RE: [CAD_CAM_EDM_DRO] reprap build started

Thanks for the run-down! I wasn't too far off then. E axis. Yes, that
would work, especially if one worked in volume, as you suggested. Good
idea!

I didn't think about the drool. Probably many other things to consider as
well.

Alan KM6VV

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Kevin Martin
> Sent: Tuesday, October 11, 2011 1:02 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] reprap build started
>
> I'm not sure how is has settled down in existing hobby 3D printers, but
> ultimately, for a given extruder/material combination, there is a ratio of

so many

> steps for a certain volume of plastic extruded. This ratio is one of the

values that

> I would like to see as a runtime-settable extruder parameter.
> At least one of the GCode models uses an additional axis (usually the E

axis) to

> control "motion" (i.e. cumulative volume extruded) of the extruder. Again,

in

> my ideal world, this E value would have definite sensible units (such as

cubic

> millimetres if the other axes count in millimetres). From this is is easy

to

> calculate, given the geometric length of a move and the change in the E

value,

> the cross-sectional area of the plastic bead laid down, and from that and

the

> height of the nozzle above the surface, the width of the bead (within a
> reasonable range of heights). By considering the E axis as just another

axis to

> accelerate and decelerate, one gets an even bead of plastic.
>
> An older and generally inferior model was to use a special command to set

the

> extruder speed (in arbitrary and probably non-linear units), and make sure

the

> geometric motion keeps up to the correct speed. To do this you have to cap

the

> top motion speed so there is only very short accel/decel at the end of

each move

> (during which time the extruded plastic will be extra-thick because the

extruder

> does not slow down). On the other hand at least one GCode generator takes
> advantage of this by generating accelerating moves which results in the
> extruded bead undergoing gradual and continuous change in thickness. This

is

> something that cannot be done with the extra-axis model.
>
> Perhaps what *should* be done is that the extra axis should not be

cumulative

> volume extruded, but cross-sectional area (i.e. volume per unit geometric
> motion). This would allow both worlds to coexist.
>
> All this glosses over the anti-drool mechanisms (on our printer, when

extrusion

> stops, the extruder actually retracts a tad to make sure the bead is cut

off, and

> when extrusion starts again, it starts with a quick feed to cancel out the
> retraction). I believe this is currently coded in the GCode (at least for

our printer)

> but again I believe this should be in the smarts of the extruder

controller with a

> handful of appropriate runtime-settable tuning parameters. Note that just
> raising Z to cut off the bead is insufficient because in the idle nozzle

the filament

> continues to melt a bit, expanding in the process, causing some extrusion
> without any stepper motion.
> -Kevin Martin
> the Papertrail Handmade Paper & Book Arts
> New Dundee, Ontario
> 518-884-7123
> www.papertrail.ca
>
> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Alan
>
> What's the basic relationship of feeding stock to the travel in X &Y?
>
> Sounds like it would be a direct relationship. So one could calculate a
> "feed distance" for the stock, and assign it to another axis to drive the
> extruder (scaled, of course)...
>
> Seems like if the X axis needed to move 5" to lay down the filament, then

5"

> of stock (or some scale of that) would need to be extruded at the same

time.

> Just like a "coordinated move"! Has someone already thought of that?
>
> I could see a little program that took normal Gcode, and generated the
> "pseudo axis" for the coordinated move.
>
>
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