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The wires are placed in the threads and a micrometer is used to measure over the top of the wires. Different thread types and thread sizes require different size wires. The "best wire size" can be calculated or looked up in the Machinerys Handbook. The three-wire method of thread measurement can be used for all types of threads including Unified, Acme, and Buttress thread forms.
To help you understand the three-wire method of thread measurement, we will use an example thread of 1.250-5 ACME-2G . The first piece of information we need to acquire is the "best wire size." The best wire size is the size of the wire that will contact the thread at the pitch diameter. The best wire size can be calculated using a formula, but you can also find it in the Machinerys Handbook. Begin by looking in the index under Thread, Acme, wire method of testing. Turn to this section and find the table which gives the wire sizes for measuring Acme threads with lead angles less than 5 degrees. Now look for the column that shows 5 threads per inch. You should find that the table gives you 0.10329 as the best wire size. The table also gives you minimum and maximum wire size limits. Make a note of the best wire size.
Now we need to find out what the measurement over the wires should be. Within the same unit you will find a paragraph entitled Three Wire Measurement of Acme and Stub Acme Thread Pitch Diameters. Within this paragraph it will tell you where to find the Approximate Three Wire Formula. The Approximate Three Wire Formula "SIMPLIFIED VERSION" should read as follows:
We really only need to find the pitch diameter to solve the formula. We have the wire size. We know the pitch is 1/5 or 0.2 . To find the pitch diameter, go back to the index in the Machinerys Handbook. Look in the index for Screw threads and threads systems. Under Screw threads and threads systems find Acme. Turn to this section and find the table that gives the Limiting Dimension for American National Standards for General Purpose Acme Single-Start Screw threads. In this table find the Class 2G, Pitch Diameter, Maximum and Minimum limiting diameters for the 1.250-5 ACME-2G thread. You should find that the table gives you 1.1210 as the minimum size and 1.1410 as the maximum size for the pitch diameter calculation. We will use the median dimension of 1.1310 for our formula calculation. Now that we have all of the information, let's plug the numbers into the formula.
When using thread wires to measure threads, the machinist should use a shop towel under the threads to catch the wires if they fall. Two rubber bands or a piece of modeling clay will also help to hold the wires in place while a measurement is being taken.