Wanting to spur a bit of discussion, I wrote up this post and I'm curious to see if it is of any use.
Metrology for the Cabinetmaker – Introduction
This post is intended as the beginning of a conversation on inspection equipment and measurement standards rather than a conclusive how-to. My equipment, techniques and process are in a continuous evolution. These techniques have been working effectively for me and so I have decided to share them, however the I do not run a metrology lab, I run a wood shop, so be aware that these techniques may be incorrect or incomplete. I introduce this only to encourage thought and spark further discussion.
The reader may wonder as to why fine measurements make any difference at all in a wood shop. After all, wood moves with change in relative humidity and so one may conclude that minor gaps are inevitable. Wood movement is one of the greater challenges of woodworking and yet we can plan for it and build accordingly, engineering considerations for wood movement into our product.
As example, looking at the typical mortise and tenon joint. Wood moves across the grain, so if the mortise were cut in the typical fashion it would shrink and swell across the width. At the point of production it may be .500″ wide, and months later it may be .510″ wide. The tenon, if produced from the same batch of material in the same grain orientation at the same time will have moved in the same fashion, so it too will have moved from .500″ wide to .510″ wide maintaining the fit between parts. Along the height of the board it will gap at the top and bottom of the mortise. This can be partially countered by lightly compressing the wood during the fitting process. The joint can draw-bored to retain it tight against the tenon shoulder and the joint can be fully shouldered joint to hide the potential gaps created at top and bottom of the tenon that are not solved by compression.
Gaps are also created by errors in setup and cutout. These are due to a host of circumstances including layout error, machine setup error, and tool bit deflection. Repair of these unexpected circumstances often requires custom fitting, remaking of parts or other time consuming approaches to repair or otherwise alleviate this circumstance. To avoid minor gaps an in consistencies created by these errors the first step is to remove uncertainty where we can find it. This requires a level of precision counted in thousandths of an inch for our machinery setup. Good technique combined with accurate equipment helps to produce accurate results, as example a saw that cuts squarely can also be used to make accurate length parts as both sides of the part can be make the same length. Accuracy is an important component in efficiency of process and it begins with the checking tools themselves before they are used to inspect the equipment or work piece.
Knowing that a square is actually square and that 12″ is actually 12″ is a worthwhile venture when the goal is efficiency. These questions become increasingly difficult to answer without the proper equipment and so it is for this reason that I feel the woodshop benefits from having a basic kit of highly accurate tools used as standards for inspection. This kit is commonly referred to as a metrology kit in the machine shop.
Metrology is the scientific study of measurements with an aim to create a common understanding and agreement of units. This agreement allows the industrialized world to create products with confidence that parts from independent sources will interact properly. For this reason many have adopted the metric system, a system of standards based on realizable values, as their countries have industrialized. If not directly replacing their own systems many countries have standardized their systems of measure on the metric system. As example, the standard for a foot (Imperial) is exactly 30.48 cm, the shaku (Japan) is 30.3cm and the chi (China) is 32cm.
Modern standards which create the basic building blocks of the metric system are derived from specific references which remain constant. As example, a meter is the length of the path travelled by light in a vacuum during a time interval of 1/299792458 of a second. These such references form the primary standards upon which international systems base their physical standards. In the US these standards are maintained by NIST. Quality measuring tools are calibrated to NIST standards and certain forms of work require periodically recalibrating tools to those standards.
In the woodshop we can compare a part to the space it must consume, mark and cut, or we can measure the space and build the part using that measurement as a reference, neither operation requires an international standard. However, if our work involves using multiple tools, each reliant upon a consistent series of measurements then we are best served by forming a shop standard and that shop standard may as well be derived from the international standards.
In a practical sense one need only compare two tape measures to realize that a lack of standards can complicate things quite rapidly. If you’re using a few different steel rules and a set of digital calipers then it’s best if they all agree with one another. This is further complicated when the machinery itself has built in gauges and those gauges are relied upon. Consistency in measuring devices is helpful in avoiding transfer errors.
Below I will detail a few of the measuring devices which are useful in the workshop, both highly accurate tools and moderate or low accuracy tools. They’re all necessary at times and their varying degree of accuracy or simplicity each make them useful in different ways.
Please note that in machine shop’s which utilize imperial measurements, the basic unit of measure is one thousandth of an inch (.001″), commonly referred to as a thousandth, or ‘a thou’. So, for example .500″ would be read out-loud as ‘five-hundred thousandths’. Finer than a thousandth is a ten-thousandth of an inch, or ‘a tenth’, this is confusing to many as a tenth is .100″, but the tenth as commonly referenced in a machine shop is one ‘tenth’ of a thousandth or .0001″. Still, it would be unusual to call out .5001″ as five-thousand and one tenths, instead it would be referred to as ‘a tenth over five hundred thou’ or similarly clear way of providing that it is not dead on .500″ but a tenth over.
Surfaces
In precise measuring, life begins with the surface plate. A surface plate is a true flat surface, often to a degree of precision measured in tenth thousandths of an inch (shop grade) or hundredth thousandths (Laboratory grade). These plates are calibrated by differential electronic level and are lapped flat, this plate arrived with the readout provided by the manufacturer. This surface is the basis for most comparative measuring so an accurate plate is a nice thing to have and fairly inexpensive.
The surface plate, along with all of the other precision tools are covered when not in use.
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Immediately following the surface plate are the straight edges. The straight edge is a crucial piece of equipment in the workshop. This simple device allows the ability to compare surfaces to a known true reference. There are many types of straight edges, shown here is a beveled edge straight edge purpose built for checking plane soles.
Taking little for granted, I have checked this straight edge for flatness and parallelism. The process is done by applying marking compound to the surface plate then taking an imprint of the edge. I followed that by taking a sweep over the straight edge with an indicator in search of errors.
Straight edges must be supported along their length when they are stored, or they can be stored flat in a toolbox so long as they’re protected from other objects banging into them.
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Another type of straight edge, the camel back. Rather than being ground flat, these are sometimes scraped by hand. This one in particular scraped by a very talented operator, by hand. This device is accurate to a few tenth-thousandths of an inch.
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Specific to woodworking are winding sticks, wooden sticks used to define twist or ‘wind’ (think wind like a watch, not like the weather). These are used to give a basic understanding of surface defects. These are compared to the surface plate to ensure that they read accurately.
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Angularity
The angles referenced most often in a typical woodshop are 90 degrees and 45 degrees. To check 90 degrees a tool know as an angle block can be used. These plates are best accurately scraped flat on their surfaces, square about their main faces and the edges square to the main faces and also flat. They can be used in conjunction with one another for comparing a square surface on three faces at one time.
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Next up is the master square, this square is a precision ground engineer’s square certified to a high degree of angular accuracy. This is an A grade Mitutoyo square, which agrees with the angle plates above. This is mainly used for comparison with other squares and occasionally brought to the work but it is never scribed against.
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A multitude of squares can be utilized in the workshop including various sizes and calibrations of try squares intended for checking work and creating layouts. Next is a smaller master square, made by Starrett, that is inspected accurate to .0001″ and it agrees with my angle plates.
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Finally, I have another precision square which is of a quality that can be scribed against and used for other similar tasks without much concern. It is highly accurate, but it is the one square in the shop that I will retune periodically to ensure it remains accurate.
Angular measurements outside of 90 degrees, are read and transferred in variety of ways depending on the degree of precision required. Measurements can be provided by protractor, angle blocks, fixed angles or sine bars. The most basic measurement kit is shown here; a bevel gauge which is used to compare and transfer angles and a simple protractor. A high quality bevel gauge can be quite reliable and this example, made by Chris Vesper has proven highly reliable.
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For the rest of the article see Page 2
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