Understanding G1 command speed with XYZU axis

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03 Nov 2011 08:55 #14536 by fma
Hello,

I'm using EMC2 to drive a reprap. A reprap has 3 standard XYZ axis, and an extruder axis to push the filament in the hotend. In my setup, I use U for the extruder axis.

Recent skeinforge (the program which slices the object to extrude) versions now generate the length of filament going into the hotend, rather than setting the speed of the extruder axis. There are a lot of parameters in Skeinforge, but this way of doing has simplified how to configure it. Now, feedrate (XYZ speed) and flowrate (extruder speed) are set equals.

What I need to understand, is how EMC2 drives the tool when using U(VW) axis...

When moving XYZ axis, I guess that the F param. is the resulting XYZ vector speed (which means that individual axis has different speeds).

But what happens when adding U(VW) axis? Is the speed of U(VW) computed indepently of XYZ? Or are they all linked together?

Thanks for your help.

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03 Nov 2011 12:58 #14543 by andypugh
fma wrote:

When moving XYZ axis, I guess that the F param. is the resulting XYZ vector speed (which means that individual axis has different speeds).
But what happens when adding U(VW) axis? Is the speed of U(VW) computed indepently of XYZ? Or are they all linked together?


ABC, UVW are not included in the vector calculations. They simply move at such a speed that they each reach their end-points at the same time as XYZ.

Basically, the path length in XYZ is calculated using Pythagoras. This distance is divided by the feed rate to get a motion-time and then each of the 9 axes moves at the correct speed to all arrive at their end points after that time.

In "inverse time" feed mode you can set the motion time explicitly.

It would be nice to consider all the motions, but is pretty much impossible without telling EMC2 an awful lot about how the axes are configured. An XY / UV hotwire cutter is completely different to a XYZ W mill with knee and quill. Rotaries really add to the confusion.

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03 Nov 2011 13:14 #14546 by fma
Ok, I see. This behaviour is fine.

Thanks!

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