Calculated sett
There are two elements to the sett: how thick are the threads with which we are weaving, and how far apart they should be spaced (both in warp and weft). Classic texts in industrial textile design [Ashenhurst; easier to read in Watson] give formulas to calculate both: the first one from what we know about the yarn; the second one from what we know about the structure.
Yarn thickness
In this context, what we really want to know about yarn thickness is how much room it occupies in the cloth, that is, its diameter. However, the information related to yarn thickness we usually get is the yarn’s grist (in yards per pound (ypp) or m/kg). The Ashenhurst formula for yarn diameter gives a way to calculate yarn diameters from the grist:
#diam/inch, or dpi = CF x √(ypp),
with the correction factor CF as follows:
CF = 0.92 for silk, cotton and linen yarns
CF = 0.9 for worsted wool
CF = 0.85 for woolen wool
Warp thread spacing
The basic idea is that the warp threads (ends) have to be spaced so that a weft thread (pick) has just enough space to travel from front to back and back to front as many times as needed for the structure.
For a detailed explanation of the merits of this idea, check the nitty-gritty sett calculation page.
To be able to use the formula, we assume:
same yarn in warp and weft
the cloth is beaten square (warp sett = weft sett, or epi = ppi)
all the picks travel the same path through the cloth,
The structure is defined by the number of ends in a repeat (R) and the number of times the pick crosses from front to back and back to front of the cloth (or intersections) in one repeat (I).
We calculate a number I call STR
STR = R/(R+I).
Calculated sett
The calculated sett for a given yarn and structure is simply
calc. sett = dpi x STR
Below is a table with the STR numbers for regular structures.
For the definition of the pick shorthand and the STR number for more structures, head this way.
Actual sett and calculated setts:
Calculated setts as a tool
I weave a lot with 8/2 cotton and cottolin. For plain weave towels, I usually sett my warp at 20 epi. How does the actual sett compare with the calculated sett?
8/2 cotton has a grist of 3360 ypp, which gives
0.92 x √3360 = 53 dpi for 8/2
The calculated sett for 8/2 in plain weave is
53 x 0.5 = 26.5 epi.
Clearly, I weave at a looser sett than the calculated sett. We can calculate by how much the actual sett is looser than the calculated sett. Let's divide the actual sett by the calculated sett to get that number:
20 epi / 26.5 epi = 0.75.
In other words, I weave 8/2 in plain weave at 75% of the calculated sett.
We can distill our experience with various yarns and structures into a percentage of the calculated setts for these yarns and structures. This is an idea I found in Ann Richard's book Weaving textiles that shape themselves, 2012, pp. 38–38.
For more about this idea, and how the setts from various sources differ, go to Calculated setts as a tool.
