Turning a part ball between shoulders

I'm having a bit of a thick moment as I can't seem to work out how to do the following and could do with prompting!

I need to turn a partial ball shape between two shoulders.in a piece of brass bar. (A in the diagram) My first thoiught it to improvise a ball turning attachment such as described numerous times in postings in these forums or available commercially. When I looked into the practicality of this there seems to be an issue that they seem fine for turning virtually a complete ball, but in my task, it would seem that the tool would catch the shoulder part way through the arc.(B in the diagram) on bothe sides.

I just don't want to spend ages fabricating a jig that doesn't do the job. Anyone have any ideas please? Ropey sketch attached.

Even if I was to grind one side of the tool, I would need both a LH and RH version which would involve problems setting up to achieve a consistent curve, These are decorative only so no great accuracy needed but still need to look right. The original items were probably cast brass. I only need to do 2 of these so I don't want to overdo the time & technology here!

Any ideas appreciated. Regards Mike

A form tool is the obvious answer ... but with the considerable risk of chatter.

Removing most of the material with a series of grooves would help !

MichaelG.

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Here’s the general idea : https://youtu.be/81lPUTYUlL8

Edited By Michael Gilligan on 20/04/2021 18:20:44

As michael says a simple form tool will do it. Drill a hole in some gauge plate and then cut in half and shape the ends to your profile

Then feed into the work, these were 9/16" dia, I did not take any roughing cuts just fed straight into the brass

Definitely form tool, as it's brass chatter shouldn't be too much of a problem. If it is slow the RPM down until it stops, also use a centre if the part allows?

Tony

My note of caution was due to not knowing what lathe Mike uses, or what grade of Brass.

MichaelG.

Sorry, yes you have a point.

Tony

If you do want to use the ball turner then you need to modify the cutter so it will cut to one shoulder and then reverse the part in the chuck or between ctrs and do the other half of the curve

Alternative if it's a one off is to work out a few co-ordinates to rough out teh shape

Then blend the steps with a file

I'd use a graver by hand for two off

I suggest you mill the spherical surface. With the milling cutter axis normal to the workpiece's axis, if the workpiece is rotated, a 'hollow' rotating cutter - or a single-point tool - will produce a sperical surface. (This is because any slice through a sphere is a disk.)

In this case, the cutter diameter has to be the distance between the shoulders. The cutter's form (in this case) has to be hollow, internally bevelled, with effectively end-cutting teeth. Think of a 'solid of rotation' formed generated by a tool like the one on the left of Jason's sketch, or think of a hole saw, with its teeth tapered so it gets as far into the aforementioned corner as is required; a single-point tool works also, of course. The workpiece is slowly rotated as the cutter is fed radially. Perhaps a good idea to remove the bulk of the material with a more hefty cutter - you can use a single-point cutter in a boring head. Milling spherical surfaces is fun, and a good talking point...

This idea, with a little elaboration, is really neat for things like 3-ball handles.

I would probably go for the form tool. While chatter is likely to be a problem, you can often clean things up at the last by turning the job very slowly, with a mandrel handle if necessary. You don't try to take a real cut, just keep some scraping pressure on as you slowly turn the job. This will usually clean up the chatter marks.

John

Many thanks for all the suggestions guys.

I had initially considered using a form tool of some kind but then thought the chatter could be excessive due to the 'length' of the cutting edge. As several of you seem to give it a thumbs up anyway, I'll give it a test, keeping in mind some of the comments on technique, and see how it goes.

I'll also try Roy's idea of using a graver by hand on a test piece just out of interest.

For the type of job this is, I don't think it will be worth my time fabricating a proper ball turning jig.

This forum never seems to fail in providing the answers (and plenty of entertainment and thought-provoking comment along the way!)

Thanks again to all. Regards Mike

The form tool worked a treat! Initially tried untreated gauge plate but even with ordinary brass it soon took the edge off surprisingly. So hardened & tempered best as I could and that was fine afterwards. Step-cut much out with a parting tool and the form tool finished it well. (Note the shape is deliberately not a perfect shere). Again, thanks for the help.

Edited By Mike Hurley on 25/04/2021 11:41:25

Edited By Mike Hurley on 25/04/2021 11:42:06

Well done, Mike

Thanks for showing us.

MichaelG.

An alternative: Create a cutter which is a hollow cylinder of the diameter between your design shoulders. Cut teeth around the sharp edge, of a shape that will cut as the cutter rotates. Harden and temper the cutting edges. Set up a machine (mill or lathe) so that the workpiece can be rotated slowly, and the cutter so it can turn more quickly - with the cutter at right angles to the axis of the rotating workpiece. Traverse the cutter slowly along its own axis, into the workpiece, as both revolve. It will cut an exact part-sphere with sharp grooves each side. It will not cut anything but a sphere, though, vase-shapes etc cannot be generated by this method. In reality it is likely that only one or two teeth of the cutter will do the work, so take your time - and do make sure that there can be no movement of cutter or work along the axis of the workpiece.

Hope this helps - it does depend, though, on whether the ball-shape you need is a sphere or not.

Cheers, Tim

Edited By Tim Stevens on 25/04/2021 17:44:54

You can do much the same thing with a single point cutter in a boring head.

regards Martin