Thread: Another lead screw repair project

Have you the capability to lock the cross slide and do you cutting with the compound.

Have you been able to assertain how much more take up you have in your cross slide nut,so as not to go to big with the cross slide screw.

Can you turn your compound around and use that instead of the cross slide?

Dave

Hi Bryan,
Just get a thick bit of plate and bolt it to where the travelling steady goes, put a drill in the chuck and drill a hole in it. Put your boring head in the headstock spindle and bore it out to fit a bronze bush that suits your screw diameter. You can modify it later to become a multipurpose travelling steady.
Thats what I would do but then I am very lazy

Phil

Originally Posted by pipeclay

Have you the capability to lock the cross slide and do you cutting with the compound.

Have you been able to assertain how much more take up you have in your cross slide nut,so as not to go to big with the cross slide screw.

I think he is talking about widening the the unworn parts of the screw to match the worn parts, so it is even all along the length.

Dave

It would work I suppose but depending on how much wear you had, you would need to make up a new screw eventually. For me the choice is to modify the existing screw and then replace it with a 'new' screw or make up a new screw the first time.
Once I had a mill that used something like a 5/8 ACME for the Y axis. It was so worn when I got it that in the middle the thread was only around a mm thick. At that spot backlash was around 80 thou.
For those of us who feel uncomfortable with the 'make it uniformly worn' plan, what are the dimensions of the current screw - length, diameter, pitch etc so we can get a feel for the size of the thing.

Michael

Is it a square or acme thread..

It would also be left hand I assume...

Hi Bryan,

I see a flaw in the master plan, when you've made the thread evenly worn, won't you then have the problem of making a matching asymmetrically threaded nut...

I'd vote for either buying some stock acme rod, (plenty of threads recently on suppliers) or making a new one from scratch..

Regards
Ray

Thanks for all the replies. I don't want to use the compound for cross feed because I'll need that set to Z so I can register the tool. I think fine X & Z will be needed to control the cut. I would like to have them anyway.

There's enough adjustment in the nut to make it too tight in the most worn area, so I figure if can make that max width I should be safe. Although a light cleanup cut along the whole thread would be nice. Maybe I should examine this a bit more. Worst case I need to mod the nut assembly for more adjustment.

I like the plate idea. Don't have a boring head but could improvise.

2 pics attached. Looks like ACME to me. Shaft length is 500, thread length 290. Major diameter 22, pitch 5TPI, left hand. Yes it's an imperial pitch on a metric diameter. Go figure. The unworn crests are about 2mm, the worn ones around 1.75. It's hard to measure accurately.

Ray, I don't understand. It has two separate nuts that are spread by a wedge.

Originally Posted by Bryan

Ray, I don't understand. It has two separate nuts that are spread by a wedge.

Hi Bryan,
Is the wedge your backlash eliminator??

Phil

Originally Posted by Steamwhisperer

Hi Bryan,
Is the wedge your backlash eliminator??

Phil

Correct.

Hi Bryan,

Ok, I see how it works, it doesn't matter if the nuts aren't the same profile. The wedge will take up the slack.

Sorry for the diversion... as you were..

Regards
Ray

I would probably attempt to make a new one. Buying rod and making a tap with extra rod to make the nuts.. Would be a fun project.. I made a new leadscrew for my slotter as well as two nuts. Although that was a square thread which is easier to cut then ACME.

Hi Bryan,
Using the compound, you could set up the tool before hand so to cut one side the tool was hard against the tool holder, then place a predetermined shim between the holder and the tool to cut the other side.
You could set the distance up by doing a dummy run beside the thread to make sure before cutting it.

To keep the tool the same depth when you move it over, you could start out by making sure the tool was level with the back of the tool holder using a strait edge, then when you move it over with the shims do the same.

I just remembered you have a QC tool post now, another way would be to set up a left and right hand tool in 2 separate holders, then just swap them over.

Dave
PS
Thanks for welcome back the other day.

That's a very snazzy bed cover you have there.

A thought - You are wanting to take some material off the flanks of the thread (I could live with only taking off 0.25 to 0.3mm - I still vote for a new screw as the better solution though). In reality, there will be clearance between the root diameter of the thread and the minor diameter of the nut, so Z adjustment could be by hand - that is, move the tool up until it touches (or close enough) lock it up and you are away. This leaves the compound free to provide fine adjustment in the X direction.
Another option is to use the compound for Z adjustment and then physically move the screw in the chuck a little in the X direction. as it is a thread with angled sides if you started say 1mm from total depth, put the screw hard against the tool and secured the screw when you fed in full depth you would get a 0.25mm DOC.

Where in country SA are you? It may be easier to put the screw on another lathe

A final complication - Metric trapezoidal threads have an angle of 30 degrees. Imperial trapezoidal threads (that is, ACME) have an included angle of 29 degrees. Given that you have an Italian lathe with a metric OD but imperial pitch, what is your thread form?

Michael