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Author Topic: G Code Skipping  (Read 4381 times)

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Offline ALR

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G Code Skipping
« on: November 09, 2012, 06:17:21 PM »
Hey guys.  It has been awhile since I posted.  I have finally gotten a drawing into Gcode that is fairly simple.  Part of the job is to make a pocket .08" deep and about 2.7" square in a piece of wood with a 1/4 inch end mill bit.  The Gcode starts in the center and spirals out to the edge.  In one of the straight x-axis moves the gantry suddenly stops and the motor makes a sound likes it is skipping.  The next Gcode step is the y-axis move and it continues with no problem.  The x-axis skip occurs in only one of the Gcode lines repeatedly.  I have run the program 3 times and the skip occurs in the same location.  Each time with the wood repositioned on the table.  I have changed feed rate from 160 to 140 thinking that maybe that would resolve the skip but it didn't.  Any ideas? :-)

Offline Tweakie.CNC

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Re: G Code Skipping
« Reply #1 on: November 10, 2012, 02:12:31 AM »
It would help if you could zip and attach your GCode file that is causing the problem.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: G Code Skipping
« Reply #2 on: November 10, 2012, 03:53:27 AM »
I had a similar problem and it turned out to be a bolt had come loose on the table and was catching in the same place on a line of code
It puzzled me until I found it.
Graham

Offline Fastest1

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G Code Skipping
« Reply #3 on: November 10, 2012, 08:09:36 AM »
I am so glad you posted this question. I just ran some repeatability tests on my mill. During the testing I was able to repeat all the way to 331ipm over and over many times. I set my velocity to 80 and accel in the 7-8 range. Still ran great. Then I pocketed a series of holes. In the move (same place each time) between 2 holes I heard a squeal and damn if it hadn't lost position! I was suspicious if the program wrote a bad code. I was using CamBam in that particular instance. I actually intended to do the same holes with BobCad to see if it made the same error. My machine is not binding, it is well lubed and constantly cared for. Fasteners are all tight. Tweakie, I am gathering you have seen Cad or Cam programs do this? I didn't see anything alarming or obvious in the code but I am truly a beginner at looking at code.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline ALR

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Re: G Code Skipping
« Reply #4 on: November 10, 2012, 11:04:30 AM »
Thanks guys for your responses.  This is interesting.  I am also using CamBam for my G code.  The drawiing was generated using Sketchup.  The SKB file was converted to DXF that CamBam used to generate the G code in a .nc file.  Mach3 readily opened the .nc file.  I had to make a few changes to the code to get the depth correct and to force a stop to make a manual tool change, but, other than that the code has worked without problem except for this one skip. 

I will look for mechanical interference but I have moved the piece I am cutting to different location on the mill to look for that type of interference.

In the meantime I will try splitting up the one line into two steps in the x axis to see if that resolves the problem.

Tweaki,  I will email you the code since I dont know how to attach directly to this post.

Thanks.   Art

Offline Fastest1

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G Code Skipping
« Reply #5 on: November 10, 2012, 11:20:47 AM »
This is interesting in that it might not be a mechanical issue. A little tidbit I forgot is that even though my velocity was in the 80's my cut feed was 12-16ipm. Of course the rapids were in the 80's. the loss was always right at the clearance plane of Z on the transition to the next hole. Oddly it only did it between like hole 6 & 7 of 14. There were no differences in the hole sizes, depths or rapids between. I can run all 3 axis with the warm up wizard at 300ipm (which actually divides the max by the amount of axis', really running 100ipm if all axis are in movement). No positional loss whatsoever after many many runs of 20-100 cycles per axis. This was not an imported file, just a simple rectangle stock with 14 evenly spaced and sized holes.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline ALR

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Re: G Code Skipping
« Reply #6 on: November 10, 2012, 11:26:34 AM »
Tweakie,  I tried to email but dont see a way to attach a file to the email system in the posting's email system.  Here are three lines of the code.  The skip only occurs in the last line.

G1 X1.2673
G1 Y3.2802
G1 X3.109

Thanks.  Art

Offline Tweakie.CNC

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Re: G Code Skipping
« Reply #7 on: November 10, 2012, 12:12:11 PM »
Hi Art,

Sorry but I can't tell much just by 3 lines of code.

Zip your GCode file.
Click the Reply box below.
Select Additional Options.
Choose File (your zipped GCode)

That should do it.  ;)

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Fastest1

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G Code Skipping
« Reply #8 on: November 10, 2012, 12:54:36 PM »
ALR, what is the last F value in the code as it is not shown here and these are feedrate moves.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline ALR

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Re: G Code Skipping
« Reply #9 on: November 10, 2012, 02:53:12 PM »
Thanks.  Here is my zipped up code.  This does not have the change made below.  I changed my code to

G1 X1.2673
G1 Y3.2802
G1 X2.5
G1 X3.109

and my machine skipped exactly in the same spot.  Just about .25" short of the end of the move from 2.5 to 3.109 the machine skips.  When I move back to machine home I am now off of my original home x axis position by the amount of the skip.

Thanks again. Art :)