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Measuring Internal Threads

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Matt in TN

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#1
I understand the thread measuring wire and thread micrometer method of measuring pitch diameter for external threads. How would one measure the pitch diameter on internal threads?

If I'm trying to cut an external thread to match an internal thread as closely as possible (like a rifle barrel to an existing receiver), what's the best way?
 

brino

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#2
39 views and no replies......that's odd!
I am sure someone here has been around that block.

I'll check a few books when I get home to see if I can find anything.

-brino
 

T Bredehoft

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#3
Cut until the barrel screws in with no binding. That's better than having a plug gauge.

Short of that, make a plug that exactly matches the barrel. Use it for your plug gauge.
 

Tony Wells

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#4
There is a commercial product on the market that will do exactly that. IMO, it's out of price range for the hobby shop. They claim (and the point can be argued) that they can replace plug gages. Indeed they can measure certain elements of threads that solid gages cannot. I have used them extensively and find that there are great applications for them, but in my thinking, the generally most representative of fit is an actual threaded, mating plug or ring. In a system as they are marketed, there are several components which do give it versatility and the ability to identify separately incorrect thread elements, but many can be satisfactorily controlled by other means and don't necessitate the need to inspect individually.
 

Holescreek

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#5
I use the commercial gauge mentioned above regularly at work. It's an internal pitch "micrometer" (for lack of better terminology) made by Starrett and has custom made anvils for each thread pitch. It has a digital gauge and is preset while the anvils are in the thread master ring gauge.

Traditionally, for cost savings as well as expediency internal threads are cut to an external thread master plug gauge. It's "good enough" to turn your master plug gauge on the lathe as long as it falls within the acceptable pitch diameter for it's size. If you turn a plug gauge while you're set up for your barrel you can use it for the next job too.

If I'm trying to cut an external thread to match an internal thread as closely as possible (like a rifle barrel to an existing receiver), what's the best way?
Take a Mauser barrel job for example. It has a 1.1"-12 pitch internal thread (large ring Mauser). It also has 55 degree threads instead of the normal 60 degree threads but it's OK to ignore that fact. You're going to turn the major diameter to between 1.0869~1.0983" or an average of 1.093". The minor diameter of the thread in case you want to cut a relief diameter at the shoulder will be .9844~1.0081". As you're getting close to the finishing the thread you'll need to measure the pitch diameter either with a thread micrometer or using the 3 wire method. For the thread to be in spec., you need the pitch to measure between 1.0385~1.0442". It's as simple as that.
 

tertiaryjim

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#6
Measure the inside diameter. Invers the calculations for determining the bore size to get the OD for a mating thread.
That should give you a good thread but if you want it real close ya gotta try the male thread in the female one or vice versa.
If you are cutting a thread to match the spindle of your lathe it's handy to have male and female try threads to check to your fit.
The try threads should be considered as priceless tools and put where you can't loose em.
 

Matt in TN

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#7
Thanks guys. I've turned threads to just barely fit and there's very little wiggle, but it just seems imprecise to guess and check and then go by feel. Not sure. I can justify that expensive tool, so I guess I'll keep practicing...
 

machinistmarty

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#8
I measure the OD with wires and then make a test plug to fit the ID I am boring. With a barrel that wouldn't be necessary. You can buy plug gauges but there very expensive.
 

4gsr

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#9
I have a excel program that I use for the threads I deal with in my job. It calculates the OD's, ID's to prepare the cutting the thread to as well as pitch diameters of internal and external threads. Also measurement over wires and best wire size for external threads. And last, creates dimensions to cut threads for soft gauges. It's only set up for class 2 threads. It maybe too much to for the average HSM. If interested, I'll see if I can post it in the down load section. I also have drawings that you just fill in the blanks for shop use in making gauges.
 

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astjp2

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#10
Thanks guys. I've turned threads to just barely fit and there's very little wiggle, but it just seems imprecise to guess and check and then go by feel. Not sure. I can justify that expensive tool, so I guess I'll keep practicing...
Unless you are an engineer, don't overthink it.....
 

Navy Chief

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#14
I have a excel program that I use for the threads I deal with in my job. It calculates the OD's, ID's to prepare the cutting the thread to as well as pitch diameters of internal and external threads. Also measurement over wires and best wire size for external threads. And last, creates dimensions to cut threads for soft gauges. It's only set up for class 2 threads. It maybe too much to for the average HSM. If interested, I'll see if I can post it in the down load section. I also have drawings that you just fill in the blanks for shop use in making gauges.
I would be interested in the spreadsheet if you don't mind sharing.
 

rdhem2

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#15
This might be a silly thought, or I do not understand fully what you are trying to do. Why not use the old barrel stub as your plug gauge? It fit once and worked well, shouldn't a well made copy suffice for your purposes? Who knows? You may get a in-accurate plug gauge to your receiver. Slim chance----but possible!
 

4gsr

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#16

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Matt in TN

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This might be a silly thought, or I do not understand fully what you are trying to do. Why not use the old barrel stub as your plug gauge? It fit once and worked well, shouldn't a well made copy suffice for your purposes? Who knows? You may get a in-accurate plug gauge to your receiver. Slim chance----but possible!
I do the reverse of that now - I use the receiver as the gauge for the new barrel threads I'm cutting. Once I get close I test fit often and take off a thousandth or so at a time until it just twists on. It works, but I have no way to quantify what "just fits" means. As an anal-retentive engineer that bothers me, so I want to find a way to measure everything and just cut to a calculated measurement instead of all that test fitting.
 

Bill C.

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#19
Thanks everyone, it was been years since I cut internal threads. I always learn new tools and methods when i am here. I have used plug and ring gages in production before. Seldom used wires and micrometer method.

I hope your threading project was successful.
 

bfd

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#20
do you have the old barrel just measure that. if not look up the specs of the thread in the machinery handbook and make a plug gauge to match the specs test fit it if too loose make another on slightly larger do this until it fits like you want it then thread your barrel bill
 

4gsr

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#21
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