As with chisels used for woodworking, those made for use with metal are precision tools capable of carrying out high quality work, depending of course on the skill of the person handling them. Because of the degree of accuracy that can be obtained from them, it is vital the right chisel is chosen for each job: it is also important to use it correctly and keep it properly sharpened to ensure the best results on the work.
TYPES OF CHISEL
Cold chisels – so called because they cut metal in its cold state – are available in different types of metal and in a range of lengths and blade widths to suit the work in hand.
MATERIAL Chisels for metalwork are usually made from tough carbon tool steel, which will stand quite rough treatment because of the carbon content in the steel. Alloy tool steel containing a proportion of nickel is sometimes also used in making chisels, since il has better cutting properties than plain carbon steel: also the chisels do not need to be tempered.
SHAPE The main body of the chisel is usually hexagonal in cross-section, giving a comfortable grip and a degree of positive positioning in use. The blade is forged to shape and then hardened and tempered if made of carbon tool steel – to give a long-lasting cutting edge. The edge is ground to shape, with the grindstone working along the length of the chisel: if ground across the edge, there is a danger it will break away in use. The edge has a very 2c vice. slight curve, which makes cutting flat surfaces much easier and helps to prevent the corners digging into the work. The curve is achieved on an electric grindstone by pivoting the chisel on the tool rest so it moves from side to side during the grinding operation.
TYPES OF BLADES
The shape of blades on metalwork chisels also varies: four in particular will cover most normal jobs you are likely to tackle in the home workshop. FLAT This is the most common blade, suitable for shearing metal held in a vice or guide, for cutting sheet metal on a cutting block, for dressing metal surfaces and for removing mill scale. It is also useful for repair and renovation work in cutting off rivet heads, nuts and bolts. When cutting sheet metal, you must work on a firm surface otherwise the chisel will bounce on the work. The cutting block is a heavy block of soft steel, but it is not readily available. You can provide a substitute by using a concrete block with a piece of sheet steel over it.
HALF-ROUND Used for grooving and channelling. this blade will also cut internal radii in corners.
DIAMOND-SHAPED This blade will cut sharp corners. particularly when making square holes in metal or cutting slots.
NARROW CROSS-CUT Unusual because the blade gels narrower behind the cutting edge, preventing the body jamming in the groove which has been cut. It is used for cutting grooves and keyways and, when dressing flat work, for cutting a number of grooves before a flat chisel is used to clear away the bits left between.
Using A chisel
To get the best out of your chisel it is important to get the correct angle of inclination between the chisel and the work. Generally a clearance of about ten degrees is needed between the work and the edge of the chisel. This means that with a chisel ground for cutting steel, the tool should be held at about a 40 degree angle to the work. This angle is reduced to about 25 degrees when cutting aluminium. As a general rule, keep the cutting angle small, but ensure there is clearance for the cutting edge. The depth of cut should not be more than 2mm, since deeper cuts are difficult to control. The chisel should be held in a steady grip, but not grasped tightly. The hammer should be held at the furthest point from its head: always keep your eye on the work and not on the striking end of the chisel and check the cutting edge of the chisel is against the chip being cut. Before you get to the end of the cut, it is good practice to reverse the direction of cutting to avoid fracturing the metal on the final cut. When using the chisel to cut strips of metal held in a vice, it is more efficient to hold the chisel at an angle to the direction of the cut as well as maintaining the clearance angle for the cut.
LUBRICATION As with any cutting tool, lubrication can be used to increase cfliciency: the type will depend on the metal you are working – oil for steel and paraffin for soft metals such as aluminium. If you do lubricate the work, take great care not to get any lubricant on either the shank of the chisel or the hammer handle; if you do, wipe it off immediately or you will run the risk of your hands slipping while you are working, which could result in injury to your fingers.
GRINDING A CHISEL
When grinding a chisel, it is most important to keep the tool cool. If the edge of the tool is allowed to become too hot, the temper will be drawn and the cutting edge will soften. To avoid overheating. the chisel must be withdrawn frequently and cooled by being dipped into water. Any discolouration of the metal during grinding is an indication that the tool is overheating.
When grinding a chisel the angle of the point should be ground according to the use to which the chisel will be put. For steel and cast iron an included angle of 60-65 degrees is suitable. For use with softer metals a smaller included angle is required. With copper, for example, the edge is ground to a 45 degree angle and with aluminium a 30 degree angle. The reason for the different edge angles is to ensure the correct wedge action when the chisel is held in the proper position.
If you are using a grinding wheel attachment on an electric drill, you must take certain precautions. Hold the drill in a stand secured to the work surface and fix a safety guard and shield. Wear tough gloves and protective spectacles to guard against injury.
The non-chiselling end of the tool will also need attention from time to time. Although the cutting edge is hardened for long life, the rest of the chisel has to be relatively soft to withstand the hammer blows and to ensure there is no damage to the hammer on impact. But because that end of the chisel is softer, it will eventually start to burr. Immediately this becomes noticeable the end of the chisel must be ground to a bevel; you should never allow the end to mushroom. If you do, chips may fly off and hit you in the face.