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Uneven finish with power feed on Jet lathe

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  • Uneven finish with power feed on Jet lathe

    The recently revived thread about Taiwanese lathes got me thinking again about an issue I have on my belt drive 13x36 Jet lathe, and hoping some of you guys can offer suggestions.

    The lathe cuts well, but when using power feed it leaves faint rings in the work. They are rings, not threads, and are approximately .050" apart, although it's very hard to measure. These rings do not vary with feed rate, or with spindle speed. They are not dependent on the cutting tool or material being cut either.

    Since the ring spacing does not vary with feed rate, I think that means the issue is not in the carriage, but might be in the gear box? If I drag my hand against the carriage wheel when it's feeding, I can't feel any skips or jerks like one might expect with a broken gear tooth somewhere. At one point I thought I could feel a faint pulsing in the wheel, but I couldn't feel it tonight.

    I don't think it would be in the headstock bearings, since rings don't show up when feeding by hand. I don't see anything obvious in the power feed gear train on the back of the lathe, but have not disassembled the quick change gear box.

    Polishing the work with emery cloth makes the rings more visible at first, but they are pretty faint so they can be sanded off without too much effort.

    Edit: Also, if it was the lead screw, I think the problem would show up in power feed on the cross slide too, but it does not.

    Here is an example of the rings in the work; I added a few arrows to point out what I mean. Any suggestions?

    Last edited by Yondering; 11-09-2016, 04:27 PM.

  • #2
    slightly bent feed rod? Not sure how you could tie that in to ring spacing, brain's a bit dead right now.

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    • #3
      It could be improper engagemnet between the rack and pinion that drives the carriage. That would give an irregular motion to the carriage. If that is the case the pitch of the markings should match the pitch of the rack or a fraction of the pitch of the rack.
      If the irregularities do match the pitch of the rack (or fractional part of the pitch) look elsewhere in the apron gearing for gears that are worn and don't mesh properly.
      A bent feed rod id a possibility especially if the worm sets in worn bearing.
      I had a similar problem with a 14 x 30 Pratt and Whitney with a badly worn bed and saddle. The pinion and rack were not engaging at the proper depth.

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      • #4
        Illinoyance, that's a good suggestion, and something I hadn't thought of, thank you! It makes sense, since the ring spacing is consistent regardless of feed & speed settings.

        This lathe isn't worn much at all, but it is of course an asian lathe, and there could be an issue with the teeth on the rack. Also, it was still packed with old grease or cosmoline in a lot of nooks and crannies (original owner apparently couldn't be bothered to clean it up), and while I did clean everything I could see, I did not do anything with the rack. It could have old dried grease packed between the teeth.

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        • #5
          Or possibly misalignment of half-nut assembly with feed screw. Precession of carriage>rings.
          Compare ring pitch with feed screw pitch.

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          • #6
            Originally posted by Illinoyance View Post
            It could be improper engagemnet between the rack and pinion that drives the carriage. That would give an irregular motion to the carriage. If that is the case the pitch of the markings should match the pitch of the rack or a fraction of the pitch of the rack.
            If the irregularities do match the pitch of the rack (or fractional part of the pitch) look elsewhere in the apron gearing for gears that are worn and don't mesh properly.
            A bent feed rod id a possibility especially if the worm sets in worn bearing.
            I had a similar problem with a 14 x 30 Pratt and Whitney with a badly worn bed and saddle. The pinion and rack were not engaging at the proper depth.
            This would be my answer as well. I have that problem on my badly worn (.035" bed wear) SB13". But it still turns straight, so I have no issues. Nothing a single cut file can't clean up

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            • #7
              Originally posted by bob_s View Post
              Or possibly misalignment of half-nut assembly with feed screw. Precession of carriage>rings.
              Compare ring pitch with feed screw pitch.
              I don't think a lathe of that size would use the halfnuts for carriage feed, but I could be mistaken. I know Craftsman 109's and some Atlas lathes do use the halfnuts for fine feeds.

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              • #8
                I have seen this on my 11 inch Sheldon. I believe it is related to a bent lead screw or something closely coupled to the screw. My machine uses keyway in the lead screw for power feeds.

                The pattern does not appear when feeding by hand, only power feed so I don't think it is the rack.

                Comment


                • #9
                  Originally posted by Andre3127 View Post
                  I don't think a lathe of that size would use the halfnuts for carriage feed, but I could be mistaken. I know Craftsman 109's and some Atlas lathes do use the halfnuts for fine feeds.
                  Correct, it does not use half nuts for power feed, only threading. It has a separate feed shaft with a keyway. The rings do not appear when using the half nuts & threading at the finest pitch. If you look at the pics in the first post of that other thread about Taiwanese lathes, mine looks just like that, only grey and says "Jet" on the front.

                  I am pretty sure the feed shaft is not bent; I haven't removed it to verify but don't see any wobble in it, even with the carriage moved all the way down by the tailstock. Also, if it was the lead screw, I think the problem would show up in power feed on the cross slide too, but it does not. Should have added that to the OP.

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                  • #10
                    It could be that the handwheel is accelerating when the handle is going down hill and slowing when it's going up hill. My lathe allows you to disengage the handwheel while powerfeeding, which suggests this is a possible issue.

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                    • #11
                      Check and snug up the gibs on the apron, particularly at the rear. I have seen similar caused by fore and aft movement or lifting of the carriage depending on how it is restrained.
                      Jim H.

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                      • #12
                        If the bands are about the same width as the pitch of the leadscrew, it could be a loose or defective thrust bearing on the end of the shaft. But if your lathe has separate shafts for feed and threading, it may indicate problems elsewhere.
                        http://pauleschoen.com/pix/PM08_P76_P54.png
                        Paul , P S Technology, Inc. and MrTibbs
                        USA Maryland 21030

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                        • #13
                          Originally posted by Robin R View Post
                          It could be that the handwheel is accelerating when the handle is going down hill and slowing when it's going up hill. My lathe allows you to disengage the handwheel while powerfeeding, which suggests this is a possible issue.
                          Yep, there is slop in the mechanism, so it can definitely change the feed slightly, or even just the "feed pressure".

                          The worm could move axially, even shift radially, the gears may have a bent shaft or other eccentricity, it may be telegraphing-through the gear tooth profile errors a the teeth engage.......... etc, etc.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            My guess would be that the shaft that drives the pinion gear is bent. Probably from someone running the saddle into a stop under power feed.

                            Let me add a little.. It could also be a gear or shaft inside the apron that is tweaked as the result of the same thing..
                            Last edited by Max Headflow; 11-09-2016, 08:37 PM. Reason: Add

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                            • #15
                              Had this problem on my 13x40 Grizzly, the cheapo third world drive belts were lumpy and when I replaced them with link belts, things got better.

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