Major Diameter Before Threading

is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
 
How dose the Machinery Handbook Pocket version compare to the Engineers Black Book?
 
is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.

The UNS standard contain 3 thread classes 1A, 2A and 3A for external threads, 1B, 2B and 3B for internal threads, the higher the number the smaller the allowance. A 1A thread will always have a Max Major Diameter smaller then the nominal diameter.
All of this data may be found here http://www.efunda.com/designstandards/screws/unified.cfm?start=1&finish=63
I have a MH but keep it at work, if I need this data at home this is where I go. It only covers up to 4-16 threads however.

For some reason this formula has been stuck in my head for the past 25 years, the Major Diameter of numbered machine screws is the number X .013 + .060", therefor a 10-24 is .130" + .060", or .190". Have forgotten more then I remember over the years but somehow this has become permanent, go figure.
 
Last edited:
is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
The percentage varies significantly depending on tpi. That's why I just use pitch × 0.859 ÷ 8 and subtract the result from the major diameter.
Dave
 
I made a chart on the wall for all the numbered threads and generally go 3-7 thou undersize on the 1/4 to 3/8 threads depending on how hard the material is.

I don't understand why all the min/max dimensions for different classes overlap:
ie: for a 5/16 external class 1A is .3113 to .2982 2A is ..3113 .3026 and 3A is .3125 to .3038

Now if one turns the shaft to .305 it fits all 3 classes???? I see the pitch diameter are different but I cannot change those with a fixed die.
 
Last edited:
martik777,
Have you ever noticed that some nut/bolt combinations are tight, loose, or sloppy?
That's the fit thing. As with all things there is a range of tolerance for both the internal and external threads. It is the combination of the two which makes for the fit.

There are three classes of external (1A, 2A, and 3A) and internal threads (1B, 2B, and 3B). The characteristics and typical application of each class are as follows:
Classes 1A and 1B: These classes are the loosest fit, in that, the largest amount play or clearance in assembly. These are used for work where ease of assembly and disassembly is most important, such as stove bolts and other general rough bolts and nuts.
Classes 2A and 2B: These classes are specified for the typical fit grade of commercial products, such as machine screws and fasteners, where interchangeability and reasonable quality and fit predictability is required. These classes are the most commonly used for commercial and aerospace applications.
Class 2AG: This application is specified to allow for rapid assembly, and to permit the application of a lubricant or for residual growth due to high-temperature expansion. In these applications, the G is specified to indicate, that when the thread is coated, the thread tolerance and size may not be exceeded by such coating.
Classes 3A and 3B: These classes are specified for high quality commercial, precision applications and products, such as machine screws, where a close or snug fit is required.
http://engineersedge.com/thread_strength/thread_classes.htm


As you have aptly identified there is likely a measurement which overlaps all the tolerances. However, the more metal you take off, the more difficult it becomes to hit the tolerance the project requires.

There is a pic on boltscience which shows me what happens when the thread tolerance is variable.
http://www.boltscience.com/pages/screw8.htm

Daryl
MN
 
is there not just a simple rule of thumb similar to ( "X" % x nominal diameter ) that will usually work ? i thought i saw or read that somewhere. something like 93% ? ? sticks in my old mind.
If you noticed for metric threads, the tap drill size is the major diameter minus the thread pitch. Fairly simple for metric.

For SAE threads, the same formula will work. You just have to convert tpi to pitch. For example for a 1/2 -20 tpi thread, 1/20 = .050", .5" -.050" = .450". The recommended drill size is 29/64 is .453". For a 1/4-20, .250" -.050" = .200"; recommended size is #7 or .251". For 1/4-28, .250"-.036" = .214. Recommended is #3 or .213". This gives drill diameter for an approximate 75% thread.

Bob
 
Machinery's Handbook is great and all, but it would be nice to have a simple chart with the rod diameter you need to use with a die (or single point) for a loose or tight fit (2A or 3A).

If I'm tapping a hole, I usually use a chart like this:

http://www.littlemachineshop.com/Reference/tapdrill.php

With this chart, I can quickly grab the drill I need: tap/clearance and loose/tight. My copy is laminated and at a handy location in the shop.

For single pointing a thread on the lathe or using a die, I haven't been able to find a chart that's as concise and easy to read as the link above. The link from Wreck is close, but the information isn't very concise nor is it in a printer-friendly chart.
 
Last edited:
Thanks Doogie, it is a well thought out chart. Gonna laminate them and keep handy.
 
Back
Top