Thread indicator dial

The position of engagement of the spUt nut on the leadscrew for each cut is important in order that the tool will travel along the same path as the previous cut. To achieve this accuracy of engagement, a thread indicator dial is fitted at the end of the apron, Fig. 9.34. The dial is mounted on a spindle at the opposite end of which is a gear in mesh with the leadscrew. 

The chart shows the gear used for a particular pitch of thread and the numbers on the thread indicator dial at which the split nut may be engaged. To cope with the different diameters of gears, the unit pivots and is locked in position when the gear is in mesh. To avoid unnecessary wear, the unit is pivoted back out of mesh when not in use for screw-cutting. 

Select the correct gear on the thread indicator dial and mesh with the leadscrew. 

Wait until the appropriate numher on the thread indicator dial comes round to the mark and engage the split nut... 

Wait for the correct number on the thread indicator dial, engage the split nut, and repeat until the final depth is reached. 

Imperial threads are designated not by their pitch but by the number of threads per inch (tp.L). The leadscrew of a metric centre lathe has a pitch of 6mm and, since the number of t.p.i. cannot be arranged as a multiple of the leadscrew pitch, the split nut, once it is engaged, must never be disengaged during the thread-cutting operation. This also means that the thread indicator dial is of no use when cutting imperial threads on a metric lathe. 

Pressure measurement for piping and vessels is achieved by the installation of dial indicators or by distributed control systems. The piping and vessels are tapped at the required location and furnished with a 3/4-in threaded or flanged conneaion and a block valve. Tbe dial indicator is screwed into the block ralve if a remote readout system is required, the valve becomes the sensing connection. Like the thermocouple, the dial can be either fixed or swivel-headed to fecilitate readout. A dual local indicator and transmitter s) em needs only one tapping point. Exhibit 14-9 depicts a typical pressure gauge and a dual-pressure system hookup. 

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