Successful drilling ot metal depends not only on using the right bit for the work, but also on adopting the right techniques to ensure accuracy. Electric drills are ideal for drilling, but those used in the home have a maximum chuck capacity of 13mm. If you are regularly drilling holes of a wider diameter, a pillar drilling machine – as used in engineering workshops – is really necessary.
TYPES OF BIT
Twist bits are most commonly used with metal, fitted to an electric or hand drill; other attachments used on metalwork are countersink bits, tank cutters and hole saws. Most bits can be used on either metal or wood but bear in mind the twist is really inadequate for boring in wood and care must be taken not to force the bit when drilling. TWIST BIT This type is made from carbon steel or high speed steel. The latter, as the name implies, can be used at higher drilling speeds than a carbon steel bit.
For precision work, particularly when tapping screw threads in holes, special bit sizes will have to be used. But for general work, where a high degree of accuracy is not needed, the following range of bits is sufficient:
Twist bits are not really suitable for sheet metal since it is difficult to maintain an accurate hole. The bit often catches and screws into the metal and excessive burring often occurs on the underside. The best method is to punch sheet metal. You can. however, work fairly successfully with a twist bit on sheet metal if the point is ground to 140 degrees. COUNTERSINK BIT Used to drill holes to take countersunk screws or rivets. The bit has a 120 degree point. TANK CUTTER Ideal for cutting into thin metal containers, such as water storage cisterns or tanks – or for making large washers. The cutter should be used in a hand drill and the blade is adjustable so you can cut holes of different diameters. Hole saw This is suitable for cutting large holes in thin metal, but should be used at slow speeds in an electric drill. The bit is encircled by a saw blade; different diameter blades are available.
The spiral of the bit, which is designed to improve the cutting action, varies according to the metal being worked. A ‘quick’ spiral is most suitable for cutting softer materials, but it is not economical to buy bits specially for hard or soft materials. If you do have sufficient bits, however, it is worth keeping one set for soft materials, such as wood, and one for metalwork.
The angle of the drill point does vary to suit the material being worked. Engineers grind the point of their bits to suit the material, but this is unnecessary if you are not tackling a lot of work.
The standard point has an included angle of 118 degrees; the normal way of defining this is to take a line through the middle of the shank and measure the angle from this line to the cutting edge – which is 59 degrees. This is a more accurate measurement because when checking the included angle the whole of the point could be tilted to one side; although still having an included angle of 118 degrees, the line through the middle of the shank might not bisect this angle. It is important not only that the lip angle is correct, but also that lip lengths are equal. If the lip angle and/or the lip lengths are unequal, the bit will cut oversize.
All cutting tools must have a clearance behind the cutting edge or the tool will merely rub on the work and not cut into it; the drill bit is no exception. The conical shape at the tip of the drill is ground back between four and 12 degrees to produce this clearance.
When drilling a hole, there must be a starting point; even if you fix the drill securely in a stand and work on a flat, rigid surface, you will still need a centre punch dot to ensure the hole is drilled in exactly the right position. The bigger the bit, the wider its web; as the drill gets older and is sharpened a number of times, the web gets thicker as the flutes are ground away towards the shank and this will affect the size of the hole being drilled. For real accuracy you should drill a pilot hole with a small size twist bit and progress size by size until the final hole size is reached.
Even when you take all the necessary precautions, it is still quite possible to drill a hole slightly off centre. The problem is that with – in particular-a small punch dot, you cannot always tell early enough whether the bit has in fact wandered off centre. The answer is to use the centre punch dot as the centre and scribe at least one, and preferably two or three, concentric circles around it on the work. This makes it easier for you to see if the hole being drilled is really central.
If it is off centre, make a small groove with a punch or chisel in the other direction. Remember that corrections can only be made when just the tip of the bit is in contact with the work; once the full diameter of the shank has entered the hole, it will be too late to make any adjustments. If using a portable electric drill, reduce the force of drilling as the bit breaks through the other side of the work. When you want to drill through more than one piece of metal, you can clamp the pieces together and, having marked the holes to be drilled in the top piece, let the bit penetrate all the pieces; as an added precaution clamp a block of wood to the back of the work to prevent damage as the bit bursts through. With two pieces you can mark the position of the holes to be drilled in the top piece and then mark one corner hole in the other piece. Drill that hole in both and secure the pieces with a bolt before drilling through the other holes in the top piece, continuing through the second piece. This guarantees the holes are in exactly the same position in both pieces of metal. DRILLING SPEED In precision work the cutting speed of the bit is all important. But the DIY metalworker is usually limited to the variations of speed on the electric drill. As a general rule, the bigger the bit being used, the slower should be the speed of rotation. For example, when using a 3.2mm bit on ordinary mild steel, you should use the top speed on the drill. For a 9.5mm bit, work at the slowest speed available. Where adjustment is possible, you will have to alter the speed by experience. The most difficult operation is using a large bit in hard materials; the work will be that much easier if you can reduce the speed of the drill to 750 revolutions per minute. Some drills are now fitted with an electronic speed reducer. Correct FEED One very important aspect of drilling is to feed the bit into the work correctly. Enough force should be applied to the drill to ensure the bit is cutting properly, but excessive force will cause overheating and alfect the cutting edge.
Lubrication is essential if you want to get maximum efficiency and life from your bits, particularly when using a powerful electric drill. The free graphite in cast iron reduces friction between the bit and the work so lubrication is not necessary; brass and copper are also drilled dry. Ferrous metals other than cast iron can be lubricated with either cutting oil or ordinary lubricating oil, while with non-ferrous metals, which include aluminium, you can use paraffin. Cutting oil should be applied with a brush or by means of an oil can; fingers should never be brought near a rotating drill bit. Lubricate at regular intervals until the drilling is finished.
With a hand drill there is normally little danger to the operator; but electric tools can be dangerous, particularly if the work is not held firmly. Always hold the work in a vice – either an engineer’s or machine vice will do, or a hand vice for small work. A imchine vice should have a chuck guard fitted; fix it securely to the drill stand wherever possible. Wear close-fitting clothing so there is no danger of it getting entangled in any moving parts. When using a portable drill, always work away from the body and ensure hands are kept well clear of any possible break-through point.
It is also vital you fit the bit correctly to the chuck of the drill. The chuck should be tightened with a chuck key and the bit mounted centrally in the chuck. Never use a bit with a damaged shank since the chuck may not tighten properly. If you do not have a vice and the work is fairly thin, fasten it with nails to a block of wood and clamp this to the work surface. Never hold the work in the hand while drilling.
A bit should never be used if the lands are worn away, since it will tend to bind in the work. If the corners of the bit wear quickly, this means the cutting speed is too high. If the cutting edges of the bit begin to chip, reduce the rate at which the bit is fed into the work or grind the bit to reduce the clearance angle. If the bit does not start cutting, this means there is no clearance on the lips. AVOIDING DAMAGE IF the bit is fed into the work too rapidly, it is liable to break. Pressure on the bit should be reduced as it reaches the point of breakthrough. When drilling deep holes, the bit should be withdrawn from time to time to prevent it being jammed by the turnings as it cuts the metal. Never let the bit overheat; a carbon steel bit will ‘blue’ when overheated and will be useless. If lubricant is not being used, withdraw the bit and let it cool if necessary. This is not so crucial with high speed steel bits. If the size of the turnings issuing from each flute are not approximately the same size, the bit is incorrectly ground.