Special shaping of tips is mandatory to avoid damage to delicate parts. In general the perimeter of the pin contact face should be rounded to avoid gouging or chiselling of bores. Various holes are bored to suit.
General contour that should be applied to all tool ends..
center hole can reduce friction when tip is turning against blunt shaft ends. In all cases the rim should be rounded to eliminate gouging.
A shallow, flat bottom bore slightly larger than the shaft diameter will help maintain alignment when applied to blunt shafts. The flat bottom can be obtained by finishing with a flat end mill bit.
A deep hole will facilitate pressing wheels on extended axles.
For tapered axle ends, the diameter should be smaller than that of the axle. Here the rounded rim also prevents jamming. A flat bottom is not required, but tip clearance is.
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