Good morning Fal Grunt:
There is a little appreciated basic geometric problem with threadmilling having to do with the fact that the cutter is not aligned to the helix angle of the thread except when the milling is done on a 5 axis machine or a dedicated thread milling machine that tilts the cutter.
That mismatch between the plane of cutter rotation and the helix angle of the thread affects the threadform to varying degrees depending on a few factors.
1) Coarse pitches are worse than fine ones.
2) Bigger cutters are worse than smaller ones
3) Square threads are worse than 60 degree vee threads and Acme threads fall between the extremes.
4) Internal threads are worse than external threads
5) Smaller diameters are worse than bigger ones for a given pitch.
The issue is clearance between the flanks of the thread and the sides of the cutter.
To visualize the extreme case, imagine a coarse pitch square thread such as might be used on a woodworker's vise.
If you slip a disc that simulates the cutter into the thread, you will see that it cannot align to the threadform without being tilted at the helix angle of the thread, and for very coarse pitches it cannot be inserted at all, unless it is very small in diameter compared to the diameter of the thread.
When you cut such a thread on a VMC the flanks will not be correct.
As you go from a square thread to a 60 degree Vee profile, the error becomes less because the cutter can clear the flanks of the thread better.
Get the combination of pitch, thread diameter and cutter diameter correct, and a 60 degree Vee profile will clear the flanks as it's driven around the developing thread and the form will be good.
The window is narrower for an Acme at 14 1/2 degree flank angle and hopeless for a square thread at zero degree flank angle.
Internal threads are worse than external threads because the thread form curves toward the periphery of the cutter.
With external threads, of course, the thread flanks curve away from the cutter so there is more clearance.
Smaller cutters curve away from the developing thread flanks more than large cutters do...so the rule of thumb is to use the smallest cutter you can, especially for internal threads.
So if I'm understanding correctly, the offset you had to input was a negative one...ie you had to program a smaller internal thread than you expected in order to get the fit correct.
If that's the case then my babble above explains your experience...if not, then I have no frickin' idea what might have gone wrong!!
Cheers
Marcus
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