Measuring Internal Threads?

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IFLYRCTOO

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I need to make a threaded retaining nut to fit an internal thread but am unsure how to measure an internal thread accurately. It has 16 TPI and the max diameter is approximately 2.125 " . I know how to measure external threads with the 3 gauge wires. How does one go about measuring an internal thread?
2-1/8"-16 is apparently a fairly common thread but how can i be sure that's what i am dealing with? Its on my old Van Norman mill.
Thanks
Tim
 
Typically with plug gauges. But, I'm assuming you don't have those, so is the male thread that goes into this nut accessible ? If so, it seems that you could get your measurements from it to determine diameter and thread pitch. Then you can refer to Machinery's Handbook for the internal thread specifications. At this point, you could also make your own thread plug gauge. Don't know if this helps at all, but food for thought.

Ted
 
The male thread is what i need to make and is not available to me. Maybe I shouldn't call it a nut. Maybe an externally threaded collar.
All I have that i can measure is the internal thread in the casting.
Tim
 
O.K., I kind of suspected you had an issue like that. Well, the only other thing I know of (other than buying very expensive internal thread micrometers) is using a casting material to make a plug of the internal thread that you could then measure. I've never done it myself, but have read of other machinists doing this using various casting materials and even very low melting point lead alloys (think fire sprinkler head type lead alloy). Maybe someone else here has a better idea. I'd like to know myself.

Ted
 
I think Ted has the right idea, using a very low melting point casting alloy. That was the first thought that came to my mind. I've used Cerrosafe for that application. It is sold by various gunsmithing outfits, but there are similar composition casting alloys sold by industrial suppliers for similar money. One thing to be careful of: some of these alloys actually expand after solidifying because of containing bismuth. Make sure you read and follow the directions, especially the use of a release agent and the timing for removal and measuring.
Good luck!
 
I agree with Randall. Some times this metal is called fixtureing metal.
 
I guess if I were doing this, I would just measure the minor diameter of the internal threads and then cut the threads on the ''nut'' to about the proper minor diameter. I would just use the dials on the lathe to get the depth of cut. I have never used thread wires if I have the mating part. Do the fit by feel.

Do a test fit. If you need to take a bit more, then put the nut back in the lathe, pick up the thread and take another cut.

Standards for a 2 1/8-16 thread
Major diameter: 2.1234 / 2.1139
Minor diameter: 2.0490 / 2.0327

I would normally make a stub arbor to do this kind of work on, that way you always can get back to concentric without having to dial in.
 
JimDawson said:
I would normally make a stub arbor to do this kind of work on, that way you always can get back to concentric without having to dial in.
Click to expand...

That's the best suggestion. Sure, it's work, but this is a hobby, so who cares.
 
I have a suggestion get the information from the machinist handbook on 2-1/8 16 threads, you will need internal and external thread dimensions, check the pitch with a thread pitch gauge check make sure it is 16 tpi measure the diameter of the hole in the casting, see what it is to what the minor diameter for inside thread of that size, find out what class of fit the thread is. then just turn the the shaft or out side thread to the major dia. and then use the wire method to get your pitch dia when cutting your threads and you should be good.
 
Inside teeth of dial caliper= minor dia.
2.125 =OD
Od - minor dia = depth of thread/divided by 2
cut within .005 of target test fit and .001 spring passes till fit. custom one off fix, don't get hung up on production style engineering calculations.
Contrary to popular belief,one can pick up a thread after removing part for a test with judicious use of compound at 29.5* angle and a Sharpie marker(then use cross slide for DOC). for the last 1 or 2 passes .
works for me when making a single part replacement
Alternate quick and dirty,measure thread(the v) pitch/depth gauge to get a target DOC
My mistake thought you cutting male to fit female
The process the same just the inverse of stated.
OD = 2.125 of male
Minor dia(of Bore) =OD - depth cut for external thread X 2
16 TPI single depth .046

Minor = 2.125-(.046 x 2) =2.033 bore in nut.
 
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