Class Of Fit
Class of fit is the standardized designation defining the functional relationship between mating parts—the amount of clearance, transition, or interference when assembled. It specifies whether a shaft slides freely, locates accurately, presses lightly, or requires force, and for Unified threads is designated 1A/1B, 2A/2B, or 3A/3B, with higher numbers indicating tighter fit.
On the shop floor, class of fit appears directly on drawings and setup sheets, steering tool selection, bore finishing, reaming, grind allowance, thread chasing, gaging, and inspection limits. ISO and ANSI B4.1 style systems translate design intent into measurable tolerance zones. A machinist reading H7/h6 knows to target a bore slightly above nominal and a shaft slightly below, yielding controlled clearance for location. H7/k6 signals a transition fit—often held near nominal—while H7/p6 demands deliberate oversizing of the shaft for press or shrink assembly. Standard running or locational fits are common defaults, but bearing seats, bushings, and permanent hubs call for interference. Threaded parts use 1A/2A/3A classes to control pitch diameter and allowance, not just major diameter. Before cutting, a machinist verifies material condition, coatings, and thermal growth, because anodizing, plating, or heat can close a clearance zone and turn a print-correct part into an interference assembly.
What is the difference between clearance, transition, and interference fits?
Clearance fits always leave positive gap, transition fits may produce either slight clearance or slight interference, and interference fits require overlap that must be assembled by press, shrink, or heavy force.
What does a fit class like H7/h6 actually communicate?
It defines the hole and shaft tolerance zones in the ISO system; H7 is the hole tolerance zone and h6 is the shaft tolerance zone, together describing a location-style assembly with controlled but minimal clearance.
Why are 1A/2A/3A thread classes important in CNC work?
They specify how tight the internal and external threads should mate; 2A/2B is the common production default, 1A/1B is looser, and 3A/3B is tighter and more precision-oriented, with correspondingly greater machining and inspection sensitivity.