Cutting tools used in wood and metalwork are generally made from medium to high carbon steel (cast steel), which is hardened and tempered’during manufacture of the tools. Mild steel, which contains very little carbon, cannot be hardened and is not used for tools. The purpose of hardening is to enable the tool to resist wear and make it hard enough to cut other materials; tempering is carried out after hardening to reduce the brittleness which results from hardening the steel.
If tools are misused, the cutting edges become overheated and they lose their temper — this is indicated by the cutting edge or point becoming blue in colour. They will need to be resharpened and then heat-treated to restore their temper. This problem does not usually arise with tools made from high speed steel (marked HSS on drills and other tools). High speed steel is alloyed with tungsten which gives it the property of holding its cutting edge even if the tool is used with that edge at red heat.
Identifying steel type You can use a grindstone wheel or emery wheel to find out the type of metal. Mild steel will produce a stream of long yellow sparks when ground; steels with a higher carbon content produce a stream of sparks which spread out like a bush with occasional bursts of sparks being given off in a different direction from the ‘bush’; high speed steels produce a stream of dull red sparks.
The cutting edge or point of the tool should be re-sharpened if necessary before it is hardened and tempered. Sharpening stones (oilstones) can be used to sharpen other metal tools such as scissors, shears and knives. If chisels, punches and scribers do not require the removal of much metal to renew their edges, sharpening stones can be used. However, if relatively large amounts of metal have to be removed, you will find a power-driven emery wheel is necessary.
Whether you are going to carry out sharpening by hand or by machine, remember if the grindstone is worn into hollows or grooves effective work will not be possible until the stone has been made true. To prevent uneven wear of grindstones you should use the full area of grinding surface. When grinding, always present the work to the stone at the correct angle. When one side of the cutting edge is flat, as with some chisels and knives, that side must be held flat against the stone when you are removing the burr produced by grinding on the bevel side. Bevelled and tapered edges must be held so the surface being ground is always parallel to the surface of the stone. A great deal of heat is produced by grinding; if you do not take proper care the edge being ground can overheat (indicated by the edge ‘bluing’) and the tool will lose its temper. (It is quite possible to overheat a penknife blade while sharpening it on an oilstone.) Always keep a small container of water close by and dip the tool frequently into the water to cool; never allow the edge to become so hot you cannot test it by touch.
You can test the sharpness of a knife or chisel by holding it under a light which provides good illumination. A properly ground edge does not reflect any light; so if you are unable to see any shiny or white spots, you have a good cutting edge.
Bench grinder The sharpening of tools for cutting metal usually requires an electric bench grinder, which is basically an electric motor with a grinding wheel mounted at each edge of the motor spindle. Alternatively you can buy accessories for electric drills. Whichever type of machine is used for tool grinding, it is essential there is a safety guard over the emery wheel to protect your eyes; you should always wear protective spectacles. It is also necessary to have an adjustable tool rest at the front of the wheel so the work can be guided during sharpening.
Warning Always stand to one side when the machine is switched on and runs up to full speed, since wheels can disintegrate whea running up to speed. Never force work against a cold wheel; if you apply the work gradually, the wheel will warm up and there will be less risk of wheel breakage. When working, all wheel guards should be in place and secure and the tool rest adjusted so it just clears the wheel with its top edge just below the centre line of the wheel; this will prevent work being jammed between the tool rest and the wheel. Wear protective spectacles, even if the machine has a safety shield, and keep your thumbs and fingers well clear.
Sharpening centre punches
Hold the punch between the thumb and index finger of one hand with this hand held steady against the tool rest. Hold the top of the punch between the thumb and index finger of the other hand. Move the tapered portion of the punch into contact with the wheel — the centre line of the punch should make an angle of 45 degrees with the
surface of the grinding wheel. With the tapered point in contact with the wheel, rotate the punch to grind the taper. The pressure on the wheel should be light and you should keep the point cool by frequently dipping it in cold water.
Grind the sides of the blade so it is symmetrical in shape. Holding the screwdriver blade flat on the rest, grind the faces of the blade so they are parallel and use a screw to test the thickness of this parallel portion will just enter the screw slot. It may not be possible to make the faces of the blade completely parallel, but they should be as near parallel as possible. Remember, the bottom edge of the blade should touch and fill the base of the screw slot.
Sharpening engineer’s cold chisels
Before grinding the edge of the chisel check the chisel head has not ‘mushroomed’; if necessary, grind the head to remove any burr which could fly off when the chisel is hammered. To sharpen the edge, hold the chisel, upwards in line with the wheel and, supporting your hand against the tool rest, tilt the chisel forward until it strikes the face of the wheel at the correct angle. For most chisels the included angle at the edge is 60 degrees; when grinding the edges, the chisel should be held so its centre line is about 30 degrees to the horizontal. If the chisel is to be used for cutting flat plates, gently swivel it from left to right during grinding so the ground edge will be slightly curved. If the chisel is to be used for shearing bolts or cutting metal in a vice, a straight cutting edge is preferable. When you want to cut harder metals the included angle of the edge can be made a little greater than 60 degrees and for softer metals the angle can be slightly reduced. Keep the edge cool when sharpening. These chisels can be sharpened with a file.
Sharpening twist drill bit
Sharpening a twist drill bit properly while grinding it by hand is a difficult job; a lot of practice is required and even then the result is not always satisfactory. If you want to sharpen your drill bit properly and you have grinding equipment, you should consult a reputable tool supplier about buying a twist drill bit grinding attachment and get advice on the possibility of mounting an attachment to your grinding machine. These attachments enable the drill bits to be firmly clamped during grinding and the various grinding angles to be set with precision. Full instructions for use are provided with the attachments.
If you want to grind twist drill bits, guiding them by hand, you should make sure the drill bit point angles are correct and equal, and the cutting lips on the drill bit are ground to equal lengths. Also, the clearance behind the cutting lips must be correctly ground and the chisel edge at the drill bit point ground to the correct angle. Use a drill bit point gauge to check the point angles and the length of the cutting edges. The clearance behind the lips has to be determined by inspection and must be the same for both lips. If the lip clearance and the chisel angle are too great, the cutting edges are weakened; little or no clearance and the drill bit will not cut.
To start grinding, set the tool rest about 2mm from the wheel and hold the drill bit so its centre line is at 59 degrees to the face of the wheel with the cutting lip horizontal. When the cutting lip is brought into contact with the grinding wheel, the drill bit has to be moved in three directions simultaneously. Move the drill bit to one side so the angle between the axis and the face of the wheel is reduced to about 50 degrees. At the same time rotate the drill bit — and move it in a downward direction — to grind away the metal behind the lip and produce the required clearance. You will find with small drill bits it will be very difficult to measure the angles and judge the clearances. If you want to practise drill bit grinding, try to obtain an old drill bit of about 13mm. With this size you will be able to inspect it more easily and it will not matter if you do overheat it. When drill bits have been repeatedly resharpened or reground after breaking, the web of the drill bit may have become thicker than it should be. You can make the web thinner by grinding on a round faced wheel, but it is probably better to buy a new drill bit.
Hardening and tempering tools
You will not need any special equipment for hardening and tempering tools. For small items sufficient heat can be obtained by using a blowtorch; alternatively items such as punches can be placed inside a piece of iron or steel pipe over a gas ring. For larger items an iron or steel sheet placed over a gas ring and heated to red heat will provide a suitable heat source. You will need tongs, a metal water container and some fine emery cloth.
Tempering colours When steel is heated in air, oxides are formed on the surface of the steel; the colour of these oxides depends upon the temperature of the metal. Colours range from a very pale yellow (straw) through brown and purple to blue; they form bands which travel along the metal away from the area which is being heated. They are most easily seen if the metal is lightly polished with emery cloth before heating and are most noticeable in daylight; distinguishing the colours in artificial light can be difficult. Before treating a tool, practise with a piece of scrap steel in order to become familiar with the colour range.
After sharpening you should harden the tool in the region of the cutting edge. Don’t harden tools over their full length since this may make them brittle and likely to break when struck with a hammer. Apply heat to the tool about 25mm (1 in) behind the cutting edge if it is a small tool and up to 75nun (3in) behind the cutting edge for larger tools such as chisels. Continue heating until the edge of the tool has reached a cherry red colour (sometimes referred to as blood red), then quench the tool by plunging it into clean, cold water. The degree of hardness obtained will depend upon how rapidly the tool is cooled, so move it about in the water to dissipate the heat. When metal is cooled rapidly there is a risk it may distort or crack; this risk is minimized if you dip the tool into the water vertically (edge or point first). If oil is used for quenching, the rate of cooling is reduced and the tool will be less hard than one quenched in water. Motor oil can be used; but if a large tool is quenched in a small quantity of oil, there is a possibility of the oil igniting.
When the tool has hardened, polish the area behind the edge or tip with emery cloth. Reheat the metal, gently applying heat behind the polished portion. After a time, which may be quite short if the tool is small, you will see a colour band moving along the polished portion from the point of application of the heat. Each type of tool has to be tempered at a particular temperature and the following table gives you a guide to the temperature and its associated colour.
Tool Oxide colour Approx Deg C
- Lathe tools Light straw 230
- Penknives Dark straw 240
- wood chisels Brown 250
- Twist drill bits, punches, rivet snaps, scissors, shears Brown purple 260
- Axes, table knives, wood turning tools Purple 270
- Cold chisels Dark purple 280
- Screwdrivers, springs Blue 300
When the appropriate colour has reached the edge. Or point, quench the tool immediately in water. Rub the tool with fine emer) cloth to remove the colour. Then test the tool. If it is not satisfactory-. you can repeat the hardening and tempering process. The temper obtained depends upon the carbon content of the tool steel. If you find the tool is not sufficiently tough after tempering, try retempering using the colour band which is next up on the scale.