I've been setting up my colchester chipmaster at last. It was an absolute dream to run, except the clutch. Pulling the handle engaged it, but as soon as I let go it disengaged. There was no clunk, click or anything to suggest any sort of locking mechanism locking in place, and I was getting pretty fed up. There was no indication of anything thant would keep pressure on the plates, either physically or in the parts book.
I joined the colchester lathes Yahoo group, and immediately found a document that explained how it works. This collar fits on the end of a shaft, and is pulled in and out by the clutch lever. Note the 3 internal lugs:
When you pull the lever, these lugs are supposed to go over these rollers, 3 off, in the ring.
This squeezes the rollers in toward the centre of the shaft, and up the ramps, which forces the two plates apart and puts pressure on the clutch.
However, you can just see there are two splined shafts, one inside the other. The collar fits on the smaller of these, which you can just see poking out in the picture above, and the clutch assembly on the larger. These splines have a different number of teeth!
So, my lathe looked like this. Look closely, and you can see that the lug in the collar is only half over the roller. So, instead of going over the roller and squeezing it in, it just presses against the plate-so the lathe starts, then immediately disengages.
You need to turn both the collar assembly and the collar on their splines (i.e. take off, advance one spline, try it), until the lugs line up with the collars like this. The rollers have been removed for clarity, since they fall out at any opportunity it is easier to line it up before you refit them.
There is a knurled adjusting collar that adjusts the slack in the clutch. You should be able to pull the collar over the rollers with a clunk-if it won't go, back off, if it doesn't clunk, tighten up.
This was one of those utter arsehole problems that seem impossible to solve, so feel free to spread this post about-I might save someone else days of stress and a headstock strip!