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Plasma cut quality problem
« on: September 24, 2009, 08:07:22 PM »
Hello, I've got a plasma cut quality problem and just can't get to the next step with it. Progress in all areas except for this. See attached photos. I've got some hesitation in arcs. No problem in circles, squares, triangles, or lines. Just at the end of one arc and the start of another arc, like in the 4in fleur-di-lis. Maybe a 15 - 20ms hesitation, just enough to see when the machine is running at 50 to 60in per minute. The faster The machine runs, the more outstanding the marks become.

I've got good repeatability to about .0005 - .001 and have good accuracy. I've worked on the motor tuning / velocity, and speeds till I'm at my wits end.

I'm using autocad 8 to make the drawing, save as a .dxf, import into sheet cam to post. I don't believe it's mechanical because of the time I've spent. I believe it's in something I've got wrong in Mach or it's the way I'm making the G-code.

Any suggestions would be greatly appreciated. I've been hammering on this problem along with other things for about a month.

Thank You!  Aj
Re: Plasma cut quality problem
« Reply #1 on: September 24, 2009, 08:26:59 PM »
I should of added the .dxf file just in case.

Aj

Offline stirling

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Re: Plasma cut quality problem
« Reply #2 on: September 25, 2009, 03:58:11 AM »
Hi ajclay

looks like you're in exact stop mode. Add this: N0025 G64 (CV On)

Cheers

Ian
Re: Plasma cut quality problem
« Reply #3 on: September 25, 2009, 05:17:57 AM »
Sterling,

I don't believe that turning CV on will help his problem.  I suspect that already is using CV.  I am having the same kind of problem with my Router CNC machine cutting solid surface material.  I have not determined how to fix it.  I think it is a signal problem.  I have ruled out the computer.  In my case the pattern is closer together and not random.  His pattern is not random.  I bet if he cuts a diagonal pattern like a diamond on its point or at a 45 degree.  He will see the pattern also.

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Re: Plasma cut quality problem
« Reply #4 on: September 25, 2009, 05:59:58 AM »
Hi SailFl - I don't do "beliefs" or "suspicions" - I give it the best shot I can with the evidence available.... I also don't gamble - sorry. ;D

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Re: Plasma cut quality problem
« Reply #5 on: September 25, 2009, 06:24:30 AM »
General config screenshot shows CV is on and code doesnt have either G61 or G64 so unless some previous code put it in Exact Stop then it should be in CV. It is however best to always have G64 at the start of your code as this always makes sure that things will be done in CV (if that is what you need of course)

The clue may be in the faster you go the worse it gets, Its hard to tell from the pic and  I know you said its not mechanical but are these gouges lining up with the axis? If so check for backlash.
Hood
Re: Plasma cut quality problem
« Reply #6 on: September 25, 2009, 07:07:17 AM »
Mach, is in CV mode.

I'm going to give adding G-64 a try today.

I don't really have any backlash. I had another problem in the past when I just got the machine up and running (two months ago) and spent a good while checking backlash. The backlash has to be less than .001.

The machine has good repeatability and accuracy..

I don't want to be quick to say everything is perfect like so many, because I do have a cutting problem.

The notches don't show up on any other shape, even 45deg cuts. They only seem to serface on the 3 point arcs or ellips and line up with the nodes in the drawings. The hessitation can be seen with the eye at the end of the arcs. The arcs don't show a big change in direction. I could understand a sharp turn because of deceleration, but this is basicly going the same direction.

There's no doubt - The job is in the details!

Thanks for the suggestions, Aj

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Re: Plasma cut quality problem
« Reply #7 on: September 25, 2009, 08:14:56 AM »
Just looked at the code in Mach and there are very short arc moves which coincide with 4 out of the 6 points you are having issues with. Afraid I have no software here to view a DXF so cant see whats in that but kind of looks like that may be an issue.
Hood
Re: Plasma cut quality problem
« Reply #8 on: September 25, 2009, 09:29:36 AM »
AJ - Don't work with plasma so I could be off base here.  I opened your dxf file in AutoCad and noticed the center of your arcs in your fleur-de-lis  (caught my attention because it is the symbol for my city) corresponded apparently exactly with the marks on your symbol.

I walked through your code and saw pauses within the arcs at their centers.  Looked at your code and saw several lines in those areas that called for an a very slight straight line movement  (G1) within the arc that should not have been there.  I have edited most (maybe all) those lines out of the attached code.

My first thought is your cad software is adding a line at the center of the arcs 

No guarantees that my observations or suggestions works.

Bob@BobsShop
Re: Plasma cut quality problem
« Reply #9 on: September 26, 2009, 09:45:06 AM »
As it turned out I finally hit the nail on the head. The guy from CandCNC - (Tom) pointed me in the right direction.

He told me to turn off all selections in "General Config" "CV Control" and make sure that "Motion Mode" was set to "constant velocity".

I did that without and change in cut quality.
 
Here's the trick.....  I had to use the "1024" screen, which I didn't have a clue about, go to the "settings tab" and turn off "CV Feedrate" .  CV Distance, on or off didn't seem to make a difference with any of my problems, so the question remains on whether this should be on/off and what value it should have.

I instantly saw the improvment in machine motion.
 
No exaggeration - my machine will run 4 times faster on that little 4in part and without any vibration or shaking... 200in/min
 
Now I'm in business......
 
That CV Feedrate setting should be on the "plasma screen" if it's that important.
 
Thank you guys, I'll talk to you later.

Aj