Results 1 to 6 of 6

Thread: repeatability errors in x axis of .002"

  1. #1
    Join Date
    Aug 2011
    Location
    Safety Hbr, Florida
    Posts
    4
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Default repeatability errors in x axis of .002"

    OKK MCH600 8 pallet 50 taper HMC with Mitsubishi Meldas 300

    I've been working on this machine for the past 8 years, machining mostly investment castings requiring true position of prcision bores on center, TP of .001 inch. For the past 4 years, I have susspected an error in my x axis but could not prove it. This past month it has finally reared it's ugly head. I put a .0005" indicator in the spindle and wrote a program with 3 offsets going to 3 indexes, b0, b90, b270. From home position in x,y,z, the indicator will rapid in position, 1." away in x, then feed to x0. I have found my b axis to be accurate in rotations. However the error observed in x to be as great as .002". A machine tech has been called out, who also observed this problem. Tightening of the gibs and purging the way lube lines was thought to be the fix, however the error is still observed. This machine is box way construction with glass scales. I was told that the control would alarm if there were a problem with the scale. Maybe someone on this forum has had a similar issue. I appreciate all replys and any advise , thanks.

  2. #2
    Join Date
    Oct 2008
    Posts
    661
    Thanks
    3
    Thanked 87 Times in 72 Posts

    Default Re: repeatability errors in x axis of .002"

    With age, most cnc machines develop mechanical play on their ballscrew-driven axis, this is completely normal. The amount of play will depend on factors like machine quality, hours of use, lubrication system, machine abuse, etc.

    The mechanical play can be electronically compensated by means of parameter adjustment, it is called Backlash Adjustment, but this method is limited to a very few thousands of an inch, depending on the machine size.

    When a large amount of mechanical play is detected on one axis, it might be necessary to disassemble the machine, take the ballscrew off and have it refurbished or replaced by a new one.

    Please note that having a variation of .002" in your X-axis measures doesn't necessarily means that you have .002" of play on that axis. Even tough these 2 things are related, you would need to first perform a full mechanical play test on all 3 axis, once you have the results on paper, they can be of great help in order to make a corrective action.

    If, after making this test, you are informed that your machine has none play on any axis (which would be very, very rare), then it might be something else. The time it takes to perform this test is very short, no more than 5 minutes on each axis.

    Hope you find this helpful.

  3. #3
    Join Date
    Aug 2011
    Location
    Safety Hbr, Florida
    Posts
    4
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Default Re: repeatability errors in x axis of .002"

    Thank you very much for your input. At this point, i'm open to just about anything. If we have to remove and replace the x axis ballscrew, then so be it. I have to be sure that is the problem. Your idea is rational and maybe I can pursuade this allternative. Thanks again! It makes sense to me.

  4. #4
    Join Date
    Sep 2005
    Location
    Wisconsin
    Posts
    999
    Thanks
    5
    Thanked 70 Times in 62 Posts

    Default Re: repeatability errors in x axis of .002"

    Hi Wayne,
    If the machine/axis has scales then the machine will position perfectly to match the scale and should therefore show the same if you check with an indicator directly over the top of the screw on the table. If you check out at the very corner as well and if this is different then the gibs might not be adjusted properly. Also check parameters to make sure scale was not turned off. If it is off then I believe the screw could be the main problem. If it only happens when cutting or there is chatter then this could be the backlash in the screw. Also if there is excessive backlash or problems with scale some times you can feel a shake in the axis as you move or start and stop the axis.

  5. #5
    Join Date
    Apr 2009
    Posts
    132
    Thanks
    0
    Thanked 5 Times in 5 Posts

    Default Re: repeatability errors in x axis of .002"

    Maho uses glass scales and as said if the machine has scales the problem is with your scales as true position is the only feedback the cnc monitors which is generated by the scale and not a motor encoder. Two thing could cause this problem. The scale is too lose or too tight. or the relationship of the read head to the scale position has changed for whatever reason. One other thing I recall is on a CMM if the feed back voltage drops below the specified value (typically 5vdc) on a measuring machine. Mechanics will have zero effect on position no matter the condition of the ball screw, bearings, etc.
    Heat, alignment of head to scale or the parallelism of the scale to the travel of the axis, tightness, turned off, etc.
    Good luck!

  6. #6
    Join Date
    Aug 2011
    Location
    Safety Hbr, Florida
    Posts
    4
    Thanks
    0
    Thanked 0 Times in 0 Posts

    Default Re: repeatability errors in x axis of .002"

    Thank you all for the input and expert advise. This has been an ongoing and progressively getting worse problem. Mechanical wear was not observed much in any but x axis, however, .008-.01" was documented with an indicator reading in the x and z axis on the rotational base. It was as if the pallet was slanted on the cones of the base. With the x axis errors still in question, we proceeded to remove all of the nessecarry componets of the BRT and inspect the cross-roller bearing. Once removed, and inspected it was determined to be dry, black, and lacking any type of oil bath. The OKK engineer informed us that this machine's design requires an oil bath for the bearing. The new bearing cost over 10k, we were a little nervous installing it. The bearing was assembled with a bp of screws which were to be loosened after insttallation.
    We noticed that the original bearing was more or less dry, after about 13 years use. There was a trochoid pump mounted to the base, which appeared to be faulty. The filter to this pump was empty, losing it's prime. The gear oil in this machine should have been changed every 6 mos. I learned that this trochoid pump has 2 purposes, to provide lube to the B axis bearing and worm bath. () This pump only circulates when the b axis rotates. It was not primed. This pump must be primed in order to circulate any lube. This condition most likely caused premature wear to the bearing and the worm gear ring..
    Now I work on a machine with a rebuilt BRT with around .005 TIR measured across each pallet. Believe it or not, my repeatabillity error in x axis has mostly dissapeared. This meaning my test prg's with indicator in spindle show's less than .0005" error. It's too early to know if this machine is cured without running several parts. To me it seems as if 80% of machine problems are gone.
    It's definately good eneugh for now!
    Wayne

Similar Threads

  1. Repeatability on CNC Lathe
    By lkindber in forum Machine Repair & Troubleshooting
    Replies: 2
    Last Post: 05-24-11, 05:39 AM
  2. Fanuc 6M "D" Y axis adjustment
    By GRUSS in forum Machine Repair & Troubleshooting
    Replies: 9
    Last Post: 08-24-10, 02:16 PM
  3. Mits FA20 WEDM "EC" or "CS" setting
    By DoubleLunger in forum Programming / Applications
    Replies: 0
    Last Post: 04-19-10, 04:12 PM
  4. Danichi 152 :fanuc 15T "OT007 z + overtravel (Hard) alarm" can't RESET.
    By jlpd in forum Machine Repair & Troubleshooting
    Replies: 0
    Last Post: 11-22-09, 09:58 AM
  5. CNC.. errors "overload" on X - Axis
    By cncman in forum Yasnac / Yaskawa Controls
    Replies: 2
    Last Post: 01-18-08, 09:30 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •