Archive for July 14th, 2011

Thing-O-Matic: Broken G91 Relative Motion

It would be handy, while doing the fast / coarse home stuff, to switch to G91 relative positioning mode and back off the switches by 2 mm by using a simple G0 X2 Y2 Z-2 that doesn’t depend on knowing the exact coordinates of the endpoint, but it seems relative positioning doesn’t work for any but the most trivial cases.

After some fiddling, this short routine produces a very fast, very long, fully coordinated XY move to some position in the +X +Y direction at the G1 X2 F100 command after the G91 command sets relative motions; it should move 2 mm away from the X switch. When the machine arrives at the new (unexpected) position, it then does the expected slow 2 mm Y and Z moves:

G21 (set units to mm)
G90 (set positioning to absolute)
(- coarse home axes)
G162 Z F1000 (home Z to get nozzle out of danger zone)
G161 Y F4000 (retract Y to get X out of front opening)
G161 X F4000 (now safe to home X)
(- back off switches)
G91
G1 X2 F100
G1 Y2 F100
G1 Z-2 F100
G90

I gave up and used an absolute move with hardcoded XYZ coordinates that should be pretty close to the stored values.

There’s a RepG ticket for that.

Obviously, I’m going where no other Thing-O-Matic operator has gone before. I do that a lot, don’t I?

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