Jigs, Fixtures and Checking Gauges

A fixture decides where a part is, and a checking gauge decides whether it is right. Both are made to the part drawing. Getting the locating right is what makes welding, machining and assembly repeatable; getting the checking right is what stops a bad part from reaching the next process.

Locating Comes First

A fixture that clamps in the wrong place will distort the part, and no amount of clamping force fixes a bad locating scheme. The usual approach is to locate on the datum surfaces named on the drawing, restrain the six degrees of freedom with the minimum number of contacts, and clamp where the part is stiff rather than where it is convenient.

Welding Fixtures and Assembly Jigs

Welding fixtures add two problems to plain locating: heat, which moves the part as it is welded, and access, because the torch has to reach the joint. Copper or heat-resistant pads at the contact points, and clamps placed away from the weld line, are the usual answers. Assembly jigs are simpler - the job is to hold the part square and leave the operator's hands free.

Checking Fixtures and Gauges

A checking fixture is built to answer one question fast: does this part pass. Pin gauges for hole position, profile blocks for edges, and dial indicators for critical faces let an operator check a part in seconds instead of sending it to a CMM. Where a CMM measurement is required, the fixture is designed so the same part can be measured on both, and the two results can be compared.

Send the part drawing and 3-5 kg of sample parts; a fixture concept and quotation follow within five working days. See also jigs, fixtures and checking gauges.