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Tough customer certification standards, ISO 9000 approvals, 100-percent feature inspection requirements, and today's "quest for quality" are putting intense pressure on manufacturers' abilities to perform precise, detailed part measurement. That's why direct computer-controlled (DCC) CMMs are being used to capture data at speeds unheard of just a few years ago. A standard, four-point check on a bore, for example, may be satisfactory only for low level inspections. For the detailed information manufacturers need to make better decisions, more than 20 data points and corresponding form analysis are probably required for that same bore--yet it must be done in the same amount of time.
So it's not unusual to find DCC CMMs that collect upwards of 5 million or more data points a year--and that's placing stress on conventional touch trigger probes.
A decade ago it would have taken months, if not years, for a CMM to do a million triggers (data points). Now, examples abound of where CMMs are doing a million triggers in a few months. The kinematic/resistive technology that most touch trigger probes use is designed to hold up for a few million triggers. While that's perfectly acceptable in many applications, an increasing number of new DCC CMM applications challenge touch trigger probe life expectancy and reliability.
The majority of CMMs use kinematic/resistive touch trigger probes. These probes use a kinematic (highly repeatable) re-seating mechanism that restores the stylus ball or tip to its original spatial position within nanometers. Such probes use a three-point kinematic mechanism that restricts stylus movement until it contacts a workpiece. Following contact, the kinematic mechanism unseats, stressing a "restoring force" spring, until a highly repeatable triggerpoint is reached. The trigger event is registered by the probe interface (usually housed in the CMM control cabinet), which monitors the probe contacts for resistance changes. Once a preset trigger resistance threshold is reached, the interface registers a trigger event and the CMM scales are latched.
After a few million triggers, it's not uncommon for probe contacts to degrade. A probe might fail to re-arm after...