It's for hammering the lug tight to the cable just in case you don't have the large crimper for 1/0 or 2/0 cable lugs handy.
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You're welcome.
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.
Enjoy, DoN.
But what is it for? what does it do, exactly?
A 1-2-3 block is a reference and pedagogical device for innumerate metalworkers.
Ned Simmons
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 crossbar.
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