What You Must Know About Calipers

USING CALIPERS
The biggest problem with calipers is the level of operator skill required to make accurate, repeatable measurements. Since the caliper is a contact instrument, the jaws must be correctly aligned on the part, and sufficient torque must be applied to the anvils to make good positive contact between the part and the instrument.

The only torque calibration in the human hand is the operator’s ‘feel.’ What feels like solid contact to one operator may not feel correct to another, so the readings may be different.

Water resistant digital calipers such as this are designed for use in extreme production environments in accordance with the IP67 standard. Built for durability, some digital calipers have specially resistant materials in the casing and keyboard, and a fluid protection system to safeguard sensitive electronic components.

Subjective or psychological factors also influence caliper measurements. Tell an inspector that the best machinist in the plant made a part – or that the boss made it – and those measurements may tend to be better (or worse!) than the part deserves.

In addition, studies show that if an operator knows what size a selection of parts ought to be before he measures them, readings will tend to be closer to that ideal than if the target dimension is unknown.

APPLICATIONS
The popularity of calipers is based both on their versatility and ease of use. Figures 1a and 1b illustrate basic OD and ID measurements. Figures 1c and 1d illustrate more complex depth and step measurements.

Other common caliper measurements include measuring the center distance between two holes, comparing a shaft diameter to a hole ID, and gaging remaining thickness when drilling a hole in a workpiece.

There are also any number of different anvils and contacts available which extend caliper use to many special measurement applications.

Ball contacts are used to measure wall thickness of tubes and other cylindrical components. Calipers are available with one or two ball/radiused contacts, and in some cases the ball contacts can be supplied as attachments for use with standard flat tipped anvils.

The attachments may be quickly and easily applied, but when using this type of attachment, the ball diameters must be taken into account by subtracting them from the caliper reading.

Similarly, other specialized anvil attachments and accessories are available to measure threads, bores and serrations, recesses, recessed IDs and ODs, grooves, and distances between grooves.

A depth measuring bridge can extend the effective diameter of the beam, allowing depth measurement of wider holes. Some models even offer an attachable device for measuring force.

However, because these accessories and attachments extend the measuring range, parallelism errors can creep into the measurement. Thus, checking the parallelism of the contacts using a precision ball is important. A discrepancy of more then a grad on the Vernier scale is a sign that the parallelism of the anvil needs to be corrected.

Even the best and most basic hand measuring tool can be made better by adapting it to special application requirements. By choosing the most appropriate accessories for the application, you will achieve faster and more accurate measurement results.