looks like something is missing what lathe is it

It looks as though you've rotated the image by 90 degrees anti-clockwise. That's just not fair

"Head" is the gear that runs on the chuck shaft.
"Inter" is probably a lower position.
"Top Stud" is probably a higher position

There will also be a fourth gear on the shaft that is connected to the leadscrew.

It says there should be a 60 tooth gear first and two 75 tooth gears. That doesn't seem to be what you have.

Some wider shots (the right way up) would help.

Ah, much better, thanks.

So the bottom right shaft is the feed shaft and the upper shaft is the leadscrew. only one of these may be in use at any given time.

My guess is that the top gear there moves from where it is for screwcutting to the other shaft for feed only.

That leaves the lower gear being an idler. It appears to be a on stub shaft set into a swing plate or "banjo" plate. It can therefore be adjusted to engage when the last gear is on the leadscrew shaft or the feed shaft.

Does that help?

I suspect you have a gear on the top stud and nothing on the bottom stud. You might find that the bottom stud is effectively a direct drive to the leadscrew for cutting odd pitch threads if the change wheels are available.

I have done a lot of turning on a DSG but didn't need to mess with change wheels, only gearbox settings so didn't even look at what was behind the cover. One interesting aspect was cutting a thread that on most lathes would mean that the screw cutting indicator wouldn't be of any use. It was on the DSG.

John

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Hi, I have recently bought a DSG 17 x 36 which appears to be from the same era as your machine i.e. motor sticking out the end rather than behind the machine.

Here's a couple of pictures of the (standard for the indicated threads in the glass panel) changewheel setup on my machine.

AFAIK, the bottom gear is the one which feeds the threading box; on my machine the feeds are a fraction of the selected thread, I have only run a 17T once, but seem to remember it being the same.