All these posts about ends per inch (epi) are very much related to a wonderful way to calculate sett invented by Mr. Thomas R. Ashenhurst for industry. (Can you image a textile engineer wrapping a thread for making bed sheets around a ruler?) I’ve written several blog posts about his formula and uses for it. You can search for any work if you look on the home page of my blog and click on the magnifying glass. However, in 2 of my books I explain it step-by-step and all its variations that make it so handy. Check out either one: Weaving for Beginners and Winding a Warp & Using a Paddle. Both books have a whole chapter on sett. The latter book may have a bit more background so if you have both books, check them out. Much is repeated in both of them however.
Allowing for Purpose
There is another issue to consider: the purpose. Do you want a firm or a medium weight cloth? The illustrations above show the same yarns, all used in a balanced plain weave (warps and wefts show equally). What it means is that there’s not one perfect sett for a given yarn and weave.
You’ve seen that there can be a variety of setts for weft-faced cloth, warp-faced, or balanced cloth.
Open up the sett (so that the wefts can pack down more between the widely spaced warps) for more weft predominance and make the sett closer (more warps per inch) for more warp predominance. So, the sett can vary greatly to serve your purpose as well as your cloth design, and choice of yarn. This is one side of the box I wove a while back where I was dealing with double weave and color choice as well as warp and weft emphasis.
Allowing for Width
A very wide warp will need to be set somewhat more open than a narrow one. With a narrow warp, the beater can beat in the wefts closer than with a wide warp where there is more resistance on the beater from having so many more warp threads. For the color blanket above, I made a narrow sample 7” wide. The blanket was 36” wide. The wefts didn’t beat in the same, and the cloth wasn’t balanced with exactly the same number of wefts per inch as warps per inch that were in the sample. It made a significant difference because the project was to be a true plaid where it was important to have exactly the same amount of warp and weft in the cloth so see the color mixing. This taught me to make a sample on the real warp before beginning weaving the project. I use the 2-stick heading when I cut off the sample to finish it so I don’t waste much warp in the process. See this post: Two-Stick Heading
Allowing for Yarn Type
These percentages refer to the Ashenhurst Rule which can be found in Book #1: Winding a Warp & Using a Paddle and Weaving for Beginners and several previous posts. However, the principles do apply in general.
For fairly slippery yarns.
80% of maximum sett is close enough for most fairly slippery silk yarns. Worsted, line linen, mercerized cotton, and Tencel can also be considered “fairly slippery.”
For very slippery silk or rayon or bamboo try 85% of maximum. You can calculate your own percentage or choose a sett a “little higher or lower” than the 80% figures given in the sett charts in Book #1.
For loftier yarns. (fat yarns that are not firm, but are somewhat spongy) You could try 75% of maximum. Yarns that expand during finishing, such as unmercerized cotton, might be in this category.
For hairy yarns the sett needs to be more open try 65-70% of the maximum sett. If the warp yarns are too close, they cannot pass one another in the headless and reed, and you won’t be able to get the sheds to open.
Woolen yarns are meant to shrink, so they should be set more open. Use 65% of maximum so the threads are far enough apart during weaving to allow for them to shrink, making a closely woven cloth after finishing.