Yet Another Limit Switch Discussion

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joebananas10
Posts: 35
Joined: Sat Jul 04, 2015 6:19 am

Yet Another Limit Switch Discussion

Post by joebananas10 » Wed Dec 02, 2015 8:11 am

I built my own set of limit switches this past weekend for my SO3 and have a few questions.

First, when I issue a home cycle command the machine will go to +X+Y+Z (right back top) position but it never checks the limit for -X-Y (front left). Which means the machine will still crash into the -X-Y limits if i'm not careful. It doesn't seem to me that it should work that way. Shouldn't the machine know the limits of itself in both - and + positions for both axes? Any thoughts.

Next issue. When I try to test probe cycle, G38.1, I keep getting a invalid command error. I can run a G38.2 with Z limit and Feed Rate but shouldn't G38.1 work without any parameters?

Next, my $20, $21, and $22 changes from 0 to 1 don't seem to stick after power cycle. I have to apply them every time I power up the machine. Shouldn't they stick?

Opps. I almost forgot. What's the point of the Feed Hold switch on the control board if there's no Cycle Start switch? I thought those two worked together. Additionally, on SO3 what would be the difference between Feed Hold and EStop since there's no spindle control?

CastIrony
Posts: 650
Joined: Mon Jul 21, 2014 10:21 pm
Location: Portland, OR

Re: Yet Another Limit Switch Discussion

Post by CastIrony » Wed Dec 02, 2015 8:52 am

First, when I issue a home cycle command the machine will go to +X+Y+Z (right back top) position but it never checks the limit for -X-Y (front left). Which means the machine will still crash into the -X-Y limits if i'm not careful. It doesn't seem to me that it should work that way. Shouldn't the machine know the limits of itself in both - and + positions for both axes? Any thoughts.
The purpose of a homing cycle is to get the spindle to a known position, not to probe all limits of the workspace.

The dimensions of the workspace should be constant, unless your switches or switch plates move. Grbl has a 'soft limit' feature where you can enter in those constant dimensions, and it'll prevent the spindle carriage from moving past those limits.

Alternately you can still use your -X and -Y switches as hard limits. I don't know if anyone's tried crashing the machine at full speed with hard limit switches, it might be quick enough to stop the motor before any damage occurs. I'll admit to crashing a few times without switches accidentally though without any apparent damage.

Next issue. When I try to test probe cycle, G38.1, I keep getting a invalid command error. I can run a G38.2 with Z limit and Feed Rate but shouldn't G38.1 work without any parameters?
I don't know what G38.1 is supposed to do, but it's not on the List of Supported G-Codes in Grbl and it's not even in the LinuxCNC docs. I believe the rest of the G38.x commands work fine for probing.

Next, my $20, $21, and $22 changes from 0 to 1 don't seem to stick after power cycle. I have to apply them every time I power up the machine. Shouldn't they stick?
They should stick. What software are you using to drive grbl? I know Easel's had issues with wiping people's grbl config settings.

Opps. I almost forgot. What's the point of the Feed Hold switch on the control board if there's no Cycle Start switch? I thought those two worked together.
You can still enable Cycle Start in software by sending a '~' over serial.

Additionally, on SO3 what would be the difference between Feed Hold and EStop since there's no spindle control?
Feed hold is basically just a pause. It will maintain position information so the machine knows where it is after the Cycle Start command is sent.

E-Stop is an emergency alarm condition. The spindle position cannot be assumed to be where it was supposed to be when the E-Stop triggered, so a full reset and re-homing will be necessary to continue.
Shapeoko 3 #192 - Complete! Needs limit switches.

joebananas10
Posts: 35
Joined: Sat Jul 04, 2015 6:19 am

Re: Yet Another Limit Switch Discussion

Post by joebananas10 » Wed Dec 02, 2015 9:15 am

Thanks for the response.
Grbl has a 'soft limit' feature where you can enter in those constant dimensions, and it'll prevent the spindle carriage from moving past those limits.
Are those values stored in $130 to $132?
I don't know what G38.1 is supposed to do, but it's not on the List of Supported G-Codes in Grbl and it's not even in the LinuxCNC docs.
It's listed in the Shapeoko Wiki. http://www.shapeoko.com/wiki/index.php/ ... on_.28G.29
They should stick. What software are you using to drive grbl?
I just started testing GRBL Panel. I don't have much experience with the program yet. My problem could be because I don't completely understand the program yet.
You can still enable Cycle Start in software by sending a '~' over serial.
Too bad the control board couldn't be switched to allow an on/off switch for Feed Hold. That way open could be Run, then close could be Hold, and open again could be resume. Oh well.

Thanks again for the response.

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