wrote:
Hello. I am trying to create this:
http://www.clotilde.com/detail.html?...a=bobbin+saver

except in a straight version, not round. I have a piece of 1/8-inch
diameter silicone tubing and a piece of 1/4-inch diameter silicone
tubing.

My question is: Is there a way to calculate how wide I should cut the
slit/opening so that the piece will grip different sizes?
Any information about this will be appreciated.

First - what sizes are your bobbins (diameter)? Unless they are much
smaller than I think, your tubing will be much too small. For instance,
my wife's bobbins are 13/16" diameter. Your 1/4" tubing would not come
close to holding them.

A somewhat different approach might be easier. If you cut pieces of
tubing long enough to hold *one* bobbin and a tubing size that with a
single slit it would open enough, you would have something like this:
http://home.comcast.net/~bobengelhardt/bobbin.jpg
The bobbin is 13/16" diameter and the tubing 9/16" *inside* diameter.

If you still wanted a single length, with a permanent opening, the ID of
the tubing would be somewhat less than the bobbin's diam, so that the
tubing would grab it. Then the strip removed would be enough to allow
the bobbin in & out readily, just so it's less than 1/2 the
circumference. E.g., for a 13/16 bobbin, I would use 3/4" ID tubing,
with a 1/2 - 3/4" strip removed.

It does depend upon the tubing material and its thickness (wall size).
Some experimentation is in order.

HTH,
Bob