LO 5-1 List and explain the factors requiring technicians’ skills related to precision measurements

LO 5-01: List and explain the factors requiring technicians’ skills related to precision measurements.

Introduction

Efficient and reliable engine operation depends on the use of precision, machined parts correctly assembled to function together. Precision measurement and machining generally refer to dimensional control of parts and components beyond the capabilities of ordinary, non-precision measuring instruments, such as the effort or “feel” for how much something is tightened and dimensional accuracies achieved by tape measures and rulers. Instead, precision measurement and machining techniques concern dimensions represented by thousandths of an inch or millimeters FIGURE 1. Assembly of components using precision techniques enables duplication of precise fits and operating clearances involving assembly techniques (such as precise control of clamping or tightening force). Examples of precision fits and clearances between rotating or moving parts include components where lubricating oil circulates between crankshafts, camshafts, and their associated bearings. Valve stem-to-guide clearance must allow movement but minimize clearance where oil can collect and burn, which can convert it to an abrasive particle. Pistons reciprocating in cylinders are constructed with operating clearances that prevent loss of compression and combustion pressure yet prevent piston-to-cylinder seizure and allow for an oil film to lubricate the cylinder walls. In other engine locations, precision machining of parts is necessary to create a fit between parts that lock components together or form a seal, such as between a cylinder liner and a block or piston pins and pistons. Incorrect fits and clearances can have consequences ranging from premature wear and noise to catastrophic failure, defined as a direct failure damaging more than 50% of engine parts or consequential damage resulting from a failure.

Accurate control of dimensions for engine components is also necessary to enable components to be used interchangeably. To produce and replace components ranging from cylinder heads and piston rings to bearings or cylinder sleeves requires precise duplication of component dimensions during production and installation. Today’s engines are produced with ever-decreasing tolerances for variations from exact dimensional standards. Efforts to minimize manufacturing tolerances are driven by the perception that precise control of component dimensions improves quality, reliability, and durability.

FIGURE 05-1: Measuring the backlash of a cam gear with a precision dial indicator.

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