On Wed, 6 Sep 2006 09:01:28 -0400, "Dave W"
wrote:

Sure is.... according to my Starrett tap and drill guide. I was a little
surprised so I measured some and found it to be correct.
Dave
"Brian Whatcott" wrote in message
.. .
On Tue, 5 Sep 2006 19:15:51 -0400, "Dave W"
wrote:

The root diameter of a 1/4-20 screw is .201 inches ...


Oh really? :-)

Brian Whatcott Altus OK


Hmmm...I'm getting an uneasy feeling that I am being pulled
in further than I want to go. Ah well: though machine screw series
are designed to break at the bolt, the failure mechanism isn't quite
via yield at some effective diameter as I suggested.

A slightly better model is to visualize shear failure at the thread
root, so this calls for computing an effective area held by the nut
times the material shear ultimate - but this ignores (at least) two
other factors:
1) you may have seen a model acrylic bolt section stressed at the
threads, and viewed through polarizers - the shading highlights the X2
to X3 stress multiplication caused by the thread root form. This is
where the cracks start.
2) fatigue life is dependent on a smooth transistion between threads
and the shank of the fixing - this too can have a /2 factor on
fatigue life.

So, the carry home message is, nobody ever got jailed for misconduct
for being either too conservative, or for knowing exactly what they
are doing if they design right to the yield limit (as aero engineers
often need to do.) But people do die if the engineering is skimped.
The choice is yours, as always.
I am of course joking about engineers being imprisoned for screwing
up - it just doesn't happen - not in the US at least, as far as know.

I think it would probably be better to pursue this kind of topic on a
group like sci.engr.mech or sci.engr.civil if you want more
Or, you can find perfectly respectable allowable or ultimate
force tables for screws of many sizes and types in web-accessible
form. Probably more than a sailor wants or needs?

Respectfully

Brian Whatcott Altus OK