Monday, September 27, 2010

Band spiral

I've known since early on that a twill band will tend to spiral on itself when not under tension, and that the way to counteract this is to have a few cards going in the other direction. EPAC mentions it (p. 68), saying:
Generally, at least two tablets at each edge are threaded in the opposite direction, or alternating S and S, to compensate for the twisting of the band which is inherent in this type of weave.
I'm sure I read elsewhere that alternating at one end of the band only was sufficient but I can't find a reference for that now. That's what I did for the "scrolling vine" band which was my first foray into twill bands. I didn't have any problem with spiralling so it seemed that doing a SSSSSSSSSSSSZ weave was sufficient to avoid any problems in that area.

However, the plain (no brocade) band I've just finished, which was threaded ZSSSSSSSSSSSSSSSSSZ, had quite an extreme twist on it. It turns out that the "reverse the edges" plan is really only sufficient for brocaded bands, which have a lot of stiffness due to the stiff metal brocade weft (not sure how well one with a fibre brocade weft would hold up, somewhere in between I guess).

So, now I know, a non-brocaded band really needs to be a near-50% mix of S and Z threading. I won't make that mistake again (This doesn't apply to bands that are twill threaded but the cards turn backwards as well as forwards, as in 3/1 broken twill, of course) .

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