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Bent leadscrew?

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Paul Ainsworth16/05/2018 13:11:59
40 forum posts
10 photos

Would a bent leadscrew make a finish like this or is it something else?

It seems to be cutting a copy of the leadscrew.

img_1147.jpg

Mick B116/05/2018 13:19:26
614 forum posts
37 photos

I doesn't look like it matches to me. 4 pitches away from the middle of the pic it's peaking halfway through the leadscrew pitch.

When something a bit like that (but less severe) happened on my Speed10, it was loose gibs in the compound- or cross slide - can't remember which. Looks like you have something quite bad somewhere, but it's not easy to see how leadscrew might be the culprit. If there's a lot of cross slide backlash, you can loosen the handle/locknut, tighten up the handwheel a bit and relock.

Edited By Mick B1 on 16/05/2018 13:25:48

HOWARDT16/05/2018 13:37:29
306 forum posts
13 photos

It may be the leedscrew is lifting the slide. The fact the part and leedscrew don't match is down to the gear train. Check all gibs and retaining plates. Also straighten the leedscrew, you can get it close enough with vee blocks and press or g clamp.

Martin Kyte16/05/2018 13:56:29
1298 forum posts
9 photos

You don't say what lathe but based on my Myford expirience:-

Does the leadscrew move (with the lathe off) when you close and open the half nut's? If it does the nuts are not closing down concentrically with the leadscrew. I assuming that your slides are nicely adjusted as has been remarked on. On most lathes there is some lind of adjustment to ensure the half nuts behave. The other question is does your leadscrew have a keyway down the middle for a power cross feed. If so disengaging the Thread Dial Indicator if fitted removes another variable.

Mick does raise an interesting point, and lead screw paterns do match the pitch of the screw which this doesn't seem to.

regards Martin

Chris Evans 616/05/2018 14:28:14
1214 forum posts

As apprentices the old turners told us to "Trail our hands" on the saddle feed wheel/handle. This was to prevent the weight of the hand wheel rising and falling giving a poor finish.

Paul Ainsworth16/05/2018 14:49:05
40 forum posts
10 photos

History: had lathe a couple of weeks it is a Myford ML10. When dog clutch was disengaged the leadscrew would catch for a second and drive cutter into work.

I had a crash the other day when dog clutch wouldnt disengage and leadscrew cover hit headstock. Following that the leadscrew was tight in right hand bearing, ok in left hand.

I stripped and cleaned all slides, saddle and apron, new bearings ordered, gibs adjusted so the parts slide but no side play. When half nuts are engaged they move the leadscrew towards the lathe bed by enough for Stevie Wonder to see it. New bearings getting fitted today after I've nipped to Sandycroft and got some v blocks.

No half nut adjustment that I can see either on apron or nut slides.

Clive Hartland16/05/2018 15:32:02
avatar
2303 forum posts
38 photos

The half nut adjustment is done with a small screw through one of the halves.

Set up the lead screw, with the lead screw engagement lever removed and the two segment pins catch the pins into the respective guides in the engagement lever. Use graphite grease to lube. Tighten the self locking nut until it just allows turning the engagement lever.

Engage the lever onto the lead screw and see what play there is and adjust the screw in the halve segment to reduce play between to two segments.

Now, mount the block loosely onto the carriage and you need to check the setting of the lead screw height by traversing to both ends of the bed with the segments engaged to ensure free run, only then tighten the screws to lock the block in place on the carriage. You can loosed the right hand lead screw bearing to facilitate the height of lead screw.

Mick B116/05/2018 17:31:57
614 forum posts
37 photos
Posted by HOWARDT on 16/05/2018 13:37:29:

It may be the leedscrew is lifting the slide. The fact the part and leedscrew don't match is down to the gear train. Check all gibs and retaining plates. Also straighten the leedscrew, you can get it close enough with vee blocks and press or g clamp.

The Myford 10s have a dovetail bed with a gib on the user side. I can't imagine that a bent leadscrew could lift the saddle unless the gib's so loose/missing as to give a whole slew of other problems.

I think mine had a slight bend in the leadscrew as well, but shutting the halfnut simply forced it into line, and it caused no issues I was aware of.

Paul Ainsworth16/05/2018 17:45:04
40 forum posts
10 photos

Leadscrew out by between 0.12 and 0.18mm, high spot is same throughout length.

The small screw does seem to hold the halves too far apart, I'll adjust as advised when I've sorted the leadscrew.

Thanks gents I'll let you know the results yes

Brian Wood16/05/2018 19:19:51
1604 forum posts
35 photos

Paul,

I didn't mention the leadscrew dog clutch in the list of things you have checked, I assume you have or will be looking at that to restore a nice crisp action to it so that it opens cleanly when told to do so.

I am puzzled though by the figure you report in the leadscrew high spot, a bent leadscrew would normally have a high spot with tapering away error 'shoulders' to either side, what you describe sounds more like a lobed leadscrew which I am quite sure would never have got past Myford inspection. It also sounds as though the clasp nuts are visibly forcing the leadscrew when engaged, a positioning fault of some sort as there should be no side movement imparted to the screw when they are engaged.

Regards

Brian

Paul Ainsworth16/05/2018 20:05:31
40 forum posts
10 photos

A good polish got rid of some score marks on both shafts that run under collar, the engagement screws also had a burr on, removed this and the clutch works as it should now.

It could have been the way I reassembled the half nuts/apron. I tightened the apron to saddle then put screws in nuts and handle on.

Figures were between low point and high point, falling off between ends and highest between headstock and mid point on the screw.

Joseph Noci 116/05/2018 20:23:05
314 forum posts
626 photos

Paul, I am intrigued by your photo- Does the leadscrew pitch actually match up with the shaft? From the photo, contrary to what Mick B1 said, I believe it does - the apparent 'mismatch' before and after the middle appears to me to be an optical illusion due to parallax.

Looking at the leadscrew, the slope of the sides of the thread is clearly seen on both sides of the crest, near the centre of the leadscrew. Higher up in the image, only the lower flank is visible, while lower down in the image, only the upper flank is visible. The machined shaft is of a larger diameter than the screw, and so this same effect is exaggerated even more.

This all due to the camera focus being close to the leadscrew, and field of view very narrow, sort of 'fish-eye' effect..

If the pitch is in fact the same, then the cause is even more intriguing! How 'deep' is the screw effect? Is it more optical, or can you feel it, say with you finger nail?

The. forgive a perhaps silly question, but it it actually a thread, ie , a spiral, and not just 'ribs' ?

As you used auto-feed to machine that shaft, what was the feed speed/gear selection at the time?

Joe

Paul Ainsworth16/05/2018 20:46:35
40 forum posts
10 photos

It is a ribbed effect, the depth is 0.01mm. On a shallower cut the effect is still there but more like a bruise than a cut.

Feed rate set to 0.0064 spindle speed 840rpm

img_1207.jpg

Edited By Paul Ainsworth on 16/05/2018 20:48:03

Mark Rand16/05/2018 21:51:55
433 forum posts

Posted by Paul Ainsworth on 16/05/2018 14:49:05:

<snip>

When half nuts are engaged they move the leadscrew towards the lathe bed by enough for Stevie Wonder to see it.

That definitely implies that either the saddle gib is loose or there is enough wear in the bed near the headstock that it can't be adjusted sufficiently to get free movement at the tailstock without slop at the headstock end.

Hopper16/05/2018 23:15:51
avatar
2794 forum posts
44 photos

If the apron on the ML10 is like that on the ML7 you should be able to loosen off the screws holding the apron to the saddle, then close the half nuts onto the leadscrew, then tighten up the screws holding the apron to saddle. This usually sets the halfnut in the right position relative to the leadscrew.

I've seen leadscrews flapping about all over the place but never one causing ridges like that. A Myford leadscrew is very long and very thin; the root diameter of the thread is only about 3/8", so the leadscrew can be grasped in the middle and flexed up and down A LOT. Usually once the halfnuts are closed on the leadscrew, this is sufficient to hold the leadscrew in place.

To get grooves like you are getting, either the saddle to bed fit or the headstock bearings is very loose and needs attention.

Check that leadscrew cover that you say crashed into the headstock. Make sure it is not fouling the leadscrew thread.

Try slowing your spindle speed down from 840 to about 400rpm.

Try turning a completely different piece of material. Just to see if the condition is universal and not just a freak combination of factors.

Zan16/05/2018 23:20:11
25 forum posts

It looks as if the grooves get deeper towards the chuck then go parallel then start again I suggest the reason is the saddle is loose on the gibs and is twisting forcing the cut deeper by rotating as it moves along before it jumps and catches up. Try without power feed and see if a smooth finish can be obtained... The gib needs adjusting, but perhaps it's a symptom of a badly worn machine.......how old is it?.... Is it generally in a good conditions? .......if the screw is bent then it will be obvious when it's rotating under power without the half nuts engaged. Have you fully cleaned the half nuts? Trapped swarf may cause problems. A lot of investigation needed this sort of problem is difficult to assess without seeing it in person. Good luck!

Journeyman17/05/2018 09:44:00
avatar
514 forum posts
71 photos

Could this be caused by the handwheel gear train tightening intermittently onto the rack as the leadscrew drives the carriage along. May be possible to test just by winding the carriage back and forth by hand and noting if there are any stiff spots.

John

SillyOldDuffer17/05/2018 09:48:18
3111 forum posts
620 photos

A lathe that's had a hard life may have multiple issues. I suggest the effect is due to two or more faults interacting.

I don't think a bent leadscrew would cause gross ribbing on it's own but might if the saddle was also loose. Other possibilities include: the work flexing under a blunt tool, a faulty/worn/gunked up chuck, sloppy bearings, broken teeth in the drive train, loose belts, loose headstock, or an unbalanced drive-train.

For starters I'd run through this list:

  • Any obvious damage, dirt, or odd noises?
  • Is the motor firmly fixed and running smoothly?
  • Is the belt loose, damaged or the adjustment wobbly?
  • Are the gears all intact (no missing teeth), meshing properly, fixed to their shafts, and the banjo clamped firmly?
  • Clamp a sturdy rod or pipe in the chuck and put a DTI on the rod close to the chuck. Then pull the rod looking for any sign of up/down or right/left movement. (If you find movement, suggests chuck, bearings, and/or headstock problem.)
  • Put the DTI on the saddle and try pulling it up and sideways. Movement that can't be fixed by adjusting the gibs needs investigation, for example, gib popped off its locating screws, worn out, saddle badly worn, or bedways worn.
  • Is there any sign of vibration not due to any of the above? If so suspect bent shafts, bearings or even a bent spindle.

As an amateur I'd approach this methodically, probably wasting time following false leads. Is there an established process for checking out a misbehaving lathe? What do experts look for before buying second-hand machines?

Dave

Mark Rand17/05/2018 10:05:59
433 forum posts
Posted by Mark Rand on 16/05/2018 21:51:55:

Posted by Paul Ainsworth on 16/05/2018 14:49:05:

<snip>

When half nuts are engaged they move the leadscrew towards the lathe bed by enough for Stevie Wonder to see it.

That definitely implies that either the saddle gib is loose or there is enough wear in the bed near the headstock that it can't be adjusted sufficiently to get free movement at the tailstock without slop at the headstock end.

Please ignore this post. I was reading it as the apron moving, not the leadscrew moving. Stupid boy!

not done it yet17/05/2018 10:15:58
1959 forum posts
11 photos

OP is either trying to take a very shallow cut or is mixing units. 0.1mm and 0.0064 in two consecutive sentences! I strongly suspect the latter.

Either way the angle of the carrige must be changing cyclically. I reckon the banding on shallow cuts would disappear if continual force were to be applied to one corner of the carriage during cutting - or the banding would change if the corner of the carriage was bumped at different times. It is likely it may be triggered by the lead screw between harder and easier turning. So IMO it is either carpy adjustment or carpy lathe.

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