I have an interesting example of the hazards of using dual units in
writing specifications. This is in the operator's manual for a Hitachi
C10FCH2 compound miter saw.
The directions for mounting the saw to a work bench specify use of 5/16
inch (8 mm) bolts, which I believe I reported earlier. What has caught
my attention now is the specification for length. It turns out that if
you follow the non-metric guidance for mounting bolts, you might have
problems!
It says:
Bold length should be at least 1-3/8" (35mm) plus the thickness of the
work bench. For example, use 2-11/32" (60mm) bolts for a 1" (25mm) thick
work bench.
Yes, they use " to mean inch and they omit the space between the numbers
and the unit symbol mm. We've seen that before in many places.
But consider this. If Hitachi were doing this in non-metric units then
converting, the last bolt length mentioned above would be
1-3/8 inches + 1 inch = 2-3/8 inches, not 2-11/32 inches. But by back
conversion from the metric sizes, they ended up with a strange minimum
specification for bolt length: 2-11/32 inches. Obviously, Hitachi
designed this in metric since 35 mm + 25 mm = 60 mm.
Instead, one would choose the next larger length if shopping for
non-metric bolts, probably 2-1/2 inches. But, what about diameter? One
can find 5/16 inch bolts, but they are not as common in my local
hardware stores as 1/4 inch or 3/8 inch bolts. A 1/4 inch bolt would be
too small; it might shear or otherwise fail. Would a 3/8 inch (0.375
inch) bolt fit the holes in the saw's baseplate? Nope!
Moral: those who design in one set of units and then try to provide
conversions to other units should be careful. Naturally, designing in
and specify only in metric is the ideal solution. I had no problem
finding 8 mm bolts of the appropriate length (M8-1.00 × 60), with
washers and nuts in my local Lowe's.
Jim
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(H) 931.657.3107
(C) 931.212.0267
