|Only one size is based on the other|
Here is a handy list of the equivalence in millimeters for the fractional inch dowel plate sizes.
1/8" = 3.15 mm
3/16"= 4.725 mm
1/4" = 6.3 mm
5/16"= 7.755 mm
3/8"= 9.45 mm
1/2"= 12.6 mm
The dowel plate holes on the metric plate are: 3, 4, 6, 8, 10, 12, 16
There is no secret code that can be revealed from staring at these numbers but I suspect that anybody who has already used dowel plates will readily see the expanded uses from having both. First, it's practical to reduce the diameter of dowels incrementally in order to produce straight and true lengths. The closer that one can space these steps, the better. As one can see, driving a dowel that just passed through a 5/8" hole into a 1/2" is a whole 1/8" reduction. A 16 mm step between these two can ease the dowel down to size a bit more gradually. The whole sequence of steps for reducing dowel diameters is thus:
5/8", 16 mm, 1/2", 12mm, 10 mm, 3/8", 8 mm, 5/16", 6 mm, 1/4", 3/16", 4 mm, 1/8", and lastly, 3 mm.
Secondly, sometimes, a slightly oversized dowel is what one needs whether to allow for wood compression, in dry or green woods, or due to unreliably accurate drilling setups. This seems to be more often the situation with hardwoods being driven into soft(er)woods. If one is working in fractional inches, an 8 mm dowel is slightly larger than a 5/16" hole (7.875 mm). The same is true for a 16 mm dowel in a 5/8" hole (15.75 mm). With the addition of some metric drill sizes, this feature can be expanded with the fractionally sized dowels being driven into metrically sized holes, too.
I don't know what's easily available in other markets, but I can readily find Japanese manufactured auger bits that use 1/4" drive shafts. This shank size seems to have become a standard for interchangeable tooling in the international market. The Japanese refer to this as a 6.35 mm hex drive. There is also a 9 mm hex drive for larger sized tooling.
|A set of Japanese drill bits and a German make|
These hex shafts can be used in both three jaw chucks for electric drills or in this amazing adapter that can be mounted into a two jaw brace. There are also adapters that accept 5/16" hex shafts and 9mm hex drill bits. Dieter Schmid Fine Tools, for one, in Germany offers a wide selection of Japanese auger bits as well as the necessary 2 jaw chuck adapters.
|Packaged set of three bits for hang hole displays|
I don't understand the Japanese sizing system. The packages of drill bits conform to what their customers want so it remains a mystery to me why drill bits might be bundled in a package with diameters: 6,8,9,10, and 12 mm. A bigger mystery is why Lie-Nielsen offers a metric dowel plate. Is it a sign that the Maine brand has reached inroads on international markets where the inch is merely a curiosity? Regardless of their reasons, I hope that this posting makes clear why both these plates ought to be in every woodworker's kit.