A worker at an assembly station fits shafts into holes drilled in a casting. Some of the shafts fit loosely and can be tipped sideways while others need tapping into place with a hammer. What’s going on? Were some of the shafts or castings machined to the wrong dimensions?

This is an example of tolerance and fit specifications being poorly applied. Tolerances define the amount by which a dimension is allowed to vary, while fits describe how closely two parts should join together (or the amount of play permissible.) Tolerances and fits are integral to precision machining. This blog provides an introduction to what they are, how they work, and the benefits of applying them correctly.

Uncertainty at the Root

Variation is unavoidable in machining. It results from factors like positioning errors, wear in slideways, and changes in material hardness that affects tool deflection.

Dealing with this requires applying tolerances to dimensions. These tolerances are set to account for both the needs of the assembly and the capabilities of the machining process. Modern CNC machine tools, with their rigid structures and high resolution positioning systems, are capable of accuracies and repeatabilities unimaginable a few decades ago, but they still have a degree of variation.

There’s also uncertainty in measurement. All measurement systems have limits to their precision, and it’s seldom possible to go beyond three or four decimal places.

The bottom line is, it’s not possible to make things exactly, and neither is it possible to measure exactly. This uncertainty is dealt with through tolerances.

Limits and Fits

When parts made in one machine shop have to fit with parts made in another shop, it’s essential that both shops follow the same system of tolerances. This is addressed with limit and fit standards.

“Limits” refers to upper and lower limits on a dimension whereas a tolerance is the width of the band of allowable variation. “Fit” is how tightly one part, typically a shaft, fits within another.

Globally, limits and fits are defined in ISO 286-2:2010. The US version is ANSI B32.100-2005.

This standard defines three types of fit: interference, clearance, and transition. An interference fit is where the shaft, even at its smallest diameter (the minimum material condition), is slightly larger than the biggest diameter of the hole. In other words, it will always take some force to push the shaft into the hole.

A clearance fit is the opposite. The largest diameter shaft is always smaller than the smallest diameter hole, so the shaft will always slide through.

A transition fit falls between the two. A large shaft will stick in a small hole, but a small shaft will fall through a large hole.

In practical terms, there aren’t many applications for a transition fit. Usually a designer wants either clearance or interference.

Looping back to the ISO standard, this ties tolerances of holes and shafts to fits. ISO and ANSI define four hole tolerances and four shaft tolerances. This gives the designer freedom to select the ones that provide the desired fit, in conjunction with ease of manufacture.

Meet Geometric Dimensioning & Tolerancing

Always referred to as GD&T, this is a system for putting limit and fit information on a part drawing. It goes beyond simple plus/minus values to define the intended function of each surface and hence which aspects are especially important. For example, concentricity and parallelism are two aspects of tolerancing that are important on almost every machined part or component.

Consistent Assembly and Interchangeability

Tolerances, fits, and GD&T are critically important in modern precision machining. They let machine shops take advantage of the capabilities of modern CNC machine tools to produce high-precision components.

When assembled, these will function smoothly, with no “slop” or excess friction, functioning accurately and repeatably, and only wearing very slowly. And when replacement is needed, the tight tolerances mean the replacement part will slot right in and work as expected, no adjustment required.

Capable and Competent

Precision machining is a highly specialized field, and mastery requires a thorough understanding of tolerances and fits, and the machining capabilities to produce whats required. As a leading machine shop operator, Impro has both. Contact us to discuss your precision machining needs.