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Harold Hall

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Reference to Sk. 3 shows the basic drill point having a chisel angle of 130°. From this, it can be seen that the chisel has to remove the centre of the hole and has a quite different cutting action to the two main edges. Actually, the chisel can hardly be called a cutting action it is rather more a case of scraping out the centre of the hole, obviously much less efficient than the cutting edges. Because of this, should the chisel length be increased as illustrate by Sk. 4, the increased angle, and therefore the longer chisel, creates a larger centre section to be removed. A result of this will be that more pressure is required to force the drill through the material being drilled. An additional,though less critical, problem with this is that the centre punch mark used to position the drill for drilling will have to be larger to enable the chisel to locate into it.

 

What then effects the chisel angle and therefore its length. Photograph 5 showed earlier how the clearance angle can vary, from this is should be obvious that a larger clearance angle will have the effect of slewing round the chisel and increasing its size. Photograph 6 shows the end view of the same two drills and clearly indicates the effect on the chisel angle, that on the left having a greater chisel angle due to too much clearance with the reverse being the case for that on the right. It has been learnt therefore that the correct clearance is when the chisel angle is correct.

 

Having established the correct chisel angle requirements, and therefore the clearance, there still remains the angle between the two cutting edges. For the vast majority of the drills sold to the home workshop owner, or even in industry, the angle required is 118°. This could be measured using an engineer's protractor of some form but as it is not critical, within a few degrees, a simple visual check will suffice as follows.

 

There are three situations that can result through the angle ground on the drill being varied. If at the correct angle (118° in almost all cases)the cutting edge will be straight, but if greater (flatter)the edge will be convex and if less  (more pointed) then concave. The reason for these situations can be easily seen if a rule is placed in a large drill's flute and moved from a flat point position to a very pointed one.

 

It can be said therefore that if the cutting edge is straight and the chisel angle correct then all other factors will automatically fall into place.

 

Returning to photograph 6 showing the end of two drills after sharpening, in this it can be seen that the one on the left has a convex cutting edge. This, even though it had been sharpened at the usual angle of 118°.

Drill Sharpening, incorrect results
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 6

Metalworking

Workshop Processes