So, awhile back I built a Lego swift, and it was workable. Then, I found a box of K’nex on sale at the toy store for $10. I couldn’t resist! The result is a swift that works more smoothly than the Lego one, which creaked and groaned quite a bit. Besides, the boys started building specialized contraptions last night, and the Lego swift was cannibalized for parts. R.I.P.
Here is version one of the K’nex swift. It’s horizontal like the Lego swift, and would have worked well with a few improvements. The last arm-joins also held the yarn. In fact, they held the yarn a little too well, causing it to tangle around the joiny bits during winding. Grrrr. I wound with it once, and then let the boys tear it up to build robots.
This morning, after the unfortunate demise of the Lego swift, I once again needed to wind yarn. So, out came the one box of K’nex I own. Version two of the swift:

This worked all right, but see how loose the yarn is? That’s a tangle waiting to happen, and it did. What’s a gal to do? Why, lengthen the arms, of course. Anyone playing along at home will notice I also had to add height to allow the longer arms to swing. This took some creative joining due to my lack of K’nex. A larger selection will yield a much prettier product. Here’s what I ended up with:

This skein of Louet Gems is sitting nice and tight now. If it were a bit longer, I’d have added orange clips to slide along the end of the arms to adjust for length. As it is, my skein was sitting on the blue rods. Here’s a close-up of the arm end. The orange clips weren’t present for my skein.
Happy building! Those of you looking to build your own (you know who you are) will be happy to know that K’nex are color-coded. So, it should be easy to follow along from the photo. If you don’t have the same colors, experiment. The K’nex system is built so that parts can be swapped out (for example, two blue rods and a joiner is the same length as a red rod.)
My husband takes all of this as a sign that he needs to buy me a real swift for Christmas. Who am I to argue?
[PS - The tub o' K'nex I bought was called the "Silver Edition Value Tub".]