What is an ISO fit like H7/g6?

H7/g6 is a fit pairing an H7 hole with a g6 shaft. The capital letter (H) is the hole tolerance and the lowercase letter (g) is the shaft tolerance; the number is the IT grade. H7/g6 is a common clearance (running) fit.

What do the deviation values in micrometres mean?

Each tolerance band has an upper deviation (ES for holes, es for shafts) and a lower deviation (EI/ei), given relative to the nominal size in micrometres (1 µm = 0.001 mm). The actual limits are nominal size plus those deviations.

What is the difference between clearance, transition and interference fits?

A clearance fit always leaves a gap (the shaft is smaller than the hole). An interference fit always has overlap (the shaft is larger and must be pressed in). A transition fit may end up with a small clearance or a small interference depending on the actual produced sizes.

What is the difference between hole-basis and shaft-basis fits?

In a hole-basis system the hole tolerance is fixed at H and the shaft is varied to obtain the fit; in a shaft-basis system the shaft is fixed at h and the hole is varied. Hole-basis is more common because standard reamers and gauges are made to H limits.

What does the IT grade number mean?

The IT (International Tolerance) grade sets the width of the tolerance zone. A smaller number means a tighter tolerance: IT6 and IT7 are typical for precision fits, while IT11 is a coarse grade. The width also grows with nominal size.

Reference

ISO Tolerance & Fits Table

Look up ISO 286 limit deviations for hole and shaft tolerance bands across nominal sizes, and tell clearance, transition and interference fits apart.