I'm guessing most of you were like I was about 6 months ago in thinking that there are only 2 types of threads used on these sort of things, Imperial and Metric. Well that is Wrong. I know of 4 main types, 5 lesser types and I'm definitely sure there are more less commonly used threads. The 4 main types that we'll all face are Metric, BSW, UNC and UNF. Here's a little ramble about them.

Metric - Designed by the International Standard Organisation from the standard German thread it was made as the a thread that'll be used around the world, as with all metric thingy's. Pretty easy to understand, out side dia is in millimeters and the pitch is measured as a distance between the tips of the grooves in millimeters. The angle of the slopes of the tips is 60 degrees. There is more, but this'll do for now. You can by nuts and bolts made with metric threads from Bunnings

BSW (British Standard Whitworth) - The standard pommy thread. Still in common use because the pom's built things to last and you can't spend millions of dollars (or pounds) just to change the threads in an 100 year old peice of equipment and still need the taps and dies to make spares (plus pommy engineers always stick with the same idea, I know, I worked with one). Because its an old thread its imperial, so the outside diameter is measured in inches (damn fractions), the pitch is measured in how many tips there are in an inch (T.P.I or Tips Per Inch) and the tread sides are 55 degrees. This is what the imperial nut and bolts from Bunnings are made with.

UNC (Unified National Corse) - The first yankee thread made the same time as the Metric theard, during the big wars when different factorys all need the same tooling to make the same tanks and planes, instead of different factorys having there own individual threads like before. And because it was made the same time as the metric thread it just help's prove how insular (good word there Philip) the yanks are. You won't find these threads in bunnings, but don't panic. Its is almost identical to the BSW thread except for the slope being 60 degrees instead of the pommy's 55 degrees (the shape of the tread is actually identical to the metric thread). If you had a UNC bolt, a BSW nut will fit on 99% of the time, but its not recomended as the nut would loosen over time.

UNF (Unified National Fine) - Guess what, these threads are on the XU1 drill style gearbox and as it turns out, on my w/w motors too. And as you can guess by the name, basically the same as the UNC thread, but finer. Where an 3/8 UNC bolt has 16 T.P.I or tips per inch a 3/8 UNF has 24 T.P.I. Basically giving you more theard over a given distance, but the tips are smaller. Useful in high speed machinerary like drills and w/w motor shafts. You won't buys these from bunnings, but can get them cheap from fastening shops.

There are other threads like Metric fine, British standard pipe thread, acme threads, ect.... But I haven't used these yet.

Did you know they don't teach ANY of this at uni.
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