Harold Hall
Whilst the standard jobber drill is by far the most common, variations are made for specific applications. One such variation are drills intended to have a much flatter point for drilling thinner materials. The advantage of this is that more of the drill will be in contact with the material before the point actually breaks through, minimising the tendency for the drill's helix to pull the drill through the workpiece.
Having purchased many years ago a quantity of war surplus drills, this one for sheet metal was amongst them, and had the drill been sharpened with a flatter angle then the requirement for the cutting edge to be straight would have been met. Possibly the 136° setting on the drilling jig would be the one to use. However, the photograph serves to show the situation that I was attempting to describe above.
Having established why the requirements for the finished drill are, a 130 ° chisel angle and a straight cutting edge, we can return to the subject of using the jig. However, neither the 130° angle nor the straight cutting edge are critical so there is no need to bring out the straight edge or the engineer's protractor. With regard to the straight edge a simple visual check will be sufficient, for the 130° then a small piece of steel scribed at 130° and placed alongside the cutting edge as shown in Sk. 5 will be more than adequate. Remember, the angle is not critical, as neither is the edge being perfectly straight! Remember, one is just attempting to produce a drill that cuts easily and close to size
Using the jig
Knowing now the requirements, there are still three conditions that have to be met when setting up the jig for the sharpened drill to conform. These are, the projection of the drill from the jig, the rotational position of the drill in the jig, and rotating the drill accurately 180° between sharpening the first and second edges. The first two factors, whilst important, are not critical but ideally the 180 ° should be met as accurately as possible. Any error in this and the drill's ability to cut size will be effected, though an error of a degree or two is unlikely to make much difference. It will though benefit by being better than just a visual setting.
Photograph 7 shows a close up of what I would call a rotational stop, this locates in the drills flute and is intended to be used for setting the drill's rotation between sharpening edges one and two. Personally, I have reservations regarding using this(more details later) and provide details of an alternative, and more positive, method later.
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