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Apr 04 2010

The Care and Feeding of Granite Surface Plates in the Shop

Published by at 8:57 pm under Metrology,Sharpening

Granite Surface Plates are the world standard for flatness in any shop. There are a number of places you can get them and the pricing on these tools varies widely. It is important to note that for most of us, they needn’t cost more than necessary.

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There are a lot of great things to know about them, but there is one thing that is really important to touch on first off.

The grading of Granite Surface Plates is of importance to the woodworker. Please have a look at the following data:

The following tolerances account for the highest and lowest differences in the flatness of the surface on the Granite Plate.

.000025” for an AA grade plate. Twenty-five hundred-thousandths of an inch.
.000050” for an A grade plate. Fifty hundred-thousandths of an inch.
.000100” for a B grade plate. One Ten-Thousandths of an inch

Now to look at the numbers, you would say that A is double the roughness of AA and B is well more than double the roughness of A. While this is true, the B grade plate is smoother than a baby’s behind and imperceptibly flat to you and I. Please don’t become mesmerized by those numbers, Grade B is amazingly, awesomely flat. I mean if one ten-thousandths is not flat enough for woodworking then, Um, I am not sure what you have planned.

The AA grade plate is a Laboratory Grade plate and is a very expensive precision tool that lives in a temperature controlled precision laboratory . It is used primarily for the calibration of tools that will do high precision or precision work. This is well beyond the needs of most end users. The A grade plate is an Inspection Grade plate which is usually kept in a clean area and used for Quality Assurance (QA/QC) comparisons and analysis. Consider it for companies that need to compare whether the work they are doing in the shop is meeting manufacturing specifications that are required to be met.

B grade is otherwise known as a “Tool Room” quality surface where us folk with grubby hands are allowed to use them. It is the plate used by those performing the work. It is the least expensive and most used, and also the least expensive to replace should it become damaged. Sounds like the one for us, and it is! The grade B plate is more accurate than we, and the tools we have in the workshop will ever need.

What’s even better is that Tool Room Grade B plates go on sale often and we can pick them up for cheaper than river rocks at a renaissance faire. Keep that in mind and just go cheap. Nothing being sold as a granite surface plate and that is cheap at the same time is a bad thing. Just buy it. It is way too easy to purchase these anymore. Surface plates come in many sizes and are on sale quite frequently at Enco. There is often free shipping if you buy enough to meet a very low priced minimum order for that

There are several ways to use a Granite Surface Plate in the woodworking shop, so lets touch on them.

First off, the top surface of the granite plate is the business surface, the sides and bottom of the plate are not certified. The bottom of the stone is generally parallel to the top, and pretty flat. Granite is hard stone, likely harder than what you have placed it on, and also likely flatter than what you have placed it on and what this can mean is that the surface plate can rock if the surfaces don’t match.

Depending on what you use the plate for, rocking may not matter to the end user, or it may matter a lot. There are a few factors happening at once here. If you are trying to work with the accuracy the plate offers, having it rock on you is not going to make for a usable experience. If you are using the plate for sharpening edge tools, having the plate rock and skid may not work well either. The solution for this is affixing rubber feet to the underside of the surface plate.

Adding rubber feet to the underside of the surface plate while solving some problems is not as straight forward as simply just doing it. Granite is rock solid of course, but if it isn’t properly supported, it can sag and warp from it’s own weight, and this can affect the accuracy of the working surface over time. So there is a proper method for locating the feet to the bottom of the plate so this all works out.

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First, it takes three feet, not four, because a tripod will not rock, and as such will stabilize the plate on nearly any surface.

Second, locating the feet in a tripod shape and within the right parameters to the underside of the plate is key. Here is the formula for this:

On one narrow side edge, or end of the plate, there will be one foot affixed. It will be located at the middle, 1/2 the width of the plate from each side, and between 1/5th and 1/4th the length of the plate from the ends. I like to affix to the middle, and have the edge of the foot at 1/5th the length from the edge.

On the other narrow side edge, or end of the plate, there will be two feet affixed. They will be located in the corners, between 1/5th and 1/4th the length, and 1/5 to 1/4 the width of the plate. Again, I like to affix them with the outermost portion of the foot just within the 1/5th realm of the spec.

If you want to have a look at the Federal Specification GGG-P-463c which is the standard for which Granite Surface Plates are made and maintained with, I have a copy of it in the Woodworks Library Hand Tool section, just click this link right here.

If you have added feet, then a couple cool things become benefits to you. The plate is now reasonably skid resistant, (great for scary sharp work) it doesn’t rock, you can clean under it, and you can easily get your fingers under it should you choose to or need to move it around in your shop to use or store it. That is great usability!

Some people like to make a carrying tray for making the moving of the plate easier, because you can then add handles etc. If you do this, it’s a great idea, but it is still beneficial to add the feet under the plate to stabilize it, even if you put it in a tray.

Once you have a granite surface plate in your shop, there are a number of uses for it. Many woodworkers buy one for use with the “Scary Sharp” method of sharpening, where sandpaper abrasives of varying grit, or micron size are used to abrade metals for grinding and honing. Even if “Scary Sharp” is not your primary method for sharpening, I have found there are times it is efficient to resort to it instead of stones.

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It is important to note here that these plates are very smooth to the touch. The smoothness is a component of the flatness. It is wise to avoid using abrasives on the plate in a way that will scratch it, and it is likely a bad idea to use loose abrasives on a surface plate, because they will abrade the plate and it will lose it’s certified flatness. Many people use a combination of sharpening methods including PSA sanding abrasives and a granite plate to maintain their tools. Wet-Dry sandpaper can also be made wet with water or light oil and the capillary action of the water will help hold the abrasives in place, but the use of a surface plate doesn’t have to stop there.

Many times, just for general sanding on woods, you can keep it simple. Just lay a piece of sandpaper on the plate and sand with one hand while you hold the paper in place with the other. It is really handy on the fly. This is particularly when you need to sand flatten a small surface or joint a short edge for glue up. Think jewelry box pieces, inlays, marquetry, fine fitment of things where close tolerences are desired.

Other uses for the surface plate include using it as the reference standard that it is to evaluate the quality of your layout tooling’s accuracy. It is great for all kinds of analysis. The plate will help reveal the accuracy of straight edges and squares by comparing them on the surface.

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If you have a high quality square, you can place it on the surface plate to compare the squareness of other squares to it. 1-2-3 blocks are great tools for comparing squareness as a for instance. Any square nudged up against a 1-2-3 block is square if there is no light seen between them. It is for woodworking, good to know which squares are highly accurate, or just good enough for the job at hand.

It is also a place where you can measure and compare parts with depth gauges and dial indicators. A magnetic stand need not be left out on a granite surface plate. Allow it to hold the dial indicator; the stand is not the accuracy. The dial indicator is set so it is perpendicular to the surface, the tip is in contact with the plate, and the dial zeroed. Anything you place on the plate under the tip of the indicator is measured for thickness. It’s just an idea of what you could use the plate for with a little imagination.

Glue joints are pretty particular about matching well. We often cut things straight and plane things flat so they will mate well. Often times when a small part is too small or delicate to work with common tools, sanding it on a granite surface plate is a great way to prep it. You can achieve flatness to greater than 0.001 while sanding things on the surface plate. You can also contour sand convex shapes on the plate, because the plate is heavy, rigid and flat, and adhesives hold sandpapers to it really well. All you have to do is move the part needing sanded as you sand. It often works with higher control than holding the work piece and sandpaper in your hands.

Finally, what about that big rock in the shop when you are not using it? Granite is a porous material, so leaving any kind of grunge on it might not be a good practice, as it can soak into the granite. Leaving adhesives on it long term can affect how easily they come off later. When it is exposed and uncovered, it is good to make certain then hard things wont be dropped on it, so as to allow it to become gouged or scratched, and piling things on top of it can render it too difficult to use when it would be nice if we could.

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Good practices include keeping it clean and dry, and cover it so the dust in the shop doesn’t land on it when it isn’t being used. Keep it ready to go and you’ll find a ton of ways to use it.

Depending on what you use it for, it can be nice to have it ready to use when you need it, and it is wise to remember that any dust on such a flat surface can easily affect the accuracy when precision measurement is desired. Unless it has a full time home where it is placed, covering it and placing it on a shelf out of harms way is always a good way to keep this precision tool ready for the next use we have for it. It is way more versatile than float glass or MDF for many reasons, and can be more versatile than a floor tile or piece of countertop in many cases as well. Well worth having!

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