8 Sheet Metalwork
This work includes, apart from tinplate, the use of sheet aluminium, brass, copper and galvanised iron, also sheet steel and terne plate. Industrially these metals are formed into useful articles by machines which can stamp, bend and form the sheet metal as required. In school we use hand processes usually on tinplate. Sometimes ternc plate is used and more rarely galvanised iron.
Galvanised iron is in fact galvanised steel and is mostly used out of doors because of its rust-resisting property. It is mild steel coated with zinc. Terne plate is sheet mild steel coated with an alloy of lead and tin. Tin plate is sheet mild steel coated with tin. It varies in thickness between 0.011 and 0.026 of an inch. We often refer to the thickness of tin plate in decimals, but tin plate manufacturers have their own method of indicating the thickness (see table in appendix), e.g. 1C = 0.012"; 3X = 0.019*.
Before a tinplate article is bent or formed into shape, the tin-plate must be cut to the correct size and shape. This shape, before it is formed, is known as the development.
Simple developments can be drawn directly on to the tin plate with a hard pencil or copper scriber. The lines to be cut, or parts to be hidden by a joint, can be scribed. It is important not to mark with a scriber any part of the job which will be exposed, because this allows rust to form where the mild steel has been exposed. Care should also be taken to prevent the surface of the tin from being scratched: for this reason it is advisable to put newspaper on the bench on which to lay the tin plate.
Complicated developments are often drawn on paper and carefully cut out. By this means it is possible to see how the finished article will look and also to check the dimensions. With care the paper development can be used as a template, i.e. it can be laid on to the tinplate and drawn round with a sharp pencil or copper scriber. The parts to be cut must be carefully checked with a rule and then scribed. Another method is to stick the pattern on to the tinplate.
Allowances must be made for joints and seams. Figure 1 shows the allowances for joints and seams, also for the beaded edge and wired edge. Figure 2 shows the allowances for a circular lap seam, a circular folded seam and a knocked up bottom.
Tools (fig. 3)
Bench Shears. There is usually one bench shearing machine bolted to a bench in each metalworkshop. The blade is curved so that it presents the same cutting angle in all positions. Any size sheet can be cut and there is a hole at the side of the blade into which a rod can be inserted for shearing. The fixed blade can be adjusted and locked in position by the screws provided.
Snips. Two kinds are shown: the universal snips can cut inside and outside curves and straight lines. The straight snips are to cut straight lines or outside curves. Curved snips are also available. These are for cutting inside curves.
Folding Bars. These are for bending and folding. The sheet metal is inserted up to the line where the bend is required and then the folding bar is gripped in the vice. The work is now firmly held and can be bent over using a mallet, or a piece of wood can be held against the sheet and this tapped with a hammer.
Extinguisher Stake and Bick Iron. These are useful for making conical work. The flat top and the square end are often used for box making.
The Mandrel. This is about four feet long. It is made of cast iron and has a rounded top at one end and a flat top at the other. There is a square hole at the flat end in which small stakes can be held. Sheet metal workers often use the mandrel on large work.
Tinmen's Anvil. The top surface is flat and one edge is curved.' It can be held in a special socket or in the hardie hole of a large anvil.
Hatchet Stake. Used for bending straight edgesDeyond a right angle.
Half Moon Stake. Used for bending curved edges.
Canister Stake. Used on the bottom of a canister for "getting in" to the corners as in making a knocked up bottom.
Bottom Stake. Similar in use to the canister stake but the sides can be used for truing up the side of a canister.
Creasing Stake. The grooves are used for finishing off wired edges and other similar operations. The square end is usually cut back slightly so that it can get into the corner of a box.
Funnel Stake. Used for large conical work such as funnels which were once made by hand by tin-smiths.
Mallets (fig. 4). These are used on tinplate to avoid damaging the surface. The tinman's mallet has a head of boxwood or lignum vitae and a handle of cane. The bossing mallet, used for hollowing, also has a boxwood or lignum vitae head and a cane handle. The rawhide mallet has a hickory handle.
Paning Hammer. Sometimes used for tucking in joints and wired edges. Care must be exercised in its use because it can easily damage the surface of the metal.
METHOD OF MAKING JOINTS
Flush Lap Joint. First make a lap joint then solder the joint. Now set the joint down on tp a flat surface using a piece of hardwood and hammer as shown in figure 5.
Corner Lap Joint. Bend lap using folding bars and mallet as shown in figure 6 then solder.
Grooved Seam Joint. The size for this is usually stated on the drawing. For most work it is normally between 1/8" and 5/16" inclusive. The allowance for this joint is twice the joint size on one side and only once on the other (fig. 1). However, to allow the grooving tool (fig. 5) to fit over the joint, slightly less than the width of the joint must be bent over. Figure 5 shows this clearly.
A reliable way to find out exactly how much to bend over for a particular thickness of metal is to make a trial joint using two small pieces of the same metal (see fig. 7 (1), (2). (3). (4). (5)). The grooving tool is offered up as shown in figure 8 and then set down using a hammer on the grooving tool. Finish off the joint by tapping down with a mallet along the top of the joint as in figure 9. For a good joint it is important that the bend in both pieces is parallel.
Capped On or Circular Lap Seam. Cut the disc for the bottom making allowance for the "turn up". Carefully set down on a canister stake with a mallet. Do not try to set down too much at each blow. The disc must be continually rotated using only light blows making sure all the time that the bend is occuring on the pencilled circle (fig. 10).
An alternative way is to have a disc or former of metal of the right diameter and a backing disc. (This is not always possible owing to the large number of discs of different sizes that would be needed.) The tinplate circle is positioned centrally between these whilst they arc held in a vice (fig. 10).
Care must be taken not to lose the position of the tinplate when it is rotated in the vice. Any small wrinkles can finally be removed by tapping with a mallet on a bottom stake. When the bottom is finished it is soldered on to the container.
Circular Folded Seam. The cylindrical body is first made then the flange is made as in figure 11 (1), (2), (3), rotating the work anti-clockwise. Usually the body distorts at the joint as shown at A. Tap this true with a mallet as shown.
The bottom is made as in figure 10 and set down as shown in figure 12. Care must be taken to avoid hammer contact with the sides of the vessel particularly when using the paning hammer.
Knocked Up Bottom. This is similar to the previous joint but it is taken a stage further as shown in figure 13. The edges of tinplate articles are usually finished either with a wired edge or a beaded edge because tinplate is seldom thick enough to withstand damage. Safety is another consideration: bare edges are often sharp.
Beaded Edge. Between 1/8 " and1/4* is usually allowed for this. Start by bending on a hatchet stake (fig. 14 (1)) and proceed, as shown in figure 14 (2), to bend the edge over a piece of metal which has a radiused ddge. Finish with a mallet as in figure 14 (3). Never flatten the edge completely otherwise it will lose its rigidity.
Wired Edge. Allow between two and a half to three times thev. diameter of the wire for this. Start by bending to the pencil line on a hatchet stake, figure 15A, then lay the wire in and tap down until the wire is trapped (B). Next, true the edge on a bick iron or similar stake as shown at C and D.
If the tin plate does not enclose the wire, "dress" with a mallet using a sweeping action to one side as shown at E. If too much metal has been bent over, "dress" in the other direction. The edge can then be finished on a creasing iron as shown at F.
Soldering. This is almost invariably done with a soldering iron as explained in Chapter 4. On good tinplate work no edge is left with the mild steel exposed as this will rust. To finish these "raw" edges they are tinned with the edge of the soldering iron. Excess solder must be "lifted" off with the soldering iron. The surface of tin plate must never be filed or emery-clothed.