Tolerance Stackup

Tolerance stackup is the cumulative effect of the components of a product being skewed towards the same side of the specification limits. Tolerance is the range between the lower specification limit and the upper limit of a part.

Imagine you have a product that lines three or four or more components up end to end. Each will have some sort of distribution and when you combine the distributions of all the parts, by something called the central limit theorem, the distribution of the system of parts will be a normal distribution.

Most of the time, the total of all the parts will fall somewhere in the middle of the range for the full product. But every once in a while, all the parts in the product will fall close to the same side of their tolerance. For example, every part will be in spec, but on the short side of average. All those shortages “stack up” and the total of the combined group of parts is too short to work in the product.

Tolerance stackup is mitigated by precise manufacturing techniques or by designing products so there are not a lot of components that can stack up in the same direction.

Lean can have a substantial impact on the quality of parts that come out of manual processes. Standardization and poka yoke both play a significant role in quality.

When addressing tolerance stackup, you have three basic avenues.

The first is to redesign products to limit the potential for stackup.

The second is to improve processes to tighten up the actual measured sizes of the product to a range that prevents stackup. In effect, you are creating a secondary spec that you know works.

Finally, you can use some form of adjustment. This might be a manual step such as grinding down a part (if that is acceptable for your product), or adding in a bushing or a spacer, or adding an adjustable part to your product, such as a turnbuckle. Make sure that you recognize that any adjustment process is decidedly un-Lean. It means that somewhere, your main process is failing. Use this as a stopgap, and always remember that stopgaps have a tendency to become permanent.