Leadscrew thrust bearings

December 2014

One of the first things I noticed during the lathe rebuild was the lack of thrust bearings on the cross-slide and carriage leadscrews.

The standard arrangement features same-metal to same-metal ‘bearings’ lubricated by light oil dripped into the oil holes on the handle bosses. The key principle of plain (non-roller) bearings is that they are made of a different material to the surface being borne. The stock design doesn’t include functional plain bearings.

Carriage leadscrew

When I first started using the lathe, I thought that something was wrong when I tried to reposition the carriage, as the handle required so much effort to move. After oiling everything, and checking the tightness of the carriage on the way, I concluded that this is just how it is. Some googleing confirmed this as other people had the same problem with the MD65.

A quick search on eBay and a thrust bearing was on the way in the post. I took off the handle and fitted the bearing – the difference was significant, the leadscrew is now much easier to turn.

brearing

The bearing I used: AXK1226. To install it, undo the locking nut and unscrew the leadscrew handle, which should have the indicator dial attached. Insert the bearing onto the shaft and reverse the process. You apply tension to the assembly by tightening the leadscrew handle.

bearing installed

The arrangement of using the ally handle to apply tension is not good – in fact after installing the bearing I noticed that the handle was not square to the leadscrew, which explained why it was hard to turn at certain points in its rotation.

As with many such temporary measures, it became permanent. I ended up simply adding some large o-rings in the gap around the bearing (that you can see in the pic above), to protect the bearing from swarf.

A better arrangement would be to either bore a recess into the boss for the bearing, or make a ‘cup’. See the section below on the cross-slide leadscrew.

Cross-slide leadscrew

After using the lathe for a few projects, I felt that the cross-slide could also do with a thrust bearing. This would make adjusting the gibs easier and improve the feel of the lathe (as the resistance from the leadscrew would be removed).

I did a better job of this than the carriage leadscrew… The bearings are 4mm thick with both washers, I bored recesses for the bearings into each side of the handwheel boss. 4.0mm deep on the cross-slide side, and 3.8mm deep on the handwheel side – so that the handwheel butts up against the bearing. A little light oil on the bearings and you’re good to go.

The bearings are AXK08021.

CAD drawing showing the bearings as installed. What you can’t see is the shoulder on the leadscrew butting up against the far side of the right-hand thrust bearing. Tightening the handwheel clamps the two bearings around the handwheel boss.

As machined…

After installing the bearings, I immediately had to tighten up the gibs as the weight of the handle was turning the wheel and moving the cross-slide! This mod will improve rigidity as I can now adjust the gibs correctly, without getting false ‘tightness’ feedback from the handwheel.