Wednesday, August 06, 2008
I've been following RepRap for a while. Like Brendan and NopHead, I am a cnc hobbiest, and last year converted my 3-axis mill to cnc. I started following reprap with the idea of adding an extruder head to my mill. I'm basically following the "EmcRepStrap" path.
I've made enough progress on my extruder, though, that I thought I'd share my experiences with the group in case someone else finds them useful.
Starting with the RepRap extruder, I wanted to make a few modifications/goal adjustments:
- I'd like to run reliably at high temperatures
- I'd like to use an off the shelf heater
- I would like to be able to "grow" into high accuracy and higher feedrate
Last week I finally finished and tested my first lashup-- it fed about 3ft of filament quite nicely so i've started configuring my EMC machine so i can start making some profiles. I'm very excited.
A couple of extra pics. The first shows the extruder block and barrel. After experimentation i found that a shorter heater would be ideal-- a 3" barrel was just too long.
This one shows the v-groove rings and skate bearings. This concept worked very well. Though you have to have a lathe, the rings are easy to make, and eliminate springs and such. ( It does take a bit to get the adjusting right though )
- Heat is a 1/4" diameter, 2" long cartridge heater
- Feed is via a threaded rod, like the normal reprap extruder
- I used skate bearings with custom v-groove rings added to minimize friction while feeding
- The bearings are mounted on eccentric drilled posts to allow adjustment to pinch the filament the correct amount.
- Temperature measurements are via K-type tcouple in a ring around the tip
- Normal acorn-nut nozzles for now
- Extruder barrell is three parts-- brass heated section ( inside the alum. block), aluminum connector with heat sinks, and stainless steel exit. This design seems to keep the filament from melting at entry
- An aluminum block transfers heat from the cartridge to the barell
- For the lashup, the hot part of the extruder is attached to a piece of cement backer board from Lowes
- Gearmotor with AS 5040-based encoder for closed loop control
- PIC 18F4331 with two PID loops, one for heat control, one for motor position control.
- Gains are set via serial interface
- MOC 3031 Triac controls the heater. duty cycle is based on cycle control. The triac is a zero-crossing one so i dont have as much noise.
- The PIC also has step/direction inputs and safety interlocks.
My plan is to treat the extruder as a rotary axis in EMC, and to control the extruder with a python-based HAL module and pyVCP Axis plugin, what i'm working on now.
The photoset is here:
Forrest has pointed out already that the gap between the last roller and the filament entry point is a bit long, which is true-- I'll fix that but for now if it works dont mess with it , right?
I've noticed a few good things and a few bad things about this design so far:
- [GOOD]The v-groove bearings work very nice-- there is very little friction wasted in feeding the filament
- [GOOD] The heater is very reliable and the PID control results in rock-solid temperature, even when feeding etc. Since its a "line current" heater it heats up very quickly.
- [GOOD] the design can operate to very high temps
- [GOOD] the barrel can be easily replaced with other materials/lengths without messing with the heater
- [BAD] for some reason with this design the t/c temp and the nozzle temp seem to be quite different, though perhaps my DMM probe is just wrong.
- [BAD] its pretty ugly
- [BAD] it requires more parts for which you need a lathe to make
- [BAD] adjusting the feed rollers takes some patience-- harder to adjust than spring loaded.
Labels: repstrap bluedirt
In order for this to work reliably, you should probably route the thermocouple wires (no extensions other than with the same TC wire!) to a junction block located in some place where the temperature is constant (i.e. room temp).
Good luck, and keep up the nice work.
i love seeing experimental designs like this.
do you happen to be using the RRRF boards? we make handy PCBs for both the AS5040 chip as well as the AD595 thermocouple interface chip.
anyway, keep up the good work.
zach: i'm using a board of my own design for the controller-- i'm using a bunch o peripherals that are not standard. I probably could use the rrrf AS5040 board but to be honest i kind of enjoy milling my own boards too ;)
I assumed that you were using something like that, but it sure looks in the images like you are changing wire types in mid-air, close to the nut. That would screw up any measurements! ;)
I understand now, good point. I'll adjust that and see if that fixes.
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