Shaft Basis System

The shaft basis system is an ISO limits-and-fits method where the shaft is the reference size with zero upper deviation; fit is created by varying the hole tolerance zone. The shaft's largest permissible diameter equals nominal size, while the bore is machined to achieve the required clearance, transition, or interference. This system is used when the shaft is a standard stock diameter or ground shank.

In a CNC cell, the shaft basis system is advantageous when the rotating member, such as an arbor, spindle shank, or purchased bar stock, must remain constant. The shaft serves as the datum with its tolerance zone locked at the zero line. The mating bore is then adjusted by boring, reaming, or toolpath offset to generate the intended fit type. This approach is particularly practical when shafts are standardized, ground, or difficult to modify, as holding the shaft fixed reduces variation crucial for interchangeability and running accuracy. On a millwork or assembly line, the same principle applies when a fixed-diameter insert or dowel must locate into a drilled feature without altering the standardized component. The design logic simplifies sourcing and inspection by keeping the shaft constant while the bore is adapted to fit.

• ⚠️Treating shaft-basis like hole-basis: If the bore is held constant instead of the shaft, the fit can shift from clearance to interference. Assemblies won’t start, require force fit, or have excessive lash because the wrong reference was used.
• ⚠️Assuming shaft can never exceed nominal: Upper deviation is zero, so oversize from tool wear or thermal growth compromises the fit window. A bore machined to nominal may bind, causing seizure, press-overload, or damaged bearing seats.
• ⚠️Ignoring hole-making process capability: Shaft-basis works only if the hole process reliably hits tolerance. Variability from out-of-round, taper, or reamer wear leads to inconsistent clearance and poor concentric running.