Monday, March 30, 2009

 

Three heater barrels and a new drive

I made some extruder barrels over the weekend:
(click on the links for more pictures, design and test results)

From renoir1_extruder_quick_v0.01

quick and dirty extruder

From renoir2_extruder_v0.01

Version 2 - Welding tip

From renoir3_extruder

version 3 - wider feed channel (3.5mm through PEEK) for the filament



And I mucked around with a minimalist drive (Keep it Simple)

From Renoir1_extruder_drive

Simple - shove a screwdriver in a hole
This worked OK - as long as it was driven at a slower speed than the screwdriver default (2.4v instead of 3.6v)
I also had problems driving the screwdriver motor from the RepRap electronics

From Renoir2_extruder_drive

Version 2
used the BfB motor. Plenty of torque, but I can't drive it slow enough!
Below about 50/255 the motor stalls: above, the speed and torque is good but it stalls the filament once it hits the end and chews up the filament.
It works well (filament speed is aprox 5mm/sec at the 3mm diameter end) and as much drive as the filament can stand. The star screwdriver bits work well and seem to drive well. Perhaps I'll have to get a stepper motor and driver circuit for the extruder to get the speed down?

I'm going to try and reconnect the original BfB screw thread drive to my version3 heater barrel, and see if that drive will screw down at a slower rate.
*embarassed* I can't exactly use the original BfB heater barrel anymore- I was drilling some out stuck filament when the drill snatched and ripped off the ends of the nichrome wire and threw the thermistor circuit across the garage. I still haven't found it...

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Comments:
Why not drive the filament from yet another stepper motor?
 
That would be ideal. however, I'll have to get one and a control circuit first. at the moment, I'm working with what i've got.
 
With one of Zach's magnetic encoders, I was able to run a GM3 as low as 1 rpm. I usually extrude at 1 mm of 3mm diameter filament per second. I hope to speed that up with a new extruder soon, but for now, shoot for 1 mm/s - that corresponds to about 16 mm/s head speed with 0.5 mm.

If you don't want to mess around with optical encoding right away (but you're going to need closed loop at some point to get your accuracy down), try pulsing the PWM drive in software; ie, figure out the minimum PWM to get the GM3 to turn, then give it a quick pulse of that PWM, then 0% PWM. The GM3 will pulse along like a clock mechanism, but the give in the extruder smoothes it out. Someone on the forums was experimenting with this; Geert I think, but my encoder controlled extruder ends up doing the same thing when I set the speed really low.
 
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