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Posted By rkapuaala on 11/02/2008 12:15 PM
2. What is the logic behind the numerical descriptions of certain smaller screws and bolts such as 00-90 and 00-80 etc... (I mean I think I understand that the lowere the last 2 digits are the larger the bolt or screw, but what is the theory behind it. What do the first 2 digits mean and the last 2 digits mean)

I don't know what the sizes themselves are, or what the logic might have been behind them, but I can tell you a little about the nomenclature.

There are a range of sizes, including 00, 0, (maybe?), and 1-12. Above that, I've always seen them referred to by size (1/4", etc.). The second part of the number is the threads per inch. So a 00-90 bolt is size 00, 90tpi. It's pretty easy to find 10-24 (coarse thread) and 10-32 (fine thread) bolts in most hardware stores. Both are #10 diameter (whatever the heck that is), but have different threads, different tap diameters, etc.

The reason for the different tap diameters is that the stated bolt diameter refers to the outside of the thread. A 1/4" bolt is cut from 1/4" rod, and will go through a 1/4" hole (in theory, anyway). the thread is a 60 degree V with the top 1/8 and botom 1/8 missing. So, the finer the thread, the shallower that V will be, and the larger the tap drill will need to be.

Aint it fun?!
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