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Inches or millimeters

In Book 1, and in all the other books on kumiko-zaiku that I’ll be writing, I use millimeters as the unit of measurement. This may be an anathema to woodworkers who have only ever used the imperial system of feet and inches, but I’ll try to explain why this is the only measurement unit that will work without the need for large calculations that increase the risk of error.

You can, of course, use the traditional
shakkan-hō (kanejaku) measurement system (shaku, sun, bu and rin) used in Japan since ancient times, and although officially discontinued in 1966, is still used today by the majority of carpenters, tateguya and many furniture-makers. This system is just as good, if not better, than the millimeter system for this kind of work, but tape measures and straightedges with these markings are not readily available outside Japan, so I used millimeters in the book.

Correct me if I’m wrong, but woodworkers who use the imperial system use fractions rather than decimal places when working below the inch unit, and herein lies the problem.

In shoji with a standard kumiko arrangement, like the first shoji in Book 1, the imperial measurement system is quite straightforward and acceptable. For example, there are three vertical kumiko, the shoji width is 515 mm and the pitch between vertical kumiko is 111.9 mm (you’ll have to look in the book to find out the calculations to achieve this).

Rather than make a direct conversion of this to inches, I’ll give rough, rounded imperial measurements, and calculate from there. Instead of 515 mm, the width of the shoji will be 20¼ inches (514.4 mm), and instead of 30.5 mm, the width of the stiles will be 1¼ inches. After going through the calculation in the book, the pitch of the vertical kumiko is 4 3/8 inches. No great drama with that — it all worked out quite well into a workable fraction, and there is a small leeway with accuracy in this kind of arrangement.

The shoji I’m making up now for Book 2, though, has 31 vertical kumiko. This will be the second-last shoji listed in the book because it is very difficult. In this shoji, the pitch between vertical kumiko is 13.9 mm (actually 13.91 mm, but 13.9 mm is acceptable).

Now try this in imperial with fractions. Again same 20¼ inch shoji width, and 1¼ inch stile width. The pitch becomes 35/64 inch. A much more difficult number to calculate and work with than 13.9 mm, and there’s no leeway with accuracy. Squares are involved with patterns, so measurement has to be precise, otherwise the squares and the patterns won’t fit — 34/64 (17/32) inch is not acceptable.

And this is just with the vertical and horizontal kumiko arrangements. The third book will go into great detail on the diagonal diamond arrangement and many variations, and three-way joints and all the intricate patterns shown in this website, and many other patterns, and for this fractions of inches just won’t cut it. The numerator and denominator numbers become too high to work with.

So my advice is that if you’re satisfied with just the simple shoji, the imperial measurement system will work OK, and stick with that if you’re more comfortable using inches.

If, however, you want to go beyond the standard simple designs, and explore the range of marvellous things that can be done with shoji and small thin pieces of wood, you will need to learn how to use millimeters. If it makes it any easier, don’t think of them as millimeters — think of them as units. So instead of 13.9 mm, think of them as 13.9 units. You’ll be surprised at how easy pitch and interval calculations become. And there is much less risk of inaccuracy and calculation errors.



As an aside, I’m old enough to have been raised on the imperial measurement system and the pre-dollar currency days of pounds, shillings and pence in Australia, so don’t use age as an excuse — it
is possible to learn to use the metric system.
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