Hello,
I often add helical threads to parts that our customers send to us. This example has a 3.75 x 12 UN 2A thread added to it based on the values listed in Machinery's Handbook (27).



Is there a recommended length for a lead-in like this? I have not been able to find any reference, but I may not be using the correct terminology.

I'm familiar with Higbee cuts for Blunt Start Threads, but I'm looking for a solution on UN threads. This is usually taken care of with a chamfer on a bolt or nut, but I think a bigger thread like this, needs a taper of some sort to start correctly. The customer wanted this thread to start/end where it does, so extending it was not an option. I think I've seen similar threads on some cast/machined components, but they are often hose fittings and couplings.

Machinery's Handbook references Higbee Cuts, Lead Threads, Thread Runout, Washout Threads, Partial Threads, and Vanish Threads, but I wasn't able to find many suggestions for the length of these features in reference to something like the major diameter. The closest was a reference under Ends of Screws, but that talks about a simple radius or chamfer on the ends of small (6mm and under) threads.

I usually create the feature in SolidWorks using a sweep along a variable pitch helix, or a revolve around a sloped and off center axis.