Z axis moving when cutting

Z axis moving when cutting

Postby kak » Tue Mar 12, 2013 4:41 pm

Hello.
I use an older 5.5 VCP version, mach3 and Rhino. When I import dxf drawing from Rhino to VCP, make the Gcodes and cut Profiling, the Z axis is moving sometimes about +/- 0.05mm while Im cutting.
So when I want to cut through something 10mm thick, sometimes its cutting 9.95mm. I can hear the Z motor moving and DRO in Mach is showing 9.95 as final path.
I didn´t notice this movement when Pocketing.
And I am moving all the dxf drawing elements to the same level in Rhino before exporting to dxf.
What could be wrong?
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Re: Z axis moving when cutting

Postby PaulRowntree » Tue Mar 12, 2013 9:31 pm

Could you post the file?
Are there tabs/ramps?
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Re: Z axis moving when cutting

Postby kak » Wed Mar 13, 2013 4:19 pm

Hi. There are no tabs and no ramps.

I was cutting today.
You can see in VCP, Cut Depth is set to 16.05mm.
Depth 16.05mm.jpg


In the Gcodes final Z is also 16.05mm
Toolpath.JPG


I took a Mach3 screenshot when I was cutting last depth pass. It shows 16.045mm. Hmmm...
Final depth 16.045.jpg


The other thing is that Z axis is moving sometimes when Profiling.
I was running the code a few times and it´s not doing it the same place everytime. And not always.
Right after the Z axis moved, I pressed Feedhold to see the G code. There was no Z movement in the code.

But I connected the 4th axis few weeks ago. Im not using it, but its connected. My PSU is 12Amp and motors are 4x 4.2Amp. Maybe thats the reason.
I will remove 4th axis again and see if it helps.

I just don´t understand that Mach3 is doing something thats not in the Gcode and showing it in the DRO.
At least its honest :-)
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Re: Z axis moving when cutting

Postby fretsman68 » Wed Mar 13, 2013 4:39 pm

I wonder if you're picking up noise somewhere and the motors are creeping around a bit.

Yes, if you noticed a change after adding the 4th, definitely disconnect and try again. Let is know how you make out,

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Re: Z axis moving when cutting

Postby PaulRowntree » Wed Mar 13, 2013 5:38 pm

kak wrote:
I just don´t understand that Mach3 is doing something thats not in the Gcode and showing it in the DRO.
At least its honest :-)


Could the difference between the gcode and the DRO (0.005mm) be less than your Z step resolution?
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Re: Z axis moving when cutting

Postby kak » Wed Mar 20, 2013 10:26 pm

I can´t find the reason. I removed the 4th axis.
My step resolution is 0.005mm.

But I think VCP sometimes is generating Gcode less than that. I changed my "Rapid Z gaps above material" today from 12.5mm to 20mm. Then VCP changed Z home position to 20.001mm So while cutting, Rapid Z hight was sometimes 9,995mm and sometimes 20.005mm.
Im not Mach3 expert, but if there is a Mach3 update and I open an older xml file in the new Mach3 version, does it affect anything?
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Re: Z axis moving when cutting

Postby Adrian » Wed Mar 20, 2013 10:44 pm

I've used Mach3 and ShopBot3 control software. Neither of them will match to that level of accuracy in the DRO readouts. It doesn't make any difference to the cutting if the display is 0.001mm out.

Are you using a mill? 0.005mm seems a very high level of accuracy for most CNC machines.
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Re: Z axis moving when cutting

Postby kak » Fri Mar 22, 2013 3:40 pm

I don´t need that accuracy, I´m cutting wood, but I havent notice that before. Maybe it´s normal.
I set the depth cut 0.05mm above the table, but sometimes it is cutting through the material and I get marks on my table.
Thats just something new I´m trying to avoid tabs.
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