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Thread Milling Issue

I'm thread milling various ID threads (1/2x28 to 13/16x16) and the all have the same problem. When I'm testing the thread fit, the test parts will screw in 2-4 full rotations and then the threads get tight until you can't thread the test part on any further. This is with all threads I'm machining. I've checked the run out on my tools, I'm doing multiple spring passes, and I've check and verified the minor diameter, thread pitch in the program is fine, everything checks out okay. I'm running out of ideas, hoping some of you might shed some light on it. I've got only one thread mill I'm doing these jobs with, so I don't know for sure if it's the thread mill or not. I've also checked my programing to see if it's threading some sort of tapper, everything looks normal (using Fusion 360.)

Let me know if there is anything else I can try. Thanks!
I would check,
are you milling the holes? if so, is the tool deflecting? measure the holes at the bottom before thread milling.
edit:
It is the only parameter I can see that would change from the top to the bottom,
especially when your using a single point for threading afterwards.
other than the machine not cutting the pitch correctly.
 
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Gradually tighter. I though it could be a pitch error in the programming, but everything checks out okay. I've got someone that's going to lend me a similar new thread mill, so I'll try that and see how it comes out.
Maybe the hole is tapered to start with before threading that would make it get tight down in the hole?
Don
 
Verify minor for size and taper prior to threading.
Verify sample for major diameter, pitch diameter, and taper on both.
Double check that you are using a proper feed rate for your different sized holes. (As the tool wears, slowing the feed rate for the proper ID will be more important as the tool will deflect more as it wears)
Double check thread spec (3A etc)

Work I do is usually odd ball one off and doesn’t justify the cost of gauges. Usually I will make a gauge, threaded over wires, to verify. Of course if it is a standard, and available, then buy a standard.
 
Making threads to match a sample is a crap shoot. Without a print or specs to define the tolerances nor any gages to check them with, it is extremely difficult to know if you are making good threads. Is the sample nominal? At the max tolerance? Minimum tolerance? Sure, your parts may fit the sample, but will they fit any of the other parts that you don’t have available to check against?

49/64 x 16p is awfully close to 20mmx1.5. Are you sure it’s not supposed to be metric?
 
Making threads to match a sample is a crap shoot. Without a print or specs to define the tolerances nor any gages to check them with, it is extremely difficult to know if you are making good threads. Is the sample nominal? At the max tolerance? Minimum tolerance? Sure, your parts may fit the sample, but will they fit any of the other parts that you don’t have available to check against?

49/64 x 16p is awfully close to 20mmx1.5. Are you sure it’s not supposed to be metric?
I was thinking along the same lines - all of the threads OP mentions are at least unusual, potentially UNEF or UN series, but odd enough to make me wonder if he's measuring them correctly...

OP, do you have a set of basic pitch gauges?
 
Kinda forgot about this thread, sorry to leave you all hanging. I figured it out. On the computer, the threads were modeled in the actual solid body of the part. Once I removed the threads from the model and manually put in the appropriate pitch and offsets in, it worked like a charm. This was a bit weird, I've modeled threads in and machined them before without issues. However, I didn't model this part, so maybe there was some accidental tapper when the guy was modeling it himself?

I only say this because this problem only happens when I'm using one of his models he provides me. It's got to be something when he designs his parts.
 








 
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