Dimensions, Tolerances, and Units

From shape to size

A view shows a part's shape; dimensions give its size and location — a length, a diameter, an angle, the distance from an edge to a hole's center. Without them a drawing is just a picture; with them anyone can build the exact part.

A few conventions you'll see constantly:

• Diameter is marked with Ø (Ø0.5 = a half-inch hole).
• Radius is marked with R (R0.125).
• Dimensions sit off the part on extension and dimension lines so they don't clutter the geometry.

Units: inches dominate FRC

Most of FRC is imperial, and your CAD should default to inches. The structure and drivetrain are inch-based: 1x1 and 1x2 aluminum tube, 1/2-inch hex shaft for drivetrains (3/8-inch hex for lighter mechanisms), 1.125-inch OD hex and flanged bearings, and fasteners like 1/4-20 and #10-32.

Metric shows up in two predictable places: sensors and electronics (a Limelight, a NavX, motor controllers) usually mount with M3 or M5 screws, and some COTS parts are metric internally. So you'll keep a few metric fasteners in stock, but you won't redraw your robot in millimeters.

When you do convert, 1 inch = exactly 25.4 mm, which converts cleanly with no rounding. Two rules save you pain: pick one unit per drawing and state it in the title block, and read that title block before you measure. A "6" you read as mm that was really 6 inches is a season-ending mistake.

Tolerances: nothing comes out exact

No machine hits a size perfectly, so a tolerance is the variation you'll allow around the target (the nominal size). Two common ways to write it:

1. Plus/minus: 0.500 ± 0.002 in accepts anything from 0.498 to 0.502.
2. Limit dimensions: list the max and min directly.

Tighter tolerances cost more — better machines, more time, more inspection. Call out tight numbers only where the part actually needs them; leave everything else loose.

Where tolerances make or break the robot

This is exactly where FRC parts fit or don't:

• A bearing needs the right fit in its pocket. Bore the hole a few thousandths too big and the bearing spins in place; too small and it won't seat. Bearing bores are a classic spot for a tight tolerance.
• Hex shaft into a hex bore must match closely — slop there shows up as backlash and sloppy drivetrain control.
• Bolt clearance holes are deliberately oversized (a loose tolerance) so a 1/4-20 bolt drops straight through instead of needing to be forced.

In CAD you set the nominal sizes; on the drawing you set the tolerances. Get them right and parts assemble first try. Get them wrong and you spend competition morning filing holes.

Practice this now

Pull up a COTS gearbox datasheet. Find a Ø dimension, confirm its units, and note any tolerance. Convert one inch value to mm (×25.4) and sanity-check it. That habit catches fit problems before anyone cuts metal.