I'm moving on to the Paxtruder because I really like the simplicity of the design. In a Paxtruder, the idler wheel is replaced by a block that presses against the filament from the side, keeping it in contact with the motor gear. The MK4, idler wheel was held in place by a bolt that acted as the axle, but it was tough to get it to stay in place and even more difficult to adjust. The block in the Paxtruder is adjusted with the turn of a screw and can be done while printing. The original Paxtruder was designed by Charles Pax and is in use on Makerbot's new Plastruders. I decided to make my own version of the Paxtruder because I didn't want to spend much money. My backup plan is to buy the Drop In Paxtruder for sale by Built To Spec.
The MK4 consists of 6 acrylic layers. I made new versions of layers 3, 4 and 5. I'm still working on it, but here are a few preview photos. They show layers 1-5 with the plastic covering intact. This is a total hack job, done with a hacksaw, a drill press, files and sandpaper. I bought the acrylic at home depot. Layers 3 and 5 are .093" thick. Layer 4 is .220".
You can see the open space to the right of the bearing, which has the motor and gear behind it. The block slides in the space, against the filament. The screw will come in horizontally from the right with the tip of the screw pushing against the back of the block. I plan to curve the front of the block to help guide the filament.
I need to cut some material off the bottom of the part with the blue tape, then make a block to fit in the slot. The last piece will sit in the far right of the open space and will need to be screwed in place. First I'm going to try tapping the acrylic from the edge to hold the screw. If that doesn't work, I will make one from scrap aluminum or figure out some other way to guide the screw. The real Paxtruder design uses delrin for the block, and the screw is guided by a nut held in place with screws above and below it.
Here you can see the filament & the new gear behind it.
I'm a drafter, so I normally use Autocad or Inventor for my designs, but this time I'm doing it all by hand. I traced the parts, measured and dimensioned them. See the scan from my notebook below. I feel this information is very valuable when designing something, even if you end up not using it. If I was using Autocad on this project, I would have just gone to the Plastruder MK4 thingiverse page and downloaded the source files.
I cut the parts with a hack saw. Next, I taped them to the old parts and used them as templates to drill the holes in the new parts. I drilled out the motor shaft hole to 1/2" because that's the largest bit I have, then filed it to size. I have more filing and sanding to do to make sure everything fits correctly. The hardest part will be making the T-slots that hold the nuts at the bottom of the assembly. I'm going to bust out the dremel for that.