Adding a 4th axis for extruder motor of a reprap

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28 Sep 2011 08:52 #13487 by fma
Hi,

I need a 4th axis to control the extruder motor of my reprap. This is a stepper motor, like the 3 others.

I tried to use the U axis, and it works, except that the GUI is now garbage, as this 4th axis also moves the head, which is not the case in the real word.

Any suggestion to solve this problem?

Thanks.

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28 Sep 2011 12:12 #13496 by andypugh
fma wrote:

I tried to use the U axis, and it works, except that the GUI is now garbage, as this 4th axis also moves the head, which is not the case in the real word.
Any suggestion to solve this problem?.


In the INI file, put the U axis in COORDINATES but not in GEOMETRY.

That should sort it out.

You might also want to make the U stepgen velocity-mode rather than position-mode, if you haven't already.

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28 Sep 2011 12:24 #13499 by fma
Cool! Really powerfull :o)

Why would you turn it to velocity-mode? I guess if I do so, I won't be able to position this axis at a given position, am I right? But I prefer to setup skeinforge so it gives the length of the extruded material, instead of the extruder speed...

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28 Sep 2011 12:47 #13501 by andypugh
fma wrote:

Why would you turn it to velocity-mode? I guess if I do so, I won't be able to position this axis at a given position, am I right? But I prefer to setup skeinforge so it gives the length of the extruded material, instead of the extruder speed...


I rather suspect that skeinforge is set up that way to cope with systems that don't handle velocity-mode stepgens.
In that case, there is no real advantage in doing as I suggested, though there must be a danger of the numbers getting very big eventually.

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28 Sep 2011 12:55 #13503 by fma
In fact, Skeinforge defaut is to generate speed,. I guess this is because old extruders used DC motors (with no feedback!). Now, they all use stepper motors, to be more accurate.

And iIn position mode, Skeinforge resets the current position of the extruder at each new layer. I was too afraid, but doing so, length never exceeds a few cm. giving -1000/1000 as limits is enough!

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28 Sep 2011 20:28 #13528 by fma
Mmmm, you were right: there is a problem with U limit...

The skeinforge code uses G92 U0, but this only shifts the coordinates; it does not really reset the value. And the limits remains the same...

Is there a way to fix that?

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28 Sep 2011 20:58 #13529 by andypugh
fma wrote:

The skeinforge code uses G92 U0, but this only shifts the coordinates; it does not really reset the value. And the limits remains the same...
Is there a way to fix that?


You could make the limits wider. I think the max is 10 ^ 308, which is more than enough length to convert the whole universe to filament and re-extrude.

Alternatively, look at G10.

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28 Sep 2011 21:05 #13530 by fma
Thank you. I will first have a look at G10 code...

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04 Nov 2011 10:35 #14566 by fma
Is it possible to configure EMC to use absolute coordinates on XYZ axis, and relative coordinates on U axis?

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04 Nov 2011 10:42 #14568 by BigJohnT
I don't think that is possible.

John

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