THREAD MILLING minor diameter problem

[Anonymous]

I was trying to make an Internal NPT 3/4 thread hole using the option of THREAD MILLING. The program gave me 2 options to do it.

• Using the hole feature
• Using the thread milling option

The first option takes care of the initial hole the second one you need to have the hole already.

Using the standard values the minor diameters aren't equal.  

• Using the hole feature (24.577 mm)
• Using the thread milling option ( 23.127mm)

I did mill the first option but the thread is to big (using a solid carbide 3/4-14 NPT 0.495 cutting diameter  ) 

Is this a problem of the program???

[Anonymous]

I don't do much thread milling so it's mostly an experiment every time to get all the numbers and dimensions correct.

A tapered pipe thread is even more difficult to machine correctly and I would never expect the first one to come out exactly the way you need it.

There are multiple possibilities for an incorrect thread size: Bad numbers in the thread dimensions/definition but it could also be the tool diameter or tool length.

I don't know how FeatureCAM comes up with 23.127 or 24.577mm as a pilot hole. The chart below has 59/64 inches as a pilot hole (=23.416mm).

http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html

According to this chart you should mill 10 revolutions of thread which is about 18.14mm deep but to start I would back-off the tool length offset by 9.07mm (=5x thread pitch) and cut only 5 threads. Increase the diameter comp until the thread gage (or mating pipe) goes into your thread 2-3 turns then you can go to full depth and do a little more "fine tuning".

...hope it helps but maybe someone else with more thread milling experience has a better method.

F.

[Anonymous]

We do quite a bit of thread milling here on a variety of materials. We use single form, partial form, and full form thread mills as well, but I can't say that I have used an NPT thread mill. I don't want to write the obvious, but like other features, having your tool drawn exactly to size is critical.

I'm not following your process so I'll write what I do.

New Feature

From Dimensions-Thread Milling

Type is ID

Select Standard Thread and choose 3/4-14 NPT

All I would adjust here is the thread depth

Next select the location

Next page I select Positive. I have found I get much better threads when I plunge to depth and climb mill. Of course this will be operation specific. I do rough and finish, and usually 1 spring pass.

There is no 3/4-14 so you have to draw one.

I got the attached dimensions using Harvey Thread Mill PN 70226

Should be straight forward from there. As always if it is an expensive part I would use some positive cutter comp and work the tolerance in until I found the right numbers. As long as I draw the tool right the program has always been within .001"-.002". Hope this helps.