Help with thread size

[litewings]

Have a male thread, 2A I'm assuming, that I'm in the process building an internal part to mate with. The male threaded part is described by the supplier as a 2 3/16 x 18.
I looked up the thread measurements using a calculator on line for a 2.1875 x 18 UN 2B. The major diameter listed for that is 2.2038 / 2.1875. Problem is measuring the male threads of the manufactured part, I get 2.2880. What the heck size is it? I can find nothing. Pitch is confirmed at 18. According to what I'm seeing, I would need to bore out an additional .084 to make this work, and its looking like my best option. What am I missing? The supplier states the 2 3/16 size, not the manufacturer. Trying to get some other info from the manufacturer but no contact yet.

 

[RJSakowski]

Travers Tool has a 2-1/4 - 18 and a 2-5/16 -18 tap listed. Those are closer to your measured size.

 

[litewings]

What I'm seeing, this is in the middle of those. I cant find anything. Checked metrics also even though the pitch is a perfect match to 18. Thought there could be a metric diameter but same thing. In the middle of two sizes. The part is a fuel cap flange. I'm making the extension that welds to a tank that this part screws into.

 

[JRaut]

Grab your thread measuring wires and give'em a whirl. Some collets I've had to make in the past used non-standard thread sizes, so matching pitch diameters (w/ a little bit of clearance) was the best way to do it.

With the as-measured pitch diameter, you can plug it into the formulas to figure out what your bored diameter should be:
H = 0.86603 x pitch = 0.0481"
Minor Diameter = Pitch Diameter - (2 x 0.25H)
So if your PD measures out to, say, 2.150", then your bore diameter would be 2.126" (add clearance as appropriate).
Start testing for fitment early and often, if you can.

Getting the actual pitch diameter is key, then it doesn't really matter what the supplier or manufacturer says.

(Double-check my math, of course.)

 

[litewings]

I'm a new guy to this. Haven't threaded anything in 40 years till last week. This is my 3rd thread job in 2 weeks since I got the machine and first internal thread. I don't have pitch wires,,,,yet. Anyway to get usable numbers without wires. Measuring the flange, I have 2.220 minor so to speak and the 2.2880 major.

 

[T Bredehoft]

Measure over the threads, what ever you get the bore size will be that number minus (1/18)x2 .

.0555 is the height of the thread. The total to subtract from the OD of the thread, on each side is .0555 x2 or .111 total.

Make a sample cut on a piece of scrap to verify this. You may have to file off the crest of the threads, sometimes a fin will form there and fool you into thinking it is still too big.

 

[Tozguy]

Tom and I were writing at the same time.
If you have the male thread on hand to measure you are home free.
Start with your measurement of 2.288"" and subtract (1/18 in decimals) .055 for each side.
The result of 2.178'' can be increased slightly if the top of the male threads are flat or rounded, say 2.200''.
Without seeing how flat the thread tips are 2.220'' looks too big to me. A photo of the male threads would help a lot.
With a bore of 2.200'' chances are the tops of the internal threads will end up a bit flat too (which is good).
Start doing trial fits with the male threads early and sneak up on final dimensions.
You can always bore it out larger during the operation if necessary but not smaller.

 

[litewings]

Thanks for the help guys. Ill give it a go using the math. Kinda intimidating doing my first internal thread and its a non-standard size. That, and I'm trying to do it in reverse mode since it starts at a shoulder with virtually no room for a relief cut.

 

[SLK001]

I don't have pitch wires,,,,yet. Anyway to get usable numbers without wires.

You need to get the pitch wires. It is the PITCH DIAMETER that is the most important dimension of a thread. Another way is to purchase a pitch micrometer (which is $$). Pitch wires are fairly cheap. TAKE SOME PICTURES AND POST THEM!!!

 

[litewings]

Threads done. Used the formulas posted from the help here and it worked perfect. Also, I'm now a big fan of threading from chuck to tailstock.
Pitch wires ordered. I have another one of these to do and will use the wires more for a comparison than anything since I have the measurements from the first. Always learning, and the help on this site has been fantastic.
Thanks everyone.