There are a number of different forms of twist drill, different base metals, different coatings and an even larger array of ways in which you should sharpen your drill bits. This article has been abbreviated from the 20 April 1916 “The Model Engineer and Electrician” and it really is a fundamental text that is worth some time reviewing as it takes us back to basics.
A drill does not differ from a lathe or other type of tool in respect of fundamental essentials regarding angles for cutting and clearance, but it suffers from two disadvantages which do not hamper a turning tool:
- the necessary width at the point, which cannot be helped, and which forms an undesirable non-cutting length that only grinds on the bottom of the hole
- congestion of cuttings in the hole that produce friction and overheating
The first issue is corrected as far as possible by thinning the point. The second issue is solved by the twist of the drill that moves the chippings away from the cutting edge and hole, along with the use of cutting fluid.
The clearance is revealed in striking fashion by looking end-on at the point. The three views show the extremes of clearance.
The angle which the cutting edges make with the longituinal axis is always 59°, the reason for this being that it gives straight cutting edges. There are two other clearances formed on a drill:
- the radial relief – to prevent the drill rubbing uselessly in the walls of the hole
- longitudinal relief – reduction in diameter from point to shank, again made to reduce the useless rubbing on the wall of the hole
The objectionable feature of the point, mentioned above: that it has an appreciable width which is non-cutting tends to increase as the drill wears and the thicker part of the web is reached.
The cure for this is the thinning of the point on a narrow grinding wheel or on the corner of the grinding wheel.
In the figure to the right the point has been ground back and thinned towards the centre of the drill. There is a limit to this thinning, too much having the effect of weakening the point so that it snips off under the pressure of drilling.
Note that evidence of too high a feed rate is seen by damage to the tip of the thinned drill.
The most perfect results are attained when the lips of the drill are of equal length, uniform angles and the point exactly in the centre of the drill axis.
If ground thus, the resulting chips will come out in two exactly similar shapes and sizes, and the drill will not shake in the hole if tried by hand.
But if the grinding is at fault, the hole may be too large, and the lips may not cut equally, a fact revealed by the difference in the cuttings from each. This throws a greater strain on one lip, and as the duty is not properly divided up, the feeding cannot be done at a proper rate, nor will the drill last for long without becoming dull, mostly on the lip that does the greater proportion of the work.