Sunday, September 14, 2008


Extruder assembly - Laser Cut parts kit

I recently purchased 2 rolls of filament, one roll of HDPE and one roll of ABS in anticipation of the completion of my RepStrap using the Laser Cut parts kit. I'm hoping to be making my celebratory cup within a week or two.

I went back to my partially assembled extruder mechanism and installed the proper bearing that I purchased this past week. I then tried to manually feed a piece of filament and found it to be a little too tight. I disassembled the extruder mechanism and found that the relief that was laser cut in item #5, the top plate, had a rough finish and was causing the binding. I took a fine file to it until the piece of filament ran smoothly through the slot.

Here's a picture of the part after filing:

I also filed the slot where the drive bolt runs while I was there. I then assembled the extruder according to the procedure but I left the 2 M5 bolts in step 1 and the 5 M3 bolts in step 6 loose. I placed a ;iece of filament stock in the slot and wiggled the extruder assembly until I got the best feel of the stock sliding in the slot. After tightening the bolts I still found a spot that bound a little when the filament is beyond item #4 in the assembly and the entering the hole in item #5. I took a small round file and filed this until there was absolutely no resistance when feeding the filament all the way down to the feed screw. The size of the slots and holes for the filament could be opened up a bit in the design. I don't think there is any requirement for the feed slot to be as tight as it is. This would also help the alignment between items #4 and #5 in the assembly.

I fired up the extruder electronics and software and voila, the drive mechanism is feeding filament! I'd like to figure out a way to measure the accuracy of the feedrate. If anyone has any ideas I'd like to hear them.

Regarding the laser cut parts, there seems to be a side effect of the laser cutting process when the cut is not through the entire part but rather to a depth less than the thickness. It appears as if the acrylic becomes brittle and rough in these depth cuts. I found that all it takes is a small file and a little careful filing to clean them up. I have done this with some of the other parts that have partial depth cuts. The nut cavities in the belt clamps are difficult to get with a file so I just scraped away any axcess with a small awl. This little bit of clean up is not a big deal and as we all know is really part of any assembly process using most types of materials. I don't consider this to be a problem at all.

The last of my parts should be here next week and I will be building the heater barrel assembly next. I'll also have the rest of my parts to finish the partially completed sub-assemblies. Almost there...

I've changed the design to use sheets of 3, 4.5(or 5) and 8mm acrylic. This leaves only flat and laser-cut surfaces in contact with the filament. I've also put a little bot of a slope just past the window for the pressure bearing, which allows easier feed-in of a new piece of filament. Designs in users/vik in the Sourceforge SVN repository.

Oh yeah, that design also fits into the Darwin head-changer.

Vik :v)
When I was assembling my extruder, I found that the main screw did not rotate completely freely and there was some sort of friction somewhere. I think caused by the top screws rubbing against the big gear. Did you get this effect too?
No chance of the top screw heads rubbing on the gear here; there's an awful lot more space under the gear in the design I'm using. Take a look in the blog st the latest photo. I also had to use gears thicker than the nuts to get a good grip as I wasn't using routing to hold them onto a bolt head.

Vik :v)
For the filament path I ran a 3.2mm drill through to open it up a bit. I then used some mini files and sand paper to smooth the cut surface. I also ended up drilling where the end blots enter the main body to go to the captive nuts, although I didn't bother with smoothing those.

letsbum00 - I found this as well and added a washer between the top bearing and the spacer for the gear. The bolt still continues beyond the filament path so I don't think that will be a problem. The gears are now a bit (1.2mm) less well aligned vertically but still seem solidly meshed.
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