A tapered oval mandrel

[Gary WoodingParticipant @garywooding25363]

I make jewellery and make many of the required tools myself. I often have to make collets for holding gemstones of various sizes. Making a tapered mandrel for truing collets for round stones is a trivial lathe turning exercise, but a mandrel for oval collets is not so simple.

Commercially produced tapered, oval mandrels, are machined on an eccentric turning lathe (or grinder) , but the product has a severe problem: the difference along the mandrel between the major and minor dimensions is maintained instead of the ratio. At the large end, the dimensions are12.5x11mm, but at the small end they are 4.5×3, which means that the ratio varies between 1.14 and 1.5.

Oval gemstones typically have a ratio of around 1.4:1, so, to be able to make various sizes of oval collets, a tapered mandrel is needed that maintains the ratio of about 1.4:1. This was an interesting challenge.

Here is a photo of the small end of the finished mandrel: it is 2.6×1.86mm, and the large end is 8.5×6.1mm. Yes, I know the large end is not as big as the commercial product, but its big enough for my needs; it maintains a ratio of about 1.4:1 all along.

Here are photos of the wide and narrow sides.

So, here's the challenge. How would you make it?

[Gary WoodingParticipant @garywooding25363]

[Ian PParticipant @ianp]

This looks like an ideal candidate for 3D printing (in metal, Titanium?)

Getting back to reality, I would make an oversize conical blank in the lathe, then mill two or four flats and then blend and dress the rest off by filing.

Ian P

[Les Jones 1Participant @lesjones1]

Hi Gary,
The only way I can think of without using a CNC machine is to use a milling machine with a rotary table set at a slight angle to the vertical. (The axis with a slight angle to the horizontal.) I suspect the geometry you require is a constant ratio between minor and major axis. This method would give a constant difference. It would really be a many sided polygon but with enough sides you would not notice. I used this method to round the edge of a 6mm plate to a semicircle. For this the Z axis did not need adjusting for each traverse along the edge. (I think I moved the rotary table in about 5 degree increments. For your requirement you would need to work out a table of Z axis positions for every step of rotation.

This is the result I got. Note the end of the plate was supported with the tailstock while machining. This would present a problem in your case.

Les.

[blowlampParticipant @blowlamp]

It sounds like the commercial product is just turned out of parallel, with a set pecking distance in/out of the cutter.

I'd draw it up in CAD and then use the rotary axis of my CNC mill to cut it. You could make it almost any shape you want that way.

 

Martin.

Edited By blowlamp on 16/08/2014 20:04:42

[Neil WyattModerator @neilwyatt]

One way would be to use a tapered template for eccentric turning AND offset the tailstock.

Don't ask me to do the maths…

Neil

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