Machinery's Handbook - 7: Measuring Over Wires
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.

Using three wires to measure the thread pitch diameter. This method of thread measurement uses three precisely matched ground steel rods (called wires) that fit on the threads as shown. A micrometer is then used to measure over the wires and calculations are made to determine the pitch diameter of the threads from the micrometer reading.. From page six above it is noted that the pitch diameter must be between .9168 and .9100 for this thread.

With a typical wire gage collection like the one shown left there will be various sets of three wire diameters to pick from (meaning there will be numerous sets of three wires which are exactly matched in diameter). The Machinery's Handbook will have a table for selecting a working diameter of wires for the 1-8 thread. As shown below, a wire set of .0700 dia to .1125 dia should be used for this thread. This means that any set of wires that are between .0700 and .1125 can be used to measure the pitch diameter of this thread. However it will take a great deal of layout and calculation to determine what the measurement over wires

should be for a given pitch diameter and wire set. Machinery's Handbook helps out doing the calculations for certain common wire sizes and placing them in a table. In the table shown below a .090 diameter wire is selected. Note that the .090 diameter is within the limits of .0700 and .1125. A .090 dia wire will be available in all common wire gage sets precisely because the Machinery's Handbook utilizes that diameter in its tables.

From the tables it will be noted that a 1-8 thread with a pitch diameter of .9314 will measure between 1.0535 to 1.0806 over three properly placed .090 diameter wires. Remembering of course that the .9314 pitch diameter is the average pitch diameter obtained in page six and that the table includes this in its calculations

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label: machiney's Handbook., blueprint, steel properties, speed and feeds, threads, measuremennt, over wires, press fits, slip fits