Mini-lathe - shimming the saddle

 This is a job I've been intending to get around to for years.

These Chinese mini lathes have a saddle to bed clearance set by two M5 screws providing a clearance between the saddle itself and it's retaining plate. These screws are more or less on the same centre line as the bolts retaining the plates and thus the plates are never parallel to the bed and the contact between the plate and the bed is more of a line than a flat area - something you can see in one of the pictures below.

This leads to poor control of the saddle clearance to the bed which can spoil finish or make your saddle stiff to move. The fix varies - since you don't adjust this very often, I think the easiest way is to shim the plates but various people have shown other options such as a sliding wedge operated by a screw which has the advantage that you can adjust the clearance from outside without having to strip off the lathe down. 

As I said we don't do this very often so we will go with shims. The first job is to measure the thickness of the saddle accurately and this is something around 24 mm - actually 24.08 mm.

The next job is to measure the distance between the bottom of the bed and the top of the saddle which will be similar to the first measurement plus a few hundredths of a millimetre (This lathe is Chinese we will be talking in metric for this post!) 

The difference between these measurements is 24.33 - 24.08 = 0.25 mm.

I don't have any shim stock but I do have various bits of sheet. I have a chunk of oil can which is 0.025 mm thick and is easily cut with the Wiss snips. Making holes however is extremely difficult - I drilled these between a piece of wood and the saddle plate but it really needs a punch.

It turned out that I had already done the apron side but with a 0.15 mm shim - but I had left the adjusting screws in for some reason. I took them out and replaced the plate with the same shim and found that the saddle wouldn't move at all, so I put a 0.25 mm shim in and reassembled the lathe.

The saddle seems to move smoothly but not have any up and down movement - but it is very hard to tell. Perhaps I'll put a DTI on it and we'll see if we can measure movement.