Centre help!!!

Christine

If your workpiece is a flat disk of some sort of plastic then I am not sure the plywood is any benefit in this case. I would be tempted to sit the job directly on the faceplate (with sheet of paper) and put a length of threaded rod through the lathe spindle to act as the central clamping bolt. Whatever the material its proportions you will need to use a large 'washer' so that the clamping pressure is applied nearer to the OD than to the centre of the part .

If the job is plastic, even if its 1" thick a central M10 bolt with a small washer will only really clamp the central part of the disk especially if it pulling up against a compressible material like plywood. Even if your workpiece was thick hardened steel the plywood would compress so the resulting gripping area would be concentrated near the bolt rather than where its needed (closer to the periphery.

The key to clamping with a central bolt is to use a large washer but they are not the sort of thing one has lying around. What you might be able to use is a large changewheel or maybe a pulley. It does not have to round, but it does need to be a reasonable size and its face (that is clamping the job) needs to be relieved.

Ian P

O.K. forgetting the four jaw chuck for now perhaps you could try this:

Get a new piece of plywood or better MDF, cut a 180 mm circle and drill it to bolt to the faceplate. Don't drill the centre hole yet. Bolt it to the faceplate and drill the centre hole from the tialstock using the highest speed and a really sharp drill. The hole will now be truly centred on the lathe spindle.  Clean off all the sawdust!

Remove the faceplate and bolt your workpiece in place firmly but not too tight. Put it back on the lathe and rotate by hand to check that it runs true. If it doesn't a few well judged taps with a soft headed hammer should true it up ( a 10 mm bolt will be slightly loose in a 10 mm hole). Remove from the lathe and fully tighten the central bolt again, replace and recheck. Hopefully it will still run true.

Russell.

Edited By Russell Eberhardt on 06/12/2016 15:27:35

If the hole you have in the part is fairly central you could pull it directly back onto your face plate with a length of all thread and then tap it to spin true and then finally tighten the all thread.

I made several things on a Taig this way that were too big for it really. It's best to make a shouldered part that fit's nicely in the rear of the spindle with a hole in it to keep the all thread fairly true to the axis of the spindle.

At the work end you will need another nut, a penny washer and a decent sized piece of ply or whatever to press onto the work. I'd suggest a 100mm square piece for a part this big but a piece of aluminium say 1/2" thick would be a better option.

I used M6 all thread and found it offered plenty of holding power. Sometimes the work could have an M6 hole in it but I don't think that would be a good idea when holding PVC.

You shouldn't willy nilly face off a surface plate. They should be produced pretty flat and your lathe will face a dish into it. Usually a concave one as lathes are generally made that way. It's done so that separate faced parts can rest on each other without rocking.

I should mention that when holding work this way both ends of the all thread need to be fairly true to the axis of the spindle. If it's at too much of an an angle it might work loose while turning. I just took a bit of care with that aspect and never had a problem so never made anything to centralise the all thread at the other end of the spindle.

John

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Edited By Ajohnw on 06/12/2016 15:48:33

Guys - thank you so much for all this info. I never even thought of using the tailstock to drill or centre it. That is a great idea.

I think too you are right about the ply compressing- I will try removing it.

You guys are the best. Thank you all.

Christine