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Transfer tools (such as spring calipers) have no reading scale. Jaws on these instruments measure the length, width, or depth in question by positive contact. The dimension measurement is then transferred to another measurement scale for direct reading.
Attribute gages are fixed gages which typically are used to make a go/no-go decision. Examples of attribute instruments are master gages, plug gages, contour gages, thread gages, limit length gages, assembly gages, etc. Attribute data indicates only whether a product is good or bad. Attribute gages are quick and easy to use but provide minimal information for production control.
Variable measuring instruments provide a physical measured dimension. Examples of variable instruments are rulers, vernier calipers, micrometers, depth indicators, run out indicators, etc. Variable information provides a measure of the extent that a product is good or bad, relative to specifications. Variable data is often useful for process capability determination and may be monitored via control charts.
A reference surface is the surface of a measuring tool that is fixed. The measuring surface is movable. Both surfaces must be free from grit or damage, secure to the part and properly aligned for an accurate measurement.
The terms measuring tool, instrument and gage are often used interchangeably in this text. Appropriate gage should be used for the required measurement. Listed in the following slides are some gage accuracies and applications:
The Micrometer (inside) has Mechanical accuracy of about 0.001 inch. Some digital models are accurate to 0.00005 inch. They are normally used to check diameter or thickness. Special models can check thread diameters. The Micrometer (outside) has Mechanical accuracy of about 0.001 inch. Some digital models are accurate to 0.00005 inch. They are Normally used to check diameter or thickness. Special models can check thread diameters. The Optical comparator has the accuracy that can be within 0.0002 inch. They measure difficult contours and part configurations. The Optical flat depending on operator skill and are accurate to a few millionths of an inch. They are used only for very precise tool room work. Best used for checking flatness. The
They have Accuracy that is very good for checking the largest or smallest hole diameter. They check the diameter or drilled or rimmed holes and will not check for out of roundness. The Precision straight edge have Visual 0.10 inch with a feeler gage 0.003 inch and are used to check flatness, waviness or squareness of a face to a reference plane. The Radius and template gages have Accuracy that is no better than 0.015 inch and are used to check small radii and contours. The Ring gages will only discriminate against diameters larger or smaller than the print specification. The best application is to approximate a mating part in assembly. Will not check for out of roundness. The Split sphere and telescope have No better than 0.00005” using a micrometer graduated in 0.0001” and are used for measuring small hole diameters. The Steel ruler or scale are No better than 0.015”. They are Used to measure heights, depths, diameters, etc. The Surface plates have Flatness expected to be no better than 0.005” between any 2 points. They are Used to measure the overall flatness of the object. The Tapered parallels are Using an accurate micrometer, the accuracy is about 0.005”. They are used to measure bore sizes in low volume applications.
The Tool maker’s flat have Accuracy that is no better than 0.0005” depending upon the instrument used to measure the height. They are used with a surface plate and gage blocks to measure height. The Vernier calipers are About 0.001”. Some digital models are accurate to 0.00005”. They are used to check diameters and thickness. Vernier depth gage are About 0.001”. Some digital models are accurate to 0.00005”. They are used to check depths.
Attribute screens are screening tests performed on a sample with the results falling into one of two categories, such as acceptable or not acceptable. Because the screen tests are conducted on either the entire population of items or on a significantly large proportion of the population, the screen test must be of a non-destructive nature.
Screening programs have the following characteristics:
A clearly defined purpose
High sensitivity to the attribute being measured (a low, false negative rate)
High speciﬁcity to the attribute being measured (a low, false positive rate)
Benefits of the program outweigh the costs
Measured attributes identify major problems (serious and common)
Results lead to useful actions
Common applications of screening tests occur in reliability assessments and in the medical screening of individuals.
In reliability assessments, an attribute screen test may be conducted to separate production units that are susceptible to high initial failure rates.
This period is also known as the infant mortality period.
The test simulates a customer use of the unit, or perhaps an accelerated condition of use.
The number of failures, per unit of time, is monitored and the screen test continues until the failure rate has reached an acceptable level.
The screen test separates acceptable items from failed items, and an analysis of the failed components is performed to find the cause of the failure.
In medical screening, a specific symptom or condition is targeted and members of a defined population are selected for evaluation.
In many cases, the members of the selected population may not be aware that they have the condition being screened. Medical screening tests have the ultimate objective of saving lives.
Measuring instruments are typically expensive and should be treated with care to preserve their accuracy and longevity. Some instruments require storage in a customized case or controlled environment when not in use Even sturdy hand tools are susceptible to wear and damage. Hardened steel tools require a light film of oil to prevent rusting. Care must be taken in the application of oil since dust particles will cause build-up on the gage‘s functional surfaces. Measuring tools must be calibrated on a scheduled basis as well as after any suspected damage.