Some notes on the way to an...

In which your narrator experiments with ways to make Nophead's GM-17/tin can stepper hack pinchwheel extruder without a lathe...

After working for a while with the Nutjob extruder, I came to the conclusion that it was too big, complicated and didn't perform well enough. I backtracked and decided to use my knock-off of Nophead's GM-17/tin can stepper hack and do a pinchwheel. I had a problem, though, in that I didn't really have a lathe that I could use to make a gripper like Nophead did.

Looking over his work I decided that I could make something like the knurled pinch wheel that he had also tried.

I took an hour off Sunday and spent time at my local hardware store seeing if there was something I could use as stock to do such a knurled concept pinch wheel. Sure enough, Ace had some nice stainless steel bolts made for hex wrenches.

I figured that I could use my little diamond cutting wheel to cut some knurling into the business end of one of these. Notice that I've gone for 3/8-16 bolts which go with the flanged, 3/8ths inch bearings that I have lying about from Tommelise 1.0 days. I'd have probably gone for an M8 bolt and skateboard bearings except they didn't have this kind of bolt in M8, this being the US, and the surf and skateboard shop where I can get M8 bearings is closed on Sunday. Pay attention! This is how critical design decisions are usually made, by me at least. :-)

I bought two of these stainless steel bolts so that I could afford some mistakes. Being Scots-Irish and the bolts costing $2.80 a pop, I wasn't excited about making a lot of mistakes. It wasn't long before I ran into design reality when I discovered that cutting knurling into what turned out to be a very hard stainless steel bolt was more than I could deal with.

Another reality I encountered was that for milling the thickest sheet stock I had was 3/8ths inch. Making a connector between the GM-17 gearbox and the bolt was going to require something a bit thicker than that. I either had to order thicker stock, which is expensive, or bolt two thicknesses together with a few #4 machine bolts. That was awkward and I had serious questions about how something like that would hold together with the torque loads that were going to be generated.

I started fooling around with some scrap that I had to see if I could solve these sorts of problems. The first thing that seemed promising was when I simply cut paths parallel to the bolt axis in the threaded part of a piece of 3/8-16 threaded rod.

Just for fun, I tried using the diamond wheel on the threads of the stainless steel bolt and discovered that I could actually cut those pretty quickly.

That cut held the filament as well as the threaded rod cut. It then occurred to me that if I took advantage of the hex pocket in the bolt I could easily make a 3/8ths inch thick connector to connect the bolt to the GM-17 gearbox.

This humble little gripper was able to handle 10+ kg before the connector between the filament and the spring scale failed.

You see only 4 kg of load here. That's all I could manage holding the scale in one hand and the camera in the other.

The moment arm on the gripper is about 4.5 mm. Combine that with a 10 kg load and you see that you are looking at a torque of...

torque = (10 kg)(1000 gm/kg) (4.5 mm)(.1 cm/mm) = 4500 gm-cm ~ 62.5 oz-in

...which is about what a low end NEMA 23 or a high end NEMA 17 can deliver.

Now, if we leave the GM-17 hack gearbox at its original 228:1 and calculate torque assuming 150 gm-cm which is the holding torque for the Jameco tin can stepper

torque = (150 gm-cm)(228) = 34200 gm-cm ~ 475 oz-in

Figuring that a 3 mm filament has a cross-sectional area of about 7.1 mm^2 and a 0.5 mm filament about 0.2 mm^2, you can extrude 35.5 mm of 0.5 mm thread for every mm of filament consumed.

Thus you'd need about 1.4 mm of filament/sec to extrude at a rate of 50 mm/sec. To do that you need to be turning the gripper at about 3 rpm. We should be able to do that pretty easily without overheating the tin can stepper.  I already know that I can get 5 rpm out of that hack at that gear ratio, so I should be okay.