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  • April 17, 2024
  • Reata

DFM Tip: Why You Should Model to the Mean, Not the Nominal

DFM

When producing high volumes of the same part, slight variations are inevitable. Tolerancing is just one way we account for these variations. 

However, a common misconception is that tolerances play the same role in prints and models. 

On prints, tolerances indicate the measurements for your ideal part, indicating how much deviation is acceptable. But when it comes to your part model, you should be thinking about tolerances a little bit differently. 

Why? Because the model isn’t supposed to represent your ideal part but rather your most average part. 

Here’s why that distinction is so important to get right.

Why the Bell Curve Matters for Contract Machining Customers

If we were to make a graph of the variations that occur during the manufacturing process, we would find a normal distribution, also known as a bell curve. This means that 95% of parts measure within two standard deviations of the intended measurement, which is most often the mean or average between tolerances.

DFM

However, when the customer’s model uses the nominal (ideal) measurement instead of the mean, the entire bell curve is skewed, and the number of usable parts decreases.

Three out of four times, it’s unclear that a customer has used the nominal measurement instead of the mean until the part reaches our quality assurance team—which is one of the last steps in our manufacturing process. 

At that point, we have to return to step one. Our engineers must adjust the model and begin the complex machining process again, costing the customer valuable time and money.

A real-world example

Recently, we machined a part with a dimension of 1” and a tolerance of +0.030”. The customer modeled the part using the nominal of 1”. Using the customer’s model, 50% of the parts would be out of tolerance.

Why? Think back to the bell curve. 

If the nominal, 1”, becomes the mean, half of the parts will be less than 1”. But because the print specifies that parts must be between 1” and 1.030”, any part under 1” can’t be used.

Our engineering team reworked the model using the mean (1.015”) to ensure the maximum amount of parts would be within tolerance. 

Our Precision Machine Shop Provides the Solution

At Reata, many of our employees have been on the engineering side of the manufacturing industry. We understand how difficult your job is. The “mean vs. nominal” rule probably seems like just one more thing you need to get right.

That’s why we’ll take care of it for you. 

Our team is well-versed in Design for Manufacturability best practices, and we are happy to review models and help make any adjustments. We believe in proactively preventing costly remakes to ensure you receive your parts on time.

Request a quote today.

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