How to use a Flatness gauge - Technical Article

  1. Calibrate flatness gauge by placing it on the master flat supplied with the gauge and zeroing the indicator dial. This is done by rotating the dial indicator. Gauges are pre-set on final inspection prior to despatch.
  2. Place the gauge on the surface to be checked.
  3. Read the deviation from perfect flatness directly on the dial.
  4. The reading from the gauge can be used to predict the flatness of a part lapped on the plate by using the graph on the flatness gauge body.
flatness gauge reading

For example

If the indicator reading shows 0.002mm, a 25mm component will have a light band reading of less than 1 light band, while a 50mm component will have a light band reading of less than 2 light bands, while a 75mm component will have a light band reading of 3 light bands.

Similarly, if you wish to achieve a light band reading of less than 2 light bands, on a 50mm component, the indictor reading must not be greater than 0.003mm. However for a 25mm component, the indicator reading can be up to 0.018mm to achieve a light band reading of 2.

A plus reading on the indicator shows that the surface being checked is convex.

A minus reading on the indicator shows that the surface being checked is concave.

For sizes beyond the range of the conversion table compare the square of the major dimension of the part with the square of the span of the gauge.

For Example, the Kemet Type L Flatness gauge reads +0.04mm on a 12” plate. The true reading is calculated as follows:

a) Type L flatness gauge has an instrument span of 9.383” = 236mm
b) (Instrument span)2 = 2362 = 55916 mm2
c) The Plate has a diameter of 12” = 304.8mm
d) (Plate diameter)2 = (304.8)2 = 92903 mm2
e) (Plate diameter)2 / (Instrument span)2 = (92903 / 55916) = 1.66
Multiple (e) by indicator reading = 1.66 x 0.04mm = 0.06mm

The true reading is +0.06mm which indicates that the plate is convex as is off the scale (Figure 2). The plate will need to be reground.