Metal Work

An Introduction

The Industrial Revolution was made possible by the abundant supply of iron ores and coal necessary to manufacture iron and steel. Modern technology still depends on the supply of cheap steel. Without it, Western society would collapse.

Because of its availability and strength, steel is the bulk material of machinery, railways, bridges and similar heavy engineering products. Through heat treatment, it can be hardened to make cutting tools which cut other metals, organic materials and steel itself. Table 1 shows the properties and uses of the basic irons and steels.

The other common metals-aluminium, copper, lead, tin and zinc-are normally used in combinations of two or more, as alloys, to complement steel. Table. 2(???) shows the properties and uses of these metals and of some common alloys.

FOUNDRY
The foundry is the place where metals are melted down, refined or alloyed, and cast into moulds. Intricate shapes can be produced by casting, without cutting away or otherwise wasting metal. The foundry is also the place where metal is recycled.

Pig iron, scrap iron and steel are melted in a cupola fire-brick lined tower, into which metal is loaded together with coke. Air is blown in at the base of the tower to support combustion and to oxidize some of the carbon and other impurities in the metal. The colour of the flame issuing from the top of the tower (or seen through a spy-hole) can be used to judge the condition of the melt but this requires a lot of experience. Alternatively small samples can be drawn off, cooled and tested. If a sample bar breaks when bent it contains a lot of carbon. But if it bends, then other simple tests, using Heat Treatment techniques described below, can be used to determine the nature of the sample more accurately.

When ready, a plug at the base of the tower is broken to allow the molten metal, which has drained down through the ashes, to flow out and into a crucible. A small, simple cupola can be made with the internal diameter of the tower ranging upwards from four inches.

The crucible furnace is a means of melting or alloying metals. It has the advantage that contamination from the fuel cannot occur and that exact quantities of base metal and alloying elements can be loaded into the clay crucible before firing. A typical charge might consist of sawn-up bike handlebars and other chromium-plated scrap. On melting, the chromium will alloy with the steel to form a lowcarbon stainless steel. A second firing is necessary, involving the addition to the melt of a small amount of aluminium. The aluminium acts as a catalyst and will boil off gas from the melt, which would otherwise produce very porous castings.

MOULDS
Sand moulds are relatively inexpensive and are widely used for the production of iron and steel and other metal castings. They can be constructed using a wooden pattern embedded in moulding sand. Cores are used to form shapes or voids that cannot be made strong enough from packed moulding sand. Alternatively, they can be made from moulding sand first formed in a wooden mould and then baked until hard in an oven.

There are many other casting techniques. One of the most interesting is investment casting, often known as the `lost wax' process. This method was well established by the fifth century BC in the Western Chou Dynasty to produce seamless castings of intricate shape and surface detail. The pattern is constructed from wax and is an exact replica of the required casting. The wax pattern is coated with a thin wall of clay to form the mould. The clay mould is then baked in order both to burn out the wax pattern and also to harden the clay. The hollow clay mould can then be filled with molten metal. When the metal has cooled the clay mould is smashed to free the casting.

FORGING
The art of the blacksmith is as old as the art of refining metal from ores. The essential equipment required- hearth, bellows, hammer, tongs and anvil- is simple and inexpensive and, once a forge is set up, any other tools required, such as flatters, punches or hot chisels, can be forged from crude lumps of steel. Modern technology only uses forgings in cases where the very high strength achieved by forging is essential and "justifies" the high tooling costs involved in mass production of forged items, such as car engine con rods and spanners.

This high strength arises because forged material has a fibrous nature which reaches its ultimate in the Samurai sword. The sword blade is beaten out from a small cube of steel and then rolled up and beaten back into a cube. This is repeated until the final blade is of an extremely fibrous nature. The forging is carried out with the metal almost white hot, so that the resulting layers weld themselves together. Horseshoes are forged rather than cut out of flat sheet, not only to save metal but also to benefit from fibrous strength. The piece of flat rectangular metal used to make the horseshoe is inherently fibrous, due to the process of its original manufacture, cold rolling. These fibres are preserved during the forging of the shoe. Similarly the tapered parts of chisels and screwdrivers are stronger if forged rather than filed.

It is possible to weld metal at the forge. The pieces to be welded must first be dipped in a flux, then heated until almost white hot and finally hammered together. The weld will not hold unless a flux is used. The flux melts at a high temperature and seals the metal surfaces from the air, so preventing the surfaces from oxidizing. A suitable flux for welding mild steel is calcined borax.

HEAT TREATMENT
If a piece of high carbon steel is heated until red hot and then quenched in water, it will become extremely hard but brittle. Strength, necessary for the cutting edge of a tool, for example, can be achieved by tempering, i.e. by heating the hardened steel to temperature of between 230° C and 300° C.The higher the tempering temperature the stronger the steel will become, but at the expense of hardness. A very simple and ancient method of judging the required tempering temperature is by the use of "oxide colours". If a polished piece of steel is heated slowly, coloured oxides will form on the polished surface. The oxide colour changes from straw to brown then to purple and finally to blue as the temperature increases from 230 C to 300°C.

The Tool Tempering page lists various cutting tools together with appropriate oxide colours for tempering their cutting edges. Fig 5 (MISSING)shows the hardening and tempering of a cold chisel. It is important that only the tip of the chisel should be hard and that the shank and the end which is hit with a hammer should be soft, otherwise the shank will crack or the end will splinter when hit.

Mild steel cannot be hardened and tempered because it does not contain enough carbon. It can, however, be case hardened. If a piece of mild steel is heated until red hot and then thrust into a pile of carbon dust, some of the carbon will be absorbed by the surfaces of the mild steel. By repeating this several times, enough carbon will be absorbed so that the steel can be hardened and tempered in the normal way, but the resulting hardness and toughness will only be skin deep. However this is useful for giving such things as screw-drivers and penknives' hard, wear-resistant surfaces.

The skin can be as thick as 1/16th inch, but to achieve this thickness the steel has to be `soaked' in carbon, ie packed in a box together with carbon-rich granules and kept at red heat in an oven for three to four hours. Hoofclippings are a traditional carbon-rich material used in this process (showing the influence of the blacksmith) but charcoal will do just as well.

When copper or aluminium are cold worked - i.e. bent, beaten or stretched - they gradually become hard and brittle, and if the cold working is to be continued, the metal has to be softened.by annealing (heating the metal, usually by gas flame, and then quenching it). If aluminium is quenched in brine, and copper in killed-spirits (dilute sulphuric acid), the oxides which form on the surface of the metal during heating will be purged to leave a dull clean surface.

FITTING AND MACHINING

*Bench Tools. A sturdy bench made from timber (or angle iron and timber) and an engineers vice, are vital when using most hand tools.
The vice, rigidly attached to the bench, has many uses. Among them are: rigidly gripping a work piece while it receives heavy blows. from a cold chisel and hammer; firmly holding work for scribing lines in it ('marking out'); holding cold or red-hot bars in order to bend them to a right angle; squeezing metal together; and pressing dowel pins into tight holes.

If something is to be made from a sheet of tin plate, copper or aluminium, the flattened out plan is first marked out on the sheet using a pencil, straightedge, compasses and a rule. When marking out work on iron or steel, pencil lines would be difficult to see and would be easily erased so lines are scratched using a scriber and dividers. Also, a 'centre-pop' is used to make small `pop' marks at regular intervals along all marked out lines, so that, when metal has been cut away down to a line, the pop marks will show even though the line has disappeared. The scriber can be held in a scribing block and used on a surface table for scribing accurate parallel lines.

A piece of metal can be cut roughly to shape with a cold chisel, or sawn with a hack-saw, and then finished to the required size and shape with files using first the large, rough variety, and ending with small fine-toothed files. When extremely flat surfaces are required, the final flathess can be achieved by using a scraper to take off `high spots' of the order of 0.0003 inches. It takes a lot of practice to produce flat surfaces even to within plus or minus 0.002 inches, and the bigger the surface the harder it is. If a smooth polished surface, rather than a flat one, is desired, then a piece of emery cloth wrapped around a file can be used. A little oil on the surface to be polished helps the cutting action of the emery cloth.

The simplest way to test the flathess of a surface is to place the straight edge of a scale on it and hold it up to the light. The low spots on the surface show light while the rule, resting on the high spots, causes the high spots to be dark.

POWER TOOLS
The main workshop power tools are the lathe, the shaper, the drilling machine, the milling machine and the universal grinding machine.

The drilling machine is a must for any workshop; it will be used more than any other machine. Secondhand machines are inexpensive and are usually a good buy since they don't wear out easily and are cheap to mend.

The next machine, in terms of usefulness, is the lathe, used to produce flat surfaces by facing, or to make splined bores and key-ways (by keeping the work still and moving the tool along the bed), or to produce circular rods and bores. When buying second-hand lathes watch out for wear of the bed slides near the head since this fault is difficult to rectify and will produce tapered work. Wear of the lead-screw can be easily rectified by removing it and reassembling it the other way round since the tailstock end will not be worn.

The shaper is used to produce flat surfaces. It is really a powered cold chisel but is much more versatile. Second-hand shapers are a good buy and wear in old machines will not reduce their performance.

The drilling machine, the shaper and the lathe are the only power tools really needed in a general workshop. shop. They are inexpensive to buy and cheap to run and maintain. The necessary cutting tools are also cheap and, apart from drills, can be home-made on the forge. The milling machine, which can also produce flat surfaces, wears out very quickly and second-hand ones cost a lot to repair. (The necessary tools are also expensive). Universal grinding machines are used to produce extremely smooth, flat or curved surfaces to very accurate dimensions, but are a bit of a luxury for a general purpose workshop.

SHEET METAL WORK
After marking out a job on sheet metal the sheet can be cut roughly to size with a hack-saw or a cold chisel and then cleaned up with files. However, a foot operated guillotine can do this much more easily. Your hands are free to position the sheet accurately and, as the foot pedal is depressed, the machine first clamps the sheet firmly and then shears the metal cleanly.

To make boxes or brackets with sharp corners the sheet can be bent using clamping bars. Position the sheet between the bars clamped in a vice, and the sheet is bent over using a soft (or hide) hammer so as not to damage the surface. Alternatively, a foot operated bending machine can be used. This works in the same way as the guillotine but bends instead of cuts.

Cylindrical and conical shapes can be made by beating the sheet, again with a soft hammer, over a circular rod held in a vice. The rolling machine makes short work of this job.
Sheet metal can be made very strong and stiff by pressing or beating it to a spherical or bowl shaped surface. The car body is a good example, being entirely built of pressed steel sheet with every surface curved, usually in more than one plane. You will find that the more a sheet is beaten the harder it will become to bend it and that it might even crack. This effect is called work hardening but it can be reversed by annealing, described in the section on heat treatment.

To make a bowl from a flat sheet, the sheet should first be annealed. The centre of the sheet is then rested on the spherical head of a ball-headed stake and a light planishing hammer used to distend the centre of the sheet by beating in concentric circles working outwards from the centre. When the sheet gets stiff, anneal it again and continue beating, turning the work as you go. You will probably need to anneal the bowl several times before it is finished.

PERMANENT FIXINGS

• Riveting: In the Victorian era most metal objectsfrom kettle and saucepan handles to the great steel ships of Brunel - were held together by rivets. Modern X-ray inspection of welded joints and development of welding techniques have, in the main, made riveted joints obsolete, but in a small workshop with modest resources, riveting is the cheapest fixing method.

The rivets themselves are made from soft metals-mild steel, aluminium or copper-and consist of a short, straight shank with a spherical or countersunk head on one end. Two pieces of metal to be riveted together are first drilled through to the diameter of the rivet, which is then pushed through the hole.The projecting shank is then beaten to form another head.

'Pop-rivet' sets, available from most hardware stores, provide a cheap, quick and easy means for light duty or general purpose riveting.

*Soldering: Solder is a low-melting point alloy, made from tin, lead and a tiny amount of antimony. Two pieces of metal are soldered together by alloying the solder with each metal surface. To do this both pieces of metal must be heated to the melting temperature of the solder. To prevent oxides forming on the metal surfaces as they are heated, a flux of killed spirits (zinc dissolved in hydrochloric acid) or resin (Fluxite) is dissolved in hydrochloric acid) or resin (Fluxite) is used. The oxides would otherwise prevent the necessary alloying of the solder to the metal surfaces.

To solder two pieces of metal together, the surfaces to be soldered must first be cleaned with a file or a piece of emery cloth and then completely coated with flux. The surfaces are then held together and a hot soldering iron is pressed onto the two pieces of metal. When the flux begins to boil a stick of solder is held to the tip of the soldering iron. The solder melts and flows between the two surfaces, drawn by capillary action. The soldering iron is then removed and the solder quickly cools and solidifies.

If the flux used was killed spirits then the joint should be thoroughly washed to prevent the remaining flux corroding the metal. Resin flux is not corrosive and requires no subsequent washing. The soldering iron can be electrically heated or heated in a gas flame. Alternatively, a small gas torch can be used instead of a soldering iron.

• Brazing: The principle of brazing is exactly the same as soldering with the stick of solder replaced by a brazing rod, made of copper/zinc alloy, which has a much higher melting point than solder and produces a much stronger joint. To reach this high melting temperature a gas torch with an air supply provided by a foot-operated bellows can be used. The flux used is Borax. This can be obtained as a solid stick and a paste is made, as required, by rubbing the stick on a glass surface with a drop of water.

• Welding: This is similar to Brazing but steel alloy welding-rods are used. To reach the high welding-temperature necessary to melt steel welding-rods, an oxy-acetylene torch is used. The oxygen and acetylene gas is supplied from a compressed oxygen cylinder and a compressed acetylene-cylinder. These cylinders must be kept several feet away from where the welding is taking place since an explosion would be fatal. Another danger with acetylene is that it is very unstable and will explode if it comes into contact with oil. Never wipe the outlet connections from an acetylene-cylinder with an oily rag.

The welding-torch is supplied with different sized gas nozzles which are used to produce a suitably sized flame for the job in hand. A really large nozzle can be used to produce a flame that will burn through steel sheet. This can be used for cutting sheets to a required shape or size or for salvaging pieces of metal from large scrap. The welding rod is coated with the flux and so no additional application of flux is required.
A welding mask, which has a very dark visor, must be used, since the light radiation from the welding will eventually cause blindness.
Electric arc-welding is a lot safer than gas welding, but it is not so portable since it needs a 3-phase electrical supply.

REFERENCES
Workshop Technology Pt I. by N.A.J. Chapman. Chapman's Workshop Technology is a bit quaint but is the best introduction to workshop practice that you can get. For specialized information, eg foundry practice, technical libraries in Technical Colleges all over the country are crammed with such information.

EXPERIENCE AND SAFETY
The potential danger element in a workshop is much greater than most people realize. Workshops can be a nightmare. Young people in particular seem to have an urge to insert their fingers into holes-no doubt a sexually based desire-but if the hole is in a machine tool the finger can get chewed up. Long hair can easily catch on the rotating shaft of a drilling machine.

Once caught, the shaft will wind up the hair and scalp the victim and new hair may not grow over the scalped area. Necklaces and neckties can catch on rotating work in a lathe. A jacket or boiler suit sleeve can be caught by the rotating chuck of a lathe or the rotating tool of a milling machine: these machines have the power to throw a person right over the machine. Another common accident is dropping heavy weights on feet. Most people in factory workshops wear shoes with steel toe caps.

Safety consciousness - the thing that stops you hurting yourself - can only be gained by experience in a workshop. To prevent yourself getting seriously hurt while you gain the necessary experience, you should limit yourself to areas of low potential danger, such as hand tools and bench work, soldering and brazing. When you find that you have stopped cutting your hands in one way or another (this may take six months to a year), you should be safe enough to use powered machines or welding machines, or to handle metal and use the forge.

Evening classes in workshops at technical colleges are good places to learn some of the skills of metal work and to learn safety consciousness.