Help - lead screw nut binding!

[raynerd]

Hi guys,
Really annoyed tonight as I’ve been working up weeks to get my lathe into a wheel cutting setup and was about to give it its first go when my saddle started to bind up on the lead screw. Slackened everything off and finally pulled the saddle off to leave only the nut. Sure enough, the nut is binding!

It’s super free as it should be for about a 4 inch section and then starts to bind at either side! I don’t want to push my way through it but it binds up really tight so dont know if I could anyway!

Any help or advice as to what I should do? I’m a bit lost as where to go. I’ve checked the lead screw with my loop on and I can’t see anything that changes at the point it binds! It’s clean as a whistle.

Made a little video to show you but doesn’t say anything more than I’ve typed above...

I appreciate your help!

[awemawson]

Chris, has this just happened or is it the first time you've traversed this bit of the thread?

Pitch error seems the obvious issue if as you say there is no physical damage visible. This could have happened if for instance  the thread was stretched by a bad jam up by a previous owner.

I'm assuming you've checked the o/d at this point?

[raynerd]

Evening, thanks for the reply. I’ve just got back in and got no further with it.

Even though the lathe is fairly new to me and I’ve been doing a lot of prep work rather than actually using it, I’m confident I have traversed past these points myself. I’ve not had a job until tonight when I immediately stopped using it. I admit, I gave it a little tight turn initially but nothing excessive and then stopped.

The only thing I have done is had a fair bit of weight on the saddle with the pinion cutter mounted and it’s been sat on there for the last week whilst on holiday!!  However, shouldn’t that have all be taken up by the saddle on the bed and no pressure on the lead screw and nut.

If it was damage, would this be to the nut thread or lead screw thread? Like I say, it also jams up on both sides of the good smooth run! If it was damage, it would be coincidental that it was damage at these two points. O/d is consistent across the leadscrew.

Any advice on what I should do from here. I’m a bit stuck what to do next!

[raynerd]

One thing I don’t make clear on the video is that it binds up on both sides of that section. So if I come back towards the tailstock it then binds up again.

So there is about a 2/3 inch perfectly smooth run and then binds on both sides of it.

[Will_D]

By disconnecting the carriage and just running the nut you have eliminated a major cause of binding: Carriage/screw mis-alignment.

So what else could cause binding?

1. Pitch error: If the screw has been stretched the pitch will change. To test this mount a dial indicator on the nut and check the actual pitch where its free and where it binds. You are looking for a difference of only a few thou.

2. Thread form error: If the thread form (crest of screw) has been damaged you should be able to see this with a magnifying glass.  Trying chasing the thread with a suitable chaser or needle file.



Hope this helps.

Will

[awemawson]

The loose area of travel is of course where it would have been most used in the past so the area most likely to wear.

It's not unknown for people to squeeze a nut to reduce the back lash when the lead screw has worn - if this was done in situ then you would have the symptoms that you are observing. Take your glass to the nut and look for evidence !

[chipenter]

I would mike the bottom of the thread with three wires to check depth of thread is consistent .

[raynerd]

It’s sorted now. It was binding over one section of thread that seemed to have a little edge to it, same on both sides. I rubbed it down and as soon as the nut went over the other side was fine! Very odd! Sorry for the panic and I appreciate the help last night and today.

Chipinter, just for further reference, what do you mean by using three wires?

 Chris

[chipenter]

The tree wire method is wires of the same size are used , two on adjacent threads and one opposite to form a triangle to measure , there are tables for this method using precision ground wires .https://littlemachineshop.com/images/gallery/instructions/ThreeWireMethod.pdf

[vtsteam]

I guess the questions I'd want to figure out if it was mine is why were those edges there? And when did it happen? It sounds like it was after the purchase.

Symptomatic, also, to have that in two places. A single spot can be the result of an accidental ding.

If there's a mechanical use cause, it may return.