Is buying a custom ground tool my only option??

If the application is horizontal with mainly vertical loads you could use an entirely different approach if you can obtain two ring magnets say 100mm diameter with an 80mm hole, magnetised so that the annular faces are the poles. Such magnets, made using powerful ferrites, used to be common in TV focusing assemblies and are available from specialists like Magnetexpert - also used on loudspeakers. This is the kind of thing I mean:

**LINK**

If you place say a dozen ball bearings on one of the faces, they will all pull themselves to the inner edge rather irregularly spaced. But now bring the second magnet down on top with the oppositely magnetised face and suddenly all the balls will position themselves roughly in the centre of the faces, equally spaced and holding the magnets apart. You will now find that you have a low friction self-aligning ball bearing.

To form accurate circular grooves I have made an adaptor for the lathe tool post which holds milling cutters ,the type which were used on horizontal mill arbours with a one inch bore,these were available with full form internal and external forms ,good used ones can be found at s/h tool dealers or auto jumbles ,usually very cheap as there is little demand .

Guy,

You say in your original post that the bearing will not be under significant load, so would a bearing made from plastic material suffice? Certainly the machining would be considerably easier and it's not without precedent. There are a lot of plastic bearings available. You'd have to get suggestions for a suitable material from someone, though - just not me. I don't have the technical expertise or knowledge.

Just a thought ........

John

Did you solve your problem Guy? It would be nice to get some feedback.

Thanks for all the further interesting suggestions. Some really interesting ones and from the tool grinding ones, I'd say that the brazing of a ball bearing and grinding half away would be my best bet with the kit I have on hand. I think grinding a rake angle onto HSS rod would leave a slot cutting edge.
I have decided what I am going to do for this next build and to be honest I have ended up shirking the challenge to some degree. Ketan from ARC put a link to some very thin section bearings and I've managed to source a 120x108x6.35 bearing for a reasonable price. I'll make a housing for that and should end up with something compact enough for this build.

Thanks John Haine for the magnet option. I'm not sure as I fully follow it. I'm thinking that the magnets would be axially magnetised (i.e. with the poles on the flat faces). I can't see why having the balls sat keeping the magnets spaced apart would cause them to space evenly. Also, would that not mean that the balls run on the faces of the ferrite? Although RPM would be very low in this application, I'd be concerned about wear if that were the case.

Just grinding the end of a round toolbit at an angle is easy, (Freehand ) and causes no problems.

I needed to cut a half round thread, so I made up a holder (just a drilling at an angle , slit with a junior hacksaw, with a small Allen capcsrew to clamp.

From memory, the drilling was at about 20 - 30 degrees, and the end of the toolbit ground to about 15 -20, so that when installed there was a positive rake of about 10 degrees.

It would probably worth, in this case, honing the ground face with an oilstone or diamond file.

Howard

Yes, they are axially magnetised. There must be some strange magnetic effect that keeps them evenly spaced, and more or less in the centre of the faces, but it's like magic when you bring the faces together that the balls suddenly jump into a stable symmetrical configuration where they are equally spaced. Yes, the balls run on the ferrite surface, but ferrite is very hard, I'd be more worried about the balls wearing TBH. I discovered this when I was about 10 and used to play around a lot with magnets and things, which I scrounged from scrap TV sets. Since then I have seen it referred to a couple of times but it isn't widely known. These magnets can be very strong so the lateral constraint is also very strong, it is very hard to push the top magnet sideways.

Thinking about this further, I believe the reason the balls space themselves evenly is that they are magnetised by the intense field in the gap. If you imagine that the underside of the top ring is N and the topside of the bottom ring is S, the balls will take up a similar field, and will repel each other. To minimise the stored energy (always what a physical system tries to do) they will space themselves equally round the face.

I never understood the reluctance by so many to grind a tool bit. A bench grinder and stones or diamond files honing are all that is required and, of course, lots of patience.