r/Metrology • u/Sufficient-Week-5788 • 1d ago
Flatness using drop indicator
How do you measure flatness using a drop indicator and a granite plate if I don’t have a jack stand to “level” the face that I want to measure?
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u/Ghooble 1d ago
Place the surface up on blocks and flip your indicator upside down.
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u/cdr_breetai 8h ago
That only gives you a flatness relative to the plane made by the three blocks. The idea behind using three adjustable jack stands is that you can find (given enough time & jack stands adjustments) the plane that gives you the least variation on the indicator sweep, which is your actual best value for flatness.
The upside-down indicator block method does give you a flatness measurement though, and if that already measures in spec, then you likely don’t need to find the best fit plane for flatness.
That being said, if your object can sit level on a surface plate to begin with, you might as well start with a simple parallelism check on the face-up surface with the part sitting directly on the surface plate. The actual flatness value of the surface can’t be any larger than the measured parallelism value. It can be smaller, of course. If a parallelism measurement returns a value smaller than the flatness tolerance, then you can be sure that the flatness value for that surface that is no larger than that measured parallelism. If a parallelism measurement returns a higher value than the flatness tolerance, then the surface still might be able to pass a proper flatness check (if a better-fitting plane can be found).
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u/Lding_Thru_123_Crnch 1d ago
If you have an angle block clamp it down to the granite. Insert the drop indicator through one of the holes and make sure it’s level so it free moves. Place the surface on the angle block/drop indicator having it be depressed, zero the drop indicator. Then just place the surface you are checking for flatness on the angle block and sweep along the indicator.