Myford cutting varied thread pitch problem

I have a Myford Super 7 with the power cross slide. I added an imperial gearbox and all seemed to be working well.

I then did the stupid thing of trying to do too much at once and ran the saddle into the end of the lath at the tailstock end and jammed it really good. I managed to get it loose and sorted out. There seemed to be very little noticeable damaged which appeared to be mainly paintwork scratches and a little adjustment of the leadscrew.

Later I tried to cut some thread and found that the pitch of the thread varied and it would not take a nut. The nut would travel a short distance of around 3mm and then stop.

I then did a test of just running the lathe on feed cutting a piece of brass. Then I could see that the saddle would travel at the correct rate, slow down, the correct rate, slow down etc. There were lines on the surface where you could see that the tool had spent more time on the surface. I measured the distance between each line and it was 1/8^th of an inch. This I think is the same as the pitch on the leadscrew.

I have checked the leadscrew and it is not bent and it does not move notably from left to right. I stripped the saddle and checked the ½ nut. That seems to be okay. I removed the power cross feed workings and still the lines remain. When I manually turn the leadscrew it seems to have no areas of resistance. I opened the cover to the gear box and it seems to be okay.

I am now at a loss! Any ideas on what the trouble could be

Just guessing here. Maybe a shear pin has failed and it is driving enough at times, before losing another rotation before catching, temporarily, again.

I had exactly the same problem on mine and checked all the things that you have done, it was play in the saddle to the bed causing it to be "crabbing" its way along.

Adjustment of the gibs cured it.

Phil

+1 on check shear pins. There is probably one at the end of the leadscrew where it joins the gearbox if it is like my ML7. Sheap pin could be sheared but still gripping sometimes and slipping other times.

Also check the change gears and make sure you have not lost a tooth off the small fibre gears on the tumbler reverse mechanism.

If still no good, check the gears in the gearbox for damaged or missing teeth from the crash you had.

Or it could be one of the woodruff keys sheared off in one of the change gears.

Edited By Hopper on 22/03/2023 08:55:59

As far as I am aware there are no shear pins on the Myford S7 however as Hopper says there is a woodruff key on the last gear thats on the lead screw and the lead screw on your machine passes right through the gearbox,

On earlier machines or on some retro fitted gearboxes as yours is its not necessarily so the lead screw may be in two sections but the woodruff key will still be present.  Whether your's is as per the factory, one piece L/screw it depends on the age of the gearbox you fitted but you will know either way.

John

Edited By JohnF on 22/03/2023 09:11:33

I retro fitted a gearbox to my S7 and as the instructions stated had to trim the length of the leadscrew. When fitting the drive gear the woodruff key was there. But i did notice this in the Myford fitting instructions with regard to fitting the leadscrew gear No.168 …….with driving pins or woodruff key……….

This made me wonder if on some of the leadscrew variants the drive was with pins?

The ML7 drove all the changewheels with keys (The mML1 to 4 used pins) so probably all m,other bdrives would be plain or woodruff keys..

A comnination of single phase motor, slack gibs and or bearing wear can ncausen problems with an unwanted extra helix on work.

Careful adjust ments should effect a cure. That includes setting backlash of the gears correctly. Too little will produce noise, and possibly nhtis problem.

Too much should not be too much of a problem, as long as the load is constant.

Ideally the gears should be meshed so that a piece of 80 grm paper ().003" 0.75 mm thick ) is trapped in the mesh as the geras rotate. .

The eassiest way is to start setting the mesh at the leadscrew or gear box end, to the Idler and then work your way up the banjo, swinging it to set the mesh, until you reach the driving gear..

Howard

Nuts not fitting cut threads suggests something severe, especially as the fault appeared after a crash.

I've had the same symptoms from a broken shear pin that caught and slipped intermittently after a crash, but if the experts say the S7 doesn't have shear pins it can't be that! (Just a thought - not even in the gearbox?))

If shear pins are dismissed and the lead-screw isn't bent or moving left-right, my chief suspect is the half-nut mechanism. The half-nut may look fine, but might not be engaging correctly because an arm or fixing was bent by the collision, or maybe an adjuster disturbed.

How good a view is there of the engaged half-nut on a S7? Might be possible to confirm it's misaligned if the half-nut can be seen to waggle whilst the lathe does a power traverse.

Dave

I drove my super 7 into the headstock ,after freeing it, I turned some bar which had ribs intermittantly along the length,I then deduced that the half nuts were damaged.I replaced the half nuts and all was well.Perhaps this is the cause of your problem ,just a thought.

Frank

Check for endfloat in the leadscrew, and whether the saddle can be moved when the halfnuts are engaged. Also endfloat in the compound slide and in the spindle.

 Can you measure the max and min pitches?

Edited By old mart on 24/03/2023 20:01:28

Could you try cutting a 8 tpi thread on a soft material perhaps pvc ect for a GOOD length the observe with tpi gauge the results, What variations do you have?

I had this on another lathe, and it was the leadscrew engagement cups riding on top of the leadscrew for a bit, Replacing those cured it. Your problem suggests something is happening later on..Is it thread pitch or depth of thread problem.....? Good luck

Vic