abrasive

A material or tool consisting of hard particles used to wear down, rub away, or machine material. Abrasive materials are often used in finishing operations.

accuracy

The difference between a measurement reading and the true value of that measurement. The less error present in the measurement, the more accurate the results.

allowance

The difference between the smallest permissible hole and the largest permissible shaft. An allowance creates a certain kind of fit between mating parts.

bilateral tolerances

A tolerance method using an equal plus and minus deviation from the specified dimension. Bilateral tolerances are preferred if deviation in either direction risks exceeding the absolute limits and can be equal or unequal.

blueprint

A document that contains the instructions necessary to manufacture and/or assemble a part. The designer includes the degree of tolerance necessary for the part to function as intended in the blueprint.

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.

clearance

A fit that allows intentional space to exist between a hole and the shaft inserted into it. For a clearance fit, the shaft diameter is designed to be slightly smaller than the hole.

dimension

The desired measurement of a feature on a part. A dimension is listed as a number given in the appropriate units.

dimensioning

The process of determining the desired measurement of a feature on a part. The ideal dimensions for a part can never be met, requiring the addition of tolerances.

drilling

The use of a drill to cut a round hole into a workpiece. The drill may rotate to cut a stationary workpiece, or a stationary drill may be fed into a rotating workpiece.

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.

interference

A tight fit designed where the smallest permissible shaft is larger than the largest permissible hole. Force is required to assemble parts with an interference fit.

lapping

An abrasive process that removes the last bit of unwanted material. A lapping process sometimes uses an abrasive paste that is rubbed across the part to gradually smooth it.

lay

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

limit dimensions

A tolerancing method using an absolute maximum and minimum allowable dimension. Limit dimensioning specifies a range of acceptable measurements instead of a target dimension.

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.

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.

precision

The degree to which an instrument will repeat the same measurement over a period of time. Precision is also called repeatability, as it will show the same results under unchanged conditions.

roughness

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

surface tolerance

The allowable deviation in surface texture. Surface tolerances should only be used for surfaces that affect the use of a part.

tolerance

The total amount a specific dimension is permitted to vary. The object will still meet specifications if it remains within its tolerance.

tolerance limits

Parts may have different tolerances for different features. A cast part can be milled to produce a flatter surface within a hair’s thickness of 0.001 in. (0.03 mm). Likewise, a machined surface can be ground afterward to meet tighter tolerance restrictions within 0.0003 in. (0.008 mm).

transition

A fit toleranced for an allowance that permits both a clearance and an interference fit. For a transition fit, a clearance fit occurs with the smallest shaft diameter and the largest hole diameter, but an inference fit occurs with the largest shaft diameter and the smallest hole diameter.

turning

A machining operation used to make cylindrical parts. During turning, a single-point cutting tool passes along the outer surface of a rotating cylindrical workpiece and gradually removes a layer of material.

unilateral tolerances

A tolerancing method using a deviation in only one direction, either plus or minus, from the specified dimension. Unilateral tolerances are used if variation in only one direction risks exceeding the absolute limits.

waviness

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