found that when a stream of water was directed onto the cutting surface, it was possible to increase the cutting speed without any increase in tool wear and with no loss of accuracy or deterioration in surface finish. But although water is an ideal coolant, a simple experiment will show that it has one major disadvantage as a cutting fluid. A few drops are placed on a steel plate, and a similar quantity is mixed with some steel chips of the type produced during drilling. The procedure is then repeated, only this time a soap solution is used. The plate is then left in a dry place. After 24 hours, the steel chips are removed and the plate is cleaned with a dry cloth. The water has caused severe corrosion of the metal, whilst the soap solution has left only a faint mark. As corrosion can seriously damage machine tools. Soap solutions were much preferable to water as cutting fluids. They have the same cooling properties and make it possible to increase cutting speeds by reducing the temperature of the tool tip. The action of a coolant is simple, as it flows over the chip and the tool, it absorbs and carries away much of the heat which has been generated. Generally speaking, the action is best when a large volume of coolant is applied at a low pressure. For some operations, however, straight coolant like soap solutions offer only a slight advantage. They have little lubricating power and therefore do not materially reduce the friction, which is the main cause of tool wear. For this reason, oils of both the petty and mineral types are used as cutting fluids. They have less cooling power than water, but by reducing friction they can increase tool life, particularly on severe operations. Cutting oils also have another advantage. This can be demonstrated by adapting a milling machine for use as a planer and setting it up to machine a steel bar. At certain cutting speeds. The surface finish is rough and obviously unsatisfactory for most purposes. The machine is run long enough to make sure that the finish is consistent, and then a single drop of oil is introduced into the rake crevice. There is an immediate and very obvious improvement in the surface finish. A result is marked as this is not necessarily achieved in all machining operations, but it does show the value of using a lubricant. In certain cases where surface finish is important. As well as the straight cutting oils. We have the special class of soluble oils, when mixed with water. The oil disperses into droplets to form an emulsion. These emulsions combine the high cooling power of water with some lubricating effect from the oil, and nowadays have almost entirely replaced the older types of aqueous fluids. Apart from the advantages gained from the use of cutting fluids, there have been great improvements in other spheres, and in particular the development of cutting tool materials. For many years, steel containing a high percentage of carbon and known therefore as high carbon steel, was the material generally used for tool manufacture. Much of the best steel of this type was produced by the Crucible method. After careful preparation in the furnace, teaming takes place. In other words, the molten metal is poured from the crucible into molds. Here it sets into ingots from which the tools are ultimately made. Heat treatment is the final stage in production. The tool is taken from the furnace, where it has been heated to the hardening temperature, and is then quenched. This gives it the necessary hardness to do its job. High carbon steel tools are quite suitable for cutting operations at low speeds. But when the cutting rate is increased, the tool softens rapidly as a result of the heat generated and may collapse entirely. Carbon steel tools impose a definite limit on machining rates, but about the same time as the first experimental work on cutting fluids was taking place, there was an important development in the production of tools steels It was found that when certain elements, especially tungsten, were added to the steel, a very much better tool material could be produced. These high speed steels, as they are called, made it possible to increase machining rates to a much higher level than previously. This is possible not only because the tool has the necessary hardness and toughness, for its job, but because it has another property sometimes known as red hardness. Unlike carbon steel, high speed steel does not lose its hardness as a result of the high temperatures produced during cutting. But because higher speeds and loads of permissible, a new problem arises when cutting certain metals with high speed tools. As the underside of the chip rubs against the breast of the tool, two chemically clean surfaces are in contact under conditions of high temperature and pressure. In these circumstances, some welding between the chip and the tool may occur. In practice, this results in tiny particles of metal torn from the underside of the chip, forming what is known as a built up edge. Up to a point, this will not matter, as the build up helps to protect the cutting edge and prolongs tool life, but when it becomes excessive and continually breaks off and reforms, both accuracy and finish suffer. In such cases, something more than ordinary straight oils or emulsions may be needed. This has led to the development of oils with special anti welding properties and special tests to measure these properties. In the four ball test for example, three steel balls are locked in a cup containing the oil and placed in contact with a fourth ball clamped to a shaft. When the machine is started, the shaft turns at high speed, spinning the fourth ball against the other three. The friction between the fourth ball and the others is recorded on a rotating drum. When the test is run using a straight mineral oil welding occurs very rapidly. The conditions which caused the four balls to weld together are similar to those which produce a build up on the tool during machining. However, if certain substances embodying sulfur or chlorine, and occasionally other elements are added to the oil welding can be prevented even under very severe conditions. In this case, an oil containing special additives is compared with the straight oil used in the previous test. After a much longer run, there is still no suggestion of welding. These extreme pressure or EP oils, as they are called, are used nowadays in many operations such as tapping, their especially useful where tools are expensive and where a good surface finish is needed. They are one more factor in the development of higher standards and higher efficiency. During the past few years. There have been many new problems in machining. On the one hand, the development of new and often exceedingly tough alloys, such as those used in the manufacture of gas turbines and turbo jets, and on the other, the continual rise in industrial output have created a demand for even better cutting materials. One result has been the production of tools fitted with tips of sintered carbide or special non-ferrous alloys. Although exceedingly hard, these materials are also brittle. For this reason, the special tips are brazed onto steel shanks, which gives the tool strength as well as hardness. These special tools can be run at very high speeds without any danger of the chip welding to the tool. Therefore, they are often run dry. When a cutting fluid is needed. An emulsion is generally selected to wash away swath and to prevent distortion of the workpiece by keeping it cool. Modern industry is always seeking greater efficiency by increasing output and reducing production costs. To this end, the use of automatic machines for the production of standard parts is continually increasing. Once set up and apart from periodical resetting, these machines operate entirely automatically and produce finished parts at the rate of hundreds each hour. This streamlining of production has been achieved as the result of ingenuity and careful planning. It is backed up by research which aims to establish precise standards and increase efficiency still further. The actual equipment for research in metal machining usually consists of standard machine tools, plus special instruments for close control and measurement. An example of this control can be seen in the preparation of tools. For instance, when this tap has been carefully ground, a wax impression is taken and projected onto a screen. Examination of the enlarged image reveals any small inaccuracies which might have a serious effect on performance. The machining tests are often mechanized to eliminate the human element. During the investigations many factors, such as the speed of the operation, the exact form of the tool, and the effect of different types of cutting fluid must be carefully controlled and their effects measured. For this reason, a complete record is made of the torque variations during this test. The accurate grinding and preparation of tools is a vital factor in machining. In this case, an investigation is being carried out on the preparation of drills. The exact relation between the two cutting lips of a drill is carefully measured and recorded. For real efficiency very accurate measurement is essential because even a slight variation can have a critical effect on drill life. Drilling test are carried out on large and very rigid machines designed to give negligible deflection under load. Data produced during the tests is recorded on special instruments. Apart from providing information on the effect of tool angles on cutting efficiency, tests of this sort can be used to determine the machine ability of different metals and for evaluating the properties of different cutting fluids. In this connection, correct application of the fluid is most important. Haphazard distribution doesn't give the fluid a chance to do its work properly, and some type of distributor which will concentrate the fluid onto the cutting point is desirable. Knowledge and experience in metal machining, coupled with basic research are the background to more efficient production. In spite of the apparent complexity of various machine tools, all machining operations depend on certain fundamentals. Among these are the physical characteristics of the workpiece, the design and nature of the tool, and the relationship between the tool and workpiece. Efficient machining is a compromise based on sound knowledge of all the factors involved, and the skill to apply that knowledge to a particular job. This is the background to the accurate machining of metal.