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First Post - Help with setup issue?
« on: May 25, 2009, 04:46:41 PM »
Hello Everyone,

I'm new here and could use some help.  I've been working on an X2 CNC conversion for some time now.  I have no prior CNC experience.  I have some machining experience so I'm comfortable using a manual mill or lathe.  I'm just starting to learn Mach3 and g-code.  

My X2 CNC conversion is almost operational however, I discovered a problem while testing this afternoon.  While checking position repeatability I discovered the X axis is traveling .006" short of what Mach3 is reporting on the DRO.    

My test setup is simple.  I've clamped blocks a fixed distance apart, 2.000".  The DTI has a 0.099" diameter ball end.  The indicator is set to read 0" at the starting block.  It travels 0.001" after contact to get to a 0" reading.  I wrote some simple g-code to move the X axis from 0.000 at the starting block to the end block (2.000" away).  By changing the distance little by little until the DTI reads 0.000" (at the end block) I found that distance needed to be 1.906".  I put a dwell in so I can read the DTI at the end block before it returns the table to the original position.  The repeatability is fine. It returns to within 0.0005"  on both ends repeatedly.  

Here's my breakdown.

Start = 0"
Reported distance traveled = 1.906"
+ DTI travel to 0" = 0.001"
+ indicator ball dia. 0.099"
Actual Distance traveled = 2.006"

Over travel of 0.006" per the Mach3 DRO

I've also tried the above test with blocks set 3.000" apart with the same 0.006" over travel reported by the Mach3 DRO.

By using my Shuttle Express to jog at 0.001" per click I found that the DTI doesn't move til about 0.006" of reported travel when changing directions.  

So, would this be the backlash on the X axis?

Thanks for the help,

Dan


 

Offline Hood

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Re: First Post - Help with setup issue?
« Reply #1 on: May 25, 2009, 05:16:43 PM »
Yes, certainly seems like backlash. Could be a loose pulley or even a sloppy bracket or could be the ballscrew.
Hood
Re: First Post - Help with setup issue?
« Reply #2 on: May 25, 2009, 06:34:49 PM »
Looks like it's backlash.  I'm surprised to see that much backlash in a ball screw.  I've got the lower cost Nook ball screws.  I've checked the table mount and the ball nut is screwed into a bracket.  I guess I'll have to tear into the table and see if the bracket is secure.
Re: First Post - Help with setup issue?
« Reply #3 on: May 25, 2009, 07:11:39 PM »
Looks like it's backlash.  I'm surprised to see that much backlash in a ball screw.  I've got the lower cost Nook ball screws.  I've checked the table mount and the ball nut is screwed into a bracket.  I guess I'll have to tear into the table and see if the bracket is secure.

0.006" backlash in a non-pre-loaded single ballnut is not at all unusual.  You can minimize, but not totally eliminate, this by loading the nut with oversize balls.  To get to very low or zero backlash requires spring-loaded double nuts.

Regards,
Ray L.
Regards,
Ray L.
Re: First Post - Help with setup issue?
« Reply #4 on: May 25, 2009, 07:52:12 PM »
Thanks for the input Ray.  I'm going to take it apart and see if I missed something.  If not, I guess I'm going to figure out how Mach3 handles backlash.

Thanks,

Dan
Re: First Post - Help with setup issue?
« Reply #5 on: May 25, 2009, 10:12:37 PM »
are you using ball screws? I didnt see them mentioned. You also need to make sure the thrust bearings are preloaded slightly to eliminate any play there.  I have done a number of x2 conversions and most of the factory screws when new run around .002 backlash
Re: First Post - Help with setup issue?
« Reply #6 on: May 26, 2009, 08:33:58 PM »
Yes, I've installed Nook ball screws.  I disassembled the table and found the X axis ball nut mount set screw wasn't tightened down.  I'm still working on getting it properly positioned.  My first try cut the backlash down to 0.003" but I still think it can be improved.  I'm going to have to look into oversized balls.  I can tell there's some slop in the nut.

Thanks,

Dan

Offline jimpinder

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Re: First Post - Help with setup issue?
« Reply #7 on: May 27, 2009, 03:58:08 AM »
Dan - I admire your tenacity - but rest assured, if all else fails (and it will - you must have SOME backlash in the system otherwise it would seize up) Mach copes with backlash compensation admirably. I recently changed my lathe to ball screws, and was pleased with the results, but I still use the Mach backlash compensation.

If you think about your manual work, you always automatically compensated for backlash by always coming up to a cut in the same direction. CNC does not do this, it cuts in any direction, hence the need for backlash compensation. I must admit I still, if writing my own GCode, tend to run past the job on the return, and still always approach the cut from the same direction. I have no problems and my work is accurate and repeatable.
« Last Edit: May 27, 2009, 04:03:24 AM by jimpinder »
Not me driving the engine - I'm better looking.
Re: First Post - Help with setup issue?
« Reply #8 on: May 27, 2009, 05:40:57 AM »
Thanks for the note.  Once I'm satisfied with the mechanical setup of each axis I plan to set the backlash compensation for each in Mach.  I've played with it a little but haven't really figured out how it works yet (noob curiosity). 

Dan

Offline simpson36

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Re: First Post - Help with setup issue?
« Reply #9 on: May 27, 2009, 07:00:30 AM »
For a milling machine, I found Mach's backlash compensation unuseable in its current implimentation for anything but drilling holes of other non-motion type of operations becuase it literally stops and takes up the backlash at a set rate at each direction change and then begins moving again at the commanded feed rate.