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Jig for training the hand to sharpen twist drill bits


There are two important criteria to watch out for when sharpening twist bits:

1. Maintain a constant, symmetrical angle that meets in the centre, such that the cutting angles are identical .... 118 degrees is the typical included angle for wood, and this is 59 degrees each side (135 degrees for steel). Meeting in the centre adds balance to the cutting action. Too sharp an angle will not cut as the sides may just rub against the material.

2. The bevel requires a relief angle, that is, there is a higher bevel angle behind the cutting edge (the heel of the bevel). Without this, the cutting edge cannot penetrate the material (as above, will just rub against the wood). To do this you will progressively raise the bit on the grinder wheel ... cutting a slight arc (instead of a single flat).

While all this sounds complicated when laid out like this, in practice it is a simple sweeping motion of the bit on the face of a grinder wheel along with a rotation of the bit on the face.

Here is a short video by Mathias Wandel, who uses the same method as I ...

https://www.youtube.com/watch?v=KkDe8IatSw0&ab_channel=MatthiasWandel


To aid those starting out to maintain the angle and facilitation the sweep, I came up with this little jig ...



The fence is set at 59 degrees. There is no specific diameter required for the dowel - just large enough to make rotation easy.



The method is simply to push the bit into the wheel, create the flat to the centre of the cutter, and then slide the bit up the wheel (lowering the end of the bit) while using the dowel as a pivot and the fence as a guide ...




After some practice runs, you should be ready to do this freehand using a finger as a rest.

Should take under a minute in all.

Regards from Perth

Derek


January 2024