Shaft, bush and sleeve tolerance basics

• Clearance: the parts always assemble with a gap. Used where relative movement is required, or where assembly must be possible by hand.
• Interference: the parts always require force to assemble and stay put afterwards. Used where the joint must transmit load without a key or fastener.
• Transition: depending on where each part lands in its tolerance, the fit may be slightly loose or slightly tight. Used for accurate location where the part is still removable.

The useful sequence is: what must this joint do, what fit delivers that, and only then what tolerances on each part produce that fit. Working the other way round — picking tolerances that feel safe — is how parts end up over-specified.

Stating the intended fit on the drawing, in words if necessary, also gives the manufacturer something to sanity-check against. If the tolerances and the stated intent disagree, that is worth catching before machining.

Every reduction in tolerance costs something: a slower process, an extra operation, more inspection, higher scrap. On a ground diameter, halving the tolerance can change the route entirely.

It also makes assembly less forgiving, not more. A joint with almost no permissible variation is one where a small amount of thermal growth, contamination or handling damage puts it out of specification.

• Heat treatment, which can change size and shape after the feature was cut
• Plating and coating, which add thickness that must be allowed for in the machined size
• Temperature at assembly and in service, particularly where the two parts are different materials
• Surface finish — two parts at the same measured size assemble differently if one is rough
• Form error: roundness and cylindricity can make a part fail to assemble while every diameter measures correctly

A diameter tolerance constrains size, not shape. A bore can be within tolerance at every measured point and still be oval enough to bind on a round shaft.

Where the joint depends on shape as well as size — a bearing seat, a seal running surface, a sliding fit — it is worth stating the form requirement explicitly rather than assuming the size tolerance implies it.

• The mating part and the intended fit, even as a note
• Which of the two components carries the tighter share of the tolerance
• Form requirements where shape matters, not only size
• Whether the stated size is before or after any coating
• Surface finish on running and sealing surfaces