A device designed to hold and maintain parts in proper relation to each other.
The terms fixture and jig have essentially the same meaning. The function of ;I fixture is to facilitate assembly of parts and to hold a workpiece assembly in proper alignment during handling and welding. If the assembly is only tack-welded together it is called a fitting jig. The use of a fixture promotes good fitting tolerances in the final product. Most fixtures fall into three major categories:
(1) assembly or fitting fixtures,
(2) precision fitting and welding fixtures designed to produce accurate fabrications, and
(3) robotic welding fixtures.
The design and manufacture of the fixture should reflect the number of parts to be produced. Small quantities may be produced on temporary jigs assembled specifically for the product. For large quantities, the fixture could be an integral part of the whole production system. In this case, the fixture may include automatic clamping on a positioner, and the fixture may accommodate welding by an industrial robot. This type of fixture is expensive to build, but if many parts are produced on it, the fixture is cost effective.
Design
There are few standard commercial fixtures, but many light and heavy duty clamping devices are available which can be incorporated into dedicated fixtures for large production runs, OK into adjustable fixtures that can be modified for several short run products.
For the most part, fixtures are designed and built by plant operations personnel to facilitate the production of one or more assemblies. A production fixture should be designed to accommodate the following:
(1) Weld joints must be accessible in the fixture.
(2)The fixture must be more rigid than the assembly.
(3)Holddowns, clamps, and threads of bolts and nuts should be protected from weld spatter.
(4)The fixture should allow assembly of the work with a minimum of temporary welds that are visible on completion.
(5) The workpiece must be easy to remove from the fixture after welding is complete.
The designer plans how many welds are to be made while the work is in the jig. For example, the second side of a complete penetration weld may be deposited after the weldment is removed from the fixture. Sufficient welds should be completed in the jig to restrain the assembly from distortion during the completion of welding outside the jig. Since most weldments are fabricated as sub-assemblies, the tolerances are critical.
However, often the intermediate dimensions are less important than the end and edge dimensions that control the fit in the final assembly.
Assembly Fixtures
Some assemblies may require stiffening fixtures to maintain shape, and some type of clamping or fixturing may be required to hold the joint alignment for welding and to prevent warpage and buckling from the heat of welding.
For assemblies that are inherently rigid, tack welding alone may suffice. Heavy section thicknesses in themselves offer considerable restraint against buckling and warpage. In intermediate cases, a combination of tack welding, fixturing, and weld sequencing may be required. For joints of low restraint in light gauge materials, clamping is needed. Clamping bars maintain alignment and remove heat to reduce or prevent warpage. Tack welds are usually necessary.
Fixtures also include the jigs and tooling used to facilitate the welding operation. Weld seam trackers and travel carriages are used to guide machine or automatic welding heads.
Precision Jigs
Jigs must be designed to produce a fabrication to close tolerances. A detailed, step-by-step assembly and welding procedure are calculated and built into the design.
Robotic Fixtures
Fixtures for robotic welding have several specific requirements. They must allow access for the robot. Most robots are not equipped with sensing systems; those having sensing systems are primitive in comparison to the vision of a human being. Therefore, the fixture should have low-profile clamps located away from the joint. The fixture should contain at least two reference points that are in a fixed relationship to the weld seams of the workpiece. The robot is then programmed to locate the reference points on the fixture. The reference points establish a coordinate system for the robot to find its way along the joint on the work- piece. The fixture should be easy to use, so that the workpieces can be loaded and removed rapidly, Fixtures are often used in conjunction with turning rolls, rotating turntables and manipulators. See POSITIONER.
Brazing Fixtures
When fixtures are needed to maintain alignment or dimensions, the mass of a fixture should be minimized. It should have pinpoint or knife-edge contact with the parts, away from the joint area. Sharp contacts minimize heat loss through conduction to the fixture. The fixture material must have adequate strength at brazing temperature to support the brazement. It must not readily alloy at elevated temperatures with the work at the points of contact. In torch brazing, extra clearance will be needed to access the joint with the torch flame as well as the brazing filler metal. In induction brazing, fixtures are generally made of ceramic materials to avoid putting extraneous metal in the field of the induction coil. Ceramic fixtures may be designed to serve as a heat shield or a heat absorber.
Reference: American Welding Society. Welding Handbook, Vol. 1, 8th Edition. Miami, Florida, 1987; American Welding Society: Welding Handbook, Vol. 2,8th Edition. Miami, Florida: American Welding Society, 199 1.