Sunday, December 13, 2020

Someone With a Threading Attachment For Your Lathe?

 I need to cut an external thread to screw into what appears to be an M40x0.5 thread.  My Bushnell scope seems like it would accept such a thread.  The ID seems to be 40.64mm, and I can measure 5mm of thread with about ten turns.  Yes, I am trying to get a 5" long sunshade and not having much luck finding one off the shelf.


  1. I'm an amateur machinist, and I've only done a few single point threading jobs.
    A recent failure while trying to make a thin wall spacer has shown me that the problem is holding the thin tubing while cutting the threads.
    It may be more feasible to turn a stepped adapter to go between a store bought sunshade and your scope, or you may consider if boring out the ID of either plastic, or thin wall aluminum would allow a friction fit on the OD of the scope, much like the way that scope covers fit.
    A quick scan of 1 1/4" PVC pipe shows that the difference between the OD and ID may allow machining a slip on sunshade.

    I know that this doesn't exactly answer your question, but it might include a possible acceptable answer.

    1. I had considered boring a 2" OD .25" wall aluminum tube to slide on the outside with nylon tipped set screws to hold it in place. I would turn the outside down to better match the size of the scope. The trick is to put a piece of aluminum inside the tube at the chuck jaws so that you can get a tight fit that does not jump out under cutting pressure. PVC might work as well and easier to turn. Is there matte black PVC pipe?

    2. I know ABS pipe is black, but I don't think it's matte finished.
      I do believe that there are paints that will stick to PVC.

      I think I could have made the thin wall spacer work if I'd bored it to the final size before parting it off.

      If you chose to turn down the 2" .25 wall aluminum tubing, there will probably be enough meat to grip with the lathe chuck.
      As I experienced, once you part the thinned out tubing off, you cannot grip it again.
      I didn't think about putting a mandrel inside.