Super7 Power Cross Feed Problem

Gentlemen I am hoping the knowledgeable forum members can throw some light or confirm my suspicions please.

My Super 7 PX was purchased as a Metric Machine and I decided to convert it to Imperial purchasing the feed screws and micrometre dials from Myford. I didn't get round to fitting them for some considerable time but when I did I had a  cyclical issue with the screw or nut. It was impossible to set the backlash satisfactorily. The nut supplied was made from streel as the original.

I refitted the metric screw and nut and it was possible to adjust the backlash to less than one micrometre division and the screw and slide were as near to perfect as one has any right to expect. I tried the imperial screw again and the variable backlash problem resurfaced.

I contacted Myford who to their credit after checking the screw and agreeing there was a defect gave a fair discount toward a replacement. The new nut  was of a bronze? material. The screw and nut operate smoothly before assembly, but when assembling the nut, screw and PX pinion the action becomes extremely stiff. Tried refitting the metric screw, perfect!

I contacted Myford again who provided fitting instructions which closely mirrored the method I had been using.

"Fasten the nut and collared gear on to the screw and then fasten that onto the saddle, screw the nut all the way in before attaching the cross slide to the mounting bracket, put the two screws for the bracket in loosely and then as you're winding the whole unit out with the loose screws then tighten as this aids self aligning. Sometimes also the apron will need slackening off as this allows for the collared gear to mesh with the apron."

I am reluctant to slacken the apron, as it will then need tightening and I would think I will be back to square one! Given the metric screw and nut align and can be adjusted to perfection and the only issue with the first imperial screw and nut was the periodic backlash error it appears there is a problem with the new bronze not and screw. My view is either the bronze nut is not concentric with the screw or apron.

This is not a New Myford bashing exercise!

Any thoughts or suggestions would be very much appreciated.

Robert Butler

Edited By Robert Butler on 13/06/2021 18:34:05

Edited By Robert Butler on 13/06/2021 18:36:19

Edited By Robert Butler on 13/06/2021 18:38:32

Edited By Robert Butler on 13/06/2021 18:51:54

Hi Robert,

You could try and the new lead screw and nut without the power cross feed gear in place and see if you can adjust the new screw backlash and see if it will work satisfactorily .

The old metric screw is presumably 2 mm pitch and .500 inch diameter the same od as the new 10 tpi imperial screw , the part about " but when I did I had a cyclical issue with the screw or nut. " seems to suggest some problem with the internal key in the drive slot in the leadscrew.

Perhaps you will try this and let us know how you get on.

John

I probably misunderstand the English "you're winding the whole unit out" but after tightening the nut I wind the whole unit in as far as possible and then tighten the bracket screws. Niko.

Robert , Suggest you may also find ex Myford Nottingham engineers Darren and Pete helpful , if they still available , via [email protected] 07790 364189 , all as described on page 2 of forum post Myford Lathe Service from 14/03/2012. Hope this helps . Trevor

Dear John & Niko thank you for your responses.

The metric screw is perfect, the cyclical issue was with the first imperial screw and nut, I think the screw and nut were inaccurately machined which meant the backlash set by the collared nut varied along the length of the screw. The PX pinion does not affect backlash.

I have just tried the set up with the the replacement imperial screw and nut but omitting the PX drive pinion and the screw operates satisfactorily. With the pinion in place the screw is fine until pushing the nut the final 1/8" or so into position. whether this is the nut or pinion which is having some effect and why is unclear. The pinion and the exposed gear in the apron are undamaged as is the key which is an integral part of the PX pinion and it slides freely on the screw.

I have not yet slackened the apron and am reluctant to do this following the doctrine of unintended consequences.

The emboldened text in inverted commas is quoted from Myford's instructions. My view is that if the nut and screw are concentric they should fit.

Puzzled.com

Thank you for your help so far.

Robert Butler

Edited By Robert Butler on 13/06/2021 21:11:13

Trevor

Thank you, I do have his contact details, he serviced the lathe back in 2017 before my conversion project. If the forum members are unable to help me solve the problem I will contact him.

Robert Butler

Posted by Robert Butler 13/06/2021 18:30:25

I have just tried the set up with the the replacement imperial screw and nut but omitting the PX drive pinion
and the screw operates satisfactorily. With the pinion in place the screw is fine until pushing the nut the
final 1/8" or so into position. whether this is the nut or pinion which is having some effect and why is
unclear. The pinion and the exposed gear in the apron are undamaged as is the key which is an integral
part of the PX pinion and it slides freely on the screw.

Hi Robert,

It seems as if you are nearly there ,try fitting the power cross feed pinion with the
new nut but without the lead screw in place .
Disconnect the the lead screw drive from the lathe ,gearbox in a neutral position
or remove a change wheel ,engage the power cross feed and turn the lead screw
hand wheel whilst doing up the nut fixing screws and see if it binds up on the final
1/8 inch into position.

While you have the lead screw out check the depth of the drive slot and compare
it with the old metric lead screw....i can't remember how much clearance there is
around the lead screw in the bore of the drive pinion,but the lead screw should
be able to maintain a central position within the bore of the pinion without the
key bottoming out in the slot.

John

Dear John

Thank you, sounds like a good suggestion but it may be the weekend before I can try this.

Robert Butler

An update of sorts.

I tried fitting following Myford's instructions exactly including reluctantly loosening the apron screws - no improvement whatsoever!

I then tried John Paces suggestion, fitting the nut with the PX pinion in place onto the apron and then operating the PX with the main leadscrew handwheel, the pinion showed no signs of binding and rotated freely. As soon as the leadscrew was inserted taking care to mesh the pinion with the leadscrew slot the screw binds. Reinstate Metric nut, screw etc perfect.

If the Imperial nut is assembled on to the screw and the pinion fitted the screw works fine and the pinion moves without any signs of sticking or binding it is only when fitted to the apron there is a problem.

I think John may be on the right track with the depth of the slot, but cannot see an easy way of establishing the slot depth on either feedscrew to confirm this. Failing this I think there may be an axial alignment issue with the feedscrew/nut as without using force for fear of causing damage the issue is cyclical.

I would appreciate any further suggestions please.

Robert Butler

Hi Robert ,
Seeing your posting it seems the problem is with the lead screw and or nut.
The depth of the slot in the lead screw only needs to be deep enough as in the
sketch "A" ,i can't remember how much clearance there is in the inner bore
of the gear bush and lead screw the last time that i had mine apart was in
2005 when i converted the machine to part cnc.
If you fit the gear /bush on to the lead screw and look endwise with the
lead screw pushed hard towards the the key the screw should be able to
touch the inner wall of the bush and just still have a gap between the bottom
of the slot and the top of the key as in "A" , if you have a situation as in
the sketch "B" where the slot is not deep enough in the lead screw then the
screw would be forced off centre ,as you have said and i would agree there
may be a problem with the axial alignment between the bush bore and the
thread in the nut if that is so with a situation as in "B" and that would probably
result in cyclical binding.
Just have a look at this to see if it is the cause.

John

Dear John thank you very much for your further advice I will investigate, hopefully tonight and report back.

Robert Butler

Dear John

A report back, I have examined the gap in the slot of the imperial leadscrew with the key having assembled the nut onto the screw and fitted the gear/bearing as you suggested and illustrated in your sketch, there is adequate clearance. I repeated the exercise with the metric screw, nut, gear bearing and the clearance is the same (no means of measuring accurately) with a clearly visible "gap" along the length of both screws.

I again refitted the metric screw and it is perfect.

For what it's worth, i think there is a lack of concentricity with with the screw, the nut or bore in the saddle?

Thank you for the diagram and assistance so far.

Robert Butler

Edited By Robert Butler on 04/07/2021 18:26:53

Hi Robert ,
On seeing this i had sort of come to same conclusion,

"For what it's worth, i think there is a lack of concentricity with the screw, the nut or bore in the saddle?"

When you look at the product on the Myford page they claim to lap the nut and the screw together
so they are a pair, there would not have to be much of a mis-alignment in these parts to give a
problem if that is so.
The bore in the end of the nut should be concentric with the outer diameter of the body
of the nut that fits within the apron similarly the threads should be the same these are the only
parts left to look for the problem and this only seems to occur when you push the nut right
home.
I don't think you have anywhere left to look.

When you refit the the screw and the bracket as their advice is to leave the screws
loose for the bracket to settle into position and then tighten the screws, you could just
try to bias the bracket up a little and tighten the screws and also try the same down
and to each side in turn to see if there is any difference.
Clutching at straws here but worth a try to see at any difference in any of these positions.

John

Hello John

Thanks once again for your reply. The imperial nut is some kind of brass rather than the metric nut which is steel. I am aware the nuts and screws are lapped but I'm not sure this is done with a brass nut as I think the abrasive would embed itself in brass thereby unintentionally continuing the lapping process! The screw and nut seem well made and before attempting to fit the nut and screw to the saddle/cross slide run smoothly.

The nut is a good fit in the saddle and I can't see anyway it can be adjusted up or down before the final tightening of the nut. I think Myford's suggestion to slacken the apron was a shot in this direction - but the apron still requires tightening, I tried this without any improvement even with the screws slack and thinking this through it is a saddle/cross slide issue not an apron issue.

Myford have promised to get back to me this week as their man was on holiday last week.

Many thanks

Robert

Hi Robert,

It is quite possible for them to lap even with a brass or bronze type nut ,yellow label Timesaver abrasive is used just for this ,quite useful stuff and does not embed and can be used on whitemetal bearings.

Hope that Myford will be able to help you ,let us know how you get on.

John

Hi John

An update, Myford suggested I return the screw and nut for appraisal, which I did. They detected tightness and suggested it was borderline on tolerance and sent a replacement "original" Myford screw and nut which I fitted today. The replacement is significantly better but lacks the silky smooth action of the metric screw and nut. I can't get it down to virtually nil backlash either.

I am reluctant to concede that it may bed in as I feel it should work out of the box.

Robert Butler

Edited By Robert Butler on 18/07/2021 21:56:17