Dimensions, Surfaces, and Their Measurements
the predominant direction or pattern of the texture of a surface is called
lay
the surface texture Ra is a measure of:
roughness
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
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; it varies depending on direction; it does not indicate lay; 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 and the changes in it
surface roughness is a measurable aspect of surface texture; what does surface roughness mean?
the average value of the vertical deviations from the nominal surface over a specified surface length
what are common methods for assessing surface roughness
1. comparison of the specimen surface with standard test blocks having known surface roughness values 2. stylus-type electronic instruments which measure average roughness
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 is precision in measurement
Precision in measurement is the degree to which random errors are minimized.
what is meant by the term graduated measuring device
a graduated measuring device has markings called graduations on a linear or angular scale to measure an object's feature of interest
what are some of the reasons why surfaces are important
aesthetics, safety, friction and wear, effect of surface on mechanical and physical properties, mating of components in assembly, and electrical contacts
what is the difference between a bilateral tolerance and a unilateral tolerance
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
name some manufacturing processes that produce poor surface finishes
casting, hot rolling, sawing, and thermal cutting
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, inter-granular attack
the surface finish of casting would be expected to be numerically _______ a finished machined part (via turning or milling)
higher than
name some manufacturing processes that produce good surface finishes
honing, lapping, polishing, and superfinishing
define nominal surface
the ideal part surface represented on an engineering drawing. It is assumed perfectly smooth; perfectly flat if referring to a planar surface; perfectly round if referring to a round surface
define surface texture
the random and repetitive deviations from the nominal surface, including roughness, waviness, lay, and flaws
what is a tolerance
the total amount by which a specified dimension is permitted to vary
a tolerance in which variance from the nominal dimension is permitted in only one direction is called:
unilateral