DRO reading is off
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  1. #1
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    Unhappy DRO reading is off

    I originally posted this in CNC since on a CNC/manual machine but got no response so hopefully someone can help me in this forum

    I have glass DRO scales on a shoptask machine (which i have spend an enormous amount of time on trying to get it running properly but thats another story). The scales on Z (carriage) read high in one direction (about 0007 per 6"). The steppers are moving properly and move exactly 6". The DRO scales seem to be parallel to the surface in both planes - the reader could be a little off from the sales due to the fact the scales are not level in the vertical plane (mounted like that from factory) but I have done my best to shim the reader head to be parallel to the scale. When I move the reader around nothing changes.

    Are the scales just low quality and not accurate or should I try something else? I dont seem to understand why if its reading high in one direction wont it read high in the opposite. the movement is the same over the same stretch.

    Thanks in advance for any help forthcoming

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    Ok so without know what readout or scale brand is on the machine... here are some things i would check.

    Flatness... needs to be more than "thinking" their flat/parallel. Acu-Rite scales say they need to be flat within like 7 or 10 thou over the length of the scale, but you should be able to get it closer to 3 thou or less quite easily. Ontop of that, you need to make sure that the reader head is mounted properly and square. I know that you said it seemed like its flat, but that can be a big thing. One of my "sayings" (not that my sayings are famous or that anyone listens) is that if you didnt do it, dont trust it. Just cause its the factory doing the install, doesnt make it right.

    Now you said that the steppers move fine. Do they read off of the scales? Or do their have a rotary encoder on them? One thing to think of is that most CNC controllers have CNC settings that dont engage or turn on when in manual mode (ie backlash, error comp). So are you getting the reading correct in a controlled commanded move and not in manual mode?

    Is it every time you move in one direction that you are off or only in a certain part of a scale?

    Jon
    H&W Machine Repair

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    read high in only one direction?
    does it always return to zero?
    If you run the carriage back and forth 100 times does the return to the same number or does the error increase?
    Teh machine is not a high accuracy machine, it is possible that everything is not in the same plane

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    Its reading high in just one direction, it goes back to zero and does not increase - I have verified movement of steppers to be correct via indicators - in fact I have tuned movement via indicators not the DRO.
    The steppers are not related to the dro and we are talking about verified movement of 6" - doesnt matter if its done via steppers or not
    I dont know what brand scales these are but they are cheap Chinese scales. They were not parallel before but after adjusting them there was no difference. The reader was not at the correct angle to the scales but me shimming it and making it square didnt seem to make a difference - what I dont understand is why the reading is off in just one direction. in other words if I move 6" one way it will read exactly 6.00000 but if I move the other way (doesnt matter the exact position) it reads 0007 over.

    This reading doesn't really change no matter how I put the head - it can be a a totally wrong angle and still read the exact same thing -0 I don notice any inaccurate other than the one i am talking about. I am suspecting that perhaps the scale is defective but am looking for input from people that have more experience in this

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    You can get this effect on something like a Bridgeport table. Since the table is longer than the saddle it slides on, the table will wear in an arc, the same way amateur astronomers can make very good concave mirrors with a simple setup. The instructions with one DRO referred to it as being on the rim of a wagon wheel. If the DRO scale is not on the same radius of the arc, the readings will differ. I don't know what your setup is like, so that is the best I can offer.

    Bill

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    to me makes no sense for it to be off in one direction though - if there is an arc it should read wrong in the other direction too - keep in mind it will do it regardless of the point of origin.
    this is on a Shopmaster / Shoptask new Mill Turn machine its the scale thats along the z axis of the lathe.

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    Quote Originally Posted by ntino View Post
    to me makes no sense for it to be off in one direction though - if there is an arc it should read wrong in the other direction too - keep in mind it will do it regardless of the point of origin.
    this is on a Shopmaster / Shoptask new Mill Turn machine its the scale thats along the z axis of the lathe.
    It depends on where you zero it. From your description, it appears that if you move one way it goes off and comes back to zero when you move back to the original point. If that is the case, it is making the same error in both directions. If you zero in the position to the right and see the error when you move to the left, then if you zero in the left position and move to the right, you should see the same error but with reversed polarity.

    Some programs have a correction function where you can assign a multiplier to the reading.

    Bill

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    I guess I got confused with the questions. - if I zero the machine and go right to left it reads high. If I zero the machine and go left to right it reads normal. The actual movement distance is exactly the same per dial indicators

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    Quote Originally Posted by ntino View Post
    I guess I got confused with the questions. - if I zero the machine and go right to left it reads high. If I zero the machine and go left to right it reads normal. The actual movement distance is exactly the same per dial indicators
    Do you have an oscilloscope? The outputs from the scale are probably two square waves with a 90 degree phase shift between them. If the phasing or the symmetry of the outputs is far off, it could be managing to count properly in one direction but sometimes missing a count in the other direction.

    Bill

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    I do have a scope, not sure which pins to test... however its always off by the exact same number - do you think its possible that the crosslide has play in the gibs perhaps so it twists at the end of movement to one direction?
    I mean i can check the waveform but if it was a case of being a bad signal I would imagine it causing more issues and not being the exact same thing every time

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    how are you able to verify .0007" in 6"?

    -you say it does not accumulate error over more cycles so I would put it down to:

    1. your cheap scale resolution

    2. to much backlash compensation value in the cnc.

    3. screws or thrust washer stiffness not being equal in both directions.
    .

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    Pound to a pinch of oriental excrement, that is a mechanical problem.

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    I am not sure if everyone understands that I am verifying movement with 2 1-2-3 blocks. The machine has manual wheels as well and the same thing is true for manual - no backlash compensation is turned on for this- I am taking out backlash manually

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    123 blocks aren't even that precisely nominal new out of the box, Unless they came from suburban tool and cost $150 a pair etc.. and then they are only +/- .0002 of nominal.

    Your chasing more precision than you can quantify with cheapo machine components.

    Bee happy and get to making chips.

    Sent from my E6810 using Tapatalk

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    Well one is suburban and one is starrett blocks and measure exact as far as my caliper knows. What bothers me is that the dro reads off in one direction and I don’t know why - it’s not the overall precision. It will be a couple of though out on a longer piece.

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    Quote Originally Posted by ntino View Post
    . . . measure exact as far as my caliper knows. . .
    If you're using a caliper instead of a good mic, you've already wasted the entire time you've spent on this issue.

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  20. #17
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    picture of a SHOPTASK cnc machine.



    ....annnnnd we're done here. move along.

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    Oh for f'ks sake

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    Quote Originally Posted by dsergison View Post
    picture of a SHOPTASK cnc machine.



    ....annnnnd we're done here. move along.
    That just made my day.

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