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?

 

[unclesyd]

You might want to check this website out

http://www.hexagon.de/dose/dose1e.htm

 

[sociallyinept]

Thanks, unclesyd, for trying. However, the website seems to have lost a lot in the translation from German. It doesn't give enough details for my question.

I think I may answer my own question. The reason the thread does not look right is because it is at the absolute least material condition (LMC). This situation will most likely never occur due to a difficulty in manufacturing such a thread. Tool wear and the small geometries involved make it impractical to build a thread at LMC. I will modify my model to be somewhere between MMC and LMC. For the calculation of backlash, I will use the theoretical geometries at MMC and LMC and know that the "truth" will be somewhere in between as well.

Further input is still appreciated.

 

[unclesyd]

Where, % thread flank, is your root radius tangent? 831/3% was the smallest allowed. Some fasteners call out 75% or even larger 55%. Neither of the latter will guage out.