page 144
Limits - These typically define a dimensional range that a measurement can be expected to fall within.
Machine Tool - Generally use to refer to a machine that performs a manufacturing operation. This is sometimes confused with the actual cutting tools, such as a drill bit, that do the cutting.
Measurement - The determination of an unknown dimension. This requires that known standards be used directly, or indirectly for comparison.
Metric System - A measurement system that has been standardized globally, and is commonly used in all modern engineering projects.
Metrology - The science of measurement. The purpose of this discipline it to establish means of determining physical quantities, such as dimensions, temperature, force, etc.
Precision - Implies a high degree of accuracy.
Repeatability - Imperfections in mechanical systems can mean that during a Mechanical cycle, a process does not stop at the same location, or move through the same spot each time. The variation range is refered to as repeatability.
Standards - a known set of dimensions, or ideals to compare others against.
Standard Sizes - a component, or a dimension that is chosen from a table of standard sizes/forms.
Tolerance - The allowable variation in a basic dimension before a part is considered unacceptable
35.3 STANDARDS
•Standards are the basis for all modern accuracy. As new methods are found to make more accurate standards, the level of accuracy possible in copies of the standard increase, and so on.
•A well known metric standard is the metric 1m rod.
•Many standards are available for measuring, and many techniques are available for comparison.
35.3.1 Scales
page 145
•The most common tool for crude measurements is the scale (also known as rules, or rulers)
•Although plastic, wood and other materials are used for common scales, precision scales use tempered steel alloys, with graduations scribed onto the surface.
•These are limited by the human eye. Basically they are used to compare two dimensions.
•The metric scales use decimal divisions, and the imperial scales use fractional divisions.
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•Some scales only use the fine scale divisions at one end of the scale.
•It is advised that the end of the scale not be used for measurement. This is because as they become worn with use, the end of the scale will no longer be at a ‘zero’ position. Instead the internal divisions of the scale should be used.
•Parallax error can be a factor when making measurements with a scale.
page 146
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If the instrument is not measured directly on, |
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then there may be some error. Note: this would |
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not occur if the scale was perfectly thin. |
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35.3.2 Calipers
•A tool used to transfer measurements from a part to a scale, or other instrument.
•calipers may be difficult to use, and they require that the operator follow a few basic rules,
-do not force them, they will bend easily, and invalidate measurements made
-try to get a feel, or personal technique for using these instruments.
-if measurements are made using calipers for comparison, one operator should make all of the measurements (this keeps the feel factor a minimal error source).
•These instruments are very useful when dealing with hard to reach locations that normal measuring instruments cannot reach.
•Obviously the added step in the measurement will significantly decrease the accuracy
35.3.3 Transfer Gauges
•Small hole gauges can be inserted into a hole, as an adjustment knob is turned, the head expands to the size of the hole. The gauge can be removed and measured to determine the diameter of the hole. The end of this gauge appears as if a sphere with a shaft in it has been split into two halves.
•Telescope gauges have two plungers that are springy, until locked in place. This can be put in holes or hard to reach locations, and used to transfer measurements to other measurement