[time-nuts] Standards for units

[Chuck Harris]

Hi Brooke,

Metric screws are spec'd by major diameter, and the number of mm between
the peaks of the threads.  English are spec'd by diameter, and the
number of threads per inch.

A 40TPI screw corresponds to a (1/40) * 25.4 = 0.635 mm pitch.  There
are standard metric pitches of 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, and 1.0 mm.

What you are seeing is probably an M3-0.5, M3-0.6, or an M4-0.7 screw.

-Chuck Harris

Brooke Clarke wrote:
> Hi:
> 
> There was a recent incident when a passenger aircraft (maybe Canada) had 
> to make an emergency landing because of a wrong metric - English 
> conversion resulted in not enough fuel to get to the destination. 
> 
> Are there metric equivalents to different series of English threads.  
> For example I recently purchased a tap for 8-40 threads after mistakenly 
> purchasing a 6-40 tap.  Most good hardware stores in the US have 4-40 
> taps, dies, various lengths screws and nuts.  Although 6-40 and 8-40 are 
> standard sizes they are not commonly stocked.  Are there a a number of 
> alternative combinations of root diameter and pitch in the metric system?
> 
> Have Fun,
> 
> Brooke Clarke