Metric Taps and Dies

 I just learned a powerful advantage that the metric system has over earlier systems.  Specifically, Taps and Dies.  When you're drilling a threaded hole in the SAE system, you need to look up in a chart what the correct size drill is, as it's not simple.  SAE uses two main systems for sizing tapped holes:  1/4" and bigger, the measurement is the bolt size, and then you have a number of threads per inch.  Smaller than 1/4", machine screw sizes are numbered 0000, 000, 00, 0, 1, 2...8, 9,10, 11, 12.   Every shop in America has a tap table, telling you what size drill to use for what size and pitch machine screw, and is it a sensible system?  No, they have numbered and lettered drills.  Some taps have the drill and clearances sizes etched on the tap, but they're often illegible and for the smaller sizes.

 Metric doesn't have this problem.  Metric screws are described by screw outer diameter and pitch length.  Standard 60 degree threads make the tap hole easy.  So an M8x1 screw is 8 mm in diameter and has threads 1 mm long.  So to drill a hole with 90% engagement, such as into plastic, you simply subtract the thread pitch from the diameter to get the hole size.  To get a clearance hole, you drill a hole a little larger than the thread outer diameter.  No tap table is needed. 

 It turns out that this is both a benefit and a curse.  You can get an amazing variety of metric taps and dies, where SAE has largely standardized on Coarse and Fine threads (UNC and UNF), which are of course different for different sized bolts.  The benefit of that is that the SAE has also made some strength definitions, which are enormously powerful if your bolt will be stressed highly.  Most metric tap and die sets will have roughly the equivalent of UNC and UNF, but you can get threads both finer and coarser.