Thursday, January 11, 2007
Cutting some corners on the Mk II
My first notion was just to rip Jim's idea, viz, replicating the Mk II parts in wood, directly. After thinking about it for a while I wondered if all the design ideas that Adrian put into the Mk II were really essential or if there would be some ways to simplify the design. While the Mk II has many fine qualities it's not particularly simple for a retard like me with two hands full of thumbs to replicate.
When my lovely 3 mm cobalt drill bits arrived today I took off a little while and did some serious experimentation. Going back to Vik Olivier's original idea, viz,
I wondered about several of its features. The most noticeable is that it has only a very short run of threads doing the polymer pumping. Dr. Bowyer's design, on the other hand, has about 40 mm of threads that the filament has to get past. Was that really necessary?
As well, Vik and Adrian both seem to have the idea that you need a pressure fit between the two halves of the pump. Why? I could understand it if you were expecting wide variations in the diameter of the filament. We solved that problem some months ago, however.
Here you can see my dumb little experiment. I took two scraps of 3/4 inch poplar. I drilled slightly overlapping 3 mm and 1/4 inch holes in one of them. The 3 mm hole is for the filament and the 1/4 hole is for the threaded drive. The second piece of poplar scrap covers part of the 1/4 inch hole so that the threaded rod actually pumps the 3 mm filament instead of just crawling out of the other side of the board.
Seating the 1/4 inch threaded rod and threading in the 3 mm polypropylene I rotated the threaded rod with a pair of pliers.
As you can see in this closeup the threaded rod bit into the filament and pumped it right through the poplar board.
Keep in mind that I am applying torque to the rod from the bottom of the metaphorical polymer pump instead of the top, so the second scrap of poplar absorbs the thrust at the top of the pump. I will need to apply torque to the 1/4 threaded rod at the top so I will have to adopt a scheme somewhat like Vik did in his drawing where he used a lock nut seated against a thrust collar at the top of his pump. Doing that there should be no need for any machining of the bottom of the threaded rod. It can simply seat against the 1/8 inch brass or aluminum plate that I will have between the poplar pump and the PTFE thermal barrier.
There you have it. No springs holding two halves of the pump together. No machining of the threaded rod. I might use a file to make some flats on the rod where it meets the gearmotor. I'll probably use some fish tank tubing to connect the gearmotor and the threaded rod, though I might simply cook up one of those coupling nut connectors that I documented some time ago.
It might not work, but it won't hurt to give it a try.
Actually though, you're right. Long before wear gets to be a problem the fricton of the threads contacting that threaded surface demands too much torque out of the gearmotor. I'm have to have a steel or brass plate at the bottom of the pump between the pump ant the PTFE thermal barrier. I'm going to tap that as a bushing for the threaded rod at the bottom so that the contact is taken up only there rather than across the whole wood housing. That will reduce both friction and the wear that you rightly worry about.
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