I originally posted what follows a few years ago on Machinists Gazette and just stumbled on the Word document that I used to create the original post and thought I should share it here.
Due to a recent post regarding the cutting of threads on the OD of a rod, this subject has come up, but I feel that it has enough merit to have a thread of its own. It deals with the age-old problem of figuring out what the real OD of a shaft should be for a given thread, which has always been a pain in the posterior. Of course, this is more of a problem when threading with a die. I am sure that we have all had that wonderful experience of trying to use a die to cut a 3/8"-16 thread on a 3/8" diameter shaft, for example. It just does not work without reducing the OD by a few thousandths. Better yet, with smaller sizes, such as 1/4" or less, it is very possible to twist the end of the rod right off before you even get a full thread cut. Tell me that isn't enough to make a grown man scream like a little girl!
When single point threading, the problem is not as apparent because we continue cutting the thread deeper and deeper until a thread gauge of some sort, such as a nut, will fit on the work. Close examination of such a thread reveals that the crest of each thread is very sharp, and due to being very thin, is often torn and full of burrs. If you then measure the major diameter of that thread you will find that it is somewhat smaller than the figure listed in all of the thread tables. If you have access to a comparator and are able to measure the minor diameter, you will find that it is also smaller than what is listed in the tables. This reduction in the minor diameter reduces the cross section and resulting strength of the part, which in most cases is not very desirable.
So, in reality we never want to cut a 100% thread. Even though it works on paper, the reality is that the mating parts need a little bit of clearance to allow for little burrs, dirt, etc. The OD of a shaft for single point threading should be the same as for cutting with a die, but there is no such dimension given in Machinery's Handbook. Instead, it lists the major diameter of a thread as a certain diameter, regardless of pitch. Using 3/8" as an example again, the major diameter, regardless of pitch, is 0.375". So just how does one determine the correct OD to start from without a bunch of guesswork? With a formula, of course.
Some time ago I found this formula somewhere on the Internet, but I do not remember where. Truth be told, I recently went looking for it again and could not easily find it. Regardless, it seems to be correct, though I have not had the opportunity to try it on many thread sizes just yet. The formula simply says that the shaft or rod OD should be equal to the major diameter of the thread, less 10% of the thread pitch. This results in better than a 95% thread on the male component without reducing the minor diameter, which works just fine for most of our work, and for which most dies are designed for.
Metric threads use a very similar formula, but is just a bit different due to the fact that metric thread pitch is already defined by distance between threads rather than number of threads over a given distance. Both formulae and examples are shown below. As you will notice, the math for metric threads is greatly reduced, and in most cases can be done quickly in one's head. Some may see this as another advantage to moving to metric measure, but that is a discussion to be had elsewhere and at a different time. You might as well be discussing politics or religion when the subject of imperial versus metric measures comes up! Those discussions can become quite heated, to say the least.
FORMULA FOR IMPERIAL THREADS: Ø = MD - ( 1 ÷ TPI × 0.1 )
WHERE: Measurements are in inches, Ø = Rod blank diameter, MD = Major Diameter of thread, and TPI = Threads Per Inch
EXAMPLE: 3/8"-16 thread. MD = 0.375" TPI = 16
Here is the math:
Ø = MD - ( 1 ÷ TPI × 0.1) (substitute real figures)
Ø = 0.375 - ( 1 ÷ 16 × 0.1 )
Ø = 0.375 - ( 0.0625 × 0.1 )
Ø = 0.375 - 0.00625
Ø = 0.36875 (round off)
Ø = 0.369"
FORMULA FOR METRIC THREADS: Ø = MD - ( TP × 0.1 )
WHERE: Measurements are in millimeters, Ø = Rod blank diameter, MD = Major Diameter of thread, and TP = Thread Pitch
EXAMPLE: M10 × 1.5 thread. MD = 10 mm TP = 1.5 mm
Here is the math:
Ø = MD - ( TP × 0.1) (substitute real figures)
Ø = 10 - ( 1.5 × 0.1 )
Ø = 10 - 0.15
Ø = 9.85 mm (round off if necessary)
As machinists, we are something of a spoiled bunch. Face it guys, we are! We are used to having charts for everything. A chart for this would be great, but I currently do not have one. Given a little time, I will produce such a chart for these formulae in both imperial and metric and post it in this thread. Creating it for all of the common bolt sizes is easy, as plugging in the numbers goes quickly working only from memory. It will take a little longer for the smaller machine screw sizes as I don't know all of their major diameters right off the top of my head; I have to look each one up before I can plug it in. Same goes for the metric stuff.
Originally posted to Machinists Gazette, 2/17/2019
The chart was created at the time of posting the original thread and it can be downloaded from our downloads section.
Stock Diameter for External Threads (https://www.homemachinistsjournal.com/index.php?action=tportal;sa=download;dl=item51)
That's very handy. Cheers Terry.
I hear yah. For a while now I have not made anything in my Hobby Shop for someone else and never for NASA...lol. And I do not miss the challenge at all! No more blueprints. No more ridiculous specs! And for topic, no more thread gages! Today/now I might use a hardware store nut to check or just make the threads screw together. If I start with a nominal size OD and single point thread it. It might have a burr at the top of the thread and if I kiss it with a file, the burr is gone and the OD is now tad under. That's my life now in my Hobby Shop, but I hear yah!
I have a handy dandy spread sheet that calculates a the thread information needed for cutting threads. It calculates the OD range for a given thread.
I'll try to post it here.
Terry, you may want to post the file somewhere else on the forum.
One thing to remember on "vee" threads, my program is only set up for calculating class 2 threads. it would be real easy to create a new tab for calculating class 3 threads. Just everything I work with is class 2 threads.
Ken