Making things to make things to make things

I recently acquired a woodturning lathe. Unfortunately, it was missing a rather important part – the centre drive head, which grips the wood being turned. It’s not huge or complex. It just looks like an elongated nut with spikes on one end which grip the wood being turned. At the other end is an internal screw thread, which fits on to the drive shaft of the lathe:
drive_centre 1
Because the lathe is old (you can tell from the black and white picture), the manufacturer doesn’t sell the spare part any more. They don’t seem to appear on eBay, presumably because anyone with a lathe isn’t going to sell such an important component. If I had any significant metalworking machinery, I could make one – but I don’t (yet). I do have a 3D printer, however.

I didn’t do any stress analysis on the component, but my instinct told me that a printed part wouldn’t going to be strong enough for this job. The centre has to hold a largish lump of wood spinning at up to 3000rpm. It has to do this while the wood is carved by a chisel, and most importantly it has to do this while I am standing very close to it. It’s not allowed to break. The body of the part needs to be reasonably strong, and the spikes on the end doubly so. It’s not really printable unless you have a SLS metal printer. If I could afford one of those, I wouldn’t be buying an old second-hand lathe.

My solution was to cast a new component, and to 3D print the mould for it. Some years ago I came across Plastic Padding Chemical Metal, which is a two-part polyester resin filled with metal powder. It’s designed for repairing things, and it’s very strong when it sets. I’ve mended various things with it over the years, and none of them have ever failed. I thought I might be able to use it to cast a new part. My wholly unscientific feeling was that it ought to be strong enough for the body of the thing, but probably not for the spikes. For that purpose, some hardened steel masonry nails might do the job. Casting would allow me to embed the nails in the part, and to create the required femall screw thread by simply casting around the male one.

I used OpenSCAD to design a three part mould, and printed it:

This is actually the Mark III version. The first two did not have a separate end piece, and I eventually realised that once I had filled the mould with resin and put the nails in, I would not be able to separate it. Doh! One thing to bear in mind is that Plastic Padding is sticky. It’s designed as a repair material, so it’s intended to stick to things. I didn’t want it to stick to the mould, or to the screw thread on the drive shaft. To stop this, I lined the mold with sellotape, and covered the shft’s screw thread with a layer of PTFE plumber’s tape. You can’t beat a good bodge. Here’s how the mould looks, half assembled and almost ready for filling:


And here it is fully assmbled and filled with sticky goo, waiting for it to set:


After 20 minutes, I opened up the mould (you’ll have noticed the carefully-planned slots on the mould join line, for levering it apart) to reveal this mess:


Which eventually delivered a component which clearly needed a bit of cleaning up:


But it works! Just for entertainment, I mounted a 2″ square section block of cheap pine, and turned it into one of the roughest spindles you’ll ever see – but the Plastic Padding component worked fine:


Now I just need to learn how to turn wood properly…

Incidentally, I’m not sponsored by Plastic Padding. I’m not averse to them sending me a free pack, either 😉