Understanding Process Tolerance

No matter how perfectly your organization’s processes may have been designed, there is always a chance that you’re going to observe some variance in their execution. This does not have to be a problem if it’s properly accounted for in the very design of those processes, and as long as you know what you can expect from each component of your facility, you should be able to avoid any complications.

Process tolerance refers to the exact limits that each of your processes has on both ends the lower and the higher. When you have a properly defined process tolerance for each part of your organization, you can easily account for every possible problem that can come up, and deal with those issues before they’ve had a chance to grow into something bigger.

Defining Process Tolerance

In more specific terms, process tolerance refers to the exact values that certain variables can have (usually related to the quality of the product) and still fall within acceptable boundaries. Acceptable is a very flexible definition here of course, and it typically comes straight from the customer. To this end, proper market analysis and ensuring that you understand the fundamental requirements of your project is obviously important.

As we mentioned above, you’ll need to define two different levels related to your process tolerance the upper and lower limits. Anything that falls between these two levels will be treated as acceptable, and you should additionally strive for a situation where the majority of products coming out of your organization are right around the average between the two.

In addition, process tolerance can refer to the actual difference between those two levels. For example, if a bag of product has an upper limit of 1.02 kg, and a lower limit of 0.98 kg, this gives you a process tolerance of 0.04 kg. This value can be useful in certain calculations, e.g. when you’re trying to figure out how much you’re deviating from the average.

The Importance of the Average

For the most part, you’re going to want to direct your attention towards the middle ground between the two tolerance points. The average is where most of your output should be around, as we pointed out above, and the closer you can maintain everything to that level, the better results you’re going to see overall.

It’s important that you apply statistical analysis tools properly here. If you simply take the average of all your products to check how much they’re deviating from the expected average, you’re probably going to see a nearly identical value. This makes sense if your upper and lower deviations are within the norm and don’t stray too far from the expected values.

This means that you should keep track of the actual numbers of products that deviate from the average, and the value of their deviations as well. There are different statistical analysis tools that can help you get a good overview of the true situation withiln your plant, and it’s important to master them as quickly as possible if you want to be a responsible leader in your organization.

Most people look at just the average (mean) value of their product or service, but fail to evaluate the variation (calculated as the standard deviation).

If the variation exceeds the limits (called “capability”), then the goal is to reduce variation and/or shift the average closer to the center of the limits.  This is the heart of Six Sigma, to understand how the average and standard deviation compares to the tolerance limits, and identify what factors are causing the variation.

Shrinking the Tolerance Limits?

For the most part, unless your client or customer specifically requests it, you probably shouldn’t be putting any additional effort into trying to shrink the gap between the tolerance limits. The goal should be to continuously reduce the variation, so you can achieve results closer to the average, which improves customer satisfaction and reduces reliability issues.

Check out this video from Ford about a transmission study they conducted on their supplier parts, that had less variation…

On the other hand, if you start receiving complaints stating that you’re deviating from the expected norms far too much, you should consider some immediate changes, such as tightening the tolerance limits.

And in those cases, keep in mind that you don’t necessarily have to shrink the process tolerance from both ends. If products are primarily jumping over the limit, for example, you can lower the upper level but keep the other one in place. That way you’ll prevent unnecessary repetition of work within the organization.

Conclusion

Understanding process tolerance and its implications for an organization is a critical skill for any responsible leader. There are many finer points to be discussed about process tolerance as well, but once you have a good grasp of the fundamental concepts, the rest should fall into place relatively easily and in a natural progression.

To learn more about Six Sigma, consider Green or Black Belt training and certification >>>