Surface Texture and Inspection 201

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

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. Average roughness is abbreviated as Ra.

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.

casting

A manufacturing process that involves pouring a heated liquid material into a hollow mold until the material cools 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.

fingernail test

A type of comparison measurement 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.

flaw

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.

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 wheel

A grinding tool made by bonding abrasive grits together and forming them into a circular shape. A grinding wheel rotates and shears away microscopic chips of material and can produce very fine but still imperfect surface finishes.

lay

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

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.

microinches

One-millionth (.000001) of the U.S. standard inch. Surface roughness is typically measured in microinches. Microinches are expressed as the greek symbol μ.

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.

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.

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.

real surface

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

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.

static surface

A surface that remains fixed in one place during its use. It does not contact other surfaces 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. 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 face of a part.

surface finish

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

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 imperfection on the surface of a part. Roughness, waviness, lay, and flaws on the surface of a part make up its surface texture.

surfometer

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

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.

waviness

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