Why Do We Use Tolerance?

The importance of machining tolerances cannot be understated. Tolerances are perhaps the most critical aspect when it comes to manufacturing a part so that it can be produced affordably, efficiently, and will operate as the designer/engineer intended.

In a nutshell, a tolerance gives the manufacturer a dimensional range that a part feature can fall within to still function properly. In some cases this may be a very wide range on non-critical or aesthetic elements where tight geometry doesn’t come into play. In other instances (i.e. fittings, bushings, mating parts) the tolerance may call for a very tight range, often times less than a thousandth of an inch, to ensure proper alignment and avoid any type of mechanical breakdown or interference.

How Much Tolerance Do You Need?

It is always best practice to include tolerances on your parts, but how much tolerance do you really need? Producing a part with a tighter tolerance will take longer to machine and inspect, which means extended lead times and increased production costs. This is why it is so important to consider how much tolerance is actually needed for each machined element.

Standards

Designers and engineers use a host of manufacturing standards to derive tolerances on their parts, and these can be found through a variety of online resources. Standards such as these help eliminate the guess work when creating part geometry and calling for certain types of fits between key elements. Many part manufacturers will also provide tolerances on their website when purchasing components to incorporate into your design (i.e. custom fittings, bushings, etc.)

Talk About Tolerance

Most machine shops will do their due diligence if a tolerance is ever in question, and will follow up with the designer. In many instances a shop may suggest opening up a tolerance to allow for an alternative machining process if it will save the customer time/money. A shop may also suggest tightening things up if they anticipate issues on a new part based on its end use. Like most things in manufacturing, tolerances are an area that benefit from open communication to ensure that parts are made with excellent quality without unnecessary time/expense.