What is the definition of "average roughness"?
The average distance between peaks and valleys of surface roughness.

Learn more about average roughness in the class Surface Measurement 140 below.

Inspection Training

Class Information
Tooling U classes are offered at the beginner, intermediate, and advanced levels. The typical class consists of 12 to 25 lessons and will take approximately one hour to complete.
Class Name:Surface Measurement 140
Description:This class identifies the different types of surface texture and describes how the surface texture of a part affects its use.
Prerequisites: none
Number of Lessons:18
Language:English, Spanish

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Below are all the competencies and job programs that contain the class Surface Measurement 140. Job programs are our traditional class lists organized according to common job functions. Competencies are our latest job-specific curricula that help tie online learning to practical, hands-on tasks.

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Class Outline
  • Objectives
  • The Importance of Surfaces
  • Static and Dynamic Surfaces
  • Static and Dynamic Properties
  • Surface Terminology
  • What Is Surface Texture?
  • Flaws
  • Lay
  • Roughness
  • Average Roughness (Ra)
  • Waviness
  • Measuring Surface Roughness
  • Surface Replica Blocks
  • Stylus-Type Instruments
  • Skidded and Skidless Gages
  • Calibrating Surface Measuring Instruments
  • How Surface Finish Affects Cost
  • Summary
Class Objectives
  • Describe how machining processes cause surface finish.
  • Distinguish between a static and dynamic surface.
  • Describe how the surface texture of a part affects its function.
  • Distinguish between the actual surface and its specifications.
  • Define surface texture.
  • Identify a flaw.
  • Identify lay.
  • Identify roughness.
  • Define average roughness.
  • Identify waviness.
  • Identify the methods used to measure roughness.
  • Describe surface replica blocks.
  • Describe the fingernail test.
  • Describe how a profilometer measures roughness.
  • Distinguish between skidded and skidless gages.
  • Explain the importance of calibration.
  • Describe how surface finish affects cost.

Class Vocabulary

Vocabulary TermDefinition
abrasive grain A small, hard particle or crystal of abrasive material used to machine, grind, or finish a workpiece.
amplified To make something larger, to exaggerate detail. A profilometer amplifies surface roughness.
average roughness The average distance between peaks and valleys of surface roughness.
ball bearing A rotating support containing metal balls that is placed between two parts allowing them to move easily with little friction.
bushing A removable sleeve or liner for a bearing, or a connection used to reduce the size of an opening. Bushings have both static and dynamic surfaces.
calibration The comparison of a device with unknown accuracy to a device with a known, accurate standard to eliminate any variation in the device being checked.
casting A metal part that is formed by pouring molten metal into a mold. The metal cools and solidifies into its final shape.
comparison measurement A measurement that compares the surface of a machined part with a standard surface. Inspectors often use their sense of sight and touch to perform comparison measurements.
comparison microscope An instrument used to magnify the surface of a machined part next to its standard surface. A comparison microscope is used for non-contact measurements.
cutoff The sample length on the surface of a part that a stylus-type instrument measures. The cutoff length is often specified on a part drawing.
diamond stylus A cone-shaped spherical point made of diamond that contacts and measures surface roughness. Diamond is the hardest available material and is wear resistant.
dimensional properties The characteristics of a surface that affect the way it fits. Rough surface texture can cause a part to become loose or not fit properly.
direct measurement A measurement that calculates the average roughness value by tracing the surface with a stylus-type instrument.
dynamic surface A surface that moves against other surfaces during its use.
fingernail test A type of comparison measurement where inspectors use their fingernails to scrape the surface of the machined part and a surface replica block to compare the roughness of the part.
flaw An unintentional surface irregularity that may be random or repeating, such as cracks or inclusions.
grinding wheel A grinding tool made by bonding abrasive grits together and forming them into a circular shape.
inspector A person who examines or compares a part with various tools and techniques to determine its conformance to specifications.
lay The overall direction of the pattern created by the production process.
machining The process of removing metal to form or finish a part, either with traditional methods like turning, drilling, cutting, and grinding, or with less traditional methods that use electricity or ultrasound.
measured surface The surface that represents the real surface after it has been measured. The measured surface determines how much the real surface deviates from the nominal surface.
microinches One-millionth (.000001) of the U.S. standard inch. Microinch is abbreviated μ.
nominal surface The surface that represents the desired specifications on a part drawing. The nominal surface does not have surface irregularities and is geometrically perfect.
non-contact measurement A measurement that uses optical methods to compare the surface of a machined part with a standard.
peak The point of maximum height on the surface of a part that lies above the average line. Inspectors measure the distance between peaks and valleys to determine average roughness.
physical properties The characteristics of a surface that affect the way it performs a task. Physical properties affect the way a surface bonds, coats, or resists corrosion.
precision reference specimen A small, square plate that has standard surface characteristics. Precision reference specimens are used to calibrate profilometers.
probe A device attached to a measuring instrument that uses a stylus tip to contact the surface of a part.
process stability The consistency of a process over a period of time.
profilometer A device that uses a stylus to trace the cutoff length of the part to determine average roughness.
real surface The actual part surface produced by a machining process. The real surface contains imperfections.
roughness The inherent, fine, closely-spaced irregularities created by the production process.
sealing properties The characteristics of a surface that affect the way it seals against liquids or gases. Rough surface texture can cause an improper seal.
skid A metal rest that is attached to the probe on a profilometer. The skid moves with the stylus to measure the average roughness of the surface.
skidded gage A type of profilometer that has a metal rest pad, or skid, that rests on the part. The stylus and skid move together to measure the average roughness.
skidless gage A type of profilometer that moves relative to an internal reference surface. Skidless gages measure the entire profile of the part.
static surface A surface that remains fixed in one place during its use.
stylus-type device A measuring instrument with a cone-shaped spherical tip connected to a probe. The stylus contacts the part and traces its surface irregularities.
surface The boundary that separates one object from another object, shape, or form. The surface is the exterior appearance of a part.
surface finish The smoothness of a machined surface after it has been measured. Surface finish is the complete, desired surface.
surface replica block A surface that contains a specific standard roughness pattern. Surface replica blocks are used in comparison measurements.
surface texture The combination of the imperfections on the surface of a part. Roughness, waviness, lay, and flaws make up surface texture.
tolerance An unwanted but acceptable deviation from a given dimension. Tolerances indicate the allowable difference between a physical feature and its intended design.
valley The point of maximum depth on the surface of a part that lies below the average line. Inspectors often measure the height from the valley to the peak.
variation A difference between two or more similar things.
waviness The repeating irregularities with spacing greater than roughness marks that result from machine deflections and vibration.