On 2018-02-20 1:57 PM, John Ruth wrote:
> There's often a big square-threaded or Acme (trapezoidal) threaded column on
an adjustable-height office chair.  And, of course, there's a matching "nut."


I have a pair of old draftsman's stools which have 1x10in square-thread 
screws and a large adjusting nut. These could be readily-adapted to 
clamping action, I think. I expect old piano stools have something similar.

FWIW

Don

-- 
"You can tell a man that boozes by the company he chooses"
The Famous Pig Song, Clarke Van Ness

The harder they come, the bigger they fall." Ry Cooder