Absolute positioning

(Stephen Gullage) #1

Is there a way to have CM report to me the absolute position of the spindle instead of the relative position from zero? I have a jig set up to hold a piece of material on the bed and intended to finish that job but now I need to cut something elsewhere on the table. Can I send the spindle to zero position, find out the absolute position, then run a job with a different zero, and send it back to the original location after so I can re-zero on my jig?

Edit: doing some research, there’s G28/G30 predefined positions in Grbl 1.1, are they in use by CM and if not can I use them to store my jig location?

(Craig) #2

Sounds like you are looking for WCS - Work Coordinate System which has five additional machine work coordinates in addition to G54 (G55-G59). I don’t believe CM supports anything other than G54.

I use Intelli-G-Code by Software Redefined, the WCS system is extremely easy to use. Next I would recommend Chilipeppr for WCS. BCNC also supports WCS.

(William Adams) #3

G54 is used by Carbide Motion — I believe you can use the others, but they have to be sent using the MDI, and possible prefixed w/ a slash.

FWIW, I just use the various rapid position points as zeros for fixtures.

(Stephen Gullage) #4

Unfortunately, my first project is too big to use any of the rapid position points, if I’d thought of it before starting lol. My other project that I need to do is 25"x22 and will fit within the jig. Would love to see a “save custom rapid position” set of buttons in the jog screen.

If I’m reading the Grbl wiki correctly, I should be able to send my spindle to the current zero, then go to the MDI screen type “$G28.1” to store the position. Then I can re-zero on my new job location, run the program there, then go back to the MDI and use “$G28” to send the spindle back to the saved position to be re-zero’d on my jig location.

All of this is dependent on CM not using G28 for anything. I came across an article that says the X-Carve uses G28 as the safe return point. Does CM use G28 to save the location the spindle goes to (NW Corner) after a job is complete? If it does, what about G30? The C3D supported gcode page says G28 is “Go to Pre-Defined Position” but doesn’t indicate if that is already set and if so, where?

Can anybody confirm any of this information? I don’t want to screw anything up in the settings, but lets just say I do… how do I reset it back to the defaults?

(William Adams) #5

To reset back to defaults, just resend the settings per:

(Neil Ferreri) #6

Can you send $# and report the values? That will show your current G28 and G30 offsets.

(Stephen Gullage) #7

I tried a bunch of commands, but CM had issues with them, and I couldn’t get any kind of report to come up… so I went ahead and experimented, knowing I could screw the pooch on my current project.

Typing /G28 caused no movement in the spindle, which I reasoned meant that it didn’t have any variables in it at all. Typing /G28.1 set the location wherever the spindle was currently sitting. Typing /G28 sent it back there from anywhere, even after re-zeroing the spindle. The only weird thing was if I tried to jog it with CM after sending it to the saved location, CM sent the spindle right back to wherever it started from when I gave it the G28 command. Presumably because the jog screen is using absolute commands to move the spindle. The good news is that I now have my jig position saved and can send it back to that spot, re-zero X/Y and then adjust Z accordingly. Totally stoked!

(mikep) #8

You’re kind of off in “more advanced than CM was designed for” and needing to work around a bunch of stuff. This is a lot easier in the other senders.

(Dan Nelson) #9

I think I asked this question a while ago as a feature request and found you can actually click on the position text in CM and it will display spindle position from home. Hope I’m understanding the question correctly, if not disregard.

Edit:not at my machine at the moment to take a screenshot.


(Stephen Gullage) #10

I agree, maybe someday I’ll look at some of the senders people have recommended. I really like CM for it’s simplicity though, which is also a negative when you’re trying to go beyond it’s capabilities. Usually I work on one project at a time so it’s not an issue. My cousin is getting married and I’ve been tasked with decorations and gifts for 8 bridal party members… and the wedding is in two weeks and she didn’t ask me until a short while ago. It’s my own fault, I can’t say no to my baby cousin (and she knows it!)


On a nomad using CM (or, I should guess, shapoko with homing), home the machine then move to the home point. If you want to recover the current coord system, note the values so you can reset them.

Zero the coords.

You are now reading absolute coords. Go where you need and zero.

To recover to old system, go to home position and enter the values you noted before.

Yes, a little clunky, but not really much moreso than on most machines.

(Neil Ferreri) #12

I think you’ll find others very simple, more intuitive, and less restrictive.

(Kelly Taylor) #13

Add a cavity the same size as your bit with the bottom at the same elevation as your top of product in your jig for a virtual zero spot. Manually move your spindle to position your bit in the cavity and then zero your positions.

(Stephen Gullage) #14

That’s an excellent idea! Keeping that one for later projects.

(Dan Nelson) #15

I just confirmed my previous thought.
On Jog/Position screen click the words “Work Position” and it will switch to “Machine Position”, click again to switch back.


(Stephen Gullage) #16

That’s a nice feature… wish I’d known about it lol