Cardon's Tools is having a fastener sale, wish I needed more small machine screws, but it started me wondering where the # screw series came from. Fractional bolts go down to 3/16 and 1/8" but those smaller sizes are little used. The number series 10-24, 8-32 etc. screws are much more common.
So, it seems the system starts with the base size "0" being .060" and the sizes go up and down from there in .013" increments, so a #6 screw is .138" diameter, .060+ (6 x 013") and #000 is .060-(2 x.013"). Over the years the odd number series have been pretty much discontinued. Seems pretty arbitrary to me but here we are 100 years later..
Where did that protocol come from? I find two references.
In 1907 the American Society of Mechanical Engineers (ASME) defined two series that used Seller's thread, numbering the sizes by gauge numbers from 0 to 30. Who was William Sellers? He was an American machine manufacturer who, when elected to the Franklin Institute in 1864, was instrumental in the adoption of a thread form different than Whitworth and also a graded series of nuts and bolts.
Also, apparently during and after WW1, the powers that be, the ABC (America, Britain, Canada) Council, decided there should be more standardization in threaded fasteners, part of that process was the decision in 1919, that small screws needed better defined, hence this # series. There have been other systems for small screws, but the main standard for screws smaller than #0 is now ANSI/ASME standard B1.10 Unified Miniature Screw Threads.
More than you wanted to know about threads at Wikipedia, here.
3 comments:
As far as odd numbered machine screws go #5-40 and not as frequently 3-48 are still used for models and sometimes called out on plans for model steam engines on this side of the pond (US).
They even had some boxes of #5s. Never seen them before.
Menards stocks a decent selection of 3-48 screws in a variety of head styles. I lost a screw out of my 1980's-vintage Mac 3/8 ratchet and they had a 1/2", 3-48 stainless screw, bagged, in stock at my local store for a couple of bucks. Beats having to buy a rebuild kit (if I could have even found one) for one screw.
Obviously I stocked up, just in case.
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