The vast majority of engineered products have multiple components that need to fit together. Sometimes you need components to slip together easily, while other times, you need components to press together and not come apart. There’s a systematic way to design parts so they fit together exactly the way you want on the first try, every time, and in this video, we show you how!
The engineering term for this consideration is “fits and tolerances”. Let’s define these terms. Most of the time, especially in CAD, we just type single, exact numbers into the computer for our dimensions. That’s called a “nominal” dimension. But when it comes time to actually make that part, we won’t be able to manufacture the features to exactly those dimensions. Each feature might be slightly larger or smaller than the number we originally typed into the computer. So, we need to specify a “tolerance” which tells the person manufacturing the part how much of a variation from our nominal dimension is acceptable.
Different manufacturing processes are capable of different tolerances. It’s important to remember that tolerance capability corresponds to a large production run, where factors like tool and machine wear, deflection, sharpening, and the natural variation of different processes apply. A very common mistake that new engineers make is selecting tolerances that are far tighter than necessary. As the tolerances become tighter, the cost to manufacture the part grows exponentially.
So how tight of a tolerance do you need? Tolerances typically come into play when designing fits. In some cases, you might want components to slip easily together, but not have a lot of perceptible play. In other cases, lots of relative movement is okay. You might even need to have two components come together permanently and not come apart. These different cases are called “fits”.
Machinery’s Handbook is the go-to reference for designing fits. Fits have a particular naming convention of a two-letter abbreviation followed by a number.
The letters denote the type of fit, and the number corresponds to the tolerance class of the fit, with larger numbers representing looser tolerances. This video deals primarily with ASME fits in the inch system, but there is a similar convention for metric. The naming convention is different, but the general concept is the same. For more information, you can reference Machinery’s handbook, or ISO 286-2.
Be sure to watch the video to see examples of six different types of common fits, and some general guidelines for how to manufacture them.