That is because their inventor was Carl E. Johannsson (I'm not
sure that I have the right number of 'n's and 's's in that name), and he
was hired to implement his set of reusable standards at Ford many years
ago. Brown & Sharpe gauge blocks are still marked with his full name,
or on the smallest just "C.E.J".
Starrett sells a different line, marked "Webber", which I
presume started when the patent that Johannsson had ran out. Both are
excellent sets. But "Jo blocks" tends to be used even when they are
marked "Webber". :-)
While a good 1-2-3 block is quite accurate, they are not as
accurate as gauge blocks. Even the cheap Chinese ones are supposed to
be accurate to 0.000050" (50 micro-inches), and the best (and most
expensive) of them are as good as (I think) 0.000002" (2 micro-inches).
At that kind of accuracy, the thermal expansion from the heat of your
hand when you hold a "Jo" block too long will introduce errors.
Agreed. I've never seen ones with blind holes, which makes me
wonder about the item in the posted puzzle set.
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It's a packing block, with a convenient range of adjustable heights,
It's also full of holes, so you can stick bolts through it.
Imagine you're trying to clamp a T-shape to the table of a milling
machine and only the central leg of the T is machined flat and
clampable. Hang it over the top of a 1-2-3 block and you can hold it
down, without resting on the as-yet unmachined bosses of the T's
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