hread note
 

thread note
once upon a time, back in the days of board drafting, we old folks were taught this is how to read a thread note:
1/4-20 unc
1/4 = major diameter in inches
20 = threads per inch
everybody in agreement so far?
how about this one:
1-8 unc
1 = major diameter in inches
8 = threads per inch
still all on the same page, are we?
well, how about this one:
4-40 unc
hmmm..., we have a situation here. does the "4" mean "major diameter in inches" or does it mean "#4", which translates to a .112 major diameter in inches?
without the "pound sign" (#), wouldn't the 4 mean a "4-inch diameter bolt"?
but perhaps the "unc" defines the "4" as meaning "#4". after all, unc is a standard unto itself.
thoughts?
it has been my habit to include the pound sign (#) for good measure, but i do not find that requirement in the asme drafting standards.
can anybody find the requirement to use the "pound sign" (#) in a standard? if so, please post.
the # symbol is not used for thread callouts. it is only used as short hand for the actual drill. however, even that isn't clear because the question can be whether one is requesting a #4 clearance hole drill, or a #4 drill (two completely different bits).
4-40 unc is id'd by the number of threads. standard 4" threads do not have 40 threads per inch, nor would that be unc...it would be uns.
nowadays, it is recommended that we use the decimal equivilent to the old numbers, but many of us still use the traditional numbering system.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
quote (asme y14.6-2001:3.2.1.1):
numbered sizes may be shown because of established practices. the decimal equivalent to threee decimal places, should be shown in parenthesis. examples are as follows:
no. 10 (.190)-32 unf-2a
or
10 (.190)-32 unf-2a
or
.190-32 unf-2a

kenat,
sorry, hit submit instead of edit.
what i was going to say was that sadly asme b1.1-2003 doesn't quite agree with 14.6.
if you look in table 2 or 3 it lists
quote:
4-40 or .112-40 unc
and if you look at section 6, screw thread designation it essentially says you can use just the screw number and has examples showing this.
if memory serves yet another inconsistent method is shown in one of the specs i mentioned but i can't find it now.
as you suggest in your case it is effectively governed by the fact that there are only certain specific sizes that are unc and 4.000-40 isn't one of them.
i don't see the '#' sign used for screw size anywhere in either standard. that said we do use it on our standard thread callouts such as "#10 (.190)-32 unf-2a" - a real belt & braces approach.
kenat,
also:
what are "braces"? is that british for suspenders?
it seems that usage of the options in y14.6 depends on the company or industry. government jobs pretty much use the decimal callout exclusively, and ignore the letter callouts for the small machine screw threads. i.e.
.190-32 unf-3b.
yeah ron, it's english for suspenders.
suspenders are a garter belt.
well off topic though.
kenat,
they straighten teeth.
chris
solidworks 08, catia v5
we here use the # sign for the smaller thread callouts. (#4-40 unc)it's standard in our cad system, which is supposedly follows asme standards. we build and design deep drawn stamping die's. plus if i saw a callout like 4-40, i would double look and double check my reference books. you never know these days
solid edge v20
just to be clear, no common standards are pound signs used as part of any thread callout.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i agree, remove the #.
chris
solidworks 08, catia v5