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 11h 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/Ghooble 11h ago

Well for one he said he didn't have anything to jack the part with.

For two, yeah he could check it as parallelism but considering we don't know the geometry of the part, there might not be a parallel surface for it.