How to make a hemispherical cutter? question (long)

I could not follow all the sizes and shapes you went through but if only cutting a soft material then why not try key steel or silver steel and then hardening it.

Silver steel is available in bar stock or various diameters is easy to machine or file and then can be readily hardend by heating to cherry and quenching.

Adrian

Adrian

I've just re-read my post and have to agree my description was a bit confusing!

What I am trying to do is cut a hemispherical recess so that a 3/4" diameter ball drops in half way, to a surface that is cut back (spotfaced/counterbored) by about 1/16" deep 1" diameter from the original face.

I suppose it might be easier if I look at it as two separate machining operations. 1" diameter spotfacing cutters are available and I could make a mandrel to suit the cutter bore and the existing 3/16 hole. A ball ended mill or a carbide burr of the correct diameter could cut the recess but since this cutter is going to be hand held and hand driven I don't hold out much hope of keeping it concentric.

Just read the above and I'm not sure its any clearer

Ian Phillips

If you have a ball or radius turning attachment, you could bore a piece of 3/4" Silver steel to 3/16" or whatever your pilot diameter, then turn a ball shape of the full 3/4" dia on the end, and drill for a couple of grub screws to fix the pilot in place. You'd do better to reduce the other end to say 1/2" dia and use that to hold the cutter in a drill chuck than to try to drive it with the pilot. Next you'll need some flutes cut, I've had success doing this sort of thing for special wood countersinks with careful use of a small angle grinder to rough it out, then maybe a dremel type grinder to put some relief on the cutting edges. If you have a mill and indexer you can do a nice neat job.Two flutes would be easiest to do, I'd try just straight flutes along the axis to begin with simply because that's the easiest to do, you can always play with the cutting angles right at the edge with a Dremel or whatever. When you've got it somewhere near, heat to dull red & quench in oil, you might then not need to temper it. An oilstone or diamond file will then give you a sharp edge. Find something really slow to drive it with, maybe even a hand brace. If your pilot is only 3/16" dia, that's going to bend easily under the side forces you inevitably get with hand powered drills, a big cordless drill with good speed control might be the most controllable way to drive it.

No guarantees, but that's how I'd try it.

I'm sure it would be possible to make up a cutter to do the spotfacing as part of the same operation, maybe a piece of 1" silver steel clamped around the 3/4" ball cutter, but is it worth the complication?

HTH

Tim

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Instead of machining back, couldn't you build up the existing form witn one of the epoxy-based repair compuunds. Possibly using a ball bearing to press a hemisphere into it. I think Devcon make several compounds for tasks like this.

regards

Kevin

Kevin

Originally I considered using SKC? slideway lining compound (used for rebuilding machine tool ways) but the cost and complicated application requirements put me off so I decided on the 'making new insert' method.

After comments on this group I've had a rethink and have gone back to using epoxy of some sort. I will enlarge and roughen up the recess before pressing a ball bearing into it. The only thing I am not sure about is what to use as the best release coating.

Devcon sounds a good idea even though I have some of the SKC stuff its time expired and has a really complicated installation procedure.

Ian Phillips