UN thread checking

[USAeng]

I am thinking about having out machinist make a 1 1/4-8 class 1A and 1B thread... I would have to order custom go, no go gages though... I am trying to figure out if these gages are really necessary to check threads... they are much easier, but can you do the same thing with a micrometer and pitch micrometer? Thanks

 

[bcd]

To fully measure all dimensions, you will need an optical comparator in addition to the micrometers. If you are just trying to pick up the tolerance difference between A and B, pitch micrometer will do.

 

[mrainey]

You can't check the thread form with micrometers.

Software For Metalworking

http://closetolerancesoftware.com

 

[Cockroach]

What you can do is check the thread pitch using a pitch gauge. Typically a Three Wire Measurement can be used to measure the pitch diameter over wires. Standard wires can be purchased from many tooling stores, places where you buy your measurement equipment. They are standardized in small packages and are available in imperial or metric sizes. The Machinist Handbook gives a fairly comprehensive discussion of methodology.

You can also find literature on the subject from technology books in machining courses from a trade school. Other sources would be with online websites.

Typically I use the micrometer to size the nominal diameter of the thread and use the pitch gauge to obtain either the TPI for imperial or modulus for metric. You can then begin the analysis by consulting standard geometry for threading sizes. I would use the three wire measure to confirm the pitch diameter thus positively identifying the thread.

In the case of a bastard thread, you can interpolate between a set of known threads of the same TPI (imperial) or modulus (metric). I prefer the old fashion hand computation, there are several decent thread calculators out on the market. I use one that I custom made several decades ago, so I have the confidence with the numbers it kicks out. In general this is the better method, but you can look at the Thread Standards for UN Threads in the ASME Catalog. This gives you the exact equations for computations to the various threads, you can then program those into your preferred platform.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[mfgenggear]

use thread wires as suggested , then use a mating part as a functional gage. too verify the it assembles. , but for production use gages.

 

[mrainey]

If the tip of the threading tool has chipped or broken down, or if a tool with the wrong radius has been selected, you'll never know it using thread wires as your only check. The pitch diameter and pitch can be good while the root geometry isn't.

Software For Metalworking

http://closetolerancesoftware.com

 

[tbuelna]

USAeng,

Unless you have some very tight cost constraints, a proper set of master GO/NO-GO thread gages is a very worthwhile expenditure. The full set would probably cost you about $2000.

These master thread gages are especially valuable if each of the mating threaded parts are being made by different manufacturers. The master gages can be used by both manufacturers and will ensure compatibility and proper fit.

Having master gages is also important if replacement parts will ever be required in the future. Making replacement parts to the master gages will ensure compatibility with mating parts in service.

While it's not a consideration for most people, I personally love the look of a precision ground, polished, and oiled set of thread or spline GO/NO-GO gages in a hardwood storage box. It just projects the classic old-world image of precision and craftsmanship.

And finally, checking screw thread and spline fits using a GO/NO-GO gage is much faster and reliable than using gage pins/balls and micrometers. So don't be cheap!

riff_raff