madscifi Wrote:
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> This last week I finished my EmcRepStrap build and
> have successfully printed a variety of objects. It
> consists of a geared stepper driven extruder on a
> CNC'd Taig and it is controlled using the approach
> discussed by David Carr (the stepper motor in the
> extruder is directly controlled by EMC2).
>
> I'm able to get what I think are reasonable
> prints, but only if I restrict the maximum
> velocity to around 15 inches/minute (6.35 mm/s).
> Otherwise, it appears that the
> acceleration/deceleration of the extruder causes
> the filament to stretch, leaving thick walls at
> the corners of an object and thin walls in
> between. As I understand it, EMC2 should modify
> the velocity of the A axis (speed of the extruder
> filament) in proportion to the velocity in XYZ,
> which in theory should result in the same volume
> of filament per unit distance, or so I would
> think.
>
> I've attached a picture that demonstrates the
> problem. It is a print of Thingiverse 198. Notice
> how the flat surfaces appear sunken, with the
> edges taking on a skeletal appearance.
>
> I am wrong to expect the filament width to
> correctly adjust as the velocity of the tool
> changes? Has anyone else seen this occur (with or
> without EMC)? Does this indicate some error in my
> configuration, or that my extruder is slipping, or
> it is to be expected for some other reason?
>
> -- Jim
>
Jim, congrats on getting your EMC setup working!
I've recently finished configuring EMC for my RepStrap and am pretty happy with the results. You are correct that if you have configured EMC properly, you will have 4-axis coordinated motion, and the extruder will accelerate and decelerate with the toolhead motion.
I think that the effects you are describing are a symptom of an oozing extruder. I've done some simple tests with my machine that demonstrate that precise control of the extruder flow is basically impossible with our current extruder design. So at the corners of your object, the toolhead is decelerating, and the filament drive is decelerating, but the plastic continues to flow unabated for a period of time leading to the heavy extrusion. As you've seen, when you slow the machine down, you are operating closer to a constant speed condition, which produces the best results since it is unaffected by ooze.
-Tim
