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Metrology for the Cabinetmaker - Page 2
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.
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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
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.
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When measuring from a 45 degree position one can use this feature of the combination square.
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When measuring from an angle outside of 90 or 45 the protractor head is used.
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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.
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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.
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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.
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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.
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Metrology for the Cabinetmaker - Page 3
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.
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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.
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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.
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The indicator can also be used in a magnetic base and mounted directly on iron or steel surfaces.
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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.
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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.
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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.
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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.
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