Sunday, August 23, 2009

Design for a pinch wheel extruder






I had some free time today so I designed and build a pinch wheel extruder out of stuff I had laying around. I thought I could either work for 5 hours to buy the pinch wheel extruder kit from makerbot or just build one from scratch from stuff I had (which was a good choice considering I dont have a job :)). Here are some pics, tell me what you think! The motor is a 6 wire stepper I will be running as bipolar, the metal pieces holding the motor are "U" extrusions hotglued to each other the plywood and the motor (like I said I was using things i had), the main piece of wood is of course plywood (dont remember the thickness just a piece I had next to my table), the piece coming off the stepper is a piece of a 1/4 -20 bolt, attached to the stepper with 3/8in silicone tubing and hotglue (i really like hotglue :)), the wooden wheel is also plywood, cut with a holesaw, and attached with a 1/4-20 machine screw, a nut below the wheel and above the plywood to allow the wheel to spin and the bolt to be tightly attached to the plywood. I had to grind the bold down slightly so that the wheel wasnt too far from the main piece of plywood. I dremeled it at an angle so that the nut could be sufficently big but the wheel could angle back to the plywood before it meets the 1/4-20 bold coming off the stepper. I drilled a slot in the back plywood so that the wheel can be moved twards and away from the bolt on the stepper (known as the bolt from now on, it is a piece of a 1/4-20 bolt that I knurled with a cutoff wheel on my dremel, I think i already said that but whatever). According to my calculations, the stepper should have to run at at speed of (kits drive from the motor is 10.3mm, mine is 1/4in (about 6.4 mm) which is close enough to %60 of the diameter of the kits gear size, so the kits drive has to run at %60 the speed of mine, so since the kits motor runs at 2RPM, mine has to run at 3.33RPM, so at 200 steps per revolution it has to run at 666 steps per minuite or 11.1 steps per second, which is (obviosly, and yes i know i spelled that wrong i just have no clue how to spell it right) 22.2 half steps per second) 22.2 half steps per second. I hope that my stepper will have enough torque at that speed without any gearing. Unfortunatly I dont have any ABS right now, and Makerbot is out of stock, so I will not be able to test it yet. Please comment with your thoughts on this design (if anyone is even reading this, I have 1 follower, lol, thanks Dave!)

5 comments:

  1. Hi Ping.

    Nice simple design.
    If I had a comment, it would be that there is a lot of force required to squash the teeth into the ABS, so the pulley wheel will be pushed away hard.

    I don't think a butterfly nut will hold it steady - it will be pushed away. (Think 10kg or more).

    According to NopHead's figures, the ABS needs to be pushed down with 10kg - so the shaft has to 'bite' into the ABS hard.
    See if you can reinforce the pulley shaft. you need about a 2mm gap between the shaft and the pulley wheel, so the filament is compressed.
    I was always thinking about a version where you can screw the gap smaller using a nut/bolt or something.

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  2. In terms of strength, you can test your stepper by winding string round the shaft - if it can pick up a heavy weight (including the flexible coupling) using the string then it's strong enough to drive the pinch wheel. The small diameter of the knurled shaft helps too - a big wheel reduces the force.

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  3. Yes I thought about the strength of a flexible coupler, but I decided that since the connection is at the end of the stepper and there is another inch before it gets to the plywood, the plywood and connection to the stepper would form 2 points keeping it from flexing. I am now going to test the strength...ok I'm back did you miss me :). I didnt have any string handy so I used 22 guage stranded wire. I also didnt have any weights so I just tied a loop in the wire and stuck my foot in it. I pushed until teh wire snapped (not my square knot, the wire snapped almost exactly between the rod and my foot. The knot was under my foot.). I don't know the strength of 22 guage wire off hand but I would guess it is more than 10KG. I might however have to revise my design for the wooden wheel, as it may not stand up to 10KG pushing it out....Ok nevermind I just tested it, this time with a thicker solid core wire (not sure of exact guage looked like maybe 16AWG), this wire was obviosly stronger, but it still snapped after I put over half my weight on it (I would guess about 70 pounds), and before the wooden wheel started sliding flexing or showing any signs of fialure. I have a slot for adjusting the wheel so I will just slide it in slightly to put more pressure on the abs. Thanks for the help!

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  4. One interesting thing to note as I was looking at the solid core wire that snapped, is there is about 1.5in of bare wire showing at the place it snapped. At first I thought that the insulation just snapped in a different place from the actual wire, but after checking it out I noticed that thw wire went all the way up to the break on the other side (the side with no bare wire showing), I also noticed that throughout the wire the insulation was loose and could be easily twisted around the core. My best guess is that the wire stretched while I was pulling on it, along with the insulaiton. The insulation however returned to its original size after the wire snapped, the core however did not. That is my best guess anyway, if I'm wrong feel free to correct me.

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  5. Your blog is posted on the reprap aggregation pipe. So I guess more than just Dave are actually reading it. ;)

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