Fusion360, LinuxCNC, not fully retracting

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23 Jun 2016 22:05 #76520 by kentavv
Hi, I'm using Fusion360 Adaptive Cleaning to create mill CAM code for LinuxCNC 2.7.4. I select the "linuxcnc.cps (Generic LinuxCNC (EMC2))" post configuration. I'm encountering a problem that retracts along Z do not retract before movement along X-Y begins, and the part gets clipped by the cutter.

I've attached a couple of pictures showing that the tool gets above the work piece, but clips the part along the way. Also, a screenshot of the section of g-code in the Fusion360 simulator and in NCPlot.

The section is

N25550 G1 Y-0.9875
...
N25630 G0 Z-0.1821
N25635 X-0.6927 Y-1.4083
N25640 Z-0.74
...

With this code, should the tool rapid to Z-0.1821 and only after reaching Z-0.1821, rapid to X-0.6927 Y-1.4083? I probably have something not set right. I've not been able to find a similar question online but I may be searching with incorrect terms. This is problem a common problem others have encountered and someone will know immediately what's wrong.

If needed, I can post a short video of the tool clipping the part. Slowing the clip down, the tool appears to be decelerating, but not fully stopped before moving along X-Y.

In case it helps, here are the first few lines of the g-code.

%
(2001)
(T4 D=1. CR=0.03 - ZMIN=-0.94 - BULLNOSE END MILL)
N10 G90 G94 G17 G91.1
N15 G20
N20 G53 G0 Z0.
(ADAPTIVE1)
N25 M9
N30 T4 M6
N35 S2500 M3
N40 G54
N45 M8
N55 G0 X0.0237 Y-3.4998
N60 G43 Z0.5906 H4
N65 G0 Z0.15
N70 G1 Z0.05 F10.
N75 X0.0238 Y-3.4995 Z0.0422

Thank you, Kent
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23 Jun 2016 23:50 #76523 by andypugh
Fusion knows nothing about the limitations of your machine and/or LinuxCNC.

LinuxCNC (unless told not to) will tend to cut corners in situations where the max velocity is relatively high and the acceleration is relatively low. You can adjust this to suit your needs with G61, G61.1 and G64 linuxcnc.org/docs/2.7/html/gcode/g-code.html#gcode:g61-g61.1
But you might want to take a look at your acceleration settings in the INI. Were they guesses or based on data? Accelleration and velocity should both be as high as possible for the machine for best performance.

The simple fix might be to just raise the retract heights in Fusion.
The following user(s) said Thank You: kentavv

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24 Jun 2016 04:04 #76534 by kentavv
Thank you Andy. I'm sorry if mine was a frequent question. Shortly after posting my question, I found G61, and that stopped the behavior, though perhaps not ideally. My mill is simply a small desktop conversion, that I run conservatively slow. I'll follow your advice and look into the acceleration settings, trajectory planning, and the rest. While this all makes sense, it also shows there is so much to learn, and that most of what I don't know, I don't know. I used Mach3 in the past. Making use of the sophistication of LinuxCNC is rewarding, and while it can be challenging, so far it's not been frustrating :)

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30 Jun 2016 09:55 #76824 by kentavv
The acceleration could be increased a good deal, though the "sudden" start-stops when rapidly jogging seemed abusive, so ACCELERATION values were increased only to 60in/s^2.

Is there is a difference between DEFAULT_ACCELERATION and MAX_ACCELERATION for a simple servo three-axis mill? Could DEFAULT_ACCELERATION be removed?

When using G61 and G61.1, the sounds of start-stop are noticeable. Movements are quieter with G64 P.001 Q.001 (G64 P.02 Q.02 if in millimeters.) Is this particular G64 call a reasonable alternative to G61?

Thank you

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30 Jun 2016 09:59 #76826 by andypugh

kentavv wrote: The acceleration could be increased a good deal, though the "sudden" start-stops when rapidly jogging seemed abusive, so ACCELERATION values were increased only to 60in/s^2.


Wheel-jogging sends a sequence of step-changes to position an is often unpleasant. There is a HAL component to address this:
linuxcnc.org/docs/2.7/html/man/man9/ilowpass.9.html

When using G61 and G61.1, the sounds of start-stop are noticeable. Movements are quieter with G64 P.001 Q.001 (G64 P.02 Q.02 if in millimeters.) Is this particular G64 call a reasonable alternative to G61?


Anything that works on your machine and gives you the result you want is reasonable.

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