Problem- cutter nicks on retract

[chuck1024]

When I mill a pocket and retract the cutter with a G0 Z 0.1 and follow that command with a G0 X xx Y yy the cutter will nick the part. The X and Y axis are starting to move before Z is above 0. It sounds like x and y axis are starting to move when Z is slowing down. It works ok if I change the first line to G0 Z 0.3

I am running EMC2 2.2.8. Hardware is a Gecko G540 on a Prolight 1000 mill.

Any idea what is wrong?

chuck

[samco]

emc by default runs g64 -blending. If your accelleration is set pretty low - you will see the next axis start moving before the previous one stops.

You could..
G64Px.xxx where x.xxx is how far you will let the blending deviate from the programmed path.

or exact stop mode - g61.

you can read about it here.
http://wiki.linuxcnc.org/cgi-bin/emc...jectoryControl

sam

[chuck1024]

Wow, that was fast.
I do have the accel set very low. I didn't think it really mattered and was save with low values. This only happens with G0 which is traveling at 60IPM. G1 commands seem to work fine but they are usually only at 5IPM.

I will test and see if I can increase the accel a bit.

thank you very much!!!

chuck

[Geof]

This kind of thing can happen with just about any controller and another way to get around it is make the Z retract a very fast feed move.

[jmelson]

Originally Posted by chuck1024 Wow, that was fast. I do have the accel set very low. I didn't think it really mattered and was save with low values. This only happens with G0 which is traveling at 60IPM. G1 commands seem to work fine but they are usually only at 5IPM. chuck

Yes, it absolutely DOES matter. The trajectory depends on the acceleration it has been told the machine can handle. When blending a right angle move, it needs to know when the first axis to move will begine slowing down. As soon as that axis (Z in this case) begins its deceleration, then next axis to move begins. On a machine that is speced out with high acceleration, the actual distances for the decel and accel are quite short. On a machine where the acceleration is set low, these distances will be much larger. Therefore, the blend region becomes bigger.

Jon

[jmelson]

Originally Posted by Geof This kind of thing can happen with just about any controller and another way to get around it is make the Z retract a very fast feed move.

That actually doesn't fix it. One way to fix it is to make the Z axis retract farther. I'm not sure what happens of the Z retract was a G00 move, maybe G00 moves never blend with cutting moves, but I don't know.

Jon

[Big John T]

Originally Posted by jmelson That actually doesn't fix it. One way to fix it is to make the Z axis retract farther. I'm not sure what happens of the Z retract was a G00 move, maybe G00 moves never blend with cutting moves, but I don't know. Jon

The G0 moves are treated the same as G1 by G64. With low acceleration the corner would follow the programmed path closer if a slow G1 move was used to retract the Z then use a G0 to move to the next X Y location. Or as you suggest just Z up more...

John

[acondit]

Originally Posted by chuck1024 When I mill a pocket and retract the cutter with a G0 Z 0.1 and follow that command with a G0 X xx Y yy the cutter will nick the part. The X and Y axis are starting to move before Z is above 0. It sounds like x and y axis are starting to move when Z is slowing down. It works ok if I change the first line to G0 Z 0.3 I am running EMC2 2.2.8. Hardware is a Gecko G540 on a Prolight 1000 mill. Any idea what is wrong? chuck

Same thing can happen when making a cut with cutter radius compensation, when you turn off compensation, if you have not moved the cutter at least the radius of the tool away from part.

Alan