Chapter 5

What is a tolerance?

A tolerance is defined as the total amount by which a specified dimension is permitted to vary.

What is precision in measurement?

Precision in measurement is the degree to which random errors are minimized.

Define surface texture.

Surface texture is the random and repetitive deviations from the nominal surface, including roughness, waviness, lay, and flaws.

What is the difference between a bilateral tolerance and a unilateral tolerance?

A bilateral tolerance allows variation in both positive and negative directions from the nominal dimension, whereas a unilateral tolerance allows the variation from the nominal dimension to be either positive or negative, but not both.

What is accuracy in measurement?

Accuracy is the degree to which the measured value agrees with the true value of the quantity of interest. It is a measurement procedure that is absent of systematic errors.

What are the common methods for assessing surface roughness?

Common methods for assessing surface roughness are (1) comparison of the specimen surface with standard test blocks having known surface roughness values and (2) stylus-type electronic instruments which measure average roughness.

What causes the various types of changes that occur in the altered layer just beneath the surface?

Energy input to the surface resulting from the manufacturing process used to generate the surface. The energy forms can be any of several types, including mechanical, thermal, chemical, and electrical.

Name some manufacturing processes that produce very poor surface finishes.

Processes that produce poor surfaces include sand casting, hot rolling, sawing, and thermal cutting (e.g., flame cutting).

Name some manufacturing processes that produce very good or excellent surface finishes.

Processes that produced very good and excellent surfaces include honing, lapping, polishing, and superfinishing.

Within the scope of surface texture, how is roughness distinguished from waviness?

Roughness consists of the finely-spaced deviations from the nominal surface, while waviness refers to the deviations of larger spacing. Roughness deviations lie within waviness deviations.

Surface roughness is a measurable aspect of surface texture; what does surface roughness mean?

Surface roughness is defined as the average value of the vertical deviations from the nominal surface over a specified surface length.

Indicate some of the limitations of using surface roughness as a measure of surface texture.

Surface roughness provides only a single measure of surface texture. Among its limitations are: (1) it varies depending on direction; (2) it does not indicate lay; (3) its value depends on the roughness width cutoff used to measure the average.

How is surface texture distinguished from surface integrity?

Surface texture refers only to the surface geometry; surface integrity includes not only surface but the subsurface layer beneath the surface and the changes in it.

Identify some of the changes and injuries that can occur at or immediately below the surface of a metal.

The changes and injuries include: cracks, craters, variations in hardness near the surface, metallurgical changes resulting from heat, residual stresses, intergranular attack, etc. (see Table 5.1).