Troubleshooting Inaccurate Cutting with Dual Tables

      Mechanical backlash or a control problem could be causing performance flaws in a dual-table CNC setup. Here are tips of finding the source of trouble. May 16, 2005

Question
I run a CMS machining center with independent V and Y tables. I have been experiencing an extremely horrible cut quality when using the V table. The rough cut is only experienced when interpolating the X and V axes. I can run the exact same program on the Y table and the cut is extremely accurate. Also, when the V and Y are slaved, the cut remains accurate and very acceptable. I am not sure what could be causing this problem, but I have a few ideas. It could be the servo drives or ball screw. I am not sure where to start in diagnosing this problem, so any help would be appreciated.

Forum Responses
(CNC Forum)
From contributor D:
Easy checks that I recommend include:
1) Check backlash on the X and V axes. To do this, use a dial indicator mounted on one of the ways and jog the machine in increments of .01mm, .1mm, and 1mm, while reading the actual movement from the indicator. Go several steps one way and then reverse. Any difference between the jog command distance and the indicator reading is positioning inaccuracy (probably, but not always mechanical backlash). Do this test on both sides of the X and V tables. If you get one reading on one side of the table and a different reading on the other side, your table is "crabbing" on the ways. With the indicator still attached and the drives still on, push and pull the table and see if the indicator moves and does not return to the original position. Failure of the indicator to return to the same reading almost certainly indicates mechanical backlash. Mechanical backlash is usually the result of wear in the screw or nut, or a problem with the support bearings. If you find backlash in excess of about .04mm, I would look into getting it fixed. Below that, you can probably compensate adequately in the control servo parameters.

2) If you find no mechanical backlash, I would slave the tables together. Mount the indicator base on one table and indicate off of the other. Jog back and forth incrementally and continuously. Ideally, the indicator should not move. If it does, note the amount of error, the situation under which the error occurs, and the characteristics of the reading (rapid oscillation, regular periodic deviation, etc.). If you have this sort of problem, then it is probably time to call in a tech to take a look at the control/servo/feedback system.

3) If you still see no problem and you can control both X and V separately, I would unslave the tables, but write a program that moves both tables back and forth the same distance at the same feed. Perform a test similar to test 2. This should yield the same results as test 2. If it does not, then the tech has some real head scratching to do, and will probably need to talk to the control folks.

As an aside, if you are covering both tables with a single spoil board when running in slave mode, the good table could very well be dragging the bad one along. I do not know if this is the case, but I have seen this sort of thing before.



From contributor D:
In previous post, please note that I was calling the axis parallel to V the "X" axis. It is obviously the "Y".

There is one more thing you might check. Do a backlash check with the tool carrier positioned over the Y table and then do one with the tool carrier positioned over the V table. It is possible that you have a mechanical problem in the X axis that is localized over the V table. You usually see this kind of problem in a rack drive axis, but it is possible in a ball screw system if there is way damage.



From contributor K:
Have you checked your hold down system? You might be loosing vacuum on your V table or you may have too much overhang of your part.

Would you like to add information to this article?
Interested in writing or submitting an article?
Have a question about this article?


Have you reviewed the related Knowledge Base areas below?
  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Computerization

  • KnowledgeBase: Computerization: CNC Machinery and Techniques


    Would you like to add information to this article? ... Click Here

    If you have a question regarding a Knowledge Base article, your best chance at uncovering an answer is to search the entire Knowledge Base for related articles or to post your question at the appropriate WOODWEB Forum. Before posting your message, be sure to
    review our Forum Guidelines.

    Questions entered in the Knowledge Base Article comment form will not generate responses! A list of WOODWEB Forums can be found at WOODWEB's Site Map.

    When you post your question at the Forum, be sure to include references to the Knowledge Base article that inspired your question. The more information you provide with your question, the better your chances are of receiving responses.

    Return to beginning of article.



    Refer a Friend || Read This Important Information || Site Map || Privacy Policy || Site User Agreement

    Letters, questions or comments? E-Mail us and let us know what you think. Be sure to review our Frequently Asked Questions page.

    Contact us to discuss advertising or to report problems with this site.

    To report a problem, send an e-mail to our Webmaster

    Copyright © 1996-2019 - WOODWEB ® Inc.
    All rights reserved. No part of this publication may be reproduced in any manner without permission of the Editor.
    Review WOODWEB's Copyright Policy.

    The editors, writers, and staff at WOODWEB try to promote safe practices. What is safe for one woodworker under certain conditions may not be safe for others in different circumstances. Readers should undertake the use of materials and methods discussed at WOODWEB after considerate evaluation, and at their own risk.

    WOODWEB, Inc.
    335 Bedell Road
    Montrose, PA 18801

    Contact WOODWEB











  • WOODWEB - the leading resource for professional woodworkers


      Home » Knowledge Base » Knowledge Base Article