What is the resolution (smallest graduation) of a standard outside micrometer?
Explanation: A standard outside micrometer reads to 0.001" on the thimble scale. Most quality micrometers include a vernier scale on the sleeve that provides an additional digit, reading to 0.0001". Digital micrometers display 0.00005" or 0.001mm resolution.
What measuring instrument is best for checking the inside diameter of a bored hole to ±0.0005" accuracy?
Explanation: Telescoping gauges are inserted into the bore, locked at the diameter, then measured with an outside micrometer. Bore gauges (also called bore micrometers or dial bore gauges) directly measure internal diameters. Both achieve ±0.0005" or better. Calipers lack the precision needed for this tolerance.
What is the purpose of a dial indicator in machining?
Explanation: Dial indicators (also called dial gauges) measure linear displacement. They are used for checking runout (TIR), aligning vises and fixtures, centering workpieces in four-jaw chucks, checking flatness, and verifying parallelism. They do not measure absolute dimensions—they measure deviations from a reference.
How should a micrometer be handled to maintain its accuracy?
Explanation: The ratchet stop ensures consistent measuring pressure (typically 2-5 lbs). Handling by the frame prevents body heat from affecting the anvil/spindle. Heavy pressure distorts readings. Storing in a case prevents damage to the precision ground surfaces.
What is the difference between accuracy and precision in measurement?
Explanation: A measurement system can be precise (giving very consistent readings) but inaccurate (consistently wrong). Or it can be accurate on average but imprecise (widely scattered readings). Good metrology requires both accuracy and precision. Calibration addresses accuracy; technique addresses precision.
What is GD&T (Geometric Dimensioning and Tolerancing)?
Explanation: GD&T (per ASME Y14.5) uses symbols on engineering drawings to communicate geometric requirements—flatness, parallelism, perpendicularity, position, runout, etc. It provides a more precise and unambiguous way to define tolerances than simple ± dimensions, especially for features like hole positions and surface relationships.
At what frequency should precision measuring instruments be calibrated?
Explanation: Precision instruments drift over time due to wear, thermal cycling, and mechanical stress. Regular calibration against traceable standards ensures measurements remain accurate. Most shops calibrate annually, with more frequent checks for heavily used or critical instruments. Any physical impact warrants immediate re-calibration.
What is the proper way to read a vernier caliper at 1.573"?
Explanation: Vernier calipers use three readings: (1) the main scale gives the integer and first decimal (1.5"), (2) the sliding scale gives the next digit (0.07"), (3) the vernier scale identifies which vernier line aligns perfectly with any main scale line, giving the thousandths digit (0.003"). Total: 1.573".
What is a gauge block (Jo block) used for?
Explanation: Gauge blocks are hardened steel, ceramic, or carbide blocks ground to extremely precise dimensions (±0.000002" or better for Grade 0). They can be wrung together (adhered by molecular attraction) to create any dimension needed. They serve as the primary calibration reference for micrometers, calipers, indicators, and other instruments.
What safety practice is most important when using compressed air to clean chips from a machined part?
Explanation: Compressed air propels metal chips at high velocity—effectively creating shrapnel. OSHA requires air guns used for cleaning to be below 30 PSI at the nozzle (with chip guards). Never direct compressed air at skin (can cause air embolism), eyes, or other people. Always wear safety glasses when using compressed air.
What should you do if you discover a measuring instrument gives readings that differ from a known standard?
Explanation: An out-of-calibration instrument can produce parts that appear to be in tolerance but are actually out of spec. It must be removed from use immediately, tagged so no one else uses it, and sent for professional calibration. Any parts measured with the suspect instrument since its last known-good calibration should be re-inspected.