View Full Version : Design of mating threads

08-21-2009, 11:31 PM
What is the relationship between mating threads, i.e. the external threads that mate with the internal threads on a threaded cap?

Is it a common Pitch Diameter?


08-21-2009, 11:37 PM
The inside thread's pitch diameter is slightly larger than the male thread so they can be assembled easily. The difference between the two pitch diameters defines the class of fit. Machinery's handbook has pitch diameter ranges for inside and outside threads for the different classes.

08-22-2009, 02:53 AM
Thanks TGTool.

For standard/common diameters, such as 1/4-20, the mating thread relationship for the three common classes is quite clear, and easily available in the Machinery's Handbook or similar in table form.

I've been looking for an example of sizing non-standard mating threads for applications that are outside the realm of the published tables. As an example, a nominal diameter of 2.2 inches.

So, if the 'link' between the two mating threads is related by class to the common Pitch Diameter, would the layout be done by establishing the thread on, for instance the interior male thread, determining the Pitch Diameter for that thread, equating that diameter to the internal female thread by adding an appropriate offset based upon the desired class of fit, using that new diameter as the internal thread's Pitch Diameter, and cutting the cap's thread form as a thread with the common pitch but with major and minor pitch diameters corresponding to the newly calculated Pitch Diameter? (I hope that's clear)

That process makes sense to me, and is what I expected was being done, but search as I may, I have not been able to locate an explanation or examples of this type of layout. If it is in Machinery's Handbook, I have not been able to find it in the threading section.

Paul Alciatore
08-22-2009, 03:43 AM
In Edition 26, table 3 in the Unified Screw Threads section gives max and min limits for the pitch diameters of both male and female threads in various classes of fit for threads up to 4" in diameter. Also given are the min and max sizes for the OD of the male threads and the ID of the female ones. 2.2" is not a listed size, but 2 1/8" and 2 1/4" are. If and ONLY IF you are using one of the pitches listed for those sizes, you should be able to use the numbers for one or the other size and add 0.075" or subtract 0.050" to arrive at what you need. Or, if you are not using one of the pitches shown, you should go to a thread with the pitch you intemnd to use and use that data with the appropriate constant added or subtracted.

Of course, for a non-standard thread, you are free to make up your own class of fit, clearances, and tolerances. You can observe the dimensions shown and work from there, either tightly or loosely, depending on the intended use of your thread. If it's a "one-off", the only thing really needed is for the male and female to fit one-another and you can do this by using the male threaded part as a gauge for cutting the female thread. Min and max limits are only really needed when parts must be completely interchangable with past and future runs and with other parts made in other shops or factories around the world.

08-22-2009, 04:00 AM
I posted this site 3 or 4 days ago, maybe it can help you. There's a section "UNS" in the "Common Bolt Types" that may or (may not) be what you are looking for. :)


08-22-2009, 04:08 AM
Thanks Paul.

Before folks focus on the 2.2" dimension, that was only an example, and not relavant to any particular design. Your comments make clear the advantage of making the design adhere to the published tables when able, which would be most of the time. I realize now that for most applications, the threaded portions could likely be made to adhere to the table independent of non-threaded features of the component.

I was thinking of a more general application, such as large diameters or, perhaps a pressure application where one may want a finer thread on a larger diameter, where the application is 'off the table'.

I think I'm good to go now, and look forward to cutting my first threads.