How to Choose Tolerance Grades for Machined Shafts
A practical guide to selecting IT grades, roughness and geometric tolerance for turned and ground shafts, based on how the shaft actually works in the assembly.
Shaft tolerance is often specified tighter than the assembly needs, which raises machining cost without improving function. The starting point is to identify what the shaft does: it either rotates in a bearing, slides against a bushing, or simply locates a part.
For a shaft running in a rolling bearing, the journal should be IT6 with Ra 0.4 and roundness within 0.003 mm. A looser journal lets the inner ring creep on the shaft and wears the fit. For a shaft sliding inside a bronze bushing, IT7 with Ra 0.8 is usually enough, because the bushing material accommodates small deviations.
Locating shoulders need perpendicularity rather than diameter accuracy. A shoulder that is 0.05 mm out of square will tilt a gear or a bearing even if the journal diameter is perfect. Specify perpendicularity to the shaft axis, typically 0.01 to 0.02 mm.
Straightness matters on long shafts. Above 500 mm, specify straightness per metre rather than total, and expect a straightening operation after heat treatment. Grinding removes only a small amount of stock, so distortion from quenching must be corrected before the finishing pass.
Surface roughness should follow the tolerance. There is little value in holding IT6 on a surface finished to Ra 3.2, because the roughness peaks are the same order as the tolerance band. As a rule, Ra 0.4 pairs with IT6, Ra 0.8 with IT7, and Ra 1.6 with IT8.
When the drawing states the function instead of only the number, the process route becomes easier to plan. Marking which section carries the bearing, which section is threaded and which is only for handling lets the factory decide where to spend machining time and where to leave it as turned.
