Chapter 4 - Dimensions, Tolerances and Surfaces Flashcards
What is a tolerance?
The total amount by which a specified dimension is permitted to vary.
What is the difference between bilateral and 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?
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 is precision in measurement?
The degree to which random errors are minimised.
What is meant by the term graduated measuring device?
It has markings on a linear or angular scale to measure an objects feature of interest.
What are some of the reasons why surfaces are important?
Aesthetics, safety, friction, wear, effect of surface on mechanical and physical properties, mating of components in assembly, electrical contacts.
Define nominal surface.
The ideal surface represented on an engineering drawing, which is assumed perfectly smooth/round/flat etc.
Define surface texture.
The random and repetitive deviations from the nominal surface, including roughness, waviness, lay and flaws.
How is surface texture distinguished from surface integrity?
Surface texture refers only to the surface geometry, surface integrity includes not only the surface but the subsurface layer beneath the surface and the changes in it.
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 is superimposed on waviness.
Indicate some of the limitations of using surface roughness as a measure of surface texture.
It varies depending on direction.
It does not indicate lay.
Value depends on the roughness width cutoff used to measure the average.
Identify some of the changes and injuries that can occur at or immediately below the surface of a metal.
Cracks, craters, variations in hardness near the surface, metallurgical changes resulting from heat, residual stresses
What are the common methods for assessing surface roughness?
1) Comparison of the specimen surface with standard test blocks having known value.
2) Stylus type electronic instruments which measure average roughness.
Name some manufacturing processes that produce very poor surface finishes.
Sand casting, sawing
Name some manufacturing processes that produce very good surface finishes.
Polishing, grinding
