To cut internal pipe threads, drill a tap hole in the stock to be tapped and, following the suggested procedure for tapping machine threads described earlier in the chapter under the heading Cutting Machine Threads With Taps, run the pipe tap into the tap hole. Notice that the first few threads on the pipe tap are ground away. This makes starting easier. Plenty of lard oil is the standard lubricant for steel. Tap copper and brass with no lubricant. The depth to which it is desirable to tap pipe threads is usually determined by turning the threaded pipe into the tapped hole for a trial. As shown in figure 6-30, the last few threads on the pipe should still be visible when the pipe is drawn up tight in the tapped hole. Figure 6-30 shows the values of dimension A (the length of thread on pipe required to make a tight joint) for various sizes of pipe up to 12".
The general practice in tapping holes for pipe threads is to drill the proper size tap hole and then start the pipe tap right into the tap hole. Some men recommend using a pipe reamer, especially when large deep holes are to be tapped. A pipe reamer has the same 3/4Tt per foot taper as a pipe tap. A reamed pipe tap hole would have the same shape as the pipe tap, and therefore would make tapping easier and reduce wear on the tap.
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