Every so often, we come across a CNC feature that has been available for a long time, yet we were unaware of its existence. And, of course, you cannot begin to apply a feature of which you are unaware. For me, one such feature is touch sensor. Thanks to Rob Alford of ASEA Brown for enlightening me about this feature.
For machining center applications, everyone knows that a spindle probe can be used to locate workpiece surfaces. If surface position is varying (as it commonly does with castings and forgings), the spindle probe can locate key surfaces and automatically adjust the program zero assignment (commonly with fixture offsets). While some probing applications require high accuracy, the location of cast surfaces is not one of them. In most cases, if we can find the cast surface location within plus or minus 0.005 inch, our machining program will adequately machine the cast surface.
While touch probes are commonly used for this application, and while they work quite nicely, they are a bit of an overkill. Believe it or not, the touch sensor feature allows you to use a cutting tool to locate surfaces. Though touch sensor is not nearly as accurate as a spindle probe, it will suffice when high accuracy is not required. The use of touch sensor will also minimize cycle time, since the additional tool change needed to call up the probe is not required. And if you have no other applications requiring higher accuracy, you can purchase touch sensor for a fraction of the cost of a full spindle probing system.
Though we must bow to your machine tool builder to explain the details of touch-sensor programming, in general it is handled much the same as a true probing system. The major difference is related to how the triggering signal is sent to the control. With a touch probe, stylus deflection sends the triggering signal. With touch sensor, a change in conductivity between the machine's spindle and table triggers the system.
Other than that, touch sensor is programmed in much the same way as a spindle probe. It must be activated (commonly with an M code), just as you must activate any spindle probe. The cutting tool must be driven into the surface to locate (commonly with the spindle running), just as you must drive the probe stylus into a surface. The same G code (G31 for most controls) is used to cause contact. The same method of attaining the machine's position (commonly with system variables) that is used when probing is used with touch sensor. Any action you can perform after probing (like setting fixture offsets) can be performed after using touch sensor. And finally, you must turn touch sensor off when you are finished.
This example will help explain more specifically how touch sensor works: Say you need to machine a spotface into a cast surface with an end mill. The spotface depth is specified as 0.090 inch deep. Since this cast surface is varying from one workpiece to another, its Z position must be determined in every cycle. While a spindle probe could be used, it would take about 15 to 30 seconds to probe the surface, depending upon tool changing and probing time. With touch sensor, the tool changing time can be eliminated, since the end mill can be used to locate the surface. And the actual act of touching the surface will be every bit as fast as today's fastest spindle probes.
With the end mill selected, the spindle will be started and touch sensor turned on. The end mill will be brought close to the surface and the command to touch (G31) is given. The feed rate can be slow enough to ensure that when the end mill touches, no damage will occur. When touch sensor determines that contact has been made, the triggering signal is sent. At this point, an incremental Z motion of 0.090 inch will send the end mill to its required depth. Note that if other tools key on this surface, a fixture offset could be set accordingly for subsequent operations.
Also note that some machine tool builders more actively promote touch sensor than others. In fact, you may find that some builders are as unaware of this feature as we were. But if you have applications, it will be well worth your time to dig in and learn more about it!