Working with Sheet Metal Tolerances

When choosing a manufacturer for your sheet metal parts, the term tolerance often comes up. Everyone wants their parts to be exact replicas of their models, but in truth, there always has to be some wiggle room in the definition of perfection. That variation is what we all call tolerance. Even though the term is used all the time, the reality of manufacturing tolerances can get quite complicated depending on the process in question. This post will compare machining, 3D printing, and sheet metal processes and how they affect achievable tolerances.

Sheet Metal Fabrication vs. Machining vs. 3D Printing

Machining and 3D printing are highly precise manufacturing methods, accurately removing or adding (respectively) material to produce a final geometry. Sheet metal is a slightly less precise method of manufacturing that relies on the cutting, bending, and stretching of thin sheets of metal to produce a final geometry. The looser tolerances achieved by the sheet metal method are a product of the highly variable processes used to achieve a geometry.

Machining provides the most understood frame of reference for manufacturing tolerances. We often see machining tolerance blocks on sheet metal prints, like this one on the right.

In engineering school, they teach that ±0.005 in. (0.127mm) for three significant figures is the baseline tolerance. This is possible because there is one machine creating features irrespective of features already created. It doesn’t matter if you drilled a hole in one place, the next hole position and size will be determined solely by the machine making the cut.

3D printing is also a precise manufacturing method though the actual level of precision is determined by materials and processes used. Keep in mind, like machining, 3D printing creates features on a part regardless of the features that came before it. It will continue to add material to the necessary places and maintain a high level of precision (assuming your design properly supports the part during manufacturing). Here at Protolabs, 3D printing tolerances vary from ± 0.002 in. to ± 0.012 in. (± 0.051mm to 0.305mm).

In a sense, sheet metal walks a line between manufacturing and artisan goods. We bend and stretch material to achieve a final geometry. We are not adding and removing metal with a single highly precise machine. In fact, sheet metal manufacturing requires almost a dozen machines, depending on the features required. Even the most basic formed part requires a cutting machine and a bending machine, each with its own tolerances and limitations.

Protolabs Sheet Metal Tolerances

We break out tolerances into two categories.

• Tolerances on one surface
• Tolerances across multiple surfaces

Tolerances on one surface are much tighter than those across multiple surfaces. When looking at a single surface, most of the features are created using one machine, a laser or punch. Like machining and 3D printing, this allows us to hold tighter tolerances. Its when we introduce bends to the geometry that things get a little less clear.

Refer to the table below for standard tolerances for each marked section of the part.

*Hardware is considered to be studs, nuts, standoffs, or other self-fastening product.

Refer to the table below for standard tolerances for each marked section of the part.

*Non-cumulative.