Controlling pipe tap depth

Modern Machine Shop, Feb, 1992

It can be difficult to calculate the programmed depth required for tapered pipe tapping due to the taper angle and tap diameter. To do so requires the use of trigonometry. Also, each tapered pipe tap size requires a different depth.

Note that this depth is usually quite critical. If the tapered pipe tap does not go deep enough, the mating pipe fitting will not fit into the hole (the pipe tap must be sent deeper). If the pipe tap goes too deep, the pipe fitting will be loose, and the joint will leak (the part may be scrapped). Since sizing must usually be done on the CNC machine with a crude gage (if a gage is available at all), it is likely each pipe tap must be run several times to determine the final depth while the operator tries to sneak up on the pipe tap depth.

Here is a technique that will assure that the pipe tap will machine the hole to the correct depth on its first time into the hole, meaning no more trial and error. It involves making a gage that will be fitted to the pipe tap during the tool's length measurement. Figure 1 shows the gage.

Notice that the gage is made to a precise length. In this case, one inch long. The tapped hole in the gage is machine to the correct depth required for pipe tapping. Of course, one gage will be required for each pipe tap size.

This gage is placed on the end of the tap as the tap's tool length is measured. The tool length measurement will be taken to the end of the gage. NOTthe tip of the tap. Figure 2 shows the tool's length.

When the program is written, the Z depth of the tapped hole will be set equal to the length of the gage (one inch in our case). The rapid plane can be set to zero, since there is ample clearance between the end gage and the tip of the tap. Here is an example of the canned cycle command to pipe tap the hole:

N055 G84 X1. Y1. R0 Z-1. F16.4

Since the gage was used during setup, this command will tap the hole to the exact depth on the very first try. No more trial and error.

Of course, the feed rate must be synchronized with the spindle speed as with all tapping. The use of this technique makes programming easier while at the same time, also enhancing setting up and operation.