I have shared my opinions regarding the usefulness of precision tools for woodworking on a couple of posts. Some dismissed the usefulness of precision measuring tools without providing reasoned explanations. But I think others might benefit, so a few of my experiences and thoughts follow.
I suggest that woodworkers that desire good results and value their time should own three tools: 1. A diemaker’s square; 2. A precision ground straightedge; 3. A precision micrometer (1” .0001”). I will explain why below, but first you need to understand what kind of tools these should be, since there is so much useless junk made in Taiwan, China and India, and even the US and Europe. The tools I am recommending are not sold at Home Depot, or even in the woodworking tool catalogues, but by retailers that specialize in supplying high quality machinist’s tools to professionals. These tools are expensive, and they come with real certificates of testing for accuracy, and a solid guarantee. Once you have your set of these tools, check out their equivalents made in China, Taiwan or India and you will immediately see what I mean about junk.
Why a diemaker’s square? Most woodworkers have never seen one. While not especially useful for layout, they excel at checking the accuracy of tools and setting up equipment. Unlike the engineer’s square, which is typically a chunky stainless steel body pinned to a thinner tongue, the diemaker’s square is a solid piece of steel, so much more rigid. A quality diemaker’s square is hardened, precision ground and certified. Besides being very straight, very parallel and very square, it has bevels cut into the tongue’s four long edges, which are chemically blackened, creating a knife edge that makes it easy to quickly and unerringly see any gap between the tongue and the tool or equipment being checked. No single tool beats it for checking for square. This is a big deal if you care about 90º.
These tools are not for layout or even for even daily use (although the diemaker’s square excels at setting up tablesaws, bandsaws, and jointer fences) but are used to check that your workaday tools remain accurate. They aren’t used that often and spend most of the time in their boxes.
An example. Some years ago I ordered a Starrett combo square to replace my old worn-out one. When it arrived, I pulled out my diemaker’s square and feeler gauges and checked it. It turned out to be .012” out of square measured at the end of the blade. That’s a lot, IMO. I was shocked that Starrett would let something so shoddy leave the factory, but sh#t happens. I contacted the retailer, explained the problem, and he exchanged the tool for one with proper tolerances. But if I had blithely assumed the combo square was accurate simply because all my other Starrett tools were excellent, not only would my quality have dropped off considerably, I would have begun to question both my eyesight and sanity by the time I finished the chest of drawers I was working on.
Another example. When Lie-Nielson first came out with their jointer plane, I bought one right away. One reason I was willing to pay the high price was because all the L-N tools I had purchased previously had been manufactured to very high tolerances. I sharpened the blade and tried a cut and was shocked to find that it would not plane a flat surface. My 24” precision straightedge showed that the 22” sole was both cupped and twisted. The problem was easily corrected using glass, W/D paper, lube and the same precision straightedge. Since I had checked my framing square and knew it to be quite accurate, I could have used it instead, but it would have slower, more difficult, and I would have less confidence in the flatness of the finished sole.
I am not relating these stories to badmouth Starrett or Lie-Nielson, but rather to point out that everything in Murphy’s World has problems that must be dealt with, and that good tools can help discover and resolve those problems before they get too far out of control.
When I was a civil engineering student, the professors teaching surveying told me that errors tend to cancel each other out. After graduation, Professor Murphy taught me the truth: errors always accumulate. Test this for yourself. Take a board at least 10 inches wide you believe has all six sides/edges square/parallel. Using a square and marking knife (a pencil/pen is too fat), spin a line around the six sides/edges. Begin each line where the previous line ended, and reference the square off a different surface when cutting each line. Does the last line meet the first line perfectly, or is there an offset? If there is an offset, the error may be the board. If so, ask yourself why the board is not square/parallel, and if that amount of unexplained error might tend to make a drawer flat or twisted. If on the other hand the error is in the square, how do you check to make sure the square is good or bad?
Assuming you used the marking knife properly, is the line it cut absolutely straight? How can you tell?
How do you make a board truly flat on all four sides with all sides truly at 90º without a square that has truly straight edges and that measures a precise 90º angle? You can’t. I don’t care if your square is steel or wood or aluminum or granite, if it is not true, all your woodworking will be guesswork. That’s not good enough for me.
The oldest method for testing a square, one already pointed out by others, is to place the body of the square against some reliably straight reference surface, draw a line along the tongue, then reverse the square and draw another line closely adjacent to, or on top of, the first line. If the two lines are absolutely parallel, all is well, but if not, the max gap between the two lines indicates twice the deviation from 90º. But how do you know the reference surface you used is straight? You checked it with your straightedge? You mean that $12 aluminum or steel square made in China you bought from Sears? Or did you use a more accurate straightedge? You mean that “Made in India" POS you bought from Lee Valley? I have already shared my experiences regarding the accuracy of some combo squares. Some guys use the edge of their tablesaw or bandsaw ASSUMING the surface is straight. Fat chance. Some guys swear by using the edge of a piece of paper. That is delusional. My in-laws have four monstrous German-made CNC paper shears in their factory in Sendai, and I have checked those long blades and know they are not perfectly straight. They don’t need to be since paper is not intended to be a precision tool.
I use my dial calipers regularly, mostly for measuring the thickness of boards, and comparing edge thickness of boards I am planing. When hand planing especially, dial calipers give quick, reliable results. But dial calipers are a rack and pinion tool, and racks and pinions wear out or get dinged or clogged. When this happens, and it always does, accuracy suffers and errors begin to accumulate. How do you check the accuracy? Feeler gauges are good, but a 1” micrometer lets you quickly and reliably check up to 1” thick.
Ideally, you would check the precision of your tools before you use them the first time (or better yet, before you even buy them), and then check them occasionally to confirm they are straight/square/within tolerance. Recognize that squares and straightedges wear out or become damaged in daily use. But how is a fellow to check the tolerances of his tools accurately using tools that are themselves goofy?
Besides my layout square, and framing square, I periodically check my planes (both wood and metal body) for flat/square. Yes, they do wear and move. I also check my shooting boards and cutting jigs frequently.
I think most people use their layout and measuring tools and jigs until they become either too damaged to use, or until the precision degrades to the point their inadequacy becomes too gross to ignore, usually after repeated mistakes make them question their eyesight.
I puzzled over this problem for a long time until I met a retired machinist who educated me. Manufacturers that have customers that rely on the accuracy of the products they make have Standard tools intended only for testing/calibrating the tools used on the factory/shop floor. They hire technicians or companies to periodically certify that these tools are still within tolerance and calibrated properly. These are expensive and massive granite and steel things. Since I don’t have access to such “Standards,” my retired machinist friend recommended I follow his example and keep a small set of high-quality, certified, precision tools in my shop, including a diemaker’s square, a 24” precision ground straightedge, and a 1” .0001” micrometer. I followed his advice, and since then, I have never had to wonder if any of my layout or measuring tools is straight, or square, or properly calibrated, or measuring properly because I have my own reliable Standards at hand. I also use them to check the accuracy of the tools belonging to the guys that work for me.
Long-term, I believe this has made my work much more accurate and efficient. Without doubt, it has caused me to replace tools that were out of tolerance despite appearing hunky dory, tools that I would have otherwise continued to use ignorant of their failings. It has also caused me to reject a lot of bad tools I would have later regretted buying due to unacceptable tolerances. I am certain it has saved me a lot of time and embarrassment over the years.
If you have been working with tools for a long time and think this is all nonsense, then I wish you well. But there may be guys without your skills that might find these ideas useful so retain your flames. I am not saying this is the only way to get the job done. After all, folks were making excellent furniture millennia before mankind could even hope to measure a thousandth of an inch. And there are lots of tricks for magnifying errors in woodworking to make them obvious. What I am saying is that, long-term, three precision tools can make the job easier and more efficient and give you greater confidence in your other tools.
Those of you that scoff at me for these suggestions should check your own tools for accuracy before long. You may be shocked.
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Originally Posted by Richard Shaefer
