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Lathe problems - bed twist?

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  • #61
    Caved in and bought a 3MT test bar (not looking forward to the end of the month as it is!) as I'm fighting more unknowns than I have the experience to handle. Turned up today. Mmmmm, that stink of freshly imported Chinesium! :-X

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    • #62
      Some quick results.
      Just shoved in the headstock I get 0.03mm over the top of the bar from end to end with the high point at the tailstock end. Over the front I get 0.14mm with the tailstock end nearer the user.
      With the tailstock supporting the end on a live centre (uggt to dig out the dead centre - will do next time) it's within 0.01mm on the front and about the same over the top.
      Bar is about 234mm long - just over 9".

      Both of these sound pretty good to me. Only thing of note is that the indicator wobbles around quite dramatically when you move the carriage along - but only when indicating the front, not the top. This is all with the DTI mounted on the cross slide.

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      • #63
        Originally posted by Cenedd View Post
        .....

        Only thing of note is that the indicator wobbles around quite dramatically when you move the carriage along - but only when indicating the front, not the top. This is all with the DTI mounted on the cross slide.
        This sounds like a problem to be looked at. After all, the cutter is going to be on the front side, and you do not want IT to be described as "wobbles around quite dramatically".

        Can you describe the issue more? Maybe it is not as it sounds.......
        CNC machines only go through the motions

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        • #64
          Will try to get some video later and post that. Should it be tailstock-supported or not?
          I believe I can stop the spindle but spin the leadscrew so I'll check if power feeding the carriage on a stationary bar makes any difference to the wobble.
          When I say wobble, I mean the needle moves as you crank the handle but when you stop it stops back near zero.

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          • #65
            I haven't read all, so disregard if this has been mentioned.

            You might want to check the rear retaining strip, attached to the rear of the carriage and running under the rear way. The design of this strip is not good and changing it is one of the first updates many owners make. The lousy design, coupled with any irregularities on the underside of the rear way would account for the jumpy needle, your problem with sticky spots in the carriage travel, and likely a few other issues.
            George
            Traverse City, MI

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            • #66
              Thanks George. No, that hasn't been mentioned - probably requires specific Sieg knowledge. Thanks, I'll definitely check that out.

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              • #67
                Yes, any gibs are a bit suspect as to possible looseness.... Although as a unit with V-ways, it really, if new, should not have that kind of wobble.

                Probably George is thinking that the carriage can tilt up at the back, and allow the tool to swing away from the work, pivoting around the front way, which is entirely possible with small and lightweight parts combined with larger hands. (or something like that, if the Sieg is built differently to what I think from the pics I have seen.).

                It needs checked, because that can also happen when actually cutting metal.
                CNC machines only go through the motions

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                • #68
                  Ok, shot a quick bit of video and it doesn't look quite as wild as I'd remembered it. It does wobble with the crank of the wheel but it's more like 0.01mm than the wild swing I thought it was - although I was probably cranking a bit slower. I guess that puts us in the ballpark for George being right on the money. I'll try to investigate the back tomorrow and see if there's anything adjustable there.

                  Shot these in this order so first with tailstock and then just removed the tailstock without adjusting the bar in any way.

                  DTI sweep with tailstock support
                  DTI sweep without tailstock support

                  Still have to check on the leadscrew shield but it's at least looking like this isn't a total car wreck after it's been levelled once and let sit for a few days.

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                  • #69
                    The design of the gib means it typically bears against the underside of the way unevenly, at an angle. Because of this I thought that any rough or uneven spots in the way might translate into a force 90° to the ways. The ways on this lathe are tiny. A recent article in HSM had the author adding a bracket and screw on the operator side of his carriage, as he found it didn't take much to shift the carriage under load. Of course, there will be some lifting involved in any shifting.

                    With the wonky design of the gib, I can see it shifting things as easily as the forces from cutting would. Taking the gib off would be a quick way to check.

                    BTW, the article should be mailing in a few days in the M/J issue of HSM. Titled, "A Mini-Lathe Carriage Guide" by Ted Hansen.
                    George
                    Traverse City, MI

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                    • #70
                      0.01mm is about .0004" isn't it? Doesn't sound so bad that way, but it shouldn't wobble like that, not with the saddle running on vee ways. If it can twist like that, it suggests to me that the saddle isn't sitting right down on the front vee, and when its cranked along, the hand action on the handlewheel makes the saddle twist a little. Now find out what is interfering with the saddle/way fit.
                      I'd be surprised if you can run the leadscrew under power without running the spindle, most leadscrews are driven off the end of the spindle.
                      'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                      • #71
                        George: I shall check out any gibs at the back asap - I say "any" not because I disbelieve you, just that I have no idea what's back there! Once I got it on the bench, the rear of it was uncharted territory!
                        I'll check out the magazine. Did look for a physical copy locally but it's all women's chat mags. Will try the digital version.

                        Richard: yes, slightly under half a thou. Not a lot I guess and definitely better than when I started! Did have trouble with the cross slide backlash so it's entirely probable that a similar issue exists with the carriage.
                        It is possible to run the leadscrew without the spindle. You'd have thought not but Sieg sell a milling add-on so the idea is that you can use the power feed for the mill without the lathe spindle killing you while you're not looking. Might even have gone with that if I'd not bought a mill first. Didn't know I wanted a lathe until I had the mill

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