microscopic projections on metal surfaces resulting from normal surface-finishing processes. Interference between opposing asperities in sliding or rolling applications is a source of friction, and can lead to metal welding and scoring. Ideally, the lubricating film between two moving surfaces should be thicker than the combined height of the opposing asperities.


Related Terms

POLISHING

1. In petroleum refining, removal of final traces of impurities, as for a lubricant, by clay adsorption or mild hydrogen treating. Smoothing and brightening a surface such as a metal or a rock through the use of abrasive materials.

POINT CONTACT

A contact between a specially prepared semiconductor surface and a metal point, usually maintained by mechanical periphery.

POINT-CONTACT DIODE

A semiconductor rectifier that uses the barrier formed between a specially prepared semiconductor surface and a metal point to produce the rectifying action.

PLUMMET

A loose-fitting metal plug in a tapered rotameter tube which moves upward (or downward) with an increase (or decrease) in fluid flow rate upward through the tube. Also known as float.

PLANISHING

Smoothing the surface of a metal by a rapid series of overlapping, light hammerlike blows or by rolling in a planishing mill.

PLASMA-ARC CUTTING

Metal cutting by melting a localized area with an arc followed by removal of metal by high-velocity, high-temperature ionized gas.

PILOT HOLE

1. In metal-forming operations, a prepunched hole in a metal strip into which the pilot component of the die enters in order to assure precise registration of the strip at each work station. 2. A small hole drilled ahead of a larger borehole. 3. A hole drilled in a piece of wood to serve as a guide for a nail or a screw or for drilling a larger hole.

PIEZOELECTRIC TRANSDUCER

A piezoelectric crystal used as a transducer, either to convert mechanical or acoustical signals to electric signals, as in a microphone, or vice versa, as in ultrasonic metal inspection.

PIKE POLE

1. A pole with a sharp metal point in one end that is used to hold utility poles upright while they are being installed. 2. See fire hook.

PEEL TEST

A test to ascertain the adhesive strength of bonded strips of metals by peeling or pulling the metal strips back and recording the adherence values.

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