I was googling around trying to find a source for some thick granite for a project I am thinking of doing, and I found multiple people selling “surface plates” some small, some HUGE! I know that they have something to do with metal/shop work, and they seem to come in different grades, which I assume has to do with the precision of the flatness of the surfaces. But what are they used for? I mean, when working in a shop, when do you pull out the granite surface plate?
A surface plate provides a flat surface from which to take measurements, such as with vertical height vernier gauge. We had two of these at a previous employer, and used them to check high-tolerance mechanical parts.
Oh, and they’re generally made of granite because of its very low thermal expansion coefficient. We also had a number of inductance standards which were wound on granite cores because of this property.
They could also be used to make sure other flat things are really flat. Put the object on the plate and see if you can slip a feeler guage in between them.
Flatness isn’t generally measured that way, though a surface plate is used. The piece to be checked is place on the surface plate with the surface to be measured for flatness facing up, and flatness gauge, itself place on the surface plate, is run over it.
A place that worked ,quite some tyme ago, had a large peice of granite polished side up,that was used to mix inks.
The large mass of the typical surface plate also helps to eliminate vibration that can interfere with obtaining accurate measurements.
Slightly off the topic, but depending on the thickness of granite that you need, I’d suggest your local kitchen counter shop (they probably have cutouts and broken chunks) or possibly a landscaping company (the kind that sells granite for curbs and steps).
As already noted, a granite surface plate is basically a known measurement absolute. The plates are ground and finished to a very high degree of flatness in both axis. A relatively cheap import might have no greater variation than half a thousandth of an inch throughout the working surface. (In other words, no spot on the surface is more than 0.0005" higher than the lowest spot.)
An inspection-grade American might be within a few millionths of an inch absolute.
What it gives you is a known reference- Starting from a near-perfect surface reduces the accumulation of measurement errors.
If I put a block of steel on the plate and used a high-quality height gauge, I could tell you whatever you needed to know- height, length and thickness to start, but also parallelism (is any one surface perfectly parallell with the opposite face) squareness (are the faces precisely 90.000° from each other) and so on.
Toss in a sine plate and some accoutrements and I could tell you angles to four significant decimals of a degree. My machine shop teacher once demonstrated, using a rather expensive height gauge and a quality American pink granite flat, the ability to measure the height of a fingerprint. I don’t recall the exact dimension, but it was only a little thicker than cigarrette smoke.
Granite is used for- as already noted- it’s thermal properties and it’s hardness. Granite is considerably harder than most metals you would be placing on the flat, and so wear is reduced. Thermally, it’s less prone to uneven heat/cool expansion and contraction due to it’s density, and when it does move, it’s uniform.
Similarly, it’s very stiff- even if improperly supported, it won’t “sag” or set, as a cast iron or fabricated steel gauge plate can.
Thanks for all the great information here. I sort of had guessed that there was some measurement involved, but didn’t quite get the context. Now I do. For anyone who wants to see what one of these things looks like, check this link.