Metrology for the cabinetmaker

[Lee Schierer]

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.
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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.
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Standardization

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.
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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.
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Next I measure the prob, while the measurement doesn’t show in this photo, it is 2.000″
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