lathe compound threads sticky in one direction only.
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    Default lathe compound threads sticky in one direction only.

    The compound on my lathe has had issues. It was tight when retracted, and got looser when extended. Sounds like a taper, yes? Anyway, I pulled it off, and set it on a surface plate with blue- it was printing just on the ends. So I scraped it flat, took very little to do so.

    Put it back together with no other surfaces scraped, and it seems to leave a mostly even contact on the mating surface- not great contact, but more or less even end to end.

    now it is slightly looser on each end than in the middle of it's travel range, when sliding it by hand-no leadscrew. Put together ,with the leadscrew leverage, it is hardly noticeable.

    So the thread title- for some reason, the compound extends easily and smoothly, but gets sticky when backing it up- and not evenly, but with a partially smooth rotation, then a stick, then easier for the rest of the rotation. Like you might feel with a bent leadscrew, but only in reverse.

    At first I figured the gib was loose, and when the leadscrew is backed out, the gib is wedging. Nope.
    Then I backed off the thread dial, to make sure a bent leadscrew was not camming the dial against the housing every half rotation. Did not help.

    Next I am going to pull out the screw and check the shoulder where the thrust bearing sit and see if there is something going on there.

    Any other suggestions?

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    Is it sticky as soon as you start to reverse it, and stay the same--or does it increase as you get closer to being fully retracted?

    When you say you backed off the thread dial, you are referring to the dial on the compound- correct?

    Is there a mechanism to adjust the compound nut for wear in the leadscrew? I am wondering if this was holding the leadscrew high or low- so the leadscrew is not parallel with the compound ways. The closer the compound nut gets to the thrust bearing, the greater the angle is, and the more binding occurs. This would be a reason for the initial tightness. Over time this would also lead to the thread angle of the leadscrew increasing closer to the thrust bearing, though not sure how this would contribute to the tightness you describe.

    What lathe is it?

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    Quote Originally Posted by J_R_Thiele View Post
    Is it sticky as soon as you start to reverse it, and stay the same--or does it increase as you get closer to being fully retracted?

    When you say you backed off the thread dial, you are referring to the dial on the compound- correct?

    Is there a mechanism to adjust the compound nut for wear in the leadscrew? I am wondering if this was holding the leadscrew high or low- so the leadscrew is not parallel with the compound ways. The closer the compound nut gets to the thrust bearing, the greater the angle is, and the more binding occurs. This would be a reason for the initial tightness. Over time this would also lead to the thread angle of the leadscrew increasing closer to the thrust bearing, though not sure how this would contribute to the tightness you describe.

    What lathe is it?
    Taiwan 1340, like a small Sharp.
    Gets sticky right away on retracting the compound.
    It is the compound dial I backed off.

    I don't think it is the leadscrew nut angle, as there has been no real change. It might be the gib clamp- there is a screw and dovetail clamp to lock the compound travel- maybe that is wedging on retraction?

    I have felt similar cyclic stickyness with a poorly machined boring head dovetail, where the adjustment screw had a burr on it and the counterbore where the screw fit had a hastily cut recess,that in effect made a cam- so every time the burr came around, it hit the high spot in the recess and came up against resistance.
    What is puzzling is this is only in one direction.

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    If the stiffness is periodic (i.e. same sector of rotation each revolution of the screw) and the screw itself is not bent, the culprit, in my opinion, is a thrust surface not been properly square with the threads.

    Paolo
    Last edited by Paolo_MD; 01-22-2020 at 05:05 PM. Reason: Correcting spelling mistakes (I should always wear my reading glasses when posting)

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    Quote Originally Posted by Paolo_MD View Post
    If the stiffness is periodic (i.e. same sector of rotation each revolution of the screw) and the screw itself is not bent, the culprit, in my opinion, is a trust surface not been properly square with the treads.

    Paolo
    Paolo,
    Your post has been the victim of spell check, I think! "trust"= thrust" ,"treads"= threads.

    But I understand it perfectly! And agree- I will check the thrust surfaces that engage when the threads are retracted.

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    Agree that Paolo is helping you narrow it.

    Sounds (to me) like a burr/fretting on the dial face or the part of the dial that rotates to 'zero' the dial graduations. Essentially a problem with the thrust surfaces against which the screw presses only in the retract direction.

    Does not sound like a problem with the gib or dovetail.

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    Quote Originally Posted by Dumpster_diving View Post
    Agree that Paolo is helping you narrow it.

    Sounds (to me) like a burr/fretting on the dial face or the part of the dial that rotates to 'zero' the dial graduations. Essentially a problem with the thrust surfaces against which the screw presses only in the retract direction.

    Does not sound like a problem with the gib or dovetail.
    I pulled it apart tonight- the leadscrew has a very small bend- that, in conjunction with a poor design of thrust bearings on one side only, causes it to get sticky with a side load of tiny force.
    So I went back to square one as long as it was apart- scraped in the other half of the dovetail and will straighten the leadscrew and add a a bronze bearing on the retract side.
    I wish they had left more meat on the leadscrew so I could turn the shoulder back and add a thrust bearing, but to do it now I need to make a new leadscrew. Not enough room.

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    Quote Originally Posted by stoneaxe View Post
    <Snip>
    I wish they had left more meat on the leadscrew so I could turn the shoulder back and add a thrust bearing, but to do it now I need to make a new leadscrew. Not enough room.
    If the screw is still out of the machine, you might put up a photos as perhaps someone here might have some ideas as to how you might be able to modify the screw to accommodate better bearings. Sometimes a new set of eyes...

    Denis

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    does the gib have a notch in it where the gib screw head fits into? Is it tight? As Paolo said if the screw is bent and if the gib is not the same size on the narrow top to bottom and if the bent screw as it is tightened or the slide changes directions the gib can slip down and tighten the slide. I have seen this happen and it is not a guess. I see you think you have the problem solved, I am telling this so other readers can check this too. Another thing you can check while it is apart. Blue up the positive side of the slide where the back of the gib rests and put the opposite slide on and rub the dovetails to be sure the positive gib side is the same angle of the side the front side of the gib rides against. Many never check this. Also check the positive side with a scraped straight edge and I bet it is or was high in the middle before you scraped it.

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    Quote Originally Posted by Richard King View Post
    does the gib have a notch in it where the gib screw head fits into? Is it tight? As Paolo said if the screw is bent and if the gib is not the same size on the narrow top to bottom and if the bent screw as it is tightened or the slide changes directions the gib can slip down and tighten the slide. I have seen this happen and it is not a guess. I see you think you have the problem solved, I am telling this so other readers can check this too. Another thing you can check while it is apart. Blue up the positive side of the slide where the back of the gib rests and put the opposite slide on and rub the dovetails to be sure the positive gib side is the same angle of the side the front side of the gib rides against. Many never check this. Also check the positive side with a scraped straight edge and I bet it is or was high in the middle before you scraped it.
    Richard, let me rephrase this, as I am slightly confused .

    Yes, the gib has a small notch in one end where a screw fits. It seems to have the function of a keeper, to hold the gib in place while the compound is apart. There is a screw on the other end of the gib as well, no notch there.
    Are you talking about the gib sliding longways, and acting like a wedge? Or saying the gib may not be a true parallelogram in cross section, so if it moves toward the top of the compound it can jam?

    "positive side"- I do not understand the term- are we talking male and female parts of the dovetail? I pulled the female top slide off, blued it on a surface plate, scraped it flat and then used it to blue the male bottom part of the compound. The top slide had an arch in it, both ends were printing on the plate.

    I am thinking of altering the leadscrew by turning off the shoulder and pressing on a new one, to gain some room for a thrust bearing - there is at least 1/2" more to work with if I do this. First I will get the fit right and bend out then see how it is.

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    Im talking about

    " Or saying the gib may not be a true parallelogram in cross section, so if it moves toward the top of the compound it can jam?"

    It probably isn't the problem if there is a 2nd gib. It is so hard to type this. Can you call me or can I call you tomorrow?

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    As usual, a few pictures would help a lot.
    In short, there are two most common types of gibs: one, "flat" that is adjusted by a series of screw along its length. The other type (a tapered gib) is essentially a wedge that is adjusted by sliding it forward or back longitudinally: it could be controlled by a single screw with the head engaging a notch in the gib and doing both the pushing and pulling, or two screws, one at each end, blocking any longitudinal movement.
    If there is significant play between the head of the screw and the slot, or the two screw are not tighten pushing the taper gib against each-other, the gib could move longitudinally tightening in one direction and loosening on the other direction (since the thicker end of the gib is generally toward the operator, retracting the compound would result in the gib wedging itself in, therefore reducing the play in the dovetail).
    If a tapered gib is not a perfect fit and doesn't contact the dovetail evenly, a lot of other head-scratching behaviors could result.

    Before modifying your existing leadscrew, I strongly suggest you to post pictures. You should also consider purchasing a new piece of (straight) Acme threaded rod of the correct pitch and diameter and build a replacement screw from that (while still using the original one).

    Paolo

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    The tapered gib type is what is on the slide, controlled by a screw on each end. One side only. Looseness there was the first check.

    This slide has yet to go back together, so I am going to hold off on any modifications till it is tested-it may work fine with straightening the screw, and scraping in the surfaces.

    I need to measure the screw and see if it is inch or metric- the main leadscrew on the lathe is inch, not sure about the compound- it has a dial mm / inch.

    Good suggestion on buying a piece of acme rod- at least I will have a lathe to work with!

    Richard, I think I have a handle on this now- let me hold your offer of a call in reserve , in case I dig a deeper hole!

    Rant alert- Pissbucket sorta got me to stop posting pics, maybe I will find a new way. I don't mind paying for a service, but retroactive blurring of a promised free service as an extortion is a cheap azz trick- they can go to hell.

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    Oh, one thing that might help someone-

    A very close inspection of the screw that tightened (pushes) the tapered gib showed that
    when it came up tight, it was jacking the gib sideways, because of the way the screw head pushed on the gib.
    A few seconds with file cured that.

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    I want to give you and the readers a tip that I use all the time when working with a lathe compound. Before assembling it flip it over and hold it in a bench vise. Then as you assemble it you can see what's going on. You can watch the gib, feed screw and nut and how they work. I have learned these tricks to the trade because I have been rebuilding lathes and other machine tools, "hands on" and got paid to do it for over 50 years.


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