I printed a whale tooth I found on Thingiverse: (Whale Tooth) because I thought it looked cool and would really test the capabilities of my machine and software tools. (Which is great for working out the kinks.) I'm happy to report success! The RepRap java tool couldn't handle the STL file, so I used Skeinforge. to create the GCode. I had to customize Skeinforge a bit before it worked right. It wanted to send Z moves in the same line as an X-Y move, and that clashed with the different speeds required. It still doesn't do acceleration right, which messed up a few layers when the X stepper skipped a few steps.

The secret to my success

Here you can see my extruder's hot-end. The bottom part is a standard welding tip, which already had a 0.6mm hole in it. I drilled it out to 3mm (well, 1/8" really.) almost to the tip, so there's maybe 1-2mm left for the nozzle. Around that is wrapped the nichrome wire for heating, with a thermistor near the tip. Kapton tape surrounds it all for insulation and to keep it all in place. The welding tip goes through a fender washer and into a nut which keeps it in the washer. The washer has 3 holes drilled for the long bolts (black thread in photo) that attach it to the extruder. The white PTFE insulator sits between the washer and the extruder. I carved out a hex nut-shaped hollow in the PTFE so everything stays centered, and in a pinch I could unscrew the nut using the insulator. Leakage between the insulator and the nut is minimal. (Only noticeable when disassembled.)
The latest change to get things working is I added a bunch of Kapton tape around the washer so it stops acting like a heatsink. I found that the PLA was cooling too much, (especially when it stopped while moving between layers after the extruder was warm,) and creating a plug in the top half of the welding tip. If I pulled out the filament, the portion between the pinch wheel and in the PTFE was a little stringy (it would stretch as I pulled) but the part below that, in the area of the nut and washer, was a perfect 3mm cylinder, which terminated in the really melted part.



And here's my first success with PLA! It's a small stand for my Droid phone, which holds it in portrait position. It fits a bit tight, but that's because I haven't had a chance to calibrate the extruder settings.

Print complete!

It finished, with only a little help here and there. Success! Now on to fine-tuning. But first, some sleep.

Printing! (crosses fingers)

As I write, I'm printing layer 7 of my best print so far.. Here's hoping!
I added Kapton tape to insulate the big washer that mounts the heater to the extruder, so it doesn't cool down so much. I'm extruding at 200C, with some minor back slipping of the stepper, but it's extruding pretty well.

or so I thought

Uhhg.
I can't get consistent feed from PLA. There were a few times it would feed perfectly for a few minutes, but now I can't even duplicate that much. Eventually, it gets jammed up in the extruder barrel. Frustrating. I can get it to squirt a foot or two of extruded plastic before it starts jamming. Then I have to either push it out with extreme pressure, or drill it out. If I'm really careful, and a bit lucky, I can pull a plug out and then squeeze out the remainder with a metal rod. I even re-built the hot end twice, with no improvement. :-(

A few months off

After a few months off this project, spending my time doing work and summer stuff, I'm back to my RepStrap project. I ended up ordering a custom extruder part from Shapeways that cut a big headache out of my design. It's a bit of a cheat, perhaps, but it solved my chicken and egg problem.
I just ran the first few layers of PLA, and it's looking good. still have some feed issues I'll have to work on, but there is no warp at all. I should be able to make good progress, and perhaps some full prints, this week.

I've tried a few prints in the last month, but all the large ones warped. I now have some PLA to fix the warping, but haven't tried it yet. Also, my extruder doesn't push the filiment the way I'd like, and I have to help it along by hand. Here's the first half of a new geared extruder which should fix that.

My new heater construction

Now that it works, I should mention the changes to my heater, compared to the RepRap standard design. (See RepRap.org for the standard.)

Instead of the brass bolt for a barrel, I use a welding tip. It already has a 0.6mm hole at the tip, and it's easy to drill a 3mm or 1/8" hole at the top in a drill press. The smooth outer surface makes for good heat transfer from the nichrome wire.

I had several kinds of trouble threading it into the PTFE thermal barrier. First, I used a tap set to thread the PTFE, which created a good thread, but it expanded the hole above the brass, so that the melted plastic would expand too much, and be too wide to pull out, and too thick to push in. For my second try, I cut slots across the threads of a throwaway tip, and used that like a tap. It worked, but the threads were lower quality. The motor pushed the tip out of the PTFE threads.

My third try worked. I use a nut and fender washer to secure the tip. Then I hollowed out the PTFE in the shape of the nut, to keep it centered on the hole. Drill 3 small holes in the fender washer for 1/8" bolts that connect to the acrylic extruder frame.

The tip can be easily swapped for another, such as when I had to replace the tape with Kapton.

A bit of success!

Printed my first object! Still have some fine tuning to do, but it prints. :) It was just a simple cube, but it came out nice. I did have to baby-sit the machine, though.

Progress

I raised the temperature to 240C, and promptly fried the insulator tape I was using. ("Rescue Tape" is not good above about 220C) I had initially tried "Rescue Tape," a silicon-based tape I found at a local store to keep the nichrome heater from shorting to anything, and it worked well at 200C. But at 240C, it would try to shrink too much, and ended up splitting down the side, exposing the nichrome. So I gave up on that, and ordered some Kapton online, like I should have done initially.

So, a week later it arrives, and I rebuild the heater. Now it goes all the way to 240C, and the ABS starts flowing like honey in a squeeze bottle. Now it extrudes quite well, in fact, faster than the X axis can move. So I just need to fine-tune the variables, and start printing...

always test your wiring

So I fix the heater barrel, which involved re-doing the heater element. I test resistance across the heater element, (8 ohms) and across the thermistor. Then I assemble everything and hook it to the controller. When I add power, the heater LED lights up even when the heater is off! :(
Turns out I have a short between a heater wire and the thermisor wire. Didn't test for that. Doh!

Uh Oh

Well, it grips now. I pushed a bit of rubber between the bearing's post and the acrylic frame, and loosened the nut. Now it grips quite well, and doesn't get stuck on the teeth, like it did when too tight. Only problem is, it pushed the extruder heater barrel right out of the PTFE insulator. So.. on to the next problem.

Getting closer

I finally got the extruder to grip the filament. I Dremeled some ratchet steps into the stepper's shaft, and now it will extrude thread out of the nozzle at about 25mm/minute. It can do 40mm/min, but it slips a lot then.
It extrudes, sort of. I can get it to go for a few minutes, but then it stops. I tried making the test part, but when the head stays in one place too long, it starts slipping, and then won't extrude again until I push the filament down for a bit. I'll have to play with the Reprap software settings. It's also slow as molasses. It didn't even finish a layer when I let it run for 4 hours.

The almost complete RepStrap frame.

Rear view of the X carriage. When the thing starts working, perhaps I'll replace all that wood with reprap'ed parts. The blue lines conect to the opto endstops.

Here is the extruder mounted to the carriage. It's a tight fit, by design, but that may haunt me later on.

Assembled extruder. This is my first extruder design. It's pretty much the minimum size for a pinch wheel extruder. This may work against me.


Without the motor, you can see the 3 layers of acrylic. The holes at the top of this picture mount the unit to the X-axis carriage. The layer closest to the motor required a larger hole centered on the motor to allow for the raised center of the motor. The other layers have just a small hole for the shaft.

The bearing is bolted to an aluminum plate, which, with the motor, sandwiches the acrylic. There is a guide hole for the filament that leads between the bearing and the motor shaft.

My RepStrap almost works...

My RepStrap almost works...
I have some pictures at my other site. I'll start posting here to share my progress.