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Sherline 2000 - Uneven Z-axis Lead Screw Resistance

Discussion in 'SHERLINE, TAIG, TITAN & SIEG MINI-MACHINES' started by j ferguson, Apr 19, 2015.

  1. j ferguson

    j ferguson United States Active User Active Member

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    My 2000 was purchased in July. I've made some things with it but was more consumed by getting my shop organized (??), and solving some of the other problems that floated to the surface during all of this. My 2000 is set up with Sherline's steppers and a non-Sherline controller running on LinuxCNC. It works great.

    X and Y axes run very smoothly. Each has a bit less than .003 backlash. I've been fooling with backlash on Z axis and with the backlash toggle adjusted so lead screw turns smoothly through the range that will see the most action, I get about .005-.006 backlash. I determined this with a B&S test indicator, feeding the head down to get a needle deflection, reading the thimble, then reversing the wheel and noting it's position when the indicator needle starts to back up.

    If I add even the slightest additional tension using the backlash toggle, I get binding here and there as i run the head up and down the column. It is easy to tighten it so much that head won't budge.

    Am I expecting too much of this lead screw? I could write a simple G-Code routine to run the head up and down the column over and over in the hope of "running in" the screw, or???

    Doe any of this make sense? What would you do?

    john
     
  2. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    I am not familiar with your backlash toggle. What adjustment is it making? If you are adjusting the free play in the ways, then the following applies.

    One concern is over tightening your lash adjustment. The lash you see is due to multiple factors. One is lash in the bearings, Another is lead screw nut lash. The third is lost motion caused by flexing of the the frame. lead screw, etc. When you tighten the ways, you reach a point where additional tightening requires more torque to drive the lead screw. that additional torque puts more stress on the ways, frame, causing a deflection which you see as lash.

    One method of adjustment consist of loosening the gibs to the point where there is definite play in the ways and measuring the lash as you have done. Now tighten the gib adjustment screw 1/2 turn. Measure the lash again. You should see about the same amount. Repeat the adjustment and measurement. At some point, you will have taken up the free play in the gibs and you will see the lash start to increase because the frame, etc. is beginning to flex. You would back the adjustment off to the point where you started to see the increase and about 1/2turn in addition.

    For a new machine, .005 -.006 backlash seems a lot; especially for a CNC mill. Unless your software is running backlash compensation, you will have a problem. My Tormach runs about .001 backlash and that is about as much as I think is tolerable.

    Bob
     
  3. j ferguson

    j ferguson United States Active User Active Member

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    Hi Bob,
    It's a Sherline. The toggle at the top of the Z is basicly a second-nut on the lead screw at its connection with the slide. What makes me think the problem is in the leadscrew itself is that the slide runs perfectly well if I reduce the tolerance on the fixed nut by backing off the toggle nut a bit. I use the term toggle because the nut has a brass handle on it to make it easy to adjust which looks like a "toggle."

    the lead screw is a 1/4-20 purpose made threaded shaft. I think the thing is susceptible to "running in" and I was hoping someone who'd contended with this issue on a Sherline could share his experience. I suppose it is possible that tightening the nut could somehow increase the slide's sensitivity to distortions in the ways, but my gut is not.

    FWIW, Sherline claims that it's possible to get backlash down to .001 on the Z axis using their device, but best I can do while avoiding binding is about .005. Sherline says .003 is standard for X and Y and I've got a little better than that. And yes, I use backlash compensation in my CNC config.

    Thanks much for your thoughts.

    John
     
  4. mikey

    mikey Active User H-M Supporter-Premium

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    I don't own a 2000 but I have a 5400 manual mill with this adjustable Z-axis adjuster and I have about 0.001" backlash. I adjusted the lead screw retaining screw in the center of my hand wheel before adjusting backlash.

    I found that the location of the headstock when adjusting the saddle nut makes a difference. Sherline recommends adjusting the two saddle screws with the head near the top of its travel. When I did this I found backlash got worse as I came down the column into the working range of the screw so I adjusted the screws with the head 6" down from the top. This resolved my issues but may be applicable only on my lathe, I'm not sure about yours. In any case, once I had the screw moving freely with no binding I was able to move the headstock all the way down and almost all the way up without any binding at all. I adjusted the backlash toggle from this point and was able to minimize backlash before locking the lever down with the knob.

    I do find that I have more success setting my depth of cut by passing the desired point and coming back up to it. Sherine recommends setting it as you come down. I find that this can allow an end mill to drill into the work up to the limits of your backlash and mess you up. My mill is a manual mill and do not know how your CNC mill adjusts the DOC so this may not apply. In any case, my mill is a lot more accurate with the backlash adjuster and I like it.
     
  5. j ferguson

    j ferguson United States Active User Active Member

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    Thanks much, mikey. I've gotta get my manual out and understand the effect of the saddle screws.
     
  6. kvt

    kvt Active User Active Member

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    I also have the 5400 manual mill, and it is about like what Mikey say. Just play around with it a bit, How old is the mill, and does it have the updated saddle setup on it.
     
  7. j ferguson

    j ferguson United States Active User Active Member

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    Hi Guys,
    It was new in July and it does have the updated saddle and after looking at it and (gasp) reading the manual, I can see that i need to fuss with the two set-screws on the saddle nut bracket. Duh.

    Thanks much. I'd add that I've been really delighted with this thing.

    best regards,

    john
     
  8. mikey

    mikey Active User H-M Supporter-Premium

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    Be sure to take out as much play as you can at the hand wheel first. On my older mill, I have to loosen the screw that holds the wheel to the leadscrew and then push and hold the headstock up with one hand and push the hand wheel firmly into contact with the mount and then tighten the screw with my third hand. This supposedly removes some backlash and actually seems to work. However, this method of locking the leadscrew down does introduce a small amount of tilt to the leadscrew. The saddle screws simply adjust the tilt of the saddle nut for free running, while the Z-axis adjuster takes out most of the backlash. Sort of hokey but it works pretty well.

    I am forever grateful that Sherline's philosophy of leaving no old machine behind allows our older machines to be updated as improvements occur. The Z-axis adjuster is one of these things.

    Let us know how you make out.
     
  9. j ferguson

    j ferguson United States Active User Active Member

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    Mikey, I have steppers. they are installed between the wheels and the ends of the lead screw shafts. the stepper shafts are connected to the leadscrews with couplings which have a lot of slots intended to accommodate some misalignment in the two shafts. I would be dollars to doughnuts that my saddle nuts is a tad (technical term) out of alignment and that I can improve it and then use the backlash adjustment to get the backlash down to something reasonable like what you have.

    Thanks again,

    john
     
  10. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    In looking at the parts diagram for the Sherline 2000, it looks like the toggle you are referring to is their part no. 28 in the diagram. I wold expect that its function would be to lock the z axis in position by acting as a jam nut. While you can reduce backlash in the z axis with it, it will be very sensitive to slight differences in the lead screw diameter which sounds like what you are experiencing. Using this lever to control backlash can cause accelerated wear on the lead screw and lead screw nut. If this were a manual machine, I would leave the lever loose while changing position and lock the lead screw once you have set your final position. This is a problem for a CNC mill though, as when you are doing 3 axis work, the head must be capable of moving up and down as required by the program.
    The following is from Sherline's manual:
    "The adjustment for the saddle nut consists of two flat set screws on either side of a 10-32 socket head cap screw. With the saddle nut located on the leadscrew close to the stepper motor mount, loosen these two screws and slide the saddle into position over the saddle nut. Put the 10-32 socket head cap screw through the saddle and screw it into the saddle nut, but do not tighten it yet.

    Adjust the set screws until the flat points touch the saddle nut, and then tighten the 10-32 socket head cap screw. Watch as you tighten to see that the leadscrew doesn't move. If it does, loosen the screw, readjust the set screws and retighten.

    What we are attempting to accomplish is to have the saddle nut ride on the leadscrew with the minimum amount of drag. You can check the drag by turning the leadscrew handwheel. If you feel drag, tighten or loosen a single set screw while moving the saddle with the handwheel until the handwheel turns freely, but keep the saddle close to the handwheel. If you adjust the saddle nut while it is in the center of the leadscrew, it may be slightly off center but will feel free until the saddle gets close to either end of its travel. Here, the leadscrew is supported and cannot deflect so it will bind. If you can't eliminate the binding, tap the saddle nut with a plastic hammer on the leadscrew side while the saddle nut is tightly attached to the saddle and readjust. Don't use the machine with a loose attachment screw as this will cause excessive wear and backlash."

    Good luck with your CNC!

    Bob
     
  11. mikey

    mikey Active User H-M Supporter-Premium

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    Heh, forgot about the fact that your machine is CNC - sorry about that. I'm sure you'll get it to a reasonable point, John.
     
  12. j ferguson

    j ferguson United States Active User Active Member

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    Hi Bob,
    I think you have the process down cold. You're right about the backlash adjuster acting like a jam nut. this is how the other two leadscrews are de-backlashed, but with very clever starwheel nuts which permit very slight adjustments.
    best,

    john
     
  13. j ferguson

    j ferguson United States Active User Active Member

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    I got into it and discovered that one of the two set screws was loose. The lock screws wasn't too tight either. The effect of this was to put some anti-alignment pressure on the saddle nut. Apparently this was aggravated by leaning on it with the jam-nut (backlash adjuster). As Bob suggested, I ran the saddle all the way up to the top where the control wheel is and adjusted the set-screws and lock screw until I got no resistance. Seems to work.

    I got the backlash down to a bit under .003. I continue to think that once the machine has run in a little more I should be able to do better. When I start cutting complex surfaces, it will get a lot more exercise than it has so far, although only over a limited part of the lead screws unless i back it off more than necessary between cuts.
     

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