Wonderful Flatness

This may not be everyone’s taste, but the black granite slab in the picture is an amazing artifact. It’s called a surface-plate. I got this one for my little home machine shop from a semi-retired machinist who deals in used tools on the side. He had it out in the back shed under a pile of junk and I had to pull it out myself, but he let me have it cheap—$80.  surface-plate

The surface plate is one of the machinist’s most basic and important measuring devices.  It is a slab of stone (or occasionally cast iron) ground and lapped to almost perfect flatness so that it can be used as a reference surface to measure things from.  Machinists and tool-and-die makers use them constantly to set up tools for taking complex measurements or for laying out precision work. Industry uses them for high-precision inspection, and laboratories find numerous uses for the accuracy provided by the finest grades. At 18”x24”x3” and 220 pounds, this is a small one. I saw a nine-ton,  4’x8’x2′ surface plate offered on eBay the other day, and that’s by no means as big as they get.  Two foot thick solid granite.

It’s the perfection of the flatness that’s fascinating. There are three standard grades: B for the tool room, A for a more controlled environment, such as an inspection area, and AA laboratory grade. There are detailed government regulations specifying exactly how flat a stone must be to qualify for each grade, and I had to read the spec a few times to be sure I was understanding it correctly. The perfection is hard to believe. See Federal Specification GGG-P-463c if you’re curious.

There are at least two main kinds of non-flatness. One is local irregularities of the surface, and the other is the overall geometric non-flatness. Picture a potato chip. If you get super close up, you’ll see that the surface of the potato chip has irregular bumps and ripples, which is the first kind of non-flatness. The chip as a whole also has a compound curvature, which is an example of the second kind.  Even the most perfect physical object is going to have some component of each kind of non-flatness.

So here’s how flat surface plates are. Imagine that the top surface of a surface plate is contained between two parallel planes, with one plane sitting on the highest spots and another plane running through the stone just kissing the lowest spots. For a lab-grade AA surface plate of this size, those two planes can be no more than 35 one-millionths of an inch apart.

Thirty-five one-millionths of an inch is less than a micron, which is smaller than a bacterium (bacteria range in size from 1 to 10 microns) or a red blood cell (at about 7 microns.) In other words, you would need a fairly powerful microscope to see a gap between the two planes. That’s the worst they’re allowed to be. In practice, they’re considerably closer still.

A shop-grade B surface plate like this mine can be up to a sloppy 110 millionths of an inch apart, which means that some bacteria could squeeze in between the two planes but nothing anywhere near as big a blood corpuscle would fit. I say it’s amazing, but only a narrow subset of people with actually be amazed. Machinists fail to be amazed because they know how this kind of perfection is achieved. Non-machinists don’t know enough to be amazed; a millimeter, a micron, what’s the difference? You have to be in the Goldilocks zone to be properly awed.

Surface plates have to be checked occasionally for accuracy and lapped back into flatness if they have departed from the standards of their nominal grade. The two kinds of flatness are measured separately. Even if every place on the stone is very flat relative to the surrounding surface, the stone as a whole can still be warped, and this is where the difference between the two kinds of flatness matters. Local non-flatness usually represents a genuine flaw, often where the surface has been heavily used and worn down, but if the stone seems to be locally flat everywhere, a degree of overall non-flatness might just represent a transitory overall change in shape because of a temperature differential.

The standards for flatness are so exacting that even the evaporation of the solvent or water used to clean the stone can chill the surface and cause the stone to become temporarily slightly concave. A sunbeam heating the surface has the opposite effect, and it can take hours for the stone’s temperature to equalize throughout. Combinations of uneven temperature can cause a temporary potato chip like deformation. Even the time of day matters. If it’s been cool all night, when the shop starts to warm up during the work day, the warming outer stone can expand slightly and bend the surface into a convex shape. Incandescent light can do the same thing. Technicians understand this, so if a surface plate is known to have once been flat, and appears to be locally flat, it’s a strong clue that the problem is uneven temperature rather than a genuine departure from flatness.

In workshops, the amount of flexing caused by minor warming and cooling may not matter but in a laboratory or inspection environment, it may be necessary to precisely control the temperature around the clock to preserve consistency.

The process of measuring and re-grinding a stone is fascinating. Tom Lipton of the OxTools YouTube machining channel,  filmed a pair of technicians giving some of his surface plates a checkup and performing some corrective lapping. The amazing thing is how simple the tools are, particularly the laps used for flattening the stones. The lapping is done entirely by hand with diamond dust and flat metal plates that are rubbed over the surface to cut away the high areas. Properly done, lapping is a self-correcting process, in which the lap and the surface plate actually keep each other flat by preferentially cutting away the spots that are in contact.  The speed with which they bring Lipton’s stones back to immaculate AA perfection is astonishing.

There is a peculiar risk inherent in these things. Machinists use a lot of tools that have similarly flat surfaces: Jo-blocks, surface gauges, 1-2-3 blocks, squares for mounting things vertically on the mill, etc., can all have super accurate flat surfaces. If you put an object like that down on a perfectly clean surface plate and accidentally bump it, it can glide silently away like an air-hockey puck and go right over the side. I’ve done this myself. The opposite happens too—once the object is fully in contact with the plate it feels like there’s a weak magnet sucking it to the plate.

Do I need such perfection? Me personally? Not really. Not yet. I’m a woodworker; our idea of accuracy doesn’t have that many significant digits. Until I got this baby I was using a scrap of granite countertop I rescued from the trash. It probably wasn’t that much flatter than a piece of birch plywood. I do use my surface plate of course, but my true pleasure in it is more in its almost Platonic perfection; a Euclidean plane sculpted out of 200 pounds of stone.


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