Measuring Flatness? Don’t make this mistake!

Measuring Flatness? Don’t make this mistake!

At LLC we do more than sell gages.  We are also Metrology fans, geeks or whatever you would like to call us.  So when we see the opportunity to perhaps provide a little clarity in our chosen field, we take it!

The reason for this blog entry is the result of a conversation I had with a customer last week.  He needed some advice with a “flatness” application due to a newly required specification from one of his customers. 

He was looking for a granite surface plate and a height gage with the intention of placing the height gage probe in contact with the part and then sliding the height gage around the surface plate to trace the probe across the top of the part.  Ultimately, he thought he would look for any changes on the display of the height gage and in so doing, he would determine his flatness measurement. 

I explained to him that since the side of the part to be measured is not on the surface plate (datum surface) and the opposite side is, the side resting on the plate will interfere with the flatness measurement of his part. 

Take a look at the example below which shows a probe, datum surface and a wedge shaped test piece. The measurement is to be taken on surface (A). 

Let’s assume both sides of the test piece are perfectly flat but not parallel to each other (which is easy to see).  Any left-right movement of the part will displace the spindle of the probe and produce a change in your measured value, even though the part is perfectly flat on that side.  What you have just measured is parallelism, not flatness.

To measure flatness (for this type of application at least) the proper way to get an accurate measurement is to undermount the probe so the spindle extends through a hole in the datum surface and then flip the part over so the surface to be measured (A), is not effected by the opposite side of the part. 

In this case any left-right movement that results in the displacement of the measuring probe will be considered a flatness measurement

This is a pretty easy concept to grasp when you can see it visually, maybe a little tougher when somebody tries to explain it verbally.

Measure Smarter!