Formula for Changing the Sett of a Warp
Is there a mathematical formula to use when you are changing the sett of a warp that has already been sleyed in the reed? For example, if I'm making the sett more open, how do I know how many dents to "move over” on the starting edge so I can successfully move all of the threads without a jam-up during the process?
I have to admit that what I usually do when I need to resley a warp to fewer ends per dent is just guess about how far to the right (I sley from right to left) to move the first threads over. And, because I guess, I have experienced that jam-up you are talking about more than once. When that happens, I just pull the remaining threads out of the reed and resley them, working from their threading order instead of their denting order. (I can only do this when I don’t have to worry about centering the resleyed warp.)
To resley more accurately if you don’t have to worry about centering the warp, simply subtract the width of the already sleyed warp from the width that the warp will become and start sleying that distance from the edge of the sleyed warp. Say I start with a warp of 240 ends sleyed 3/dent in a 12 dent reed for for a sett of 36 ends per inch. This warp width is 8 inches. If I decide that a better sett would be 24 ends per inch, the warp width for the new sett would be 10 inches, two inches wider. I would therefore take the first 2 ends out of their former dent and move them two inches to the right, and continue. (I’d actually move a dent or two farther than that one inch, just to be sure my math has been right.)
However, if I need to center the warp, I would have to do much more careful math and counting. The best way to figure this would be to determine the number of dents occupied by the first sett and the number of dents by the new sett, subtract the first from the second, and divide by 2. To sley, move the first thread over exactly that many dents and resley to the center of the warp. Then, repeat at the other edge and sley back toward the center. This method would require perfect accuracy, which is not a slam dunk, especially for irregular sleying orders. You could resley the second half and find that you are off one dent when you get to the center. (The thought of this makes me think that with complicated denting orders, I’d just pull the threads out of the reed and start over.)
To resley a warp so that there are more ends per dent than the original sett is much easier, of course. You can just start in the center and move out to one edge and then go back to the center and work out to the other.