What is a fit between hole and shaft?

A fit is the relationship between two mating parts, usually a shaft in a hole, set by a tolerance class on each. In the ISO system a capital letter stands for the hole and a lower-case letter for the shaft; the letter gives the position of the tolerance zone relative to the nominal size, the number gives the IT grade and so the width. At ⌀20 H7/g6 the hole lies between 20.000 and 20.021 mm and the shaft between 19.980 and 19.993 mm, which leaves a clearance of 0.007 to 0.041 mm. There are three types: a clearance fit, with clearance at every permitted size; a transition fit, such as H7/k6, which can end up with clearance or interference; and an interference fit, such as H7/s6, with interference at every permitted size.

What does a fit depend on?

Which fit a designer chooses depends on the function: rotating or sliding, accurate location, or carrying load without a key; length of engagement, materials, operating temperature and assembly method count as well. Many drawings use the hole-basis system, with an H hole and the fit chosen through the shaft, because holes are made and checked with fixed-size tools such as reamers and plug gauges. The shaft-basis system, with an h shaft, suits drawn bar used without further machining, or one shaft that carries several parts with different fits.

What does the standard say about a fit?

Codes and limit deviations are set in ISO 286-1, the basis of tolerances, deviations and fits, and ISO 286-2, the tables per tolerance class; the current edition of both is from 2010. The values hold at the reference temperature of 20 °C. Under the independency principle of ISO 8015, a size tolerance limits only the local sizes, by default the two-point size of ISO 14405-1, not the form as a whole: a shaft can be within size at every point and still be bent or lobed. Where function demands it, the drawing often adds the envelope requirement, as in ⌀20 H7 Ⓔ, or a separate form tolerance. Older German drawings under DIN 7167 apply the envelope requirement without any indication; which rule holds follows from the standard the drawing refers to.

How does a fit work in practice?

A fit shapes the route. A hole at IT7 is often pre-drilled and then reamed, bored or circular-milled to size; a shaft at IT6 often needs a fine finishing cut or cylindrical grinding. What a single pass can hold depends on machine, tooling, size, material and temperature. Measuring often becomes a step of its own too, with a plug gauge or a bore micrometer. Whether fits add time and cost depends on how many there are, the operations already in the route, how often you measure and the batch size.

What is a fit often confused with?

A fit is not an IT grade: the grade only gives the width of one tolerance zone, while the fit pairs two zones with their positions. A tolerance class such as H7 is not yet a fit either; only with the mating part's class is it settled whether there is clearance or interference. And a fit is not a form requirement: without Ⓔ or a form tolerance, size alone does not limit straightness and limits roundness only in part.

How do you see a fit in Blake?

In Blake, fits and other tolerances sit with the part as proposals from the drawing, together with material, roughness, finishing and threads. You confirm or correct them; who did so and when stays visible, with a note if you add one. The Machine fit in Blake is a different thing: a first check per machine on envelope and axes, naming the deciding limit. More on requests.