Tolerance and Fit: Clearance, Transition and Interference

A drawing that says 25 mm is asking for something impossible, and everyone knows it. What it means is 25 mm within some allowance — and the size of that allowance decides the cost of the part, so it is an engineering decision rather than a formality. What it decides beyond cost is how the parts BEHAVE together. A shaft slightly smaller than its hole spins; slightly larger, it has to be pressed in and will never come out. The gap between those two cases is a few tens of micrometres, and inside it sits every bearing, every dowel, every keyed hub and every press-fitted bush ever made. This rung is where the measurements of the previous ones acquire a purpose: you now know how to find the size, and this is how to know what size to want.