Fits & Clearances

Nothing is made to size, only to limits. A drawing gives a basic size and then a tolerance zone either side of it, and the part is acceptable anywhere inside that zone. The fit is therefore not one number but a range, running from the loosest possible combination (largest hole with smallest shaft) to the tightest (smallest hole with largest shaft). That is why both a maximum and a minimum clearance are quoted.

In the basic hole system used here, the hole is the fixed member: H means its lower limit sits exactly on the basic size, so the hole is never smaller than nominal. The fit is then changed by moving the shaft zone up or down, which is far cheaper than reaming a bore to a new size for every fit. The letter gives the position of the shaft zone and the number gives the IT grade, which is the width of the zone: a smaller number means a tighter, more expensive tolerance.

Three outcomes are possible. A clearance fit always leaves a gap, so the parts assemble by hand and can rotate or slide. An interference fit always has the shaft larger than the hole, so it needs a press, or heating the housing and chilling the shaft, and it holds by friction alone. A transition fit sits across the boundary: depending on where each part actually finished within its zone, the same drawing can give either a small clearance or a small interference, which is exactly what you want for a located part that must still be removable.

• Try this: Compare H7/g6 and H7/p6 at 25 mm. Both zones are the same width, so what has actually changed, and why does that change how the parts go together?
• Try this: Keep H7/k6 and sweep the basic size from 4 mm to 110 mm. Why do the tolerances get wider as the part gets bigger?
• Try this: On H7/k6, watch how the fit can come out either way. What does that mean for a mechanic who finds one bush a push fit and the next one on the same drawing needing a tap?