The terms jig and fixture have essentially the same meaning. Jigs, or fixtures, are designed to hold pieces of an assembly in correct relationship during welding, and to expedite removal of the completed parts after welding. Sometimes simple jigs, toggle clamps, C-clamps or wedges are sufficient to hold the alignment. In a manufacturing setting, elaborate jigs designed to hold large sheet or plate metal might be required.
In industrial production of welded parts, close dimensional control and correct alignment are critical when planning for high rates of production. Specifications must be precisely followed to produce parts that are interchangeable and readily assembled.
In repair work it is particularly necessary to hold the parts in alignment to bring the broken item to its original shape, especially if it is part of an assembly.
Jig design requires mechanical ingenuity and a knowledge of the laws of expansion and contraction of metal. When steel is heated to a welding temperature, it has very little strength and ductility. For this reason a crack or tear is very easily started by any stress due to warping contraction. In using a jig, the several parts required for a welded assembly are cut to length and fitted so that there is only a small clearance
between the abutting members. This clearance should be as uniform as possible, for example, when a truss member is welded into the sidewall of a tube, the end of the member should be milled to fit the contour of the tube.
For arc or oxyfuel gas welding of thin sheet metal ranging from, for example, 10 gauge (3.6 mm [0.141 in.]) and thinner sections, the need to use a jig is more critical than when welding the heavier plate metals. As a general rule, the thinner the sheet metal, the greater the need for a jig. Greater changes occur in the edge contours of thin sheets matched up for butt welding than in heavier sheet or plate metal. The jig must provide a means to control warping and edge movements by absorbing heat or forcibly restraining
the parts to some degree.
Some welding jigs are designed to hold the parts in a level position convenient for welding, with capability of rotation in a horizontal or vertical plane.
Tack-welding jigs are used in laying transmission pipelines to assure concentricity of the adjoining pipe ends and good alignment. They are essentially welding jigs, which are removed immediately after the tack-welds have been made. The fact that the pipe
ends no longer require the support of the jig during the welding operation means that the function of the jig has been transferred to the tack-welds.
Jig Design
Simplicity should be the first consideration in the design and construction of the shop-made fixture. Sometimes the design can accommodate set-up and welding in the same fixture.
Convenience in reaching the welded surface and visibility are two important factors. For arc welding, the design usually includes copper backing bars with machined grooves to permit complete penetration of the weld metal. The grooves should be extremely shallow (0. 4to 0.8 mm [0.015 to 0.030 in.]), and comparatively narrow (4.5 to 6.4 mm [0.18 to 0.25 in.]), and should not be square cornered.
Allowances for heat control must be made to prevent misalignment, buckling or overlapping of the parts. The jig should be constructed so that it carries heat away from the weld. Clamping pressures will largely depend on the type of structure being welded.
See also FIXTURE and POSITIONER.