Thou, tenth and skookum: how machinists talk about size, fit and strength

The setup man holds the part up to the light, then reaches for the mic. "Bore's a thou and a half small. Bump it two tenths at a time and sneak up on it." A newcomer hears a tool, two units of measure and a stealth maneuver. A machinist hears a complete plan. Shop talk about size is compressed and exact, and once the units are straight, the rest of the vocabulary falls into place.

The units, from a thou down to nothing

A thou is one thousandth of an inch, about 25 microns. It rhymes with "how," and nobody in an inch shop ever says "one one-thousandth." A reading of .503 on a half-inch pin is "three thou over." A human hair runs two to four thou thick, which gives a sense of how much room a typical part has to be wrong.

A tenth is where newcomers trip. In a machine shop it is never a tenth of an inch. It is a tenth of a thou: 0.0001 inch, about 2.5 microns. Bearing fits, ground pins and precision bores live in tenths, and the people who hold them talk about a temperature change in the shop the way sailors talk about weather.

Past the numbers, the vocabulary goes vague on purpose. A gnat's whisker is an amount too small for anyone to bother putting a number on, usually under a thou. To kiss a surface is to touch it with the tool or wheel so lightly that almost nothing comes off. And when nobody has a gauge handy, there is always the eyecrometer, the calibrated eyeball, and its partner Kentucky windage, the art of adjusting by feel.

Hitting the number

Every dimension starts as a nominal size, the target printed on the drawing. The tolerance is how far a part can wander from it and still be good. A half-inch bore at plus or minus a thou can measure anywhere from .499 to .501. A part that lands exactly on the number is dead nuts, the highest compliment a measurement can get.

Getting there is a matter of patience. The bump in the opening scene is a tiny change to a tool offset, and sneaking up on it means taking smaller and smaller cuts, measuring between each one. A skim cut cleans up a surface without moving much metal. A spring pass runs the same cut again at the same setting, to take off whatever the tool left behind when it flexed. The whole ritual exists because of one law of the trade: material comes off easily and never goes back on. Cut past the size, undersize on a shaft or oversize on a bore, and the part is scrap.

Production adds a formal step. The first article off a new setup gets a full inspection before anyone runs the rest of the batch. Quick checks on the floor use a go/no-go gauge: one end must fit and the other must not, and no reading is required.

Setting up straight

Size means nothing if the part is crooked. Before a cut, a machinist will indicate the vise, running a test indicator along the fixed jaw until the needle barely moves end to end. On a knee mill, someone has to tram the head so the spindle stands square to the table, and to find the center of a bore they sweep it, swinging the indicator around the circle until it reads the same all the way around.

Height comes from ground steel. Parallels lift a part in the vise to a known level, and 1-2-3 blocks stack into setups and squares. In the inspection room, everything sits on a granite surface plate, and heights are set with jo blocks, steel blocks lapped so flat they wring together and hang off each other. Anything that ends up out of square, from a setup to a building, is cattywampus.

Fit: the language of threads

Threads carry their own tolerance talk. Inch threads are named by diameter and TPI, so a half-inch UNC bolt is a 1/2-13 and its UNF cousin is a 1/2-20. Metric threads use pitch in millimeters instead. The number that controls whether a nut spins on smoothly or binds is the pitch diameter, not the outside size.

How loose or tight that fit runs is the thread class. Most hardware is 2A and 2B, with a little clearance built in. Precision work calls for Class 3A/3B, with no allowance at all on the bolt. A machinist checks a tapped hole with a thread plug gauge: the GO end has to screw in all the way, the NO GO end may start but must not run in. Start a nut crooked and you cross-thread it. Find a dented thread and you chase it with a tap or die to clean it up without cutting it bigger.

How strong is strong

When the talk turns from size to strength, the words get warmer. Skookum means stout, strong and built right, the top praise for a weld, a fixture or a person who can lift the other end. Grade 8 is the strongest common inch bolt, six lines on the head, and it doubles as an adjective for anything overbuilt. At the other end sits hardware-store grade: unmarked, soft, zinc-plated and of unknown parentage. The head marking, grade lines plus a maker's mark, is the main clue to which one is in your hand.

A bolt does its job through preload, the stretch that clamps the joint shut. Torque only stands in for it. The torque-tension relationship is dominated by friction, which is why a dry thread and an oiled one give very different clamp at the same wrench setting. So a click wrench setting is only as good as the lube spec behind it, and joints where preload really matters get their stretch measured instead. Once a fastener is done, a paint line called a torque stripe shows it was checked and reveals any later movement.

The informal specs are funnier. Gutentight means tightened by feel until it seems good and tight. Ugga dugga means run up with an impact gun until it sounded done. Push either too far and the bolt comes off necked down, or simply twisted off, with the stub still in the hole.

Five words to learn first

1. Thou: one thousandth of an inch, the everyday unit of shop talk.
2. Tenth: one ten-thousandth of an inch, never a tenth of an inch.
3. Dead nuts: exactly on size, nothing left to fix.
4. Sneak up on it: smaller and smaller cuts, measuring between each, so you never cut past the size.
5. Skookum: strong, stout and built right, the best thing anyone can say about your work.