Colchester Bantam cross slide lead screw binding with new nut

[Mark Davison]

I recently bought a new cross slide lead nut for my bantam. It has eliminate pretty much all of the play but seems to have revealed another issue. The lead screw appears bent and the end wonders when rotated. Only the leadscrew shaft itself is straight and true.

The Bantam's cross slide lead screw is in two parts, an inner shaft which has the lead screw thread and a male spline on the other. The male spline slides inside a hollow shaft with a corresponding female spline. The hollow shaft incorporates the drive gear and a male spline at the other end for the micrometer dial. This hollow shaft then runs inside a bush in the block that mounts to the apron. I guess this is to enable it to be disconnected when used with a taper turning attachment.

Anyway, the 'wobble' appears to be a result of the O/D of the hollow outer shaft not being concentric with the female splined inner bore. The linked videos show the issue.

The reslt is tight spots when turning the leadscrew as the cross slide nears the micrometer dial end of the travel.

The question is, how would you go about correcting this? It is tight enough that it is making the new nut unusable in the last one or two inches of cross slide travel. I assume Taking a skim off the OD of the outer hollow shaft whilst mounted on the innershaft with that between centres as per video (and re-bushing the bearing/mounting block) will no doubt affect the micrometer dials fit on the hollow shafts male splines? Not that I can use the lathe without the cross slide leadscrew in it anyway.

 

[Agroshield]

Referring to the exploded diagram, 31818, screw keep is an engineered fit in the saddle casting, retained by screws 31919.

Pinion 31863 OD should be a good fit in and concentric with ID of 31818.

I do not think the OD of the leadscrew (the exact part of it pointed to by the 31894 label) needs to be a good fit, or necessarily very concentric with the ID of 31863. Nor do the splines need to have any OD/ID close fit, as they drive on the flanks.

In normal use, the two have no relative movement, the leadscrew being retained by nut 31843. As you say, it is only when taper turning that there is relative movement, and this is sliding in and out. So long as the splines remain engaged, it could be wobbly and still work.

Given that the machine has worked for years with those parts, and the only new part is the leadscrew nut, I would be suspecting that part.

Is it possible that the dimension from the centreline of the nut to the locating face is different to the old nut? Screw both onto the leadscrew and compare.

Make a rod of OD to just fit the minor diameter of the nut. Make a bush with ID to suit that OD and OD to suit hole in saddle. Slide rod through bush with cross slide towards you as much as possible and check it enters nut.

That is the most important check to make, that the hole/thread in the nut is concentric with the hole in the saddle.

When the nut is far from that hole, flex/spring in the leadscrew would hide any misalignment. When the nut is near to the hole, and thus the unsupported length is much shorter, it cannot flex to align itself, so binds up.

Does the cross slide become tighter if you move towards you as much as possible with no screw in it? As that part of the travel is less used, it will have worn less and if someone has tightened the gibs to suit the middle of its travel, that might exacerbate the problem or mislead you in determing its cause.

You can work without a cross slide. Position the one you have roughly where you want it and then lock it it place. Use the compound slide to put on the cut.

 

[dave h 2]

Mark,
Does the nut run easily along the screw when out of the lathe or are there tight spots at each end?
Do you still get some backlash with the nut in the middle
Does the issue still occur when the gibs adjustment is backed off.
You can also have wear in middle of the screw it's self as well as the gibs as above

 

[Mark Davison]

Gibs are fine, screw is very snug at the end (there is a very slight amount of wear in the middle but not enough to create backlash with the new nut) but no where near as tight as it all is when assembled. Im almost positive the tightness is caused by the 'wobble' in the screw caused by the lack of concentricity between the inner and out shafts

 

[Mark Davison]

It did try to measure the position of the top face of the nut vs what was required by the top slide. As you say it must have been OK when new. I didnt find anything concerning. I like the idea of making up a plain spindle that just fits the ID of the nut and checking that against the hole in the saddle. Ill give that a go.

The old nut had quite a bit of wear in it. I'm concerned that at least some of that wear has been caused by the 'wobble' in the screw.