Bernd asked for a pic of the Potts drill jig so:
The jig
with large drill in place
adjusting the drill size using the caliper jaws of the jig
the jig set up close to the wheel in grinding position
The jig's rotation axis is at an angle to the vertical so the surface ground on the drill point is a small section of a cone, pointy end up. The point of this conical surface is where the rotation axis intersects the flat surface of the wheel. The adjustments for drill size cause small drills to get a more curved surface from near the point of the cone and larger drills a less curved surface from further down the cone. The jig requires a wheel intended for grinding on the side.
Like most such jigs placement of the drill in the V trough is fussy and of course the jig and it's mounting is more or less subject to deflection so results can vary. Sometimes the drills ground will even fit the holes they just made!
I got the kit from Woking Precision Models many years ago. A Google search for Pott's drill jig castings gave several hits.
I hope this helps
Dave
