Sheet Metal Tolerances
Q: What's Your Tolerance for Tolerance?
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A: Often we find that the title block used on sheet metal part prints contains a "boiler plate" tolerance section, which has typical machined part tolerances called out. Usually these tolerances will look something like this: .XXX +
.005, .XX + .010, .X + .030. The print will also have all the dimensions showing three places (.XXX).
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The problem occurs when a three place dimension (meaning a tolerance of ± .005) is used for features that have multiple bends between them. This kind of tolerancing is either not achievable or adds considerable cost.
One solution is to not use three place dimensions everywhere. However, this can obscure the actual nominal location of a feature in the CAD. So we recommend using three place dimension and using a sheet metal tolerance scheme that looks something like this:
• feature to feature in the flat: ± .005"
• features to or through a standard bend: ± .010"
(add .005" for each additional standard bend)
• features to or through a special bend: ± .020"
(add .010" for each additional special bend)
(things like hems, larger radii bends, etc...)
• weldment overall dimensions: ± .031"
(this can vary based on the weldment)
• bend radii: = to matl thickness +.062 as as max (no min)
• special/specific bend radius: ± .062" (may vary)
• hole diameter or cutout size: ± .005"
• machining dims: ± .010
• angles ± 1°
This scheme may need additional points to address concerns for some parts and circumstances (and the tolerances may need to be opened up for larger and/or more difficult parts), but it is a good starting point.
We should mention that some industries are moving more and more towards Geometric Dimensioning and Tolerancing (GD&T). In our experience much of what we have said here is also true when GD&T is used (the resulting tolerance is often too tight for typical sheet metal production).
