A gear is useless until it is locked to its shaft, and in most industrial drives that lock is a keyway or a spline. These features look simple on a drawing — a rectangle cut into the bore or the shaft — but the fit they provide determines how much torque the drive can transmit without fretting, backlash or noise. Buyers who understand keyway callouts get faster quotes and avoid the classic "the key fits, the gear rocks" complaint.
Two families dominate industrial practice. A keyway is a single slot cut in the bore or on the shaft, mated with a parallel or tapered key; it is cheap, easy to measure and standardised around the world (GB/T 1095, DIN 6885, ANSI B17.1). A spline is a series of teeth cut around the bore or shaft — involute splines for gears, serrations or parallel-sided splines for lighter duties — which distributes torque over many surfaces and allows sliding engagement. The choice is usually made by the application, not by preference.
Reading the callout is straightforward once the system is familiar. In a drawing such as 12.5h8, the number is the nominal width of the slot and h8 is the ISO tolerance of that width. For a parallel key seat in the hub, the slot width is often made to a Js9 or D10 tolerance while the shaft keyway is held to h9 or N9; the h8 class in this example is a controlled width that the key is selected against. The depth of the keyway, its position relative to the bore axis and its symmetry are just as functional as the width.
Machining differs with the part and the volume. Broaching is the fastest and most consistent way to cut internal keyways and splines, and it is the standard for production. For single pieces or large-diameter parts, keyways are slot-milled or wire-cut on a machining centre or EDM. External shaft keyways are milled or ground; splines on shafts are hobbed or shaped, then ground when the fit class demands it. Each method leaves a different surface finish, which matters for press fits and sliding fits.
Verification is where most problems surface. Width is checked with plug gauges, depth and symmetry with a micrometer or a keyway gauge, and splines with GO/NO-GO gauges or a coordinate measuring machine. For a mating pair, the practical test is assembly: a parallel key should tap in with a light interference and the hub slide over the shaft with controlled clearance. A supplier that reports the actual measured values instead of "conforms" is one you can trust.
At our factory, internal keyways and splines are broached, external keyways are milled or ground, and every slot is gauged before dispatch. Send us your drawing with the keyway or spline callouts marked — we will confirm the method and give you a quotation within 24 hours.