In manufacturing, a fixture is a specialized work-holding device designed to securely hold and position a workpiece during a production process. These tools are fundamental to mass production, where consistency and quality are paramount. By ensuring that every part is held in the exact same orientation, fixtures allow for repeatable and precise operations, whether that involves machining, welding, or assembly.
The Core Function of a Manufacturing Fixture
A fixture is like a frame that holds a canvas still for a painter, preventing shifts that would flaw the image. It provides a stable reference point for manufacturing, ensuring every component is a near-identical copy of the last. This consistency minimizes human error and can reduce the time it takes to set up a machine for a new part.
A fixture’s design directly impacts accuracy. It must hold the part without causing damage or distortion while providing clear access for the manufacturing operation. The precision of the fixture is transferred to every workpiece it holds, leading to higher quality products.
Distinguishing Fixtures from Jigs
In manufacturing, the terms “jig” and “fixture” are often used together, but they serve distinct functions. The fundamental difference lies in how each device interacts with the tool and the workpiece. A fixture’s sole responsibility is to hold, support, and locate the workpiece securely in a fixed position. It does not guide the manufacturing tool in any way.
Conversely, a jig is a device that not only holds the workpiece but also guides the cutting or processing tool to a specific location. Imagine needing to drill a hole in the exact center of a wooden block. A fixture would clamp the block firmly to the table so it cannot move. A jig would do the same, but it would also have a built-in guide, like a hardened steel bushing, that directs the drill bit to the precise spot, ensuring every hole is drilled in the same location on every block.
A fixture is a positioning device for the part, while a jig is both a positioning device for the part and a guiding device for the tool. Fixtures are often heavier and more robustly constructed than jigs because they must absorb all the forces of the manufacturing operation without the aid of a tool guide.
Common Types of Manufacturing Fixtures
Welding Fixtures
Welding fixtures are designed to hold multiple metal components in the correct alignment and position as they are being joined. These devices are built to withstand the intense heat generated during welding, which can cause metal parts to warp or distort. By securely clamping the components, these fixtures ensure the final welded assembly meets precise dimensional specifications.
Machining Fixtures
Machining fixtures are used to secure a part on a machine tool for operations like milling, drilling, turning, or grinding. For example, a milling fixture is mounted to the table of a milling machine and holds the workpiece in the correct orientation relative to the cutting tool. These fixtures must be extremely rigid to resist the cutting forces and vibrations produced during machining, ensuring accuracy and a good surface finish on the part.
Assembly Fixtures
Assembly fixtures are used to hold multiple components in their correct relative positions while they are being joined together. This could involve processes like bolting, riveting, or bonding. In industries like aerospace and automotive manufacturing, assembly fixtures are used to construct complex products, from engine components to entire vehicle bodies, ensuring that all parts fit together as intended.
Inspection Fixtures
Inspection fixtures are used in quality control to hold a finished part securely for measurement and verification. These fixtures ensure that each part is checked in the same orientation, which is necessary for accurate and repeatable measurements. They are often used in conjunction with a Coordinate Measuring Machine (CMM), which uses a probe to check the dimensional accuracy of the part against its design specifications.
Key Components and Materials
A typical fixture consists of several components working together. The foundation is the base plate, a sturdy plate or frame-like structure that provides stability and a mounting surface for the other elements. Locators, such as pins, buttons, or surfaces, are used to position the workpiece accurately within the fixture. Finally, clamps are used to apply force and hold the workpiece securely against the locators, preventing any movement during the operation.
The materials used to construct fixtures are chosen based on factors like the manufacturing process, the material of the workpiece, and the required durability. Hardened steel is a common choice for its strength and resistance to wear, making it suitable for high-volume production. Cast iron is valued for its stability and ability to dampen vibrations, while aluminum is often used for lighter-weight fixtures where portability is a consideration. In some cases, plastics or composites are used for applications where the workpiece has a delicate surface that could be damaged by a metal fixture.