We’ve all been impressed, at one point or another, by a part that was made for aerospace. They usually have tight tolerances, weird geometries, and of course they’re made from ridiculous materials like Inconel or a crazy titanium alloy

You probably already know this, but tolerances are important. If your product is a standalone part with only one component, tolerances may not be as important.

This post originally appeared on the In The Loupe blog.

Tolerance stacking, also known as tolerance stack-up, refers to the combination of various part dimension tolerances. After a tolerance is identified on the dimension of a part, it is important to test whether that tolerance would work with the tool’s tolerances: either the upper end or lower end. A part or assembly can be subject to inaccuracies when its tolerances are stacked up incorrectly.

Every machinist knows things work a whole lot better when turning if you can get your tool tip right on the centerline. During a trip to Tormach to test their CNC lathe before I got one years ago, I saw this first hand. I “eyeballed” a tool tip to center and made an OD turning pass. The cut was fine, but the finish seemed poor. So, we set the tool on centerline using the ruler trick:

We’ve all heard the stories about part drawings coming through that specify ridiculously tight tolerances. We know that holding tight tolerances is harder, and we know it is more expensive to make a part the tighter the tolerances are. But, how much does it really cost to have tighter tolerances?