Specifying thread root for an internal thread.

[sociallyinept]

I am modeling both a screw and a nut to visibly show the interactions between them, including backlash. The thread I must use is uncommon, a .125-48 Unified Form, class 3A and 3B. Therefore, I am going off of the thread form equations in the standard, ASME B1.1. What I am having trouble with is determining the dimensions of the root of the internal thread. I need to have the major dia. of the nut at its maximum to get the worst case for backlash. Figure 3 of the spec (pg. 5) indicates that the root of the nut will be at the maximum major diameter of internal (which is 0.125) and the width of the flat will be at 0.041667*P (or P/24). When I make the model at these dimensions (and the maximum minor diameter at 0.1092) it looks wrong.

Any ideas?

 

[toolhandler]

There's a formula in the Machinerys Handbook for thread cutting internal and external. I don't have mine handy, or I would give it to you. Sorry, toolhandler

 

[sociallyinept]

Yes, Machinery's Handbook has formulas for thread form. However, they simply repeat what is given in the standard, ASME B1.1.

What I will probably end up using is the basic diameter plus the major diameter tolerance for internal threads.

If anyone has a better idea, I am open to suggestions. Thanks.

 

[BillPSU]

The worst case for backlash is not caused by the major or minor diameter of the thread but the external thread pitch diameter being at minimum size and the external thread pitch diameter being at maximum size. In the class 3 fits at MMC the fit is a metal to metal and at minimum material condition the fit shows the greatest backlash. I have made some special threads with a controlled root radius and their specification is UNJ. The spec. is available but hard to come by. The concern when making most threads is the percent of thread and not the root radius as the thread should always be in tension and the loading not causing the fastener to go beyond that loading.

 

[unclesyd]

Check out his program

http://www.downlinx.com/proghtml/412/41299.htm