Engr. Midterm ch.5
flaws
irregularities that occur occasionally on the surface
dimensions
linear or angular sizes of a component specified on the part drawing
bevel protractor with vernier scale
measures angles
nongraduated measuring devices
no scale and are used to compare dimensions or to transfer a dimension for measurement by graduated device
cutoff length
parameter used as filter to separate waviness from roughness deviations
limit dimensions
permissible variation in a part feature size consists of max and min dimensions allowed
systematic errors
positive or negative deviations that are consistent from one measurement to the next
lay
predominant direction or pattern of the surface texture
measurement
procedure in which an unknown quantity is compared to a known standard
good precision
random errors in the measurement procedure are minimized
surface texture
rough, wavy, and has flaws; top part of metallic part; geometry of the surface, commonly measured as surface roughness
roughness
small, finely-spaced deviations from nominal surface
precision gage blocks
standards against which other dimensional measuring instruments and gages are compared
precision
the degree of repeatability in the measurement process
accuracy
the degree to which a measured value agrees with the true value of the quantity of interest
why surfaces are important
1. aesthetic reasons 2. surfaces affect safety 3. friction and wear depend on surface characteristics 4. surfaces affect mechanical and physical properties 5. assembly of parts is affected by their surfaces 6. smooth surfaces make better electrical contacts
surface changes caused by electrical energy
1. changes in conductivity and/or magnetism 2. craters resulting from short circuits during certain electrical processing techniques
measuring instrument types
1. graduated measuring devices 2. nongraduated measuring devices
surface changes by caused chemical energy
1. intergranular attack 2. chemical contamination 3. absorption of certain elements in metal surface 4. corrosion, pitting, and etching 5. dissolving or microconstituents 6. alloy depletion and resulting hardness changes
energy forms that affect surface integrity
1. mechanical energy 2. thermal energy 3. chemical energy 4. electrical energy
surface changes caused by thermal energy
1. metallurgical changes 2. redeposited or resolidified material 3. heat-affected zone in welding 4. hardness changes
surface changes caused by mechanical energy
1. residual stresses 2. cracks 3. voids or inclusions 4. hardness variations
four elements of surface texture
1. roughness 2. waviness 3. lay 4. flaws
methods to measure surface roughness
1. subjective comparison with standard test surface 2. stylus electronic instruments 3. optical techniques
surface roughness
a measurable characteristic based on roughness deviations
surface finish
a more subjective term denoting smoothness and general quality of a surface
graduated measuring devices
a set of markings on a linear or angular scale to which the object's feature of interest can be compared for measurement
tolerances
allowable variations from the specified part dimensions that are permitted in manufacturing
average roughness formula
average roughness= integral (from 0 to specified distance) y/specified distance over
surface changes
caused by the application of various forms or energy during processing
surface technology
concerned with defining the characteristics of a surface, surface texture, surface integrity, relationship between manufacturing processes and characteristics of resulting surface
stylus instruments
cone-shaped diamond stylus is traversed across test surface at slow speed; moves vertically to follow surface deviations; vertical movement is converted into electronic signal
dial indicator
converts and amplifies the linear movement of a contact pointer into rotation of a dial
mechanical gages
designed to mechanically magnify the deviation to permit observation
nominal surface
designer's intended surface contour of part, defined by lines in the engineering drawing
waviness
deviations of much larger spacing
electronic gages
family of measuring and gaging instruments based on transducers capable of converting a linear displacement into an electrical signal
plug gage
gaging of a hole diameter
surface integrity
the study and control of this subsurface layer and the changes in it that occur during processing which may influence the performance of the finished part or product; deals with material characteristics immediately beneath the surface and changes to this subsurface
GO limit
used to check the dimension at its maximum material condition
NO-GO limit
used to inspect the minimum material condition of the dimension in question
unilateral tolerance
variation from the specified dimension permitted in only one direction
bilateral tolerance
variation permitted in both positive and negative directions from nominal dimension
