Inspection of Depth of Threaded Holes

[CG1017]

Hey all,

I have a part with some threaded holes....The callout on the drawing is .086-56 UNC 2B minimum .19 deep. I took the measurement from the end of a plug gauge to the other end of the part, and subtracted from the total length of the plug gauge un-inserted to get the depth of the thread, and got .215. Just for comparison, I counted the turns once bottomed out (10.5 turns), and when multiplied by the pitch (1/56), I got .187.

Why the two different readings?

Which is the correct measurement?

Is there a more accurate way of inspecting thread depth that doesnt involve special tools? With respect to threads/taps, I only have access to basic plug gauges, a Micro-Hite, and verniers; we dont have CMMs, comparators, or any other advanced equipment at my job.

Thanks for any advice, this is my first time posting.

 

[3DDave]

Depth is to the final complete thread. So you'd use a thread gage that is ground flat on the end with non-truncated thread at the end to determine that depth.

The difference between the two measurements is rather large, but if you have a lead-in chamfer on the hole and an incomplete thread on the gauge then the number of turns would not match the total depth measured otherwise. It doesn't sound like you checked the initial extension of the gauge with it about to start engaging the mating thread, but a lead-in of .011 on both parts gets you that difference. [sup][/sup]

 

[CG1017]

I tried so many different ways and I'm getting different answers: the first way I mentioned I got .215, aside from counting the turns, the second way I did the same but with measuring from the top of the part from where the gage is inserted to the top of the gage (no-go end), taking the difference from the length of the gage and also got .215. I asked our lead machinist to check it, he just zeroed out a vernier to the length of the gage, inserted and measured to get .270 (with their own gage by the way). Had the lead inspector check it, he got .195 by counting the number of rotations with ANOTHER gage and did the same also with a screw. Everyone has different gages, but I went around with the particular gage I used to compare the answer they were getting with their own gage. I also tried adding/subtracting a full pitch, adding/subtracting 1/2 a pitch to my answers for different numbers as well.

The part is not chamfered. Where did you get the .011 from??

 

[3DDave]

Maybe mistyped .195-.187 into a calculator or, more likely, did the math in my head incorrectly. So use .014 instead.

I find it unlikely the end of the gauge is mirror flat with no lead in and the tapped hole is to a mirror flat with no chamfer. If it is there is a knife sharp edge on both parts.

I would set up a dial indicator and zero it on the top of the gauge with the gauge next to the hole and then move the gauge to start into the hole, turning it backwards until it just drops to the first thread. That should be a drop of .018 or so. You can then watch the dial indicator as the gauge is turned until you get to the full engagement as you count the turns to install it.

If that doesn't work out your shop is likely near a transdimensional worm hole.