Steel rods with Acme threads are used in applications where a heavy load will be applied to the rod or in circumstances where a great seal of accuracy is needed when moving an object along the threads. As an example, Acme threads are commonly used with vices. A vice requires the steel threaded rod that connects the vice jaws to remain under heavy load for a long periods. When measuring the outside of Acme threads, you will want to know the overall diameter and the thread spacing.
Open the inside jaws on your dial caliper or your digital caliper to a point that will allow you to fit the Acme threaded rod between the jaws.
Clamp the jaws over the outside diameter of the Acme threads. Do not place the jaws into the valleys of the threads as this will provide an inaccurate measurement. Angle the jaws so they rest over several thread tops.
Inspect the number on the dial caliper. First, look at the number on the shaft of the caliper where the back jaw rests. It goes in increments of 1. Each number denotes 1/100,000 of an inch. Second, look at the number on the dial. Each number denotes 1/1000 of an inch. Add the two numbers together to obtain the total number. For example, if the number exposed on the shaft is a 3 and the number on the dial is a 30, the outside diameter of the Acme thread is 350/1000 inches.
Spacing and Pitch
Pull an Acme thread gauge out of the gauge cluster.
Position the Acme thread gauge over the Acme threads. The edge of the gauge resembles the profile of an Acme thread. The gauge has peaks and valleys cut into it that match a specific pitch and thread spacing for an Acme thread. Rest the peaks on the gauge in the valleys of the Acme thread.
Inspect the mating surface between the gauge and threads for any gaps. If you see any gaps, the gauge is not the correct gauge for the Acme thread. Place the gauge back into the gauge cluster and then pull out a new gauge and test in the same manner. Do this until you locate the gauge that matches the thread profile exactly. Once you locate it, the side of the gauge will tell you the thread spacing and pitch.