# Measuring Round-Over Radius Using a Roller Gage

Quote of the Day

Of course her kids come before you! Hell, her kids come before her.

— I saw this on Pinterest, but I once heard a manager say something similar to an employee. I always work to keep my priorities properly set – family is first.

## Introduction

This blog post shows how to measure the radius on a rounded corner (a.k.a. bullnose) using a roller gage. I saw this method being used on this web page, and I wanted to document it here for future reference.

You might wonder why an electrical engineer is so interested in mechanical metrology. Unfortunately, I regularly deal with issues involving precision mechanical measurement. For example, I just left a meeting where we discussed flatness issues with connectors that have been reworked onto printed circuit boards. We have discovered that a mounting problem introduced an angle on a connector that results in failures at a specific temperature. This type of precision work justifies the granite slab and metrology gear that I keep in our workshop.

In this post, I will work through the basic geometry associated with this measurement and will work an example.

## Analysis

### Definitions

Figure 2 shows the key variables in the problem.  The blue circle represents the radius of the round-over that we wish to measure. We can derive an expression for the radius R in terms of h1, H, and D1 by applying the Pythagorean theorem to the red-legged right triangle.

Figure 2: Definitions of Terms.

### Derivation

Figure 3 derives an expression for the radius of the round-over.

Figure 3: Derivation of Round-Over Radius Formula.

### Example

Figure 4 shows how I computed the round-over radius for the example of Figure 1. The formula and the scale drawing agree.

Figure 4: Worked Figure 1 Example.

## Conclusion

I plan on using this measurement method on a project very soon. I have one more metrology example to work through and it will include determining the level of accuracy (i.e. tolerance) of these measurements.

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