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Calipers, either spring or firm-joint types, are convenient tools for measuring a number of jobs which are not required to be extremely accurate. Firm-joint calipers have two legs fastened together with a rivet or bolt of special design. To give a smooth joint, thin fiber-washers are interposed between the legs, and when purchasing calipers this point should be looked for. The legs of inside calipers are curved outward at the extremities to facilitate measuring small holes, while outside caliper legs have a large curve inward to increase their capacity for large work. Firm-joint calipers are adjusted approximately by the hands and then set to the feel of the work by tapping them on a metal surface. It is a common practice when opening calipers by this means to tap the top of the joint. It is better, if possible, to tap the inside of the legs, as repeated blows (though light) tend to burr the edges of the joint. Care should be taken when using calipers to hold them square across the job, or an incorrect reading will be obtained. The interference between the work and the instrument should be very slight, as distortion of the legs occurs if force is used. The application of calipers for good results calls for a certain amount of practice. Spring calipers are provided with an adjusting screw which moves the legs against the tension of the spring. The chief advantage of this over the firm-joint type is ease of adjustment. Quick nuts are fitted to many spring dividers, and they consist of a nut which is split completely in two and held by a conical sleeve under the pressure of the spring. If this pressure is removed by holding the legs together with the fingers, the nut is released from the screw and can be moved into any position along its length without being rotated. This is particularly advantageous in inspection work, where many varying diameters have to be checked. Either inside or outside calipers can be set to a ruler or to a standard part, the latter being the most accurate. If the standard has a very smooth surface and the work surface is less smooth, more resistance will be offered by the work to the movement of the calipers, and allowance has to be made for this. Mistakes can easily arise by accidentally altering the setting of these tools when laying them down after checking the job, and it is a good plan, particularly with spring dividers, to check the setting each time before checking the work. The use of calipers is not confined to cylindrical work. They can often be employed to advantage in checking rectangular holes, parallel faces and so on. Convenient sizes for the tool kit are 3 inch and 6 inch.
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