The SkeinTwister's first major milestone has been achieved! The prototype unit is now with a fellow dyer, who is putting it through its paces even as I write this. I had the opportunity to demo it to a few more dyers at Vogue Knitting LIve than I anticipated, which was fantastic for getting varied feedback early on. It's always interesting to see how other people use it for the first time, the new motions definitely take a little practice to master, and dyers who have twisted thousands of skeins by hand have a lot of muscle memory to overcome. Despite that, the feedback was very positive. It's amazing how prevalent repetitive motion injuries are in this line of work - one dyer showed me scars on both hands from surgery due to stress injuries from skein twisting. The relief that several people expressed to me, and the difference it will make in their daily lives is hugely motivating. I know this is not curing cancer, but I'm happy I can make an improvement in a community I care about.
Onto more mechanical thoughts - the hook! If you'd think you buy a hook in just about any shape & size, you'd be wrong. I found this out early on, as I scoured both the internet and my local hardware store. Screw hooks are about the closest thing, but they're meant to carry large loads. They don't have a large radius, and the bigger ones are made from pretty thick wire so they don't deform. This application sees smaller loads, and needs a larger radius to accommodate fat skeins of yarn. Here's a selection from the hardware store. As you can see, they're a little too small for the end of this fatty skein to easily fit in.
Bicycle hooks are the next size up, and just too large. Plus, they tend to be plastic-coated, which is too grabby. The yarn should be able to slide off the hook easily. A shallower angle on the tip is also preferable, it should only be bent back enough to hold the yarn under tension. An "L" at first seems ideal, except that you don't want the yarn to rotate around the axis of rotation (and wobble), the yarn should rotate exactly on the axis. So a hook that only doubles-back slightly is best. This is a draft sketch of the hook and shaft coupler (more on the shaft coupler in a few.):
At first, I thought I may need a small hook for mini-skeins and a large hook for fatty skeins, and make them interchangeable. Toollessly, of course. The more I thought about this, the more I realized that mini skeins didn't actually need smaller hooks. People are used to winding them around a finger, which is much larger than the hook size I'd use for fattie skeins. So really, the same hook can be used as long as it's big enough for fatty skeins to fit on. I'm currently thinking around 0.2" wire OD is about right to provide a stiff enough hook.
The hook will be attached to the motor via a shaft coupler. Many motors come with a flat shaft designed to be used with a set screw, but most motor applications are also designed solely for torsional loads. This has both a torsion and axial load component. The dyer has to hold a fair bit of tension on the skein, to prevent it from doubling up on itself until _after_ the twisting is done. So there's a fair bit of load coming straight off of the shaft. A set screw only relies on friction to keep it in place axially, and there were a few mishaps early on where the hook did come flying off at me. I needed to devise a way to fix it permanently to the shaft in the presence of this axial load, so I decided to pin it. And since one hook will suffice, the shaft coupler can be permanently pinned to the shaft, and the hook can be permanently pinned to the shaft coupler. I'll use standard spring roll pins which are easy to install. I can have the motor shafts drilled by the manufacturer, have the shaft coupler made by a machine shop, and the hook made by a wire forming company. The next step will be to make more formal drawings, and send the hook and shaft coupler out to be quoted.
The enclosure & clamping mechanism is the last big mechanical task. I do want to integrate clamps into the product, because it's just kind of annoying to have to furnish your own. And C-clamps are pretty large and clunky, and since yarn likes to snag on everything in sight, they're just not a good choice. I'm thinking of mounting the motor (and maybe circuit board) to a metal baseplate, and having a threaded L clamping mechanism, similar to the yarn guide and ball winder pictured below. The enclosure could then be a vacu-formed or molded part that would only need holes for the threaded Ls. A threaded knob and metal or plastic piece would form the bottom of the clamp, also similar to below.
This ball winder uses an annoyingly small wing nut to adjust the clamp:
This yarn guide uses a much friendlier large knob to adjust the clamp:
I'm making good progress, it's exciting to flesh out some of these concepts!