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Thread: Table Top Not Flat

  1. #31
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    Oct 2009
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    Hello Jerry. I don't have an operational vacuum source,,,,,,,, but thanks for your comment. Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 8.5, Rhino 5

  2. #32
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    Thanks Ed:

    I think my brain was broken when I stood over my machine's Z housing and only put the weight on top, as I thought I would need a giant helium balloon to pull UP on the Z housing. It dawned on me this morning that I could hook up a pulley to the ceiling to reverse the direction of my spindle loading.

    That is an interesting thought on the spindle bearings. I hadn't even thought of that. I'll have to cogitate on that, as having the spindle attached to everything else may make it a bit confusing as to isolating where the play comes from.... But first I'll hook up the pulley, and get back with measurements later this afternoon.

    Thanks again, Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 8.5, Rhino 5

  3. #33
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    May 2014
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    You don't need a pulley... Just set your calipers up like you had them and lift up and down on your Z.

  4. #34
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    Hello Eric, I just got back home... The point for the pulley, is to be able to apply a known force. When pushing by hand, I pushed hard enough on the spindle that I could move it maybe an inch.... but I may have been pushing with 60 pounds. From what I read in the Shopbot back-lash measuring guide, they said to use 25 pounds. If I push by hand, I can only guess at what force I am applying............ Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 8.5, Rhino 5

  5. #35
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    Hello Ed:

    ++++++++++++++++++++++++++++++++++++++++++++++
    Downward lash with gantry sag:
    I realized that when I first measured the lash in the downward direction with the 25 pound bucket on top of the spindle mounting plate, that the spindle was in the middle of the gantry so that I was getting gantry deflection mixed into my measurements. So I re-measured the downward lash with the bucket on top of the spindle mounting plate, with the spindle in the middle of the gantry three times, (of course resetting the height gauge each time). I got these three readings: .005", .006" and .006".

    ++++++++++++++++++++++++++++++++++++++++++++++
    Downward lash with minimized gantry sag:
    Then I rolled the Z car over to the far right, where the gantry sag would be the least and put the 25 pound bucket on top of the spindle mounting plate three more times and I got the following three readings: .003", .002", and .003" So these are the readings I would say I have for the lash in the downward direction. (I pushed hard by hand, and was able to get a reading of over one inch! So pushing by hand could give any reading.)

    ++++++++++++++++++++++++++++++++++++++++++++++
    Upward lash with minimized gantry sag:
    I measured the upward lash, using the 25 pound weight, and two pulleys, so the force applied to the spindle, would be straight up. I measured the lash three times, and got three readings: .003", .003", and .003".

    ++++++++++++++++++++++++++++++++++++++++++++++
    Rack and pinion lash or stepper motor slippage?
    Thinking more, I watched down inside the spindle mounting housing to watch the rack, pinion and motor shaft, as pictured earlier. Pushing lightly, trying to simulate the 25 pound load, I still did not see anything move independently of the motor shaft. And when I would go to push the digital height gauge back to re-engage the collet (for both the upwards and downwards deflection tests), the collet was every time, either back at .000" or once, at .001" and so I feel confident in saying that the measured deflection of my spindle was due to the motor shaft moving as I was beginning to overcome the static holding torque limit of the motor. The motor, as soon as the 25 pound bucket was removed, was able to put the spindle back up where it belonged!

    It seems to me that IF there would have been mechanical slippage between the rack and the pinion, (IE slop/play), the spindle would not have returned to the exact same zero reference point from pulling the spindle up, or pushing it down. But because there is effectively no lash in the pinion/rack assembly, the spindle returns to its static/rest position when momentarily pushed either up or down with a 25 pound force.

    +++++++++++++++++++++++++++++++++++++++++++++
    Gantry Play:
    I experimented with putting the 25 pound bucket on the middle of the gantry and found I had .001" to .002" of deflection.

    ++++++++++++++++++++++++++++++++++++++++++++
    Summary: I still have no idea why my table is so uneven. I'll rebuild it on the hope it has delaminated. And I'll order a new pinion, just in case. I suppose Shopbot is the only place to purchase one?

    Any other thoughts? Thank you again to all who have commented so far. Chuck

    PS: I also have a feeling I have gone "too far into the weeds" on this, but if anyone's brain, including my own, can figure this out, it seems as though lots of details would be required.
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 8.5, Rhino 5

  6. #36
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    You're closer but you're testing for too many variables at once. There are helper springs that lift the Z axis and essentially make it weigh nothing...

    Do this: Power your machine on. Zero out your caliper and put it between the table in a fixed spot (doesn't matter where) and push up and press down on the Z with your finger. Maybe with 5 lbs of force. See if the reading changes. If your Pinion or gear box is warn you'll see movement here. Fix this before moving on to any other problem.



    Next if you suspect that your motor is weak you can do that bucket test but I doubt that's the problem. They usually either work or they don't.

    Dropping weight on the gantry or pulling up on it aren't typical forces it sees in cutting... Especially it you're saying a v bit has Z issues. There's hardly any forces here.

    You'd be better served by tightening up all the bolts you can find around that gantry.

    Next check for poor data. Get a dial indicator and put it on the top on your Z and zero it. Jog around. Does it change position? It shouldn't.

    Next take the file you had the problem with. Run it again with that dial indicator and the job air cutting. When the job is over it should return either to Z zero or whatever you had it set to do at the end of the file. Watch is as it goes along to so you can see if it's at the Z height it's suppose to be.

    If all if this checks out you've now eliminated a loose Z, a wobbly gantry and a data problem. The last variable you have is your table. Post some pictures of it for us. I suspect it might be the culprit.

    At this point I'd surface every square inch of it and try again. Or replace the table.

    Moral of the story is one variable at a time. Pinions are wear items. Start there.

  7. #37
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    Addendum:

    You should also check your X rails and make sure they're perfectly level. Also, make sure they're leve not only along X, but to each other along Y.

    Lastly, make sure your gantry isn't racking really badly and lifting off of the tracks...

  8. #38
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    Thanks Eric. I have printed out your directions and will bring them into the shop. Thanks, Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 8.5, Rhino 5

  9. #39
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    South Elgin, IL
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    Chuck there is a local source for pinions - I'll see if I can find the receipt from when I got replacements.

  10. #40
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    Thanks Mayo!

    Eric, I started on your list of things to check, but got bogged down. The 5 pound lash test showed .001" when pushed down, and when pushed up.

    In checking the bolt tightness, I was shocked! Two smaller bolts were just snug. The others were around 10 to 20 ft-lbs. I tightened all of the larger bolts to 60 ft-lbs, and there is a series of smaller bolts, where the head is hidden inside struts, and they slip when tightened significantly. But all the big bolts tightened up well. The bad news was that when I was all tightened up, my carriage was no longer square when I pulled it up against the stops. I did multiple cycles of loosening all the big corner bolts, which made the car totally floppy, drawing the car up against the stops, and re-torqueing in a gradual sequence. Yet every time, I was no longer perfectly square against the stops. So that is where I am stuck now as I used to pull up perfectly square against my stop blocks....... And today I have to do other work........

    Thanks, Chuck
    Chuck Keysor (circa 1956)
    PRT Alpha 60" x 144" (circa 2004)
    Columbo 5HP spindle
    Aspire 8.5, Rhino 5

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