Unimat 3 watchmakers collet adapter

Emco made a few watchmakers accessories for the Unimat 3, which, apart from the hand tool rest, can be hard to track down. Perhaps watchmakers wanted a lathe designed specifically for their line of work rather than adapting a larger machine, which would explain the scarcity of these accessories. Apart from the hand tool rest, there was a turning tool holder of the type most popular with watchmakers, a dividing plate for gear cutting (which fits directly onto the headstock spindle), and, a watchmakers collet adapter. I've only seen two of the latter despite a saved search on that popular internet auction site, and one of those I now own.

Here's the business end, which fits into the lathe spindle and holds the collets;

And fitted to the lathe;

All surfaces are hardened and ground.

There should be a drawbar with it, but mine came without one. So, I bought a generic adjustable drawbar. As supplied it was 150mm long, which was fine but with the drawbar fitted I couldn't open or close the belt cover, so I cut 30mm off the non-threaded end. To hold it central at the pulley end I made up a simple sleeve from brass, the same as the back spindle steadies that I made earlier;

Fitted;

Tightening the drawbar by a fraction of a turn closes the collet by pulling it back into the taper, the drawbar having a threaded hole in one end which mates with the collet thread.

The fitting is designed to use 8mm watchmakers collets, easily obtainable in a large variety of sizes from 0.2mm(!) to 7mm. Here's a small variety;

And one fitted into the adapter;

Collets are numbered, the number corresponding to the size in tenths of a millimetre. So, a number 8 is 0.8mm and a number 21 is 2.1mm.
Also available are more specialist collets, including step collets which have a series of steps in 2mm increments to hold round flat objects such as gear wheels;

Step collets come in 5 sizes, each collet having steps of a slightly different diameters. They range from 5.4mm to 23.0mm.
Here's one holding a wheel;

You'll note that there's a keyway (slot) in the collets, this locates in a key inside the adapter. The key is not there to drive the collet, friction takes care of that, but to stop the collet rotating when the drawbar is tightened.

Each lathe manufacturer had different ideas about how collets should be made. The body diameters and the taper are constants, but body length and threads varied between manufacturers. There are two thread sizes in use, B8 which is European and has a 40 TPI pitch, and WW which is American and uses a .625mm pitch. Both thread sizes are very close, and B8 collets can be used in WW drawbars, but WW collets can bind when used in B8 drawbars. Luckily my drawbar works with both thread types giving me more options when buying collets.
Here's some collets from different manufacturers side by side showing the difference in body lengths;

Left to right; Lorch, Pultra, Boley-Lienen, Wolf-Jahn.

And the advantages over ER collets? Firstly having a set of step collets will come in very useful for holding wheels for face profiling, much easier to use than a separate step chuck held in a collet or 3 jaw chuck. Also, as you only need to tighten the drawbar by a fraction of a turn rather than manipulate two tommy bars to hold the workpiece steady it will make jobs like facing axles to length quicker. The disadvantages are that each collet is only suitable to hold material of its stated size, unlike an ER collet which has a clamping range of 1mm. However, I won't need a large selection of sizes for the work I intend to do which these, which will mainly be facing axles and turning pin points, and facing to length and profiling wheels. Another reason for wanting one is that I like collecting Unimat accessories! (And I have a fascination with watchmakers lathes).

A very useful reference is 'The Watchmaker's and Model Engineer's Lathe' by Donald de Carle.

Paul.