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Glossary
Common terms used in jig and fixture design, grouped by locating, clamping, fixtures & jigs, GD&T and power clamping. Terms are shown in the selected language.
Locating & Datums
| Term | Definition |
|---|---|
| Datum | A reference point, line or plane from which dimensions and locations are measured. |
| Six-point principle | A body is fully constrained by six contact points: three on the primary plane, two on the secondary, one on the tertiary. |
| 3-2-1 locating | Locating scheme using three points on the primary datum, two on the secondary and one on the tertiary. |
| Locator | A fixed element that positions the workpiece against the fixture body. |
| V-block | A locating element with a V-shaped notch that centers cylindrical workpieces. |
| Diamond pin | A relieved locator pin that locates in one direction only, avoiding over-constraint on a second hole. |
| Rest pad | An adjustable or fixed element that supports the workpiece at the primary datum plane. |
| Degree of freedom | One of six independent motions (three translations, three rotations) that a free body has. |
Clamping
| Term | Definition |
|---|---|
| Clamp force | The force a clamp applies to hold the workpiece against the locators. |
| Strap clamp | A clamp using a lever strap over the work, tightened by a screw or stud. |
| Screw clamp | A clamp that applies force through a screw thread directly or via a pad. |
| Cam clamp | A clamp using an eccentric or cam profile for quick action. |
| Toggle clamp | A quick-action clamp using an over-center linkage that locks in the closed position. |
| Friction coefficient | Ratio of friction force to normal force between the clamp pad and the work surface. |
| Self-locking | Screw, cam or wedge geometry that stays locked under load without external holding force. |
| Quick-release | Design that allows rapid loading and unloading of the workpiece. |
Fixtures & Jigs
| Term | Definition |
|---|---|
| Jig | A workholding device that also guides the tool, for example through a drill bushing. |
| Fixture | A workholding device that locates and clamps the work but does not guide the tool. |
| Drill jig | A jig with bushings that guide drills to precise hole positions. |
| Milling fixture | A fixture for milling that must resist large cutting forces. |
| Turning fixture | A lathe chuck or faceplate fixture balanced for rotation. |
| Boring fixture | A fixture that holds the work while a boring bar machines large, accurate holes. |
| Modular fixture | A fixture assembled from a standard set of modular elements such as base, blocks, locators and clamps. |
| Checking fixture | A gauge fixture used to verify part geometry quickly on the shop floor. |
GD&T & Tolerances
| Term | Definition |
|---|---|
| Position tolerance | The zone within which the center, axis or surface of a feature must lie. |
| Flatness | A form tolerance controlling deviation from a perfect plane. |
| Perpendicularity | An orientation tolerance controlling how close a feature is to 90° to a datum. |
| Runout | A composite tolerance controlling circular and total deviation of a rotating surface. |
| Maximum material condition (MMC) | The condition of maximum material (smallest hole / largest shaft), which can allow a bonus tolerance. |
| Datum reference frame | A coordinate system established by three mutually perpendicular datum planes. |
| IT grade | ISO tolerance grade (IT01 to IT18) defining the magnitude of the tolerance zone. |
| Fit | The relationship between hole and shaft sizes: clearance, transition or interference. |
Power Clamping & Inspection
| Term | Definition |
|---|---|
| Actuator | A cylinder or device that converts fluid power into clamping force. |
| Hydraulic cylinder | A device with piston, rod and barrel producing linear force from fluid pressure. |
| Accumulator | A hydraulic component that stores pressure energy to keep clamps engaged during power loss. |
| Check valve | A valve that allows flow in one direction only, holding clamp pressure. |
| Pressure | Force per unit area; typical hydraulic clamping uses 5–7 MPa. |
| GO/NO-GO gage | A gage whose GO and NO-GO members accept or reject parts by size. |
| Plug gage | A GO/NO-GO gage used to check holes. |
| Ring gage | A GO/NO-GO gage used to check external diameters. |