Sewing thread question

[ReflexBlueHorror]

If you operate a sewing binding machine, hopefully you can enlighten me here.

I have friends who work in traditional (hand) bookbinding and the topic of thread 'swell' came up. They aim for the sewing to add 20-25% to the thickness of the book at the spine edge. This swell is desired for rounding & backing (with a hammer). This swell is controlled by the size of thread. Medium thread is considered to be 18/3 or 18/2.

The thing which perplexes me is that they have absolutely no calculation for choosing the thread. They say (and they say their manuals say) that multiple factors 'need to be considered' around the fact that thicker sections (and thicker paper) 'absorb' thicker thread, and so ultimately choosing thread 'comes with experience'. With experience? I smell something really wrong here.

I'm used to creating print specifications for printeries/commercial binderies, and using the paper alone to calculate the spine thickness of a rounded book.

It's never occurred to me till now that the thread (and folds) must add to the thickness. But I've not seen a Smyth binding with a 20% swell. So now I'm completely puzzled!

Does Smyth sewing come with a one-size-fits-all approach? If so, what is it? Are there thread options? What guides their selection? Can you increase the swell on a Smyth binding?

I hope you guys may be able to enlighten me!

 

[mrserge]

If you correctly calculate swell based on signatures (therefore keeping in mind folds), thread thickness doesn't play a role anymore. Unless you will use very thick, decorative thread that most likely will be applied in hand process.
Deviation in hand process (including spine rounding process) is so big, that every bookbinder will give you calculations based on their own approach and experience.
Meanwhile if you are sewing book blocks on any sewing machine, this data is usually provided by the manufacturer of the machine and is more or less industry-wide similar.