What's new
What's new

Thread forming tube without deformation of OD

I had never heard of bspf before this post. I found the explanation bspf was changed from bspp because that was too similar sounding to bspt. Here is maybe a more plausible explanation of bspf.

  • Like
Reactions: Ox
Hi folks,
I am making thousands of internal 1/2" BSPF threads 1" long in aluminium tubes (1" OD 3/4" ID) on some CNC lathes.
Currently I am thread milling these threads but this is causing significant fine swarf issues, and I would like some stronger threads too.
I would be thread forming them, but the OD of the tube needs to not deform from the pressure of the thread forming. Is there a way to avoid the OD of a tube this size deforming? Gripping the OD in a collet chuck is not an option.
Keen to hear any ideas!
years ago we had a similar job, forming ID threads in thin wall AL tube - AND- keep OD within tolerance range. We made a Box Tool with two 180° follow rest bearings that attached to the form tap's holder. I attached a sketch of the tool we made. Ball brgs mounted on adjustable, eccentric, hex-head "axles"; locked in place with set screws.
edit: other OD turning operations prevented us from using a steady rest.


  • box tool.pdf
    11 KB · Views: 18
  • Like
Reactions: Ox