Jo blocks
Jo blocks turns up in 2 fields: machine shop, metrology. Here is what it means to the people in each one.
Jo blocks in machine shop
Shop name for gauge blocks: lapped steel or ceramic blocks, accurate to millionths, stacked to make any length.
Precision gauge blocks named after their Swedish inventor, sold in sets such as 81 or 88 pieces that can be combined to build almost any length in steps of a tenth. Their lapped faces are so flat they wring together, held by a thin film of oil or moisture and molecular attraction. Jo blocks calibrate mics, height gauges and indicators, and set sine bars and comparators.
"Wring up a 2.4375 stack out of the Jo blocks and check the mic against it before you touch the bearing journals."
Jo blocks in metrology
Shop name for gauge blocks, precision steel or ceramic blocks wrung together to build reference lengths.
Machinist and cal-lab nickname for gauge blocks, taken from the Swedish inventor of the stacked block system. They are lapped to length and flatness within a fraction of a micrometre and can be wrung together into almost any length from a set of 81 or more blocks. Labs calibrate them by mechanical comparison against master blocks or by interferometry, and they carry dimensional traceability down to the shop floor.
"Wring up a 37.5 mm stack from the Jo blocks and check the micrometer at three points."