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In this post I share the beginning of the chapter about shed geometry which applies to all types of looms. In future posts, I’ll briefly explain how different looms work. The complete chapter in the book has comprehensive directions on how to adjust jack, counterbalance, and countermarch looms.
Part I: Why to put the most threads on the first shafts.
Before the loom is ready for weaving, it may be necessary to adjust it so the warp, treadles, and lams are in the best position to make good sheds, to make the best quality cloth, and to make your weaving comfortable.
If the bottom of the shed isn’t flat, the shuttle will skip threads as it passes through. If the warp isn’t the correct starting position, ridges can appear in the cloth.
Many weavers know counterbalance and countermarch looms need special adjusting, but they don’t know that jack looms can need some adjusting, too. Before you can adjust anything though, you need to understand how things ought to be and why. A little bit of loom geometry also helps in many situations.
Weavers know that when the shed is open to receive the shuttle, some warp threads are up and some down. But it’s also important where the bottom and top of the shed are located, and where the shed itself is open the most. If you visualize the open shed, you know that it is open the widest at the heddle eyes, where the individual warp threads are being held up and down. The size of the shed gets smaller and smaller going away from that point, until it barely opens at all at the fell of the cloth (the last weft woven) and at the back beam. In the illustration you see a shed but there is also an extra cord with a weight at each end going through the center of the shed. I call that a temporary diagnostic string. It can help clarify where the top and bottom of a shed are. For a clearer view see the final illustration in this post.
To help to understand lams, treadles, and sheds, think of a railroad crossing gate. Compare the size of a gate crossing a country lane to the size of a gate crossing a wide boulevard. When the longer gate swings up and down, its far end must travel a great deal more distance than when the short gate swings up. But the gates are alike where they’re attached at the pivot—neither moves much distance at all. When either gate swings up or down, its far end moves a much greater distance than the pivot end. Another way to help visualize the different distances travelled whether you’re close or far from the pivot point is to think of ice skaters making a pinwheel. The ones nearest the center of the circle move very little, while those at the outside have to skate like mad and skate much further to keep up.
I use these images in teaching whenever there is an angle or a pivot on the loom: sheds make angles; treadles, lams, and jacks have pivots. These principles can guide you through setting up and adjusting any kind of loom.
The application of the gate idea explains why in some looms (especially those with many shafts), the shaft that is the farthest away from the fell (the “last” shaft) is designed to raise or lower the threads more than the front shaft that is closest to the fell. See the illustration. This means that all the warps threaded on the back shaft travel more than the other warp threads—taking more effort from you to lift or lower them.
For this reason, if some shafts in a weave draft have many more threads than others, put those threads on shafts near the front of the loom, e.g. shafts 1 and 2. This creates less strain on you and the threads. You won’t have to lift the threads so high, and the threads won’t have to move so far.
Some looms raise and/or lower all the shafts the same distance, and the threads lifted by the back shaft aren’t raised higher than the front ones. See the illustration. Notice that the threads on the last shaft are getting lower and lower as they approach the fell of the cloth, and at the position of the beater where you throw the shuttle, the threads on shaft one are lifted higher than those on the back shaft. This is another reason to put the most threads on the first shafts. Notice also that the height of the shed is reduced by the threads from the back shafts. This also might be a feature to keep in mind when buying a multi-shaft loom.
The depth of the loom from the shafts to the back beam allows the shed geometry to work or not. Remember, the pivot or stationary place at the back of the loom is the back beam and the moving end of the railroad gate is at the heddle eyes, where the warp threads go up and down. If there are many shafts, there needs to be enough room for the threads to move the distance required. If the loom is too shallow, it puts too much strain on the warp threads and tends to prevent the shafts from moving.
My point is that some looms simply don’t work very well because their designer didn’t understand loom geometry. For this reason, I don’t recommend building a homemade loom. A lot of effort could be put into a loom that won’t work well.
As promised: an illustration of just the temporary diagnostic string. It is used when adjusting looms.