وبلاگ مهندسی مکانیک ( مکانیک خودرو-حرارت و سیالات- تاسیسات)
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In this method the same mould is used for large numbers of castings. Each casting is released by opening the mould rather than by destroying it. Permanent moulds need to be made of a material which can withstand the temperature fluctuations and wear associated with repeated casting. A good example of a product made with methods such of this is the ubiquitous ‘die-cast’ child’s toy (‘die’ is another word for ‘mould’).
Figure 18: Die-cast toy
Expendable mould and pattern
With this type of casting, a pattern is made from a low melting point material and the mould is built around it. The pattern is then melted or burnt out as the metal is poured in. The mould has to be destroyed to retrieve the casting.
This method is used to make moulds for casting high melting-point alloys like those used for jet engine turbine blades (Figure 19). A model (the pattern) of the blade is made in wax. The pattern is then coated in a thick slurry containing ceramic particles. The slurry dries, and is then fired in an oven: this hardens the ceramic (like firing a pot) and melts out the wax, leaving a hollow ceramic mould. The metal is then poured in to the mould, which is broken away after the metal has solidified and cooled.
Figure 19: A turbine blade
2.4 Casting processes
Casting is used to produce ingots which are then used as the raw materials for forming processes such as rolling or extrusion. As an intermediate processing step, casting needs to be less carefully regulated (than other processes to make engineering products), as the properties of the final product are controlled by the forming processes which follow casting. Therefore we will concentrate on casting processes which make components.
Casting processes vary depending on the type of solid to be produced and the type of fluid used to fill the mould. The type of mould required depends on the material to be cast, and in particular, on the temperature at which it is sufficiently viscous to flow into the mould. Metals are cast when molten, so we should consider their melting point, whereas for polymers (whose fluidity can change markedly with temperature) we need to know the temperature where viscosity is low enough for reasonable flow.
2.5 Casting metals
Sand casting is illustrated in Figure 20. A solid replica of the required object is made: the ‘pattern’. Sand is then rammed around the pattern in a ‘moulding box’. When the pattern is removed it leaves a shaped cavity behind. The runners (where the fluid is poured in) and risers (where excess fluid can escape) also act as reservoirs of liquid to top up the casting as the metal contracts on cooling.
Figure 20: The sand-casting process
The process can be used, perhaps surprisingly, to make hollow castings. To do this, ‘cores’ are inserted into moulds to produce shapes that would be difficult or impossible to make by just using a pattern.
The mould is destroyed when the solid casting is removed. The surface of the castings produced by this method tend to be rather rough, even though quite fine-grained sand is used for the moulds. So some machining (cutting) of the surface is generally required before a finished product is made from a sand-cast route. Certainly the runners and risers need to be cut away.
Sand casting is particularly useful for casting complex 3D shapes such as automobile cylinder heads or large castings as shown in Figure 21
Figure 21: Sand cast (a) cylinder head (b) oil rig joints
Gravity-die casting, Figure 22, is similar to sand casting except that the mould is machined from solid metal, usually cast iron. This means that the mould and cavity are permanent. Being metal, the mould can be machined accurately and, having good thermal conductivity, it allows the casting to cool quickly. The surface finish is better than can be produced by sand casting, but as metal moulds are required, product sizes are generally smaller than those possible with sand casting (because a metal mould will cool the liquid faster than a sand mould would, making it harder to fill the mould evenly if it was too large). Typical products include bicycle cranks and engine pistons. Of course, the metal being cast must have a lower melting point than the mould metal!
Figure 22: Gravity-die casting
Pressure-die casting, (Figure 23) is a development of gravity-die casting in which the molten metal is injected into a steel mould under pressure; it is the metal equivalent of injection moulding (which we will discuss shortly). Again, the metal being cast must have a lower melting point than the mould material. Pressure-die casting is quicker than sand- and gravity-die casting and because the fluid is under pressure, finer surface details can be replicated. It is commonly used for door handles, electric iron bases and hollow sections requiring fine detail such as carburettor bodies.
Figure 23: Pressure-die casting
2.6 Casting plastics
Injection moulding is used mainly for thermoplastic polymer materials. When heated, thermoplastics do not become as fluid as metals so they cannot be shaped by gravity-fed casting methods. The injection moulding process has been developed specifically for thermoplastics.
The process is illustrated in Figure 24, which shows the main features of an injection-moulding machine. The raw polymer, in the form of solid granules, falls under gravity from a hopper into a cylinder where it is propelled along by a rotating screw into an electrically heated section. As the material is heated, it softens and flows. When the cylinder contains enough material to fill the mould, the screw action is stopped. In the final stage, the screw moves axially, acting as a ram, injecting the material through a small nozzle, and down channels (runners) into the shaped cavity within a cooled mould. When heated, most polymers start to degrade before they reach a sufficiently high temperature to fill a mould adequately under gravity alone. Injection moulding imposes high shear flow rates on the polymer as it is squirted at high pressure into the die. This tends to align the long polymeric molecules and increase the fluidity of the polymer substantially. This shear thinning of the molten polymer is essential to injection moulding and can only be achieved if high injection pressures are used.
Figure 24: Injection moulding
2.7 Casting microstructure and defects
Metal castings have very specific microstructures. When a liquid metal cools and begins to solidify in a mould, grains (crystals) of the metal start to form, both on the mould walls and in the bulk of the liquid metal. The way they grow is shown schematically in Figure 25(a). As the metal solidifies, it forms curious tree-like dendrites (from dendron: the Greek for tree). This structure is maintained after the casting is fully solidified, as can be seen from Figure 25(b), which shows a typical casting microstructure. (The image is created by polishing the surface of the metal, immersing it for a short while in a dilute acid and viewing it under an optical microscope.) In addition to the dendritic structure, there are two other common defects that can be found in a cast microstructure: particles of impurities known as inclusions, and porosity which is small holes in the casting.
Figure 25: Castings (a) dendritic formation (b) a typical cast microstructure
Some inclusions can be removed by heating the casting to a temperature somewhat below its melting point to anneal it and ‘dissolve’ the inclusions in the metal; but the porosity is more difficult to remove. The porosity occurs because the casting has shrunk on solidification. Most materials contract on solidification (water is one of the few liquids that expands on solidification, so that ice floats on water; bad news for the Titanic, but good news for polar bears) and this shrinkage is not always uniform, so that substantial holes and voids can be left in the casting. This reduces the load-bearing capability of the component, and in highly stressed products, where the full strength of the material is being utilised, voids can lead to failure. The shrinkage on solidification can be large, and is generally a greater effect than the thermal contraction of the solid material as it cools to room temperature.
In many casting processes, runners and risers are used as reservoirs of molten metal to prevent voids from developing in the casting as it solidifies. The runners and risers are parts of the casting which contain a ‘reserve’ of extra liquid to feed into the mould as the cast product contracts during cooling. However, if a volume of liquid material becomes surrounded by solid material, then a void is formed when the liquid solidifies and contracts. Figure 26 shows a section through a gravity-die casting in which the effects of this contraction can clearly be seen. The chimney-like feature is the runner, down which liquid aluminium alloy was poured into the mould. There is a hollow in the top of the runner caused by liquid flowing from the runner into the mould as the casting solidified. As well as the hollow at the top, you can see some holes in the runner and one hole within the casting itself. The runners and risers will later be cut off and discarded.
Figure 26: Section though a gravity die-cast microscope body
When we are using casting to form the final shape of a product, we have to live with the microstructure of our casting, including its defects. But if we are casting ingots in order to produce sheet or bar metal for further processing, then a mixture of large deformations and high temperatures is typically used to ‘break down’ the cast structure, remove the porosity, and create a far more uniform microstructure. Such material is the typical raw material for the forming processes we will look at in the next section.
Polymers do not produce the same cast microstructures as are seen in metals, as they are composed of long-chain molecules, rather than grains built up from an atomic lattice of metal atoms. However, polymers do shrink on solidification and in injection-moulded products, shrinkage holes can form, particularly within thick sections. Figure 27 shows such holes in an injection-moulded nylon gear. Alternatively, the contraction may take the form of depressions on the surface (‘sink marks’). In an effort to ‘feed’ shrinkage holes with liquid, the pressure is maintained for a short time after the thermoplastic has been injected. Similar holes are found in pressure-die castings.
Figure 27: Section though a moulded nylon gear showing three large shrinkage holes
2.8 Casting our gearwheel
Let’s now consider the problem of how best to make the food mixer gearwheel we discussed in Section 1. Could it be made by any of these casting processes? Since it is a simple solid object the answer must be yes, of course, but which processes are feasible? If the gearwheel is to be made from metal then we can consider all of the casting processes we have described:
Let’s consider each of these in turn.
Sand casting is unattractive for volume production of this shape and size. The cast wheel would have a rough surface which would need to be machined, and a new sand mould would be needed for every product. Because the wheel is such a small component, there would be a lot of scrap, and so this makes the process rather expensive and time consuming when you need to produce of the order of 100,000 gearwheels. However, sand casting can be used for mass production of parts such as engine blocks for cars, where it is more economical than other processes. We can probably rule it out for production of the gearwheel, though!
Gravity-die casting gives a better surface finish and the die is reusable almost indefinitely. However, even this surface would need some machining in order to achieve the required accuracy of tooth shape of the gear, and to remove the runner and any little sheets of extra material (‘flash’) at the splits in the mould. In addition, it is a slow process, so gravity-die casting is probably not the best option.
Pressure-die casting looks promising. Here the as-cast surface needs little or no finishing, and provided the casting has the required strength for the application then this would a feasible option.
Next we consider injection moulding. Provided a thermoplastic is acceptable for the gearwheel, this process can be used. It produces an excellent surface finish (better than pressure-die casting) and has a short cycle time. Again, provided the moulding is strong enough, then injection moulding is a feasible option. So at this point in our analysis the first two candidate processes to manufacture our gearwheel are pressure-die casting and injection moulding.
Forming processes involve shaping materials which are solid. As mentioned before, a simple example is moulding with Plasticene. However, metals can be moulded using forming processes as well, as long as their yield stress is not too high and enough force is used. One way to lower a metal’s yield stress is to heat it up. So we can shape metals without melting them; think of the blacksmith working on a horseshoe, heated, but still solid.
However, we have identified a key quandary in forming that actually applies to materials processing in general. The properties you want from a material during processing often conflict with the material properties you require for the product in service. If you have decided that the best route to make something is to squeeze it into shape, then the properties that are required to make the product will clearly be different from the properties required when in use. For easy forming, a material needs to be soft, with a low flow (or yield) stress. These are not properties that are generally attractive or useful in finished products. More often, high strength is required; so some way must be found to make the forming of such products easier – often through the use of high temperatures (see Properties for processing – forming).
3.2 Properties for processing – forming
Forming processes involve applying forces to the material being shaped. A good way of telling how a given material responds to applied force is to look at diagrams representing its stress-strain behaviour. Figure 28(a) shows the stress-strain curves, at room temperature, for two different metals. The two important things for the feasibility of squeezing-type processes are the point at which the solid starts to flow and the extent to which it can be persuaded to flow before it separates (i.e. fails). This is described by two properties, yield stress (or flow stress) and ductility. Remember that the yield stress is a good measure of the strength of a ductile material.
Figure 28(a): Schematic stress-strain curves for: A – a steel, B – a ductile metal such as lead
If you look at the curves, you can see that the one for the steel (curve A), after the elastic region, shows plastic deformation up to a strain of about 40 per cent. This provides a measure of the ductility of the steel, and the extent to which it can be squeezed, stretched or bent at this temperature. Curve B (for lead) shows much higher ductility and a much lower yield stress.
However, 40 per cent is not a lot of strain for many manufacturing purposes. Being able to change a material’s dimensions by only 40 per cent would mean that forming would be virtually a waste of time. In addition, once the steel in Figure 28(a) has been strained plastically, it has also increased in yield stress and become harder (Figure 1.28(b)).
Figure 28(b): The effect of progressively straining a material
The steel can be loaded elastically up to its yield strength, point A in Figure 28(b). Removing the load below this level leaves no permanent extension, i.e. the steel can return to its original size and shape. If the material is taken above its yield strength, say to point B, then even after unloading, the steel is permanently deformed. What’s more, when the material is then reloaded, it has to be loaded to a stress equal to point B – i.e. more than its original yield point at A – before it continues to flow. This is called work hardening: the plastic deformation causes an increase in strength (and hardness). But further plasticity is then limited. If the material is loaded to point C, where about half the available plasticity has been ‘used up’, then on reloading only the remaining plasticity from C to D is available before the material breaks.
However, all is not lost. The work hardening effect can be eliminated, and the original, softer, condition restored, by annealing the metal. This involves heating to a temperature where the atoms in the material become more mobile, so that the material softens. This type of process is widely applied in manufacturing whenever a part-formed product has been worked so much that it is in danger of cracking; and is why blacksmiths keep reheating a horseshoe during working. The precise mechanism of softening varies from material to material but it occurs for most materials at a high homologous temperature. Homologous temperature,T H, is the ratio of the operating temperature of a material to its melting point (in kelvins, K).
In the context of forming, if a metal is ‘hot worked’, it is deformed at a temperature where there is virtually no work hardening, and this resoftening effect carries on continuously. Very high strains can be imposed during the forming process.
Conversely ‘cold working’ does result in work hardening, so that the material gets harder the more it is deformed. ‘Cold working’ does not literally mean cold, though – it is all measured relative to the homologous temperature. The rule of thumb usually employed is that cold forming occurs at homologous temperatures below 0.3 and hot forming occurs at homologous temperatures above 0.6. In between the two, there is a region known as warm forming. This means that if tungsten is worked at 1000°C (1273K) it is being cold formed, as its melting point is 3410°C (3683K). Conversely, at room temperature, solder can be hot worked! So working temperature is all about being able to deform the metal without failure.
3.3 Forming v casting
As the stresses needed to make solids flow are considerably higher than those required for liquids, forming processes normally require a lot of energy and strong, resilient tooling. This means high expenditure on capital equipment as well as tooling and energy. As a result, forming is often economically viable only for production volumes large enough to justify the high tooling costs.
So when do we use forming rather than casting? There are three reasons why, for many products, forming is preferable to casting.
Why are car-body panels produced by forming and not casting? (Hint: think in terms of the shape of the final product and the form of the starting material.)
Producing thin products such as a car-body panel is very difficult by casting. To fill the mould would require the casting liquid to be very fluid and to stay that way while it filled all of the mould cavities. This is difficult to achieve. Hence it is easier to form the body panels, probably starting with a sheet of material that is pressed into shape.
3.4 Forming processes
Forming processes are used to convert cast ingots into basic product forms such as sheets, rods and plates, as was noted in the previous section. However, here we will concentrate on forming processes that produce end products or components. There are some basic shapes that lend themselves to manufacture by forming. Forming processes are particularly good at manufacturing ‘linear’ objects, that is, long thin ones, where the product has a constant cross section. Forming processes involve moving the material through an opening with the desired shape. These processes are used for making components such as fibres, wires, tubes and products such as curtain rails. The plastic ink tube in your ballpoint pen was almost certainly produced by this method.
The principle of this process is very similar to squeezing toothpaste from a tube. Material is forced through a shaped hollow die in such a way that it is plastically deformed and takes up the shape of the die. The hole in the die can have almost any shape, so if the die is circular, for example, a wire or rod is produced (Figure 29).
Figure 29: Metal extrusion
It is also possible to produce hollow sections using extrusion. In this case, the die contains a short piece (or mandrel) in the shape of the hole. This mandrel is attached to the die by one or more ‘bridges’. As the extruded material encounters the bridges it is forced to separate, but it flows around the bridges and joins up again, much the same as water flowing around the piers of a bridge. Figure 30 shows such a ‘bridge die’. This works successfully even for processing of solid metals.
Figure 30: Extrusion bridge die making a hollow section product. Note that in the picture the die has been split to show the material passing through it. In reality, the die and the ring fit together, with a gap for the extruded material to flow through
Extrusion can be used on most materials that can plastically flow as a solid, and solid metals and alloys are frequently extruded. To reduce the stresses required, and therefore the size and cost of the extrusion machine, and also to ensure hot working conditions, a metal is usually extruded at a high homologous temperature, usually between 0.65 and 0.9. This allows large changes in the shape of the material – and hence large strains – without fracture. During metal extrusion the raw material in the form of a metal ingot, known as the billet, is heated and pushed through the die by a simple sliding piston or ram.
The mechanism for extruding thermoplastics is illustrated in Figure 31. In this case a rotating screw is used to transfer the raw material in the form of granules through a heated cylinder to the die, just like in the case of injection moulding for polymers. The thermoplastic granules are compressed and mixed by the screw (the granules may contain a second constituent such as colouring). The material softens and melts as the temperature rises due to heating through the walls of the cylinder, and also from the heat generated within the thermoplastic as it is sheared by the screw. The thermoplastic flows through the die and emerges with a constant cross section in the shape of the die aperture. An almost infinite variety of cross-sectional shapes can be produced.
Figure 31: Thermoplastics extrusion
Could extrusion be used for the following products?
In rolling (Figure 32), material is passed through the gap between two rotating rollers that squeeze the material as it passes between them. The rolled material emerges with a thickness roughly equal to the gap between the rollers. When the rollers are cylindrical, rolling produces material in the form of plate or sheet. Sheet steel and aluminium for the bodies of cars and domestic appliances is made this way. Rolled sheet is often termed a ‘semifinished’ product, as it requires further processing to shape it into the final product.
Figure 32: Rolling
Rolling is not restricted to flat sheets, though. If the desired product has a contoured surface, then by using profiled rollers the contour can be rolled on. If the surface pattern needs to be deeper than is possible during one rolling pass then multiple rollers can be used; for example, railway tracks are made by rolling between pairs of progressively deeper contoured rollers. The various stages for rails are shown in Figure 33.
Figure 33: Stages in rolling railway track
In common with other forming processes, metals may be hot or cold rolled. The significant differences between hot and cold rolling are in the amount of energy needed to roll a given volume of material and in the resulting microstructures. The cooler the metal, the higher its yield stress and the more energy has to be supplied in order to shape it. As in extrusion, metals in large lumps are often hot rolled at homologous temperatures above 0.6. At this temperature the yield stress and work hardening are reduced. Railway lines require hot rolling in order to achieve the large change in shape from a rectangular bar. However, a major disadvantage of hot rolling is that the surface of the material becomes oxidised by the air, resulting in a poor surface finish.
If the metal is ductile then it may be cold rolled using smaller strains. This has some advantages: the work hardening at these temperatures can give the product a useful increase in strength. During cold rolling, oxidation is reduced and a good surface finish can be produced by using polished rollers. So, cold rolling is a good finishing treatment in the production of plate and sheet. The sheets of steel for car bodies are finished by cold rolling because a good surface finish is essential in this product.
3.7 Metal forging
Forging is typified by countless generations of blacksmiths with their hammers and anvils. Besides still being used for special ‘hand-made’ items, this type of forging is similar to that used, on a somewhat larger scale, for the initial rough shaping of hot metal ingots. Forging is particularly good at making 3D solid shapes. The basic types of forging processes are shown in Figure 34.
Figure 34: Forging processes
For example, the first stage in making a large roller or shaft would be to forge a large billet as sketched in Figure 34. The process consists of a large succession of bites between a pair of dies in an hydraulic press, with the ingot moved between each bite. As the ingot is moved through the dies it is reduced to a more manageable size before final shaping – this process is known as open die forging.
In closed die forging components are made in one action, being squeezed between upper and lower shaped dies as shown in Figure 34. There is usually a small amount of excess material which is forced out of the die cavity as flash. This must then be removed from the component. The force needed to close the dies together is dependent both on the size of the component and the temperature, since as we noted earlier, the flow stress reduces as the forging temperature increases. The quality of the surface finish of the forging decreases with temperature, however, because of increased surface oxidation.
3.8 Forming our gearwheel
We have just seen that simple 2D ‘linear’ objects can be produced by rolling, drawing or extrusion. So could our gearwheel be made using these techniques?
Because the gearwheel has a constant section it is geometrically feasible (actually the section is not quite constant because it is countersunk on one side, but let’s ignore this fine detail for the purpose of this discussion). Drawing is confined to small reductions in cross-sectional area during each pass, so this process does not look promising, but there is no such limit on extrusion. By extruding a deformable material such as metal or thermoplastic through a bridge die containing a gear-shaped hole and a square bridge it should be possible to produce a very ‘thick’ (or long, depending on how you want to describe it!) gearwheel (see Figure 35). This can then be cut up into identical gearwheels of the required thickness, so this might be a possibility.
Figure 35: Extruding a gearwheel
Would rolling be an alternative manufacturing process? It would be difficult to roll the gear teeth profile, but you could use rolled steel sheet as a starting material. You could then punch out a gear-shaped blank using sheet metal forming (Figure 36). This would be expensive, but is possible.
Figure 36: Sheet metal forming of a car body panel
However both extrusion and the sheet-metal route would produce flow lines not suitable for an engineering gear, as described in Failure of replacement gears.
So let’s go back to our gearwheel again and decide whether a gearwheel can be made by forging. The answer is yes, but only partly. Some other processing would be needed either before or after the forging process.
One approach is to start with steel bar (itself produced by rolling between contoured rolls). The bar contains longitudinal flow lines in its microstructure, produced from the rolling operation. This bar is forged into a circular, gear-sized, disc by compressing the bar parallel to its length (Figure 37).
Figure 37: Forging a gear
The forging ‘folds over’ the longitudinal flow lines into the radial direction. So in this case, all the gear teeth have the optimum orientation of flow lines and the final product is stronger than that made from rolling. Failure of replacement gears describes why the microstructure of gears can be of critical important to their performance.
Which of the processes (including both casting and forming) covered so far would be the most appropriate to manufacture the following products? Use the shape hierarchy for both casting and forming to help guide you through some of the options.
3.9 Failure of replacement gears
A heavy commercial vehicle company announced that it would no longer supply spare parts for one of its vehicles. This was a problem for customers, as the two largest gears in the gearbox tended to wear faster than the others and it became impossible to replace them even when the other parts still had a great deal of useful life. A specialist firm set out to manufacture spare parts and soon had orders for dozens of sets of these two particular gears.
This firm had measured the gears and manufactured new ones of identical dimensions, using one of the strongest steels available. It machined the gears and sent them out for heat treatment as, in addition to hardening and tempering the whole gear, the teeth had to be surface hardened to match the hardness of the originals.
All seemed to be well until a pair of gears was returned, having failed by teeth breaking off in just over three weeks’ service. The failure was assumed to be due to poor driving. So another pair was put in the gearbox, but a similar failure occurred in less than one week.
The problem was then easily identified by comparing the new gears to the originals.
The gears were made of steel that started off as cast billets. In the as-cast state, the steel has no directionality to its microstructure but, as it is worked down to billet or bar, directionality appears as grains and inclusions are extended in the direction of working. The directionality is revealed as ‘flow lines’ which resemble the grain in timber. For any given quality of steel, the strength is much better across the flow lines than in parallel with them).
Figure 38 is a view of the gear showing the inner upper ring of teeth which were breaking off. A radial section was cut from the original gear and a similar one from an unused new gear. These were polished and the flow lines revealed by etching. Figure 39 shows the old gear at the left and the new gear at the right. The teeth which were breaking off were the small ones which are at the top in the sections of Figure 38. In the old gear they are worn down, which is why the gears needed replacing. The unused new gear shows the teeth in side profile. The section has been cut so as to include the full section of the large teeth around the outside.
Figure 38: An unused gear (on the right) showing the ring of teeth which were breaking off – the smaller teeth at the top of the gear. The gear on the left was a used gear where the teeth have been worn away.
Figure 39: Sections etched to reveal flow lines: (left) original gear, inner teeth worn now; (right) new gear, inner teeth full profile. The teeth are on the bottom left of the cross-sections – compare the sections to the cuts in Figure 38.
What difference is there in the flow lines in these two sections?
In the new gear they all run parallel to the axis and, if this were made of wood you would expect them to break off easily. In the old gear the flow lines are in a looped, radial pattern tending to run at right angles to the gear’s axis. If you look carefully at the small teeth you will see that they run pretty well at right angles to the direction of the flow lines in the new gear.
The original gears had been forged before the teeth were cut, whereas the replacements had been machined directly from round bar. The flow lines tell us that the old gears had started off as a length of round bar about twice as long as the gear is thick, but of smaller diameter. This was hot forged to squash it down, causing the outside to spread out so that the shape finished up with a much larger diameter and shorter length than when it started. This was done specifically to develop the flow line pattern you can observe. When the teeth were cut, the grain flow was oriented at right angles to the applied bending stresses in service. The teeth were thus as tough as they could be for this type of steel and the flow lines were oriented in directions which imparted the maximum resistance to failure.
In contrast, the new gears had been machined from stock of the full diameter the gear required. There was no forging, so the flow lines remained parallel to the gear axis.
The result was that the flow lines in the teeth were in the most unfavourable orientation to resist fatigue and brittle fracture. This is why they broke off so quickly under service loads, despite having the same hardness (and tensile strength) as the original gears.
3.10 Powder processing techniques
Before we leave forming we should consider powder processing techniques, (Figure 40), which have elements of both casting and forming. Essentially all powder processing routes involve filling a mould with powder (this is similar to filling the mould during casting, as powders flow as slurries), which is then compressed in between dies to begin the process of reducing the space between the powder particles. The powder compact produced is then heated to a high temperature to produce a solid component.
Figure 40: Powder processing
The process of sintering starts with the material in powder form and is used for a wide variety of materials, especially those with properties which preclude shaping by melting and casting. Examples are ceramics such as alumina and silicon nitride; brittle metals with high melting temperatures (over 2300K), such as tungsten; and the polymer PTFE (polytetrafluoroethylene), which has a viscosity too high to suit other moulding methods.
The starting powder is mixed with a lubricant/binder and is then moulded to shape by compressing it in a die to form what is called a ‘green’ compact. Although this is highly porous, it has enough rigidity to support its own weight and permit gentle handling. The compact is then sintered – that is, heated at a high temperature and sometimes under pressure – for a prolonged time. During sintering, the powder particles coalesce (small particles join together to form larger ones) and grow to fill the pore spaces. The compact shrinks correspondingly, forming a solid homogeneous (uniform) mass in the shape of the original mould. Depending on the sintering process used, the final component can have various degrees of porosity and therefore strength, as any porosity can act as defects in the form of cracks. Very high temperatures and/or pressures during sintering are used to minimise porosity.
In powder processing, the volume of the workpiece does not stay constant. As the powder fuses together, most of the spaces between the powder disappear and the volume of the finished component is considerably reduced.
Sintering is sometimes economically competitive with alternative methods of shaping; for volume production of complex parts it is often cheaper than machining. A wide variety of components can be manufactured using powder processing: these can range from processing domestic ceramics for applications such as bathroom sinks to the insulating sleeve in a spark plug.
So can we consider production of the gearwheel using powder processing? As you have seen powders of many materials can be used for this process and this includes metals. This would appear to be an attractive idea because the wheel can be made in one piece, with little or no waste of material and with a modest expenditure of energy and labour. The only major problem is the need for a shaped punch and die. This is expensive, but if the price of the punch and die can be spread over a long production run, the cost of the product may be quite reasonable. So as well as extrusion and forging, which we identified earlier, we can add powder processing to our list of candidate processes to make our gearwheel.
Cutting is perhaps the most familiar type of manufacturing process. Whilst few of us have cast polymers or formed metal, shaping material by cutting is part of everyday experience. I am sure you have used scissors, saws, files, chisels or even sandpaper at some time in order to remove unwanted material. These are all mechanical methods where a force is applied through the cutting tool (whether it is the grit in sandpaper or the metal edge of a saw) to the material, and a cut is made on a macroscopic or microscopic scale.
Cutting is often used as a secondary or finishing process where the product to be cut will have been made by one of the processes described earlier. On a similar basis, if you do any DIY at home using wood then you will purchase ready prepared timber as a starting point rather than manufacturing it from the raw material, in this case trees. You may have to cut the timber to size to do a particular job, but a number of previous processes will have produced material of suitable dimensions, saving time and a lot of hard labour.
There are a number of reasons for using cutting as a secondary manufacturing operation in the production of a particular artefact:
Indeed, the majority of components produced by forming and casting require some subsequent material removal before reaching service.
However, in some circumstances it can be more economical to produce the basic product shape by cutting from solid rod or plate (although some form of shaping will have been used to manufacture even these basic starting shapes), than by any other process. As cutting typically uses machines with little dedicated tooling, this is particularly true for low production runs.
If the material costs for the product are low, then the waste from cutting will constitute only a minor part of the overall cost. The inherently poor material utilisation of cutting processes can be tolerated and automation can make cutting attractive at much higher production volumes. Cutting can then compete directly with casting and forming for the manufacture of some products.
4.2 Cutting processes
The most common cutting operations are carried out on electrically-driven machine tools; hand tools, which include electric drills, orbital sanders and the like, are also used extensively, but because of the limited number of items that can be produced economically it is not a viable manufacturing method for mass production. In industry, cutting was traditionally performed on large machines with a whole host of specialist names such as lathes, mills, broachers, shapers and many others. Each type of machine was capable of one particular method of cutting. Somewhat confusingly, these cutting machines are collectively known as machine tools and the processes are known as machining. The skilled operators of these machine tools had a significant influence on both engineering practice and labour relations for many years. However, the last few decades have seen substantial growth in the use of flexible, computer controlled machining centres (Figure 41) that incorporate both tool- and workpiece-changing facilities. The introduction of such machines, together with their reduced need for a skilled workforce to operate them, have reduced operating costs so that cutting can now be considered for production volumes that would previously be considered uneconomic.
Figure 41: Typical computer-controlled machining centre
4.3 The mechanics of machining
During any machining operation, the cutting tool comes in contact with the material to be cut, called the workpiece. The cutting machine has to hold both the tool and workpiece, and to move one relative to the other for the cutting operation to be performed.
All cutting processes are essentially complex arrangements of a simple single-point cutting operation: a saw blade is just a collection of single-point cutting tools; sandpaper is lots of single-point cutting grits stuck onto a piece of paper.
When the cutting tool is brought into contact with the workpiece, it detaches a thin layer of unwanted material (the ‘chip’). Figure 42 shows the relative position of tool and workpiece during a machining operation. The tip of the tool is shaped like a wedge so that the faces of the tool are always inclined to the machined surface of the workpiece in order to limit the area of rubbing between tool and workpiece. This rubbing is undesirable because it wastes energy and it causes the tool to wear at an increased rate.
Figure 42: Chip formation during machining
In deforming the chip, work is done by the tool on the workpiece and more than 90 per cent of this energy is transformed into heat. This heat is concentrated in a small volume of the workpiece near the tip of the tool. The actual temperature profile near the cutting point depends on the thermal and mechanical properties of both the tool and workpiece but it is common for temperatures to reach 700°C locally, even when liquid coolants are used.
At these temperatures, a state of partial seizure or bonding exists at the tool/workpiece interface, as the chip starts to weld to the tool. This is highly undesirable, since it contributes both to wear of the tool and to the consumption of energy during machining. Lubrication does help relieve the situation, but the careful design of the cutting tool is of paramount importance. There are specific tool geometries and cutting angles for different materials which help to keep temperatures and energies to a minimum while they are being cut.
The main attraction of machining as a shaping process is its ability to produce almost any shape accurately. However, machining requires expensive capital equipment and can also produce a lot of waste material, and for large production volumes it often proves more expensive than alternative methods of shaping.
As we see from the above, there are two materials involved in machining: one is the tool and the other the workpiece. During machining, the material of the workpiece should deform plastically (or ‘flow’) while that of the tool remains rigid. So the tool material must be much harder (measured in terms of Hardness) and stronger than the workpiece at the temperatures that exist near the tip of the tool. Also, the tool should be stable at the cutting temperature; it should not oxidise or undergo microstructural changes which may decrease the strength of the tool. Wear of the tool is inevitable under machining conditions, but by using material that has a high wear resistance, tool life is maximised.
Hardness is related to the strength of a material and is a measure of a material’s resistance to plastic deformation by scratching or indentation. Scratching the point of a pin across a material can be used to give a rough indication of hardness, but this is purely qualitative as it indicates only whether the material is harder or softer than the pin.
Hardness is one of the oldest ways of comparing the properties of materials. German mineralogist Friedrich Moh originally developed a scale for minerals which ran from 1 to 10 based on which would scratch each other. On this scale, diamond, the hardest, ranked 10 and talc, the softest, ranked 1, with other minerals in between. This scale is sometimes still used but its drawback is that the steps in between the different minerals are too large to be useful to discriminate between similar materials and the steps are not equal.
Thus, a range of other techniques to measure hardness have been developed. Some of these are based on pressing an indenter of a known geometry, such as a hardened steel ball or a pyramid, into a surface under a known load and then measuring the size of the residual impression. One of the more popular methods for both metals and plastics is the Vickers hardness test (Figure 43). This test uses a pyramid-shaped diamond to indent the surface, the size of the indent being related to the hardness; the softer the material the larger the indentation. The Vickers hardness number, HV, is the force divided by the surface area of the indentation. The values of HV for a range of materials are shown in Appendix 1.
Figure 43: The Vickers hardness test
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