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Aug 19 2008

The Foibles of Tape Measures. The Best, and getting the most from one.

Published by at 2:35 am under Layout Strategy,Metrology,Skill Development

For the average user of a retractable tape, there can be some usages of a tape measure that unwittingly reduce its accuracy. Basically, many people are not even aware of these details. I did say usages, but there are also problems inherent with the way a tape measure is made, that for fine work, render it a tool which is not always the best tool for the job. When we ask a tool to wear too many hats, it fails to do as well by us as we may think it should. It is good to know what some of the weaknesses are so we can learn to accept what are and are not good practices for the tape measure.

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It may seem funny to have thought about tape measures as much as I have. As a life long woodworker, and over 25 years of civil construction and surveying experience, I have literally worn out a bunch of measuring equipment from using these tools all day every day. After using tapes in dirty environments where I took well over 100 measurements a day, daily and weekly, I have learned what I value and what has risen to be the best for me. I am just sharing what I have learned with you.

When you are buying a tape measure, there are several available features that you can consider. For shop use, furniture making and cabinet making, you will rarely need a long tape, but the long tapes have features that enhance accuracy. They come with 1-inch wide tapes, which are easier to read for eye relief, harder to distort and are more rigid. Often the 1-inch tapes include more rivets on the hook, which lend themselves to resistance to wear and stretch. Unless you need a shorter tape for handy reasons, I recommend the bigger tape just for its added stability.

Consider the hook of the tape measure. The hook can be worn, bent, or the rivet holes that hold it can be stretched. This can unwittingly induce errors, and many people are simply unaware of this frailty of tape measures. Lufkin has several models with 1-inch wide tapes that include four rivets on the hook. They are the only maker I have ever encountered that installs hooks with four rivets.

All the four rivet Lufkins I have tested and compared have always compared exactly to Starrett steel rulers, even after very extensive use, and are trusted most exclusively by surveyors and engineers, meaning, I feel they are trustworthy in your woodworking shop also. I also have a Lufkin twelve footer that is a useful length, as accurate as any when new, but it uses 3/4 wide tape, and has only 2 rivets attaching the hook to the tape. It is accurate when new and well cared for, but I don’t expect its accuracy to be as sustainable as the bigger tapes. I limit its use to the “Handy” instances.

Getting back to tape measures in general, even when trying to be careful, The hook can make any measurement not taken from an edge inherently inaccurate, because the hook will not allow the tape to lay flat on the surface of what it is measuring. When the tape cannot lay flat, it induces an error through forcing a trigonometric path that is like a hypotenuse length, which is not the actual surface; it is instead an independent path above that of the flat surface one. While usually subtle, this is similar to the error induced when you measure from an edge but are not perpendicular to the edge when you read the tape.

parallax1.jpgThat gap we see can equal measurement FAIL.

Speaking of the devil, from the hook on, any measurement pulled which is not perpendicular to the edge of the board will introduce a trigonometric error, which is going to pivot from either edge of the hook. It also can induce a Parallax error when the marks are not right on the surface being measured. Either way, it is slight, but can be enough to be annoying in fine work, especially when the error can become cumulative.

Just when you think you understand all the gremlin’s and their ways, the side to side curve of the tape, which is designed to stiffen it, holds the markings up off the surface being measured by about 1/4th inch, on a 1 inch wide tape, and unless the user is careful to push the marks on the tape down flat to the wood, this too can induce a parallax error when marking.

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As an exercise, grab your tape and pull out 2-3 feet. Lock it, and hook it over the edge of the bench. Now examine how flat it lays, and how close the markings are to the surface it will be measuring. See? The tape case itself holds the tape of the surface at least 1/4th inch. It can twist, bend and flex and it needs pulled taught and laid flat to be usable at all. You really need to apply more English to it than a cue ball in a game of snooker. I’ll not even discuss how often I wish I had a third hand or wish I didn’t have to use it upside down and backwards.

For those who wonder, what I mean by parallax error is this. I am referring to parallax in terms of visual perception, particularly related to instruments. We all have a strong eye, which we favor over the other. We do this via habit, we do not think about it. Interestingly, we have two eyes though, and one can tend to throw the other one off when we are looking at a single point at close range when we need to do something precisely.

What this means, is that when we look at the markings on a rule, unless we are super careful, we my actually be looking at the rule markings from a slight angle. In order to measure anything with total accuracy, the measuring tool must have its marks absolutely flat to the surface being measured, the closer to the surface, the better, and the rule must be viewed at a 90-degree angle to the markings. This may mean favoring the use of just one eye when you measure.

If the marks on the measuring device are not absolutely against the surface needing measured, then the difficulty in determining parallax is not being overcome, and will create difficulty for the person measuring to determine.

It is hard to be really certain if the pencil is marking perfectly on line, when it is marking 1/4 inch below the 1/32 line, it is marking for on a tape. Harder if you don’t have 20/20 vision, harder still if there are shadows in the lighting, harder again if it is a precarious situation in the first place.

In carpentry, many things can be fine with some parallax error induced; things close enough are close enough. Much of carpentry is accurate enough if you work to the eighth of an inch. A good bit more of it will forgive 1/16th. It isn’t always critical. In fine woodwork, it is a situation that can leave a board to short or long, depending on the favored eye and the side the line was meant to be cut on. It happens because when we are marking and laying out, parallax errors become cumulative. After a cloud of these errors, which are commonly as small as 1/128th in size, we can easily find we are 1/32nd or more out of alignment. Worse still, the layout is the most accurate part of the work; the cutting will rarely be as precise as the layout, especially with hand tools. This makes good layout all the more important.

Remember that at the fine woodworking level, wood is often more expensive than the tools we work it with, and we may be many surfacing and dimensioning procedures put into a board or boards for fine fit and finish on a project. When errors are induced, starting over isn’t always as simple as grabbing a new board and taking up where we left off. Care and attention to details can make or break our finished product.

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Please notice in the photos. The ruler lies flat and close to the work. It can be used to measure anywhere it can fit. This is why it is useful and helpful, though not necessary to have the rulers available in various lengths. I prefer using the shortest ruler possible to measure the length needed. I use the 6-inch rules the most and as long as the measurements are shorter than six inches, they are the best fit. If the measurement I need is greater than six but less than twelve inches then I step to the next size up and so on.

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The hook rule is not meant to be used laid flat, it is meant to sit on edge and have it’s hook lay over an edge to reference the measurement, but again, the marks come all the way to the surface of the work. It is the surest way to be measuring exactly from an edge and it cleared up a lot of small errors for me once I started using them.

The trick to this is learning when a steel ruler or a tape measure is the best tool for the job, and how it is liable to induce unwanted errors simply through it’s use. Some projects present these critical situations, and knowing how to overcome them makes your final product better.

Overall, this is why I advocate using steel rulers wherever possible. They are inherently more accurate than tapes not only by virtue of how they are made and what they are made from, but also by the way the way they are made forces them to be used. They are straight, flat, and rigid. The lack of flex and curvature make them inherently more accurate. It is accurate tools and practices, used with repeatable procedures, which create repeatable results. It is by this combination of virtues they are inherently more accurate, even when you are not trying to be. Consider using the tape measure with more care, and the addition of some steel hook and non-hook rulers to enhance the accuracy of your work in the shop.

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