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Rotary carving skewed
« on: April 26, 2015, 01:36:37 AM »
So I was running a 3d carving file on my rotary axis and well all is good EXCEPT I noticed that it is slightly skewed.
I was machining a skull at a shallow depth (think of a relief carving wrapped around a cylinder) and i noticed that the
whole thing is skewed a little just enough that i noticed it and I just cant let it go to the customer this way.
When I say skewed Picture a skull made out of clay and some one put there hands on the top of the skull and the bottom and twisted it
in opposite directions its very little but enough to notice if you study it and this will be studied heavily so any ideas whats happening.
  
It looks great and stock is true so I'm thinking maybe a cv thing (as a side note I noticed the last time I did a profile cut of text that at a production speed
most of the letters had a slight skew at the bottoms like a lower case l where the bottom should be 2 equal length vertical parallel lines connected with a horizontal line what i got was 2 vertical parallel lines but one slightly longer than the other and the horizontal line was angled they all touched but not right hope that makes sense I cant remember if it was top and bottom or just the bottoms it was a few weeks ago.)

Oh and I am actually machining around the A axis so the tool path wraps around the cylinder so its constantly going 0 to 360 to 0 etc the step over is on the linear axis not the rotary.

I am running in constant velocity mode and I have not made any changes from whatever is set when Mach is installed.

Thanks for any help you guys can give.
« Last Edit: April 26, 2015, 01:41:53 AM by bearwen.us »
Re: Rotary carving skewed
« Reply #1 on: April 26, 2015, 10:20:09 AM »
Have you manually checked your A-axis motor step settings by marking zero then running it through several thousand degrees and checking zero is still aligned?

Offline ger21

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Re: Rotary carving skewed
« Reply #2 on: April 26, 2015, 10:34:02 AM »
Sounds like maybe the  axis is losing steps?
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Rotary carving skewed
« Reply #3 on: April 26, 2015, 10:38:55 AM »
as a side note I noticed the last time I did a profile cut of text that at a production speed
most of the letters had a slight skew at the bottoms like a lower case l where the bottom should be 2 equal length vertical parallel lines connected with a horizontal line what i got was 2 vertical parallel lines but one slightly longer than the other and the horizontal line was angled they all touched but not right hope that makes sense I cant remember if it was top and bottom or just the bottoms it was a few weeks ago.

I can cause this issue when diamond-drag engraving both 2D and wrapped by running faster than the machine/pc can handle, slowing it down restores normality,
Regards,
Nick

Offline BR549

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Re: Rotary carving skewed
« Reply #4 on: April 26, 2015, 12:06:27 PM »
Some stepper drives will loose a step on reversal IF the polarity of the signal is wrong.

Just a thought, (;-) TP
Re: Rotary carving skewed
« Reply #5 on: April 26, 2015, 08:51:00 PM »
Ok it is not loosing steps its way to consistent and the axis is dialed in with returning to zero after many many revs.

and the drivers are gecko 203 vampires so all good there.

Its pretty odd I have not had a chance to mess with it this weekend I will try to get some pics and do some tests and post if I find the fix.

I have not tried slowing way down as that will take forever but I will try it. I know when I went to fast with the engraving it altered the text so maybe its just to fast and there is runout in the cutter I dont know.
Re: Rotary carving skewed
« Reply #6 on: April 26, 2015, 08:57:31 PM »
as a side note I noticed the last time I did a profile cut of text that at a production speed
most of the letters had a slight skew at the bottoms like a lower case l where the bottom should be 2 equal length vertical parallel lines connected with a horizontal line what i got was 2 vertical parallel lines but one slightly longer than the other and the horizontal line was angled they all touched but not right hope that makes sense I cant remember if it was top and bottom or just the bottoms it was a few weeks ago.

I can cause this issue when diamond-drag engraving both 2D and wrapped by running faster than the machine/pc can handle, slowing it down restores normality,
Regards,
Nick
I am thinking this is what the problem is and it is really bad in 3D  I am not going fast enough for the PC to have a problem but maybe its to fast for the cutter and its flexing a little and causing this I will slow it down and give it a try hopefully in the next few days to busy at the moment.
Thanks for the replies so far. will keep you posted

Offline BR549

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Re: Rotary carving skewed
« Reply #7 on: April 26, 2015, 09:01:25 PM »
When you LOAD the program do you see that pattern in the toolpath display ?? 

(;-) TP

Offline ger21

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Re: Rotary carving skewed
« Reply #8 on: April 26, 2015, 09:08:53 PM »
Quote
Ok it is not loosing steps its way to consistent and the axis is dialed in with returning to zero after many many revs.

Mark the axis and do 50 rotations. (A18000). Is it in the same place as A0? If it is, then most likely you're losing steps. If not, your steps/unit is wrong.

Otherwise, maybe a mechanical issue. But a mechanical issue shouldn't be skewed.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html

Offline Greolt

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Re: Rotary carving skewed
« Reply #9 on: April 26, 2015, 10:28:59 PM »
Sounds like losing or gaining one  or more steps on every reversal.

As TP said, check your signal polarity against drivers specs.