Coordinates change and scale when G-code is loaded

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10-02-2013, 04:30 AM
frare bear

Coordinates change and scale when G-code is loaded

Hey everyone! I basically built a CNC recently, it consists of:

8020 frame (1530)
Steel rails with extended linear carriages from cncrouterparts.com
R&P system from cncrouterparts for X&Y axes
Homebuilt threaded rod Z axis
Gecko G540
Big Power Supply
Mach3

Basically all the electronics are from this kit HERE

I loaded Gecko's .xml file for Mach3. Changed the motor tuning to 1909.861 steps per for X&Y, and 35000 for Z. This results in accurate movements... UNTIL I try to load some G-code in. At this point the origin moves and the motor tuning somehow changes (although all the settings remain the same). All I can think of is that the G-code has something weird in it... I don't know much about G-code though.

The odd thing is that before I changed out the electronics the same code worked just fine (the old setup had some ghetto DIY hobbycnc board and motors that stalled out easily).

Here's the start of my G-code:

"( Mach2/3 Postprocessor )
N10G00G20G17G20G90G40G49G80
N20G70
N30T1M06
N40G00G43Z0.2000H1
N50S12000M03
N60G94
N70X0.0000Y0.0000F250.0
N80G00X11.9818Y42.4337Z0.2000
N90G01Z-0.1000F60.0
N100G01Y0.7643F250.0"

Maybe I'm missing something really simple, but any and all help would be appreciated!!

Frary
10-02-2013, 11:49 AM
CarveOne

I think the default in a new Mach3 installation is metric units. Change it to inches, then be sure to save it under the Config menu list.
10-02-2013, 04:15 PM
frare bear

Quote:

Originally Posted by CarveOne

I think the default in a new Mach3 installation is metric units. Change it to inches, then be sure to save it under the Config menu list.

Can G-Code affect this setting? I've had Mach3 installed for several months now, running in inches. When I open Mach3 everything works perfect, jog 6 inches and the machine tells me 6.00". Then I load some G-code and when you move 6 inches it reads out way way less, around 1.00" on the display screen
10-02-2013, 04:53 PM
ger21

No, g-code doesn't affect the native units. However, make sure that your native units are set to inches, and your currently in inch mode (G20) when you jog the 6".
It's possible to set up the machine with mm Native Units, but you set the step/s unit so that it appears it moves in inches. When you load a g-code with a G20 in this situation, the distances will be incorrect.
10-02-2013, 08:55 PM
frare bear

After reading up on G-code for awhile it seems to me that the "G90" on the first line may be better if changed to "G91".... If I'm reading this right, G90 references absolute coordinates, whereas the G91 will go off of wherever the machine was moved to and zero'd at?

When I get home tonight I'll load the new version and see what happens
10-02-2013, 09:47 PM
ger21

You can't just change the G90 to G91. The code won't run.

And no, that's not how it works. When your CAM software creates code in Absolute format, it's in absolute positions from where you zero'd it.

When g-code is in Incremental mode (G91), the position of each line of code is relative to the previous line of code.

You're post processor adds the G90 to make sure that it is run in Absolute mode.
10-02-2013, 10:04 PM
frare bear

I see. I have much more learning to do. Thanks Gerry.

So G90 is what I want. I zero it right at the origin of the part, then load G-code, and all of a sudden the display changes so it's not zero'd and the tuning gets weird.

I'll try messing with the cam software tonight, triple check units, and maybe reinstall Mach3...
10-02-2013, 11:46 PM
ger21

In Mach3, what are your native units set to?
10-03-2013, 02:58 PM
frare bear

Quote:

Originally Posted by ger21

In Mach3, what are your native units set to?

I got it all worked out yesterday, I guess the .xml changed the native units to mm. I just re-generated the g-code in mm format and was able to get a nice looking part from it! Thanks.

Wish I would have thought of that before posting... oh well, learned some things.
10-04-2013, 12:13 AM
ger21

Mach3 defaults to mm. My guess is that you set up the machine to run in inches, but left the native Units setting on mm. That will give you the results you saw.
If you want to run in inches, then set the Native Units to inches, and re enter the proper steps/unit, and accel and velocity.
10-04-2013, 01:10 AM
frare bear

The only odd thing is that I didn't install Mach3 when I built this machine, I ran it for months beforehand with all my files in inches. Doesn't matter what happened I guess.

I may switch back, but I read somewhere that doing things in mm will have slightly higher resolution.... which is probably offset by my machine anyways haha.

Thanks for the help Gerry! CNC's are amazing tools, and this is by far one of the most extensive resources of information that I've found. Gotta love forums.
08-27-2021, 03:23 AM
Grblcncuser

Quote:

Originally Posted by ger21

No, g-code doesn't affect the native units. However, make sure that your native units are set to inches, and your currently in inch mode (G20) when you jog the 6".
It's possible to set up the machine with mm Native Units, but you set the step/s unit so that it appears it moves in inches. When you load a g-code with a G20 in this situation, the distances will be incorrect.

O too have a similar situation. Haven't ran my cnc for about a week. I open everything up, load my parts, and git the start button. Machine set to run a 3 inch circle....and it tries to make a 9 inch circle and I have not changed a thing. Any ideas?