Thread: How to turn a thin disk

HI surfdabbler, maybe you could build up the segments a fair bit thicker than you need. If the final ones want to be 5-10mm why not build em up say 20-30mm then part off from outside diameter once you have the ring turned to fit the clock face recess. Not sure how you plan to hold the segments to faceplate? but there are ways.

Yes, you don't turn a thin disc, you make a thick one, glue it to the body and turn it down. ..... mostly...

Hmm, yes, I could possibly start thicker and part it off once the front is shaped...except the timber is starting off at 40mm thick, and I'm bookmatching the pieces, so I will have less than 20mm to start with. I might be able to get the parting tool in there, but I'm not sure I have the skill to part it off neatly with not much room for error. The disk is about 50mm from outer diameter to inner diameter, so that's a fair way to part. I suspect I won't be able to get a flat back surface for glueing. Could be messy trying to get it nice and flat again.

I could use an MDF backing, and part that off.

What's likely to happen when I get to the inner edge when I'm parting it off like this? Will it fly off and smash? Will I have to measure carefully and stop before I reach the very end, and do the last bit with a saw and sandpaper?

@pommyphil - Yes, if I could turn the inner/outer edges, glue the disk into the body, and then turn the front, that would simplify it, but infortunately the main slab is too big for the lathe. I wanted to turn out the recess to inlay the disc, but ran into the same problem. I'm going to have to route the recess to sit the disc in place.

Try this, use 19mm particle board as a sacrificial board in the lathe. Glue the ring to the particle board and turn the face flat and to the right diameter. Cut in with a parting tool as far as you are game and finish off with a hand saw. No stress on the finished ring. Glue the ring onto the clock face, sand and finish, simple
I often use this technique on thin rings for segmented bowls.
Jim

Old metal turning trick for thin disks, washers, shims etc was to shellac them to a faceplate and turn them, then disolve the shellac with solvent to free them. Worked OK for metal down to thousandths of an inch thick.


I think something similar should work for your job provided you glue up the segmented blank with something like epoxy that will not be affected by the solvent. Personally I would start with a sacrificial MDF backer/faceplate and face it off to get the hard skin off the face, then drill a series of holes through that would be covered by the segmented blank. One coat of shellac on back of blank and on sacrificial face, mate up and allign while damp. Turn as required and remove sacrificial backer from faceplate. Lay face down and place solvent in holes in the backer. This will seep through the MDF core and soften the shellac bond allowing you to slide the backer off the finished work. Shellac residue should be able to be removed with a scraper after the solvent evaporates.

Suggest that you try on scrap before committing to your final material, just in case, as I haven't seen this technique suggested for timber elsewhere.

Woodturners are welcome to shoot down the idea if they have prior negative experience with anything similar, cos I are not a woodturner.

@malb, Ooh, interesting idea with the shellac. That could work too. The holes in the back got me thinking - to save parting off the mdf backing, why not use MDF board that is pre-shaped to be maybe 5-10mm larger than the inner diameter of the ring, so it is only glued to a small part of the ring, and possibly a second outer baseplate that has a hole cutout slightly smaller than the ring. That way, the ring is fairly well glued in place on it's final inner and outer edges, and the MDF can be screwed onto the faceplate, but when the time comes to remove it, there won't be too much to remove off the front piece, because it will already be cut away for most of the space under the ring. This, combined with using paper between the MDF and the ring, should make the ring pretty easy to get off the backing board when I'm done, and I won't have to try to get the parting tool in there. I could even possibly soak off the glue pretty quick given it will only be a thin circle that is glued. Sounds like a goer!

Originally Posted by malb

Old metal turning trick for thin disks, washers, shims etc was to shellac them to a faceplate and turn them, then disolve the shellac with solvent to free them. Worked OK for metal down to thousandths of an inch thick.

Woodturners are welcome to shoot down the idea if they have prior negative experience with anything similar, cos I are not a woodturner.

Very interesting idea. Must try that.
Jim

Originally Posted by surfdabbler

This, combined with using paper between the MDF and the ring,

Not a good idea to use paper in a glued joint, would not recommend that.
Jim

Originally Posted by powderpost

Not a good idea to use paper in a glued joint, would not recommend that.
Jim

Oh, I heard that's what most woodturners do to make it easier to break it off at the end...

Originally Posted by surfdabbler

Oh, I heard that's what most woodturners do to make it easier to break it off at the end...

I dunno about 'most', but I certainly do. For solid timbers anyway.

However, I think that Jim is pointing out that it's a thin segmented disk being parted off - with glue lines - and that it's more likely to break a 'seg joint' than it is to break the paper joint. (Due to the relative sizes of glued areas.)

An option:
Do you have a worm-screw for your chuck/faceplate? An option would be to glue/turn using the sacrificial disk, then flip the disk around and screw it onto the worm-screw (thru the small opening in the centre where the hands will go) so you can turn away the mdf.

You'd need to be very careful that the seg-work sits up nice & flush to the chuck/faceplate 'face' to ensure that you keep the disk faces truly squared to the lathe. Otherwise it'd come out as a round wedge... DAMHIKT.

However, you shouldn't need to worry too much about getting it perfectly 'central & balanced' as it's not the rim you're turning, just the face.

Another option:
I haven't tried the shellac method before, but I think I'm going to. The one difference/improvement that springs to mind concerns the sacrificial disk... Me, I'd make it from mdf, OK. Then I'd drill a helluva lot of pairs of holes through it, so it looks a bit like swiss cheese or a disc brake... but done so that it stays balanced, of course. Then glue on the seg work and start turning.

Why? So that when it comes time to dissolve the shallac, you can just flip the disk finished face down and drop some solvent into each hole, to speed up the dissolution of the shellac instead of waiting for it to soak in from the edges of the disks, if that makes sense?

Why not glue the segmented face to an MDF mounting board with hot melt glue?When you want to remove it,place in a microwave oven and zap it till the glue softens.Clean up any residual glue with turps.
Ted

I would go with the double sided tape but a good quality one. I put the two together then clamp to get good stick take light cuts.

Originally Posted by Skew ChiDAMN!!

Another option:
I haven't tried the shellac method before, but I think I'm going to. The one difference/improvement that springs to mind concerns the sacrificial disk... Me, I'd make it from mdf, OK. Then I'd drill a helluva lot of pairs of holes through it, so it looks a bit like swiss cheese or a disc brake... but done so that it stays balanced, of course. Then glue on the seg work and start turning.

Why? So that when it comes time to dissolve the shallac, you can just flip the disk finished face down and drop some solvent into each hole, to speed up the dissolution of the shellac instead of waiting for it to soak in from the edges of the disks, if that makes sense?

Skew, my suggestion was to use an MDF Backer with the holes and the skin faced off, specifically so that the solvent placed in the holes can spread to dissolve the shellac. Facing off makes the backer pourous over its entire surface to allow good spread, while the holes act as reservoirs for the solvent. Just not sure how many holes and what size to go for, obviously needs enough meat left to support the work while turning. Lots of 6mm holes or a quarter as many 12mm, or even fewer 19mm, not sure at my end, which is why I put in the disclaimer about not being a woodturner, but happy to be shot down by experienced people.

Making progress! I glued it onto an MDF base, but one that only reaches the centre of the disk. That way, I don't have as much work to do. In the 3rd photo, I've turned up the front face, but I haven't reduced it to final thickness. I'll do that from the back.

I was a little worried about the wood, whether it would look nice, and the colour looks terrible in the photos, but in real life, it's come up really nicely. Can't wait to get to the finishing stage! I'm going to try a poly finish, and see how it compares to some danish oil pieces of this same tree that I've done in the past, and then make the decision for which way to go. The main piece will be a poly finish, and I'll polish it to a high gloss, but the disk and centre of the face will get a satin finish, so that reflections don't get in the way of reading the numbers and the time.