Units, Measurement, and Tolerances

Why measurement comes first#

Every part on your robot starts as a number on a drawing or in CAD. If two people read that number differently, the gearbox plate won't line up and the bearing won't seat. Measuring parts and writing down dimensions everyone agrees on is the skill the whole build depends on.

FRC runs on inches (mostly)#

Most FRC structure is dimensioned in inches: 1x1 and 2x1 aluminum tube, hex shaft, gear bores, sprocket pitch. Get comfortable with fractions and decimals of an inch, because that's the world your tube stock and shafts live in.

Metric shows up too, almost always on commercial off-the-shelf parts: small sensors, electronics, and 550-class motors (like the NEO 550) commonly use M3 screws. Full-size FRC motors — the NEO, Falcon 500, and Kraken X60 — actually mount with imperial #10-32 screws, not metric. This distinction matters: an M5 will start threading into a #10-32 hole and destroy it, so never assume a motor is metric — check its datasheet. So a real robot is a mix. The fix is simple discipline: always write the unit next to the number, and convert deliberately when worlds meet:

• 1 inch = exactly 25.4 mm
• Don't "round" a 1/2 inch hole to 12 mm. It's 12.7 mm, and that 0.7 mm is the difference between a press fit and a rattle.

The numbers you'll actually reuse#

A handful of FRC standards repeat on nearly every robot:

• Hex shaft: 3/8 inch and 1/2 inch hex are the two standard sizes; 1/2 inch hex is the workhorse for drivetrains and most gearboxes. Gears, sprockets, and wheels are sold with matching hex bores so they key onto the shaft and can't slip.
• Fasteners: #10-32 is the default structural screw in FRC. Use #8-32 for lighter brackets, electronics, and sensors, and 1/4-20 for heavier structural joints and rivet nuts.
• Gears: 20 diametral pitch (20DP) is the standard tooth size, so any two 20DP gears mesh.

Knowing these means you can spec a part without re-deriving it every time.

Accuracy, precision, and your caliper#

• Accuracy is how close a measurement is to the true value.
• Precision is how repeatable your measurements are.

A tape measure reads to roughly 1/32 inch; a caliper reads to about 0.0005 inch (0.01 mm) and is the tool you reach for whenever a fit matters. Don't write more digits than your instrument supports. Calling a tape-measured hole "0.5000 inch" claims a precision you don't have.

Tolerances and fits#

Nothing machines or prints to a perfect size, so you specify a tolerance: the range a dimension is allowed to vary. A shaft drawn 0.500 / -0.002 inch is good anywhere from 0.498 to 0.500 inch.

Tolerance choices decide how mating parts behave:

1. Clearance fit — the hole is always bigger than the shaft, so it slides. A #10-32 screw through a 0.196 inch clearance hole is a clearance fit.
2. Press (interference) fit — the part is slightly bigger than the hole and has to be pressed in. A bearing in a gearbox plate is the classic example: too loose and it spins in the pocket and wallows it out; too tight and it binds or cracks the plate.
3. Transition fit — in between; snug but not locked.

This bites hardest on 3D-printed and laser-cut parts, which come out a few thousandths off nominal. Printed holes shrink, so teams oversize them or drill them to final size. When you design a bearing pocket, you pick a number and a tolerance, because real holes wander. Get it right and parts assemble like they were meant to.