Two-piece circular knitting machine for six needles

This model is my first time playing around with 3D printing. Did I go a little overboard? Yes. But consider that before I had a project to go overboard on, my 3D printer (which was gifted to me) spent at least a year sitting in its box, exactly the way it came in the mail. But anyway, expect some newbie wonkiness.

This is a relatively simple knitting machine that just has two parts (aside from the needles, which you have to buy). It is symmetrical, so you can use it for either clockwise or counter-clockwise knitting. It is designed to fit replacement needles for the LK150/LK360 knitting machines (look for them on eBay, amazon, etc.). I chose those needles to fit my knitting project, which uses "bulky" gauge yarn. 

Specifically, this whole project started when I wanted to knit jumbo yarn out of knitted “easy yarn” because it would be funny to knit things out of something that is already knitted to the second degree. I use yarn that is similar to “Easy Peasy” yarn from The Woobles and  “Home Dec” yarn from Bernat. If you try this, be careful buy “easy yarn” from other brands, because it can often be even bulkier than these, and may not fit super well. So I would try those two brands first so you have a good… gauge on the gauge of yarn you need.

How it works

When the two parts are rotated against each other, each needle goes up and down. Each “needle” is actually more of a latched hook. On the way up, the needle passes all the way through the current loop on that needle, so that the loop goes down below the latch. Then, on the way down, the needle picks up the free yarn which is being held up by one of the yarn holder prongs on the outer part. Going even further down, the needle pulls this new bit of yarn through the current (now old) loop. The old loop passes over the latch and comes off the hook, while the newly formed loop is safely latched in.

Practical considerations

You probably want to start the knit (a plain circular knit with six stitches in the round) by hand and knit it to be at least a little bit longer than the machine. Then pull your existing work through the machine and hook each loop onto its own hook. Then hook the yarn into one of the holder bits, depending on the direction of the knit (see pictures for how it's supposed to go).

You don't need to manually tension the yarn after that, in my experience. You do want to tension the work that's coming out of the bottom of the machine. You can probably clip something mildly heavy to it, but I just kinda hold and tension it with the same hand I'm using to hold the machine at the bottom. Or sometimes I forget and just yank on it lightly once in a while, whenever I see it bunching up at the top.

Ideally, you want to hold the inner piece in place and rotate the outer piece around it. Otherwise the output will come out all twisted up (since you are continuously twisting it). I hold the inner piece by its very bottom, which is what lets me use the same hand to also tension the work coming out of it.

Even more ideally, try to have your spool of yarn somewhere above the whole machine so it doesn't interfere and get all tangled.

You don't want to go too fast, and you do want to keep an eye on each needle as it crests the top, to make sure that the old loop actually cleared the latch. This is where tensioning the work is important - it helps the loops come down and not get stuck on the latch. That said, I found this is still way faster than knitting a tiny circular cord by hand.

To release the work from the machine, just rotate backwards for 1-2 whole turns. Conversely, if you do end up rotating it backwards to fix a stitch, make sure you haven't released any of the other stitches from the other loops. 

Files and printing

I included the model at 3 different levels:

The OpenSCAD file has both parts of the model, as well as representations of the needles and yarn for reference. Lines 12-16 contain the boolean flags that turn each part on and off. To generate an STL file, you want to just have the inner or the outer part turned on (whichever one you're generating), and everything else turned off.

There are lots of other parameters. In particular, all of the needle dimensions are parameterized, so in theory you can easily rescale it to work with other needles. 

However, the number of needles is not parameterized. Both because I was lazy, and because I'm not sure whether the functionality will work as well with a different number of needles.

The STL files for each part are generated from the current version of the OpenSCAD file, as described above. 

The previous time I was generating them, both OpenSCAD and CURA gave me transient errors about the geometry being wrong, but I couldn't reproduce that when I generated these for publishing so hopefully it's all good now. I think OpenSCAD just gets overwhelmed with the amount of stuff I'm asking it to generate, In particular, it started acting funny after I added  the “crown” part on top of the inner piece.

The GCODE files show approximately how I printed each part on my Ender 3 (I do not know why plain old Ender 3 is not available as one of the options in the dropdown). I printed them both positioned vertically, with brim and a small amount of support. The support is configured at 10% density and 60 degrees overhang angle. This generates the support so that it's only where it's actually needed.

This model should have (almost) no infill (make sure your wall thickness is set to at least 1.6 mm) so infill settings shouldn't matter. I made a small tweak to reinforce the yarn holder prongs, so that part has a little bit of infill but it's so tiny that the settings don't matter much, I think.

This gcode is generated for PETG purely because that's what I have. 

Assembly

Basically, you just have to make sure that the main body of each needle is inside one of the slots in the inner part, and the lever (perpendicular bit sticking out) is in the diagonal groove/opening in the outer part. Then once all the needles are in place, snap the two parts together.

That can be a little finicky to achieve. I usually start by putting one needle through the groove in the outer piece, at the very top of the groove. Then stick that same needle through a slot on the inner part. Repeat going from top to bottom, pushing the two pieces together slightly as you go. Once all the needles are in correctly, push together until it snaps.

Disassembly

Erm. Sorry.

OK, actually, I found that if you stick a slim strip of paper under each “flap” in the snap-fit portion, you can just gently push it open and un-snap it. Then just pull apart until the needles fall out all over the place.

Known Issues

If you print the outside piece with generated support, the top side of the diagonal slot will be a bit rough where the supports are. This is because the generated support is purposefully printed 1-2 layers below the actual part that's being printed, so that there is some separation and it's not one piece. This makes the edges droop as they are printed (see pictures below). Over time, as the needle lever travels through that slot and the edge applies pressure to it to bring it down, the needle makes a dent which both makes the machine get stuck, and is likely to eventually tear away a big chunk of the outer piece.

So I would sand the rough edges before using the machine, and/or try using the “experimental” hand-created supports in the SCAD file (although that one also has a small gap currently, and of course is also harder to remove than the generated support designed for ease of removal).

I actually have an idea for a redesign that may fix this by printing the outer part diagonally, so the gap is totally horizontal; that way any pressure points at least won't be chipping away at the layer separation.

I recently updated the model to reinforce the yarn-holder prongs and the fins on the outer part, but they may still be a little weak.

The outer part has a tendency to “ride up”, which then adds lots of friction to everything and jams things up. I tend to apply downward force on it when rotating it. Maybe the snap-fit part at the bottom can be redesigned to limit the travel from above (in addition to the way it currently limits it from below)

The most common failure mode for the actual knit is when the current loop doesn't actually go all the way down and clear the latch like it's supposed to. This is probably a combination of not enough vertical travel, and the loop being too tight (which adds friction and makes it ride up). Both could be tweaked in the parameters and calculated variables.

Occasionally, the needle pulls the new loop through the old loop successfully on the way down, but then goes back through the old loop on the way up, undoing everything. This seems fairly rare on the current iteration of the machine, though.

Sometimes the hook and/or the latch actually goes through the middle of the yarn.

Both of the above may also be caused by the loop being too tight/too small.

If you've actually read all the way down to here, and also tried this thing out, let me know whether you do or do not encounter these issues.