Quality Magazine

Probe has a soft touch

May 19, 2003
Measuring flexible or fragile parts with a coordinate measuring machine touch probe can be problematic because a simple touch can change the shape of the part--throwing off accuracy--or it may damage the piece upon contact.

While some companies use non-contact methods to measure such parts, many companies want to reach the extreme accuracy levels that touch probes provide.

However, on soft plastic, rubber, thin sheet metal or other flexible components, this may backfire. While the measurements may be repeatable, they may not always be accurate. When measured with a traditional touch probe, a flexible part may give, or change shape, upon contact before the measurement can be taken. It maybe repeatable because the force applied to the part can remain the same, but it may not be a true reflection of the part's dimension.

For example, when a vinyl molded part such as a flexible vinyl cap or plug is measured with a traditional probe, the part can flex. If an operator wants to measure the position of a wire on a mechanical part, most touch probes will bend or deform the wire before a measurement can be taken.

A solution is to capture the touch data as soon as possible after the probe contacts the part. Most touch probes use a mechanical contact switch or a strain-gage type of sensing system that requires a force of a gram or more to activate the switch that triggers the measurement. The need was there to develop CMM touch probe with a lower trigger force and a higher sensitivity, says Patrick Flanagan, Ph.D, a developer of a new low force trigger probe.

That is where the new CMM touch probe developed by Flanagan and Apollo Research Corp. (West Seneca, NY) comes in. The Model 1015 CMM Touch Probe uses a high-frequency stylus with a trigger force of less than 10 milligrams to detect touch. The probe body has a crystal inside of it that creates a vibration in the stylus, which the company calls a structural dynamic. The probe stylus resonates at 20 to 25 kilohertz and a second crystal detects this imperceptible-to-the-naked-eye structural dynamic. When the stylus contacts the part, the motion changes and the change is detected by a microprocessor in the electronics box, which signifies this change as a touch.

The analogy, says Flanagan, is to a tuning fork. "With a tuning fork, you hit it and it rings and if you touch it, the ringing stops," he says. "That is a simplified way of looking at it. With a tuning fork you can hear it, but with this probe, the amplitudes are so low and frequencies are so high that you wouldn't hear it. You won't see it and you won't hear it, but the probe can."

The result is that the probe touches the part and triggers before the part flexes. At that point, a measurement is captured. This is compared to a traditional CMM touch probe that has a trigger force of a gram or more and won't take the measurement until it reaches that trigger force.

The probe also addresses a second problem; false triggering caused by movement of the CMM head. The "seismic force" generated by moving the probe around the CMM can generate a false trigger. Because the Model 1015 probe measures the change in the structural dynamics, as opposed to the trigger force, it eliminates this problem.

The probe has a five-way sense direction: +/-X, +/-Y and +Z. It has uni-direction repeatability of 20 micro- inches and a bi-directional repeatability of 50 microinches. The stylus' overtravel on the X and Y plane is I15 degrees. The probe comes standard with a ruby tipped stylus, and a Teflon-tipped stylus, for use on products that can't be marred, is also available.

The high-frequency stylus is meant to be a tool to use for specific applications in conjunction with other touch probes in a quality control program. An 8-millimeter mounting thread allows the touch probe to be interchangeable with other commonly used touch probes.

"This probe isn't meant to replace other probes," says David Jaros, marketing manager for Apollo Research. "We are not replacing the other probes that are on the market. It is the kind of product that gives you extended capabilities for your CMM. If you are doing rigid parts, there is no reason that you want to select this probe. A standard touch probe with a mechanical switch or strain gage would work fine. Plus, the other manufacturers have useful accessories and quick-change methods for their probes. This was meant as a specialty item to be used when other probes won't work."