Question 1

What is the most commonly used fit in mechanical engineering?

Accepted Answer

H7/h6 (sliding fit) is the most universal fit for locating parts that need to slide or be assembled by hand. H7/f6 gives a free-running clearance fit for bearings. H7/p6 gives a medium press fit for permanent assemblies. These three cover roughly 80% of typical engineering fit requirements.

Question 2

What is the difference between hole basis and shaft basis systems?

Accepted Answer

In hole basis (most common), the hole is always 'H' and different shaft letters create the fit type. This is practical because standard reamers and broaches produce H-tolerance holes. In shaft basis, the shaft is always 'h' and different hole letters are varied. Shaft basis is used when a single shaft must mate with multiple features, such as a stepped shaft with different fits along its length.