Surface Texture and Inspection 201

3D laser scanning microscopes

A measurement instrument that uses a laser to measure a surface in three dimensions. 3D laser scanning microscopes can measure a variety of surface characteristics.

abrasive grains

A small hard particle or crystal of material used to machine, grind, or finish a workpiece. Abrasive grains are capable of producing a very smooth surface finish but still leave marks on the surface of a part.

average roughness

Ra. The average distance between the peaks and valleys that characterize a particular surface. Average roughness describes the quality of a surface but does not detect waviness or flaws.

bushing

A hardened steel tube, either fixed or removable, that is used to constrain, guide, or reduce friction. During use, the inside of a bushing is a dynamic surface, while the outside is static.

calibrated

To compare and adjust a device with unknown accuracy to a device with a known, accurate standard. Inspectors calibrate devices to eliminate any variation between measurements .

casting

A manufacturing process that involves pouring a heated liquid material into a hollow mold and allowing it to cool into a solidified shape. Casting creates a part surface with no clear lay.

comparison measurements

A type of measurement method that involves comparing an unknown measurement with a known measurement. In surface inspection, the surface of a machined part is compared to a standard surface.

cutoff

The sample length on the surface of a part that a stylus-type instrument measures. Cutoff length is often marked on a part drawing.

direct measurements

A type of measurement method that allows an inspector or operator to use a hand-held instrument to directly measure a part feature. For surface inspection, direct measurement calculates the average roughness value by tracing the surface with a stylus-type instrument.

dynamic surface

A surface that moves or makes contact with other surfaces during use. For dynamic surfaces, surface texture may affect how the surface rolls or slides against another surface.

electromagnetic

Relating to the interference between magnetic fields. Electromagnetic measurements can measure material quality and detect surface defects.

fingernail test

A type of comparison test during which inspectors use a fingernail to scrape the surface of the machined part. The inspectors then run that same fingernail along a surface replica block to compare its surface roughness to the roughness of the part.

flaws

An unintentional irregularity that may be random or repeating. In surface inspection, flaws are random surface defects that are generally not included in the measurement of the surface.

graphic output

A visual display of data or measurement results. Graphic outputs from measurement devices can be read by inspectors.

grinding

The use of an abrasive tool or wheel to wear away at the surface of a workpiece and change its shape. Grinding produces a finish that is smoother than both sawing and milling.

grinding wheels

o A grinding tool made by bonding abrasive grits together and forming them into a circular shape. Grinding wheels rotate and shear away microscopic chips of material and can produce very fine, but still imperfect, surface finishes.

grit size

The size of the abrasive particles in a grinding wheel or other abrasive tool. Grit size is a factor that determines surface finish.

infrared

A wavelength of light that is beyond, or longer than, red on the visible light spectrum. Infrared light can be used to capture a visual representation of an object's surface geometry.

lay

The overall direction of the pattern created by the production process. Lay cannot be measured because it indicates only a direction.

lay symbols

A representation that indicates the direction of the pattern on a surface. Lay symbols are a standardized way for designers, engineers, and manufacturers to communicate the desired surface texture of a part.

machining

The process of removing metal to form or finish a part. Machining can occur using traditional methods, like turning, drilling, milling, and grinding, or with less traditional methods that use electricity, heat, or chemical reaction.

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.

metrology

The science of measurement. Metrology strives for accuracy, precision, and repeatability.

microinches

µin. One-millionth (.000001) of the U.S. standard inch. Surface roughness is typically measured in microinches and expressed as the symbol µin.

micrometers

µm. A unit of measurement that is equal to one-millionth (0.000001) of the metric standard meter. Micrometers, also called microns, are typically used to measure average roughness in the metric system.

microns

µm. A unit of measurement that is equal to one-millionth (0.000001) of the metric standard meter. Microns, also called micrometers, are typically used to measure average roughness in the metric system.

milling

The use of a rotating multi-point cutting tool to machine flat surfaces, slots, or internal recesses into a workpiece. Milling produces a finish that is smoother than sawing but rougher than grinding.

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.

noncontact measurements

A measurement method involving inspecting a part without actually making physical contact with it. Noncontact instruments often measure the surface of a part optically.

optical profilometers

A noncontact instrument that measures the 3D profile of a surface by projecting light onto it and measuring the interference. Optical profilometers can measure surface roughness, warpage, and other surface textures.

peaks

The point of maximum height. On the surface of a part, peaks lie above the average line, and the distance between peaks and valleys determines average roughness.

polishing

An abrasive finishing process. Polishing results in a part with a smooth, refined, and high-luster surface finish.

precision reference specimen

A small square plate that has standard surface characteristics. Precision reference specimens are used to calibrate stylus-type instruments used to inspect surfaces.

probe

A device that gathers measurement data from the workpiece. On a stylus-type instrument, the probe uses a stylus tip to contact the surface of a part.

process stability

The consistency of a process over a period of time. Average roughness effectively monitors how consistently a process produces surface roughness.

profilometer

A stylus-type device that measures surface roughness. It amplifies its signal to compensate for waviness and indicate only roughness.

Ra

Average roughness. The average distance between the peaks and valleys that characterize a particular surface. Ra describes the quality of a surface but does not detect waviness or flaws.

real surface

The actual part surface produced by a machining process. The real surface contains imperfections.

reverse engineering

A process that captures geometric data of an existing object to convert it to a three-dimensional (3D) computer-aided design (CAD) model. Reverse engineering can be used to create a reproduction based on a part's composition.

rework

The process of running a part through an additional and unplanned manufacturing step. Rework is necessary when a part is not produced within the correct tolerance, has poor surface finish, or has other defects.

roughness

The inherent, fine, closely-spaced irregularities remaining on a part surface after manufacturing. Roughness is created by the production process.

sawing

A basic metal cutting process that uses a blade with a series of teeth on its edge to cut a narrow opening in a workpiece. Sawing produces a rough surface finish.

scrap

A nonconforming product that is not used for its originally intended use. Scrapped products are recycled or destroyed.

signal

A current that carries data from one device to another. Signals are sent to stylus-type devices to record surface roughness.

standard

An established policy regarding specific product requirements or a particular practice or method. Standards cover a range of topics, from the required properties of a material to the documentation necessary across the supply chain.

static surface

A surface that remains fixed in one place during its use. A static surface does not contact other surfaces that are in motion.

stylus

The precision tip that records measurements. On a stylus-type instrument, the stylus is usually made of diamond and traces surface irregularities to measure surface roughness.

stylus-type device

A measuring instrument with a cone-shaped spherical top connected to a probe. On a stylus-type device, the stylus contacts the part and traces its surface irregularities.

superfinishing

A finishing process that uses fine abrasive stones to improve surface finish on the exterior of parts of various shapes. Superfinishing, also known as short-stroke honing or microfinishing, is used to create surface finishes with a less roughness than grinding.

surface

The boundary that separates one object from another object, shape, or form. The surface is the exterior face of a part.

surface finish

The degree of roughness and variation on the surface of a part after it has been manufactured. Surface finish cannot be perfectly smooth due to irregularities created when machining a part.

surface replica blocks

A surface that contains a specific standard roughness pattern. Surface replica blocks are used in comparison measurements.

surface roughness comparators

A tool providing a reference to compare and assess the surface finish of a part against design requirements. Surface roughness comparators can be used to inspect surface visually or by touch.

surface texture

The combination of the imperfections on the surface of a part. Surface texture is determined by the roughness, waviness, lay, and flaws on the surface of a part.

surfometer

A stylus-type device that measures surface roughness. Portable surfometer models can be carried in a pocket on the production floor.

three-dimensional

3D. Having or appearing to have length, breadth, and depth. Three-dimensional models show an object's shape better than two-dimensional (2D) drawings.

tolerances

An acceptable deviation from a given dimension or geometry. Tolerances indicate the allowable difference between a physical feature and its standard design.

valleys

The point of maximum depth. On the surface of a part, valleys lie below the average line, and the distance between valleys and peaks determines average roughness.

variation

Any change or difference from the standard. Variation in the surface of a part is what creates surface texture.

vibration

A rapid, continued, repeated motion in a machine or other structure. Vibration is caused by small shifts in the workpiece that can negatively affect tolerance and surface finish.

warping

To physically twist or deform. Warping is often a result of internal stress within a material.

waviness

The repeating widely-spaced irregularities of surface texture. Waviness is the result of machine deflections and vibration.

witness marks

A pattern created by machining on the surface of a part. Witness marks are sometimes used to indicate the location and direction for part assembly or how a part should move once assembled.

workpiece

A material that is being machined or undergoing another type of processing. Workpieces can be shaped by processes such as cutting, welding, and grinding.