| 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. |