Was doing some research on new extruder designs when I found this: http://vimeo.com/87940579
This is a setup of two nuts twisting in opposite directions and feeding the filament. You could also subsequently take a Nema 14 motor with a hollow shaft , thread it, and put it on the right side instead of on the bottom. This goes back to BerryBot’s idea of using a screw to thread the filament since it has more teeth in contact with the filament at any given time than a hobbed pulley. Two nuts are needed to keep the filament from twisting.
This concept (if works) would be great for delta bots so that you could set up a direct drive extruder and it won’t interfere with the arms.
Any thoughts?
Interesting the video didn’t work but the Vimeo link does : http://vimeo.com/87940579 I had a similar thought that using an interior thread like that might make for a more reliable extruder however you do still have the problem of it breaking near the entry point. If you machined a decreasing thread you could build what would be a “filament pump” which would pressurize an extended melt region.
@Chuck_McManis I was also thinking, you could take two nema 14 motors, or even smaller motors lets say (assuming they could be customized to have a hollow shaft via special order) and stack the two motors opposite to each other. Then you have direct contact between both internal threads (no gap like the video) and maybe more torque.
@Shai_Schechter the problem with the two shafts, that the differential solves, is mis-matched rotation. At the most basic level even two stepper motors don’t turn at exactly the same speed and so between them they would be pulling and squeezing the filament as it passed from one to the other. Also you really wouldn’t need custom motors to try that out, you could do it with two geared tubes that were internally threaded. The logical end point is the filament pump where the plastic is melted and a screw pumps it out through the nozzle.
@Mark_Rehorst Have you tried constructing it out of bent steel or CNC aluminum for added rigidity?
This is another good video of using a gear worm. The fact that it’s on a merlin is another added bonus! Those hotend are usually a bit harder to extrude out of, but this video’s merlin is slightly modified.
This is much like how we used to drive filament back in the bad old days, pre 2010. Like in the darwin and early rapman extruders which used a screw thread pressed against the filament to drive the filament downwards.
This had a few distinct issues, the snakebite extruder neatly solves one of the big issues, which was that the filament would twist and kink off the spool as it was extruded. The counter-rotating nuts of snakebite solve this though.
The biggest issue though, is that this method of applying force is really quite inefficient and you can’t get the same amount of filament force per unit torque. This is because you waste large amounts of your motor input power on rotating the screw against the considerable friction being applied to the screw as is has to slide through the filament.
They also result in a very high reduction ratio. Steps per mm in this example is over 4000. In a normal geared extruder this is around 500. In a direct drive extruder we are talking around 100. This means that you are going to have big issues with speed.
The other issue is that they grind up the filament a whole bunch and cause plastic dust to end up everywhere.
Sorry to be a debbie-downer, but using an Archimedes screw type extruder has been tried at length, and found wanting.
I started with a single driver like this with my repstrap a year ago, it was a spring inside a gear and the spring was the threads that drove the filament, it had the twisting filament problem, but the idea for using a spring made the threads adjustable by stretching the spring thereby changing the driving speed. It worked enough to print some crappy parts, but I was able to hand feed and do better… Lol. I am using a basic wades driver now and am mostly happy, but have a design for dual roller driven by worm gear that keeps getting put off.