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.
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.
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.
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.
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.
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.
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.
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.
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.
Moving toward higher accuracy, the digital protractor which reads out to a tenth of a degree. This protractor is useful for machine setup in less demanding situations.
In higher precision machine setup and for checking angularity, it is often done one of two ways, angle blocks or a sine bar. Angle blocks are precisely ground small blocks which can be stacked together to form a very accurate representation of a given angle. A sine bar is a precision bar with two round portions. The sine bar provides a fixed distance for the hypotenuse allowing the user to calculate any angle and bring the bar to that angle by using gauge blocks.
Distance measurements are done with varying degree of accuracy depending on their need. Different tools for different tasks, we separate shop standards and high precision measuring from basic layout devices which are referenced to those shop standards for quality then used for lower precision tasks.
The most basic tool is the scale, or rule, depending on your preference. The scale can be relied upon for measuring in scenarios where the accuracy needed is less than the width of the tick marks and they’re often available up to about 60″ in length in steel.
Used in conjunction with a scale, the combination square can used for measuring from an edge.
When measuring from a 45 degree position one can use this feature of the combination square.
When measuring from an angle outside of 90 or 45 the protractor head is used.
When measuring from a square corner toward center, or finding center of square corner one may chose to use another part of the combination square kit, the center head.
Next is the double square, featuring a shorter blade for getting into tighter spaces. The double square is a wonderful device for small scale layout and useful in many scenarios where high precision is unnecessary.
Followed by a higher precision, but smaller double square made by Starrett and featuring a series of blades which allow it to be used in tight spaces.
Another way to measure from an edge is a kegaki scribing gauge, this one manufactured by Matsui Measure. This provides a similar degree of precision as the combination square, but it has a longer reference face which is handy to scribe against.
Following is a piece that has a role in both the carpenter’s kit along with those of the cabinet shop, this is the square rule. In this case it is a Sashigane, or Japanese carpentry square.
Processing toward a greater degree of accuracy we first move into the vernier caliper, with digital readout in this case. This particular model is provided with its own standard, which is intended to be measured at 20 degrees celcius.
From there we move into the indicator, also digital in this case. Indicators are available which measure very finely. This particular one measures to five tenths (.0005″). Indicators are a multi function tool, depending on how they’re fixtured, they’re a great device for taking measurements directly from a machine or for use in a comparator.
The indicator can also be used in a magnetic base and mounted directly on iron or steel surfaces.
A finer reading indicator known as the test indicator is often used for more detailed work, it has a much smaller range of motion and it can provide a higher degree of accuracy. These commonly read to a tenth of a thousandth of an inch. When combined with fixturing devices such as the Magnetic stand, spindle mount or comparator it is very useful for a variety of indicating needs. I typically use a test indicator for tramming work.
For height, the height gage is used, digital in this case. This can be used for precise comparison in addition to gauging of height. This is a wonderful device for checking tooling heights but must be used with extreme care as the tip is carbide.
More accurate still we move into the micrometer. This digital mic can read down to microns in metric or hundred thousandths in imperial, I don’t use a lot of micrometers in my work, so I have a 1″ model only.
Depth is best measured by a depth micrometer, this one included a set of six probes and allow it to measure from between 0-6″ of depth.
Johansson Blocks or ‘Jo’ blocks are a gauge block system for producing accurate lengths. These are lapped to size and calibrated by Mitutoyo, the deviation from standard is provided in a sheet and measured in millionths of an inch, they’re effectively perfect to my ability to measure.
1,2,3 and 4,5,6 blocks are similar to Jo blocks but they’re not quite as accurate in most cases. These blocks are ground square and to size. These are a fairly inexpensive set of blocks that I use for setup on the milling machine (mainly). They’re also handy for other forms of setup and reference. The block is being used as a stop in this situation.
With our inspection tools in place we can begin the process of checking them to ensure that they are reading accurately. ASME outlines exactly how to calibrate inspection equipment, however for the purpose of this post I will detail my own more basic process. The goal of this exercise is to ensure that all of the tools are reading within a reasonable degree of one another. That degree varies depending on the tool class and its manufacturer spec along with the requirements of the workshop.
Initial testing will be conducted with the use of gauge blocks. I make assumption that the gauge blocks are accurate and one can reinforce the quality of that assumption by measuring multiple blocks with each instrument. These blocks are certified and come from a manufacture held to international quality standards for accuracy, they’re graded for accuracy and provided with a certification.
These blocks are made to be accurate to size at 20 degrees Celsius (68 degrees Fahrenheit), so the first thing to be aware of is the temperature being measured it. Steel expands at a coefficient of 0.00000645 per inch degree F, so a 4″ block could increase in size by .0005″ if it were measured at 90 degrees rather than 68 degrees. Doesn’t sound like much, but that always depends on what your after and it should be calculated for when measuring outside of 68 degrees.
In order to eliminate possible variables I decided to check a few things before getting started with comparison testing. My first step was to apply compound to my granite plate and check my indicator bases for contact. My indicator bases are flat so I won’t need to worry about them rocking and causing a false read during any of the tests. I’ve verified all squares against each other and against the angle plate, they all agree with one another, save for the combination squares which deviate by a couple thousandths except the mini Starrett.
Using the standards I begin by checking my calipers. It’s important to have the feel for measuring down pat, but having a number of years on micrometers and calipers I have a consistent feel. The calipers much be clean where they will touch measuring devices and the standards must also be clean. I oil things between uses and so I wipe the oil off before starting. This should be done with multiple standards to see if the error remains constant or if it increases.
The main jaws I measure in three places using the thin side of the measuring block to ensure that the jaws are parallel to one another. These are spot on, so far, measuring exactly as they should.
Next I measure the standard using the back of the main jaw, this provides a deviation of .0015″. More than I prefer, but it’s within a reasonable standard, especially for a part of the tool I don’t much use.
Next I measure the prob, while the measurement doesn’t show in this photo, it is 2.000″
Next I check the depth micrometer and the outside micrometer to ensure that they’re reading accurately. The first step I take is to ensure an accurate zero position. This is one in which the dial stops at exactly 0.00000″ when the spindle closes against the anvil on the micrometer and when the spindle is flush against the surface plate on the depth micrometer. The digital aspect of the gauge can be easy reset to zero if it reads otherwise. I use the micrometer’s ratchet’s to ensure that the positions are closed down to accurately.
After which I want to establish that the micrometer is reading distances accurately, so I use two blocks, one in the middle of the range and one at the extreme of the range to ensure that these measurements are accurate.
It’s important that the tools be in good order as well, in this case the depth mic shows staining on the side of the spindle, this is due to how it was stored before I received it. The spindle is in the process of being replaced and the new spindle will be calibrated again once it arrives. Its important that precision instruments be kept in good condition, their tolerances can be deteriorated by rust.
The same procedure is applied to the height gauge, first setting it to zero using the surface plate.
The digital indicator is affixed to a comparator stand. The probe is mounted so that it contacts the top of the anvil, then the indicator is set to zero. The probe is lifted, checking blocks are slid into place and the probe placed on the blocks. This measures the amount of travel. A comparator stand maintains a square position for the indicator so the travel is accurate.
In the next post of this series I will begin detailing my process for standardizing machine fence read outs and specifying a few cases where transferring measurements is an improvement in accuracy and expedience. Thank you for reading this post and I look forward to your comments.