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Harold Hall

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Multi facet drill forms

A method of sharpening standard jobber drills that has frequently been featured in  magazines is the four facet method. As the name implies it is a drill point system that has four faces, two on each cutting edge.

 

This would appear to be the equivalent of an end mill with one narrow face at a shallow angle to produce the cutting edge and a steeper face behind it just to provide extra clearance. This is not so, the steeper angle is an important part of the drills cutting ability as I will  explain.

 

Either facet can be ground first but I will go with producing the cutting edge first. The primary angle should be in the order of 10°, but no more, say 8° to  10°, and ground on each cutting edge as a flat surface. That is, there is no increasing clearance as the face moves away from the cutting edge as on the more normal drill. The drill will still have a chisel point but this will be less than 130°.

 

The secondary clearance face will have the effect of slewing round the chisel in the same way as the increasing clearance did with a drill sharpened normally. However, the amount that the chisel rotates will be dependant on the angle of the secondary face, this angle cannot therefore be chosen arbitrarily. For a chisel angle of about 130° a secondary clearance of 25° should be suitable, but in this case, no less, say 25° to 30°.

 

Unfortunately, grinding the secondary clearance is rather more complex than just grinding on a clearance of 25°. With too little ground off there will still be some of the original chisel left resulting in it having a jagged formation, see Sk. 7 If on the other hand, too much is ground away, then the line between the two secondary clearances will pass each other and the drill will end up with a forked chisel as seen in Sk. 8. What is required, ideally, is that the two lines meet in the middle as Sk. 9 shows and illustrating that the result is for the chisel to be converted into a shallow point. This all adds up to the whole process being a critical one. As a concession to accuracy, it is normal for the two lines to stop just short of lining up leaving just a small portion of the initial chisel, say 0.1 to 0.2mm long, a magnifying glass will help.

Drill Sharpening, four facet form.   Harold Hall

Metalworking

Workshop Processes

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