Is buying a custom ground tool my only option??

[Guy ThornhillParticipant @guythornhill65637]

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.

[Howard LewisParticipant @howardlewis46836]

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

[John HaineParticipant @johnhaine32865]

Posted by Guy Thornhill on 03/07/2021 11:45:05:

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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.

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.

[John HaineParticipant @johnhaine32865]

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.

[John ReeseParticipant @johnreese12848]

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.

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