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Author Topic: Position Errors Only on Diagonal Moves  (Read 10273 times)

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Position Errors Only on Diagonal Moves
« on: January 14, 2015, 12:55:08 AM »
Someone I know is having a strange problem that really has me stumped.  The machine is a bed mill, with AC servos.  It will do unlimited, perfect, single-axis moves, cutting exactly to correct dimension every time.  But when cutting a square, rotated 45 degrees so diagonals drawn between opposite corners would be parallel to the X and Y axes, the square becomes a rectangle, with on pair of sides about 0.04" too close, and the other pair of sides about 0.040" too far apart.  There is NEVER any position loss.  It returns to exactly X0, Y0 perfectly, every time.  A square cut so the sides are parallel to X/Y will come out exactly on dimension every time.  Steps per is set correctly, and confirmed by measurement.  Backlash is well under 0.001" on all axes.

As I said, I am stumped.  We've tried it running with a UC100 motion controller, and with the PP, and results are exactly the same.  Reducing velocity and acceleration, even dramatically, makes no difference whatsoever.  Running exactly stop makes no difference.  DROs always show the "correct" position (i.e. - where the machine should be, as opposed to where it actually is).  We've tried several different versions of Mach3 from 3.043.066 down to somewhere in the 050s, and get the same result with all of them.

Any ideas or suggestions?

Regards,
Ray L.
Regards,
Ray L.

Online Tweakie.CNC

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Re: Position Errors Only on Diagonal Moves
« Reply #1 on: January 14, 2015, 02:11:12 AM »
Hi Ray,

Are you re-drawing the 45 deg. rotated square in CAD/CAM or are you using the Mach3 'Local System Rotate' function and the existing Gcode ??

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Position Errors Only on Diagonal Moves
« Reply #2 on: January 14, 2015, 10:51:23 AM »
Tweakie,

All code is generated either by CAM, or hand.  No Mach3 scaling or rotation is in effect.  It appears to me like Mach3 is giving incorrect move commands only for coordinated moves..  I can't think of anything in the hardware that coudl behave this way.  Yet behavior is identical with several different Mach3 versions.  Very odd....

Regards,
Ray L.
Regards,
Ray L.

Offline BR549

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Re: Position Errors Only on Diagonal Moves
« Reply #3 on: January 14, 2015, 11:36:36 AM »
Ray how are you generating the math to calculate the diagonal points ??

Just a thought, (;-0 TP
Re: Position Errors Only on Diagonal Moves
« Reply #4 on: January 14, 2015, 11:43:25 AM »
Terry,

No math.  Draw a square, CAM the square.  The problem is NOT the g-code.  It has been verified by hand, and by simulation.  The square is a perfect 1.25" x 1.25" square in all cases.  It has been verified correct 12-ways from Sunday, and hand-generated code gets the same result.

Regards,
Ray L.
Regards,
Ray L.

Offline BR549

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Re: Position Errors Only on Diagonal Moves
« Reply #5 on: January 14, 2015, 11:49:24 AM »
Have you checked the BED to make sure IT is square ??

Are the DRO values correct when it moves out of square ??

(;-) TP
Re: Position Errors Only on Diagonal Moves
« Reply #6 on: January 14, 2015, 11:52:54 AM »
Terry,

It cuts absolutely PERFECT squares, EXACTLY on-dimension, and EXACTLY square, as long as they are aligned to the X/Y axes.  So, yes, the bed is square.  Yes, the DRO values are correct.

Regards,
Ray L.
Regards,
Ray L.

Offline BR549

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Re: Position Errors Only on Diagonal Moves
« Reply #7 on: January 14, 2015, 12:00:01 PM »
OK next test Does it cut exactly the same IF you reverse the order of cut From  CW to CCW. IS the offsetting of the cut the same ??

(;-) TP

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Re: Position Errors Only on Diagonal Moves
« Reply #8 on: January 14, 2015, 12:13:53 PM »
If it’s any help to the discussion I have just been and cut two 50mm squares, one on the X-Y axis and one at 45 deg. They were both cut ‘on the line’ with a laser so turned out 49.4mm x 49.4mm but they both measure identical.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Position Errors Only on Diagonal Moves
« Reply #9 on: January 14, 2015, 12:56:10 PM »
Is the 0,0 part position at the center of the square ? or elsewhere ? (at a corner)
Might try it both ways just to see if there's a difference there.
I would, but can't right now.
Russ