Small CMM Redefines Portability

Manufacturing Engineering, Jul 2004

Manufacturing aerospace parts presents 3-D complexities that are sometimes beyond the scope of conventional mechanical measuring devices. For example, weldments between windswept surfaces and internal braces are rarely straight, but usually conform to an airfoil or duct shape. The same is true for the fixtures used to make the parts.

But complexity is only one of two basic obstacles of aerospace manufacturing. The other is precision-and precision measuring is the key to making complex aerospace structures perform as designed.

Skill-Metric (Delray Beach, FL) is a job shop that has prospered by becoming familiar with the specialty market of support for military and government aircraft. The company builds flight hardware-parts for everything but engines. Skill-Metric also manufactures and overhauls production tooling and evaluates fixtures used to prepare aircraft for missions, such as those used to position ordnance on the underside of wings.

Because Skill-Metric often makes or reconditions only a single part or tool for a given order, there are no prototypes or production parts on which tooling can be dialed in and processes honed. But, the item must be right the first time, and tolerances are typically ±0.005'' (0.13 mm).

Holding tolerances this close with mechanical measuring instruments was difficult, if not nearly impossible, on complex 3-D parts, according to quality manager Ken Bentz. "We used almost all the conventional measuring techniques you can think of, from a surface plane to gage blocks to dial indicators," he says.

Measuring throughput and reproducibility were issues, Bentz reports. "With conventional implements, it's possible for two technicians to measure the same dimension and get two different numbers," he says. "And, errors can be additive from one measurement to the next. We needed something more certain, more absolute, and faster."

What they found was the Faro Gage, from Faro Technologies (Lake Mary, FL). A smaller version of the company's FaroArm portable articulating coordinate measuring machine, the device's small size is an advantage in machine shops or production environments.

The Gage extends with a spherical reach of 24'' (610 mm) to allow measurement of frame points, hole locations, flatness, parallelism, and other characteristics. With measuring accuracy of 0.0002" (5 µm), it's more precise than mechanical measuring implements. Accuracy is not operator-dependent, and measurements are communicated directly to a companion laptop computer where they are stored for reports or for comparison to existing designs.

Designed to be portable, the device can be mounted to a flat plate or rectangular bar and be ready to measure in seconds. It can be used just about anywhere one would carry a caliper or micrometer. Skill-Metric technicians, for example, attach it to the ways of a lathe to measure shafts that are being turned, then bring it to a milling machine and check a part being cut there. In the course of a day, the Gage may be moved many times.

The device is an integral part of every assembly project at Skill-Metric. Structures such as engine nacelles or forming tools are measured as they are assembled. Technicians create a digital template of the part in the system's computer and check the location of critical points on the on-going assembly to ensure that they are positioned correctly.

"This is when the ability of the Gage to accurately measure in three dimensions becomes important to us," Bentz said. "The CMM and its software allow us to evaluate 3-D relationships between parts with much greater certainty than we could before."

To measure the geometry of a surface that changes shape over three axes, for example, the unit's stylus is dragged over the surface to obtain data points are taken at the required density-more for tight curves, fewer for flat areas. In this way, an engine duct or stabilizer fairing can be precisely measured in a minute or two.

One of the more creative applications of the device at Skill-Metric is to evaluate how well support hardware works. Technicians often take the Gage out of the shop to check the alignment of fixtures that load ordnance beneath aircraft wings. "The loading fixture must lift and place bombs or other ordnance precisely on the attachment hardware. If the lift is skewed, then the ordnance may not hang right on the plane and may not release properly in the air," Bentz explains.

Technicians mount the gage on the frame of the lifting system, then measure the 3-D gap between the attachment hardware, recreating the space between the mating surfaces. Using a digital template in the computer that shows the ideal alignment of parts, the technicians can quickly determine if the alignment is correct.

"Few things have so positively affected our business as the Faro Gage," Bentz concludes. "It has greatly reduced the time that we spend qualifying parts, and given us absolute certainty that they are qualified correctly." Circle 223