Am I the Only One Confused by Thread Milling?

[dkaustin]

Time for a reality check, please. I've been trying to set up a thread milling project, re-watching Eric's video tutorial, poring over the Sprut instructions, and it just wasn't coming together.

Luckily, Mcmaster-Carr provides realistic CAD files of most of their threaded doo-dads. I downloaded the file for a 3/4-14 NPT coupler and fiddled with my project until the machining result approximated the thread form in the CAD file.

I suspect that my problem is a semantic one. Either something is being garbled in translation from Russian, or my comprehension is just off. The Sprut7 help file states:

"The <Thread depth> defines the distance between the inner and outer diameters of the thread. It works differently for the inner and outer threads. If hole is machined (inner thread), then the diameter that is defined in the job assignment defines the inner diameter of the thread. The outer diameter is calculated as the sum of the inner diameter and the thread depth. If the boss is machined (outer thread) then the diameter that is defined in the job assignment defines the outer diameter of the thread. The inner diameter of the thread is calculated as the difference between the outer diameter and the thread depth. For the outer thread the <Thread depth> defines also the safe distance, that is used to go around the boss if necessary."

By my reading of these instructions, for an internal thread the hole diameter specified in the job assignment should be the minor diameter, i.e., the crest diameter of the internal thread. In fact, the hole diameter specified in the job assignment has to be the ROOT diameter of the internal thread to give the proper thread form in the machining result.

For an external thread, it works as I would expect it to from reading the instructions, i.e., the diameter specified in the job assignment is the OD of the boss to be threaded.

Is this consistent with others' experience, or have I stepped thru the looking glass here?

[Gerry Sweetland]

Sorry, I have no experience with thread milling yet but am going to have to figure it out for a future project, looking forward to following this post.
I have been doing some reading and web searching in preparation.
And I need to decide on and buy a tool or three.

Have you checked out the thread-milling video on this sight?
http://www.download.sprut.co.uk/Supp...p?showforum=23

You might have to register to the forum to see that particular video

Gerry

[Geof]

I think you have followed Alice.

The explanation you quoted uses a lot of words to say that when you cut an external thread you start with a diameter that is the same as the major diameter of the thread and cut in to the minor diameter of the thread. For in internal thread you need a hole with a diameter the same as the minor diameter of the thread and cut out to the major diameter. The depth of cut in each case, measured on the diameter is twice the thread depth. (And I used almost as many words.)

Of course the boss diameter can be a little less than the theoretical major diameter because a small flat is permissible on the crest of the thread. Similarly the hole diameter can be slightly larger than the minor diameter because a small flat is permissble. Actually if you are tapping, the tap drill size is quite a bit larger than the minor diameter because the torque requirements are too much to cut an absolutely full form thread.

If you are starting with a slightly undersize boss, or oversize hole the depth of cut is less than the theoretical thread depth.