Flatness Testing by A Simple Accurate Method

Every machine shop has a surface plate. By distributing three or four of our “#75 Flatness Evaluation Pedestals” in a pattern that will support the part to be tested for flatness, the entire set up is complete. Parts can be measured in a matter of minutes. Place the flat surface to be evaluated up side down on top of the three or four “#75 Flatness Evaluation Pedestals.”

Extremely fine flatness evaluation can be made using this technique; but without special attention to cleanliness, temperature control and correction for sag due to elastic deformation, the finest measurement made is usually five ten thousands that is one half of one thousandths of an inch.

Tolerances as fine as 20 micro inches have been evaluated using the “#75 Flatness Evaluation Pedestals,” but this requires a nearly perfect surface plate, extreme cleanliness and a temperature of exactly 68 degrees Fahrenheit (20C). The accuracy of this flatness test method has been confirmed using optical flats and a monochromatic light. Remember elasticity, there will be at least some deformation of the part due to gravity.

We make individual Ball Wands for each half thousandths of an inch plus and minus. These increments are plus one half thousandths (+0.7505”), minus one half thousandths (-0.7495”), plus one thousandths (+0.751”), minus one thousandths (-0.749”), and we will manufacture any custom size Ball Wands to suit our customers’ specific needs. In operation, choose the Ball Wand that matches the desired part’s tolerance. The flat surface of the Ball Wand is slipped under the test part and an effort is made to rotate it. Please see our paper on Ball Wands on this website.

For example, using the minus five ten thousands of an inch (0.7495”) Ball Wand when rotation is attempted, the Ball Wand rotates freely.  This means that the part is either flat, within less than five ten thousandths, of an inch or the part is concave. The next test is to use the five ten thousandths of an inch plus (0.7505”) Ball Wand. Slip this Ball Wand under the part and attempt to rotate it. The “#75 Flatness Evaluation Pedestals” are very low friction, so if the Ball Wand barely runs into the Part, it will move the part slightly when attempting to rotate the Ball Wand. This movement means that the ball on that wand is touching the part. In order for the part to be flat within five ten thousandths tolerance, this ball should touch everywhere. If able to rotate this Ball Wand anywhere, it means that the part is concave more than five ten thousandths of an inch.

A different pair of Ball Wands is needed for checking each flatness tolerance. The Ball Wand that is usually used with the flatness tester has the plus ball on the end and the minus ball on the opposite end. This keeps the two balls together. The ball that is not being used acts as a handle when rotation is attempted. This design saves considerable expense, but the really big savings is that this gage is only one half as long as it would be if two individual gages were installed in a conventional (AGS) American Gage Society handle.

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