Metrology Monday: Surface Plate

This week I am talking about the humble block of stone known as a surface plate.

What Is A Surface Plate For?

When evaluating the dimensional properties of various machining related items, one quickly arrives at the need for something to compare to. A standard or reference as it were. One of the basic results one hopes machine tools produce is straightness and it’s two dimensional version, flatness. The surface plate is the general standard upon which evaluations of flatness rest. For what is basically a simple block of rock, there are some details I find fascinating.

As I mentioned last week, you can set things on a surface plate and evaluate how parallel a top surface is with the side resting on the plate. With additional tools, you can also check for squareness.

There is also a way to check for flatness of a surface. One applies a marking fluid (ink/paint) to the plate and rubs the surface to be evaluated around on it. The result is paint on the high spots. What you do with that is whole other post but basically you can scrape, grind or machine things to improve flatness. Scraping is the basic manual way of getting something really flat and a skill I hope to acquire eventually.

Options

Modern surface plates are commonly made of granite although cast iron and glass ones are also made. (I have never seen either for sale by vendors I frequent). Granite plates are produced and serviced! by a variety of vendors big and small. The history of surface plates is an inversion of the usual progression of technology because they were originally made from cast iron and granite took over when wartime metal shortages made trying alternatives such as glass and granite attractive. It turned out that granite was good enough for most uses. Cast iron is apparently preferred for really high end work.

Plates come in an assortment of sizes from 6″x 12″-ish up to huge multi-ton monsters. As you would expect, with greater size comes greater weight and things quickly get out of reasonable reach of the hobbyist. My plate is 12×18 and 3 inches thick and weighs 72lbs. I would have gotten a smaller one but this was on sale.

Speaking of on sale, plates come in a variety of grades with increasing levels of flatness. Mine is alleged to be flat within .0001″ which is quite good for an inexpensive (~$60CDN) but I have my doubts. Plates come with inspection certificates but the vendor didn’t bother to actually fill it out… I am not too worried because it is flat enough that I can’t detect any variation using the best methods I have available, a dial test indicator. (Teaser for next week!)

Alternatives

Unless you are aspiring to finicky levels of accuracy, a surface plate could be overkill. A 70lb block of granite is not something you just whip out of a drawer for a quick measurement. The best alternative I have used in the past is a piece of plate glass, in my case a discarded glass shelf. It still sits handy to my workbench for use as a reliably flat assembly surface.

I have also seen at least one Youtuber using a piece of granite counter top. I have no idea how flat that is relative to an official plate but with modern processes, probably close enough for many. Actual granite surface plate manufacture involves leaving the blanks in temperature controlled rooms for months for the internal temperature to even out to avoid the minute distortions it causes. I doubt countertop companies do that except by accident.

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