Half nut skipping still

OK Steve,

My pins measure 0.250" -.001 and the slots take a letter E drill shank (0.250" end to end whilst a letter F drill (0.254"is tight in spots. With the lever assembly off it's pivot pin the two pegs slide freely up and down as long as I align the nuts and leadscrew. Move the saddle a 1/16", lifting the top nut up out of mesh while the.bottom one falls out of mesh under it's own weight and the two half nuts are restricted in movement until I move further and realign with the leadscrew.
I've never had occasion to remove the apron but I cannot detect any shims or need for any. Remember, adding excessive shims also affects the engagement of the rack and pinion associated with the manual feeding of the saddle. With the half nut mechanism reassembled and the threads aligned to the leadscrew I can engage and disengage the half nuts and detect no movement or strain on the leadscrew anywhere along the length of the saddle movement.
The earlier thread of yours about wonky threads on the pins that screw into the half nuts may have some bearing on the problem . There has to be some symmetry about those pins, the half nuts and the leadscrew. A wonky screw will cause one half nut to engage before the other and that, together with excessive clearances to the two slots and you will get the sort of disengagement problems you are seeing.

Get your slots uniform in size and round particularly at the nut closed position. then produce two pins of a diameter to suit your slot size less 0.002" to 0.005"and get the thread as concentric as possible . I would remove shims at initial tryout as I doubt that it left the factory with shims. After all they cost less than £20 in the thirties so didn't get much individual attention.
Clear out any burrs or dross that may still be in the dodgy half nut and whilst loose, check it's engagement with the leadscrew. Put it all back together and with the nuts/ leadscrew aligned check to see if the leadscrew is being lifted/ lowered as you engage the nuts.
Once again I would say, not as a recommendation but as matter of fact, your machine should be able to screwcut with one half nut just a plain bore to suit the O/D of the leadsrew. The other half would do all the work and feed the saddle along. After all your quick release engineers vise only has a half nut and see what forces that can stand!!

Regards Bob D

Edited By Robert Dodds on 27/11/2021 21:56:25

Robert has confirmed what I suspected, that the 3/16 pins were a later, incorrect, modification.

The problem with any pre owned machine, particularly a really old one, is that there is little way ofknowing what incorrect modification shave been done.

The half nuts jumping out, probably caused by the incorrect pins being fitted, may be the reason why the lathe was sold,

Getting it back to original specification, as closely as possible, has to be the first step along the road to correct operation.

With regard to Leadscrew alignment, may i suggest the following as means of effecting an improvement?

(Having made two new 1/4" diameter pins, with 3/16 BSW threads, hopefully the half nuts will engage satisfactorily. )

1 ) Slacken the bolts retaining the Leadscrew bearings at both ends of the bed.

This will allow the Leadscrew to move in the vertical plane.

2 ) Move the Saddle to the Headstock end and engage the half nuts. It may be necessary to rock the chuck to and fro to obtain engagement.

3 ) Tighten the bolts

4 ) Disengage the half nuts, and move the Saddle to the Tailstock end.

5 ) Engage half nuts.

6 ) Tighten bolts.

Disengage half nuts and recheck Leadscrew alignment measurements. Hopefully they will now be more alike!

This may be the time to reshim the Apron to ensure optimum alignment with the Leadscrew.

These processes might need to be repeated to get the best fit and operation.

If the banjo is tight to move, that is another problem to investigate and solve, but AFTER the above has been completed and finalised., without any further movement of the Leadscrew bearings!

Eliminate one problem at a time, or you will merely confuse matters and not know what is causing what.

Howard

Edited By Howard Lewis on 28/11/2021 06:55:49

Edited By Howard Lewis on 28/11/2021 06:56:35

That makes a lot of sense Howard. The thing that’s perplexing me though, is that there is no adjustment at the tailstock end - the support for the lead screw is cast into the bed. There is adjustment at the headstock end, but I unbolted it last night, but surprisingly the amount of adjustment available was very, very limited. Maybe 1mm of play up and down, essentially the play in the holes that hold the bolts. I was able to lift it slightly and re-bolt it, which seemed to place the lead screw parallel with the bed, as confirmed by a depth gauge.

I came to the conclusion that the adjustment available was only to enable the lead screw and bed to be parallel, and the vertical placing of the half nuts was done by shimming the apron.

Or something else, but I’m running out of ideas!

Pic shows the bolts I unscrewed,

Bob,

Thanks very much for the suggestion. I have filed out the slots. I don’t have any letter drills but a 6mm drill bit seemed to be a sensible option, and so I have filed it so the bit is snug but runs freely throughout the length of the slot. They were not at all even or consistent previously.

Next step, make up a couple of 6mm pins with 3/16” threads on the end. I will probably go to hell for making a metric modification to an old imperial lathe, but needs must I suppose.

Cheers

Steve

If you want to avoid the trip to hell, make the pins 0.236 inch diameter! Imperial and metric are only different ways of expressing the same thing.

As for your threading problem, to make the new pins, a temporary solution might be to keep the nuts engaged and wind the cross slide back under power or by hand, if your chuck screws on.

John

Steve,

In making those 6mm pins do make sure that the 3/16" thread is concentric as possible with the 6mm O/D.
If you are struggling with that please PM with your dimensions and I'll help out.

As a last resort to avoid shims on the Apron the 6mm pins could intentionally be made eccentric to the threaded portion and thereby vary the engagement position of the half nuts, but that's not for the faint hearted.

Steve,

Will your 6mm rod pass through the vertical slots in the apron or are they designed for 3/16" rod.in which case you will need to make them look like cheese head screws.

Reference the symmetry I wrote about, check the two dimensions indicated on your edited photo and see if they are near equal. They will determine which of your half nuts moves most when you rotate the "banjo"

Bob D

Here we go…l

So….. it feels much more solid and decisive. It hasn’t skipped at all. Much better.

This is a .236” (as suggested) pin with the end turned down further to 3/16 for the thread into the half nut.

it does still flex the lead screw upwards slightly as it engages. Not sure why - might be because I haven’t got the measurement Bob pointed out quite right. Or something. I will try to cut a thread before work tomorrow.

Right now I need a rest from the damn thing!

Edited By Steve355 on 29/11/2021 21:41:25

Edited By Steve355 on 29/11/2021 21:42:00

Steve,

That looks more hopeful

You have a PM

Howard

Looking much more hopeful…

Steve355:

Congratulations on your success!

When you are rested, do consider making some soft clams for your vice.

Persistence pays off!

Hurrah!

Howard

Thanks Howard and thanks for all your advice. 😎

Steve

Hi Steve!

Only too glad if I have been of any assistance.

Isn't it marvelous when, after all the work and frustration, it all goes back together and works as you want!

Definitely Job Satisfaction.

Howard

7/8 9 TPI BSW in steel

Unfortunately it’s not 7/8” cos I had to start again and turn off the first thread, but once I’d solved the problem (play in lead screw) it started working properly. But very nearly right, getting better every time. It actually holds a 7/8 nut albeit rather sloppy.

Edited By Steve355 on 02/12/2021 00:19:26

Check that that engaging the half nuts flexes the lead screw the same at both ends. It may be that the bearing on the outboard end needs to have its mount adjusted so the lead screw is parallel to the bed.