Basics of Tolerance 121

abrasive finishing

The use of an abrasive to remove small amounts of material from a part to create a smooth, polished surface. Abrasive finishing is commonly used to improve the surface finish of machined and additively manufactured parts.

accuracy

The difference between a part's actual measurements and the part's nominal dimensions. High accuracy means there is a smaller difference between the actual and nominal part dimensions.

allowance

The difference between the smallest acceptable hole and the largest acceptable shaft. The amount of allowance determines the type of fit that exists between mating parts.

amplitude

The maximum height or maximum depth of a wave. Amplitude is used to indicate the peaks and valleys in roughness patterns on a part surface.

ball bearings

A type of anti-friction bearing containing metal balls that is placed between two parts allowing them to move easily with little friction.

bar stock

Raw material in the form of long bars or cylinders. Bar stock is commonly used in metal manufacturing processes.

bilateral tolerance

A type of tolerance that indicates acceptable deviation in two directions from a nominal dimension, both higher and lower. Bilateral tolerances are preferred if deviation in either direction risks affecting part functionality.

casting

A manufacturing process that involves pouring a heated liquid material into a hollow mold and allowing the material to cool into a solidified shape. Casting processes alone are unsuitable for products that require tighter tolerances.

clearance fit

A fit that allows space to exist between mating parts. With a clearance fit, the largest acceptable shaft is slightly smaller than the smallest acceptable hole.

coordinate tolerances

Design specifications that describe the design of a part by comparing its features to distances along three linear axes. Coordinate tolerances may include plus symbols, minus symbols, or both to indicate a tolerance zone based on the nominal dimension.

coupling

A component that connects two shafts in a mechanical, hydraulic, or pneumatic system. Couplings require precision machining in order to fit and function properly.

cross-hatched

Marked by very fine intersecting lines. A cross-hatched surface finish helps to lubricate a part more evenly.

cylinder

A linear actuator that houses a piston attached to a movable piston rod. Cylinders exert force in a linear direction.

datums

A point of reference for machine tools, programs, and fixtures from which measurements are taken. A datum can be a hole, line, or any three-dimensional shape.

dial calipers

A type of caliper that uses main scale markings on the beam and markings on a round dial to indicate measurements. Dial calipers are easier to read than basic slide calipers.

diameter

The distance from one edge of a circle to the opposite edge that passes through the center. Round or cylindrical features require diameter measurements.

disc brake

A friction brake that stops motion when a stationary element is pressed against a rotating disk. Disc brakes are commonly used in automobiles and in some industrial machinery, especially in older machines.

drilling

A machining operation that uses a multi-point cutting tool to create round holes in a workpiece. Drilling may use a rotating tool to cut a stationary workpiece or a stationary tool to cut a rotating workpiece.

drive train

A group of mechanical components used to transmit power from a motor to an output device. Drive trains connect the shaft of the motor to the shaft of the driven unit.

equal bilateral tolerance

A bilateral tolerance that specifies positive and negative tolerances that have the same value. Equal bilateral tolerances are used when a part feature must be equally accurate in both directions to ensure proper functionality.

feature control frame

FCF. A series of compartments containing symbols and values that describe the geometric tolerance of a feature. Feature control frame compartments are standardized to communicate information consistently.

flaw

An unintentional irregularity on a part. Flaws are random surface defects such as dents, gouges, and scratches.

form tolerance

A geometric tolerance that limits the amount of error in the shape of a feature. The form tolerances include straightness, flatness, circularity, and cylindricity.

gear train

A system of gears used to transmit rotary motion from one part of a mechanical system to another. Gear trains can alter the speed, torque, and direction of energy in a mechanical system.

gears

A round or cylindrical mechanical component with teeth that is used to transmit power. Gears are designed to mesh with each other to alter the speed, torque, or direction of mechanical energy.

geometric dimensioning and tolerancing

GD&T. An international standard for communicating instructions about the design and manufacturing of parts. Geometric dimensioning and tolerancing uses universal symbols and emphasizes the function of the part.

grinding

A process that uses an abrasive tool or wheel to remove material from the surface of a workpiece and change its shape. Grinding produces a finish that is smoother than both sawing and milling.

honing

A precision abrasion process in which a relatively small amount of material is removed from a surface by means of an abrasive. Honing is used to obtain a desired finish or produce extremely accurate features.

interference fit

A fit between slightly overlapping mating parts that requires force to assemble. With an interference fit, the smallest acceptable shaft is slightly larger than the largest acceptable hole.

key slots

A groove running parallel to the axis of a shaft that is designed to accept a key. Key slots often require a transition fit for optimal functionality.

keyways

A groove inside the hole of a gear, wheel, or similar shaft component that is designed to accept a key. Keyways often require a transition fit for optimal functionality.

lay

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

limit dimensions

A type of tolerance that indicates the absolute maximum and minimum allowable dimensions. Limit dimensions specify the range of acceptable measurements without referencing a nominal dimension.

location tolerance

A geometric tolerance that limits the location or placement of features relative to a datum or other features. Location tolerances include position, concentricity, and symmetry.

lubricant

A substance used to prevent friction between two surfaces in relative motion. Lubricants such as oil and grease reduce resistance, wear, and heat.

machining

The process of removing metal to form or finish a part. Machining operations includes turning, drilling, and milling.

machining allowance

A value representing the amount of material expected to be removed from the surface of a part. Machining allowance typically indicates the size of an unfinished part prior to the final finishing operations.

material condition modifiers

Defines the tolerance of a feature in relation to its acceptable size limits. There are three material condition modifiers used in GD&T: maximum material condition, least material condition, and regardless of feature size.

milling

A machining operation that uses 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 dimensions

The ideal measurement of a part or part feature that is specified in the part print. Nominal dimensions are geometrically perfect and are used to establish tolerance values.

nominal surface

The ideal surface texture specified in a part print. The nominal surface does not include irregularities and is geometrically perfect.

orientation tolerance

A geometric tolerance that limits the direction, or orientation, of a feature in relation to other features. Orientation tolerances include angularity, perpendicularity, and parallelism.

piston

A cylindrical component inside a hollow cylinder that is moved by hydraulic or pneumatic pressure. Pistons transmit or receive motion when pressure acts on the connected rod.

plane surface

A flat surface that extends infinitely in any direction in three dimensions. A plane surface is perfectly flat.

precision

The degree to which parts with the same dimensions can be produced repeatedly under unchanged conditions. Precision requires high quality production equipment that produces repeatable results.

press fit

A fit between slightly overlapping mating parts that requires force to assemble. With a press fit, the smallest acceptable shaft is slightly larger than the largest acceptable hole.

print

A document containing all the instructions necessary to manufacture a part. A print includes a part drawing, dimensions, and notes.

radial

Extending outward in all directions from a central point. Radial lay patterns are created with machining tools and are often visible at the ends of cylindrical shafts.

rotor

A rotating part of an electrical or mechanical device. The rotor on a disk brake stops when a stationary friction plate is forced against the rotor's surface.

roughness

A pattern of fine, closely spaced irregularities on a part surface. Roughness generally occurs as the result of a machining process.

specifications

A description of the essential physical and technical properties of a finished part. Specifications outline important information including finished part dimensions and acceptable tolerance ranges.

steel rule

A simple measuring instrument consisting of a long, thin metal strip with a marked scale of unit divisions. Steel rules are also called machinist's rules.

surface finish

The degree of roughness and variation on the surface of a manufactured part. Surface finish can affect the functionality of a part.

surface texture

The combination of the imperfections on the surface of a part. Roughness, waviness, lay, and flaws make up surface texture.

surface tolerance

The amount of allowable deviation between a part's ideal, nominal surface and the surface texture on the actual part. Surface tolerances should only be used for surfaces that affect a part's functionality.

title block

A portion of a print that contains basic identifying information. Title blocks include information such as the company name, part name, part number, and engineer.

tolerance limits

The highest potential dimensional accuracy a manufacturing process can achieve. Tolerance limits vary based on the type and quality of the machine tool.

tolerances

An unwanted but acceptable deviation from a specified dimension. Tolerances indicate the maximum allowable difference between a physical feature and its intended design.

transition fit

A fit that allows both a clearance fit and an interference fit to exist between mating parts. With a transition fit, a clearance fit occurs between the smallest acceptable shaft and the largest acceptable hole, and an interference fit occurs with the largest acceptable shaft and the smallest acceptable hole.

turning

A machining operation that uses a single-point cutting tool to remove material from the outer surface of a rotating cylindrical workpiece. Turning is performed on a lathe.

unequal bilateral tolerance

A bilateral tolerance that specifies positive and negative tolerances that have different values. Unequal bilateral tolerances are used when a dimension must be more accurate in one direction than the other.

unilateral tolerance

A type of tolerance that indicates acceptable deviation in only one direction from a nominal dimensions, either higher or lower. Unilateral tolerances are used if variation risks affecting part functionality in one but not the other.

variance

The degree of deviation between one measurement result and the next when the same object is measured. High variance between consecutive inspection measurements indicates low measurement precision.

waviness

A pattern of repeating, widely spaced irregularities on a part surface. Waviness results from machine deflections and vibration.