What stones are best for maintaing edge when sharpening drills?

[graffian]

I used to use my valve grinder for sharpening drills to drill out spot welds. the cup stone on it had a permanent corner on it. Touch most grindstone corners and they disappear, the cup wheel on the valve side of the valve grinder was fine and the edge put up with me split pointing drills and grinding spot weld drills for about 40 years and still had a corners on it. I have just cut a load of spot welds then started using the treepany drills for drilling sheet metal. The 1/4" I just sharpened goes through 18swg sheet in seconds, makes a round hole neat hole. Took a pic but it's not great. I sharpened this with a 1mm cutting disc. What sorta stones hold their corners best?

 

[E T]

CBN I guess. My Chinese drill grinder has a small CBN wheel and I bought a 150mm one for one of my bench grinders a couple of weeks ago and I like it.

 

[Agroshield]

Can you find out the specification of the wheel on the valve grinder (look for a direct replacement, find one sold for the machine on eBay, read the instruction manual).

You could also approach it from the drill grinding perspective and see what specification wheel a Brierley or equivalent uses.

They should have a code on the blotter.

This tells you what each letter of the code means:

How to Read a Grinding Wheel Specification

Explore How to Read a Grinding Wheel Specification. It includes bond type, grain spacing, wheel grades, materials, and more.

forturetools.com

Norton's 'handbook on toolroom grinding' (from vintagemachinery.org) goes into more detail on the above.

I think a 'hard' wheel would retain its shape for longer than a 'soft' wheel.