New generation of optical micrometers provide more accurate measurement of surface defects on critical parts

High-brightness LEDs with fiber optics, CCD array sensors, archiving of defect images and data, and real-time PC graphic-user interface are some of the latest features being designed into next-generation handheld optical micrometers used to determine the severity of damage caused to critical parts from scratches, blemishes, and corrosion pits.

By Jim McMahon

Whether it is scratches on a helicopter's rotor blades or on the fuselage skin of an aircraft, or corrosion pitting on industrial couplings, castings, transmissions, or engines, it is critical to evaluate the severity of mechanical damage to stress-bearing parts. These surface defects represent stress concentrations which are more susceptible to failure than the other areas of the component.

Very specific maximum allowances often apply to surface defects, which are essential to determine whether or not a part should be kept in service. To repair or replace the part can have significant structural and financial impact. For example, a helicopter's mast, which connects the chopper's motor to the main rotor, has a defect depth allowance of .012 to .015 of an inch.

If the depth of the defect exceeds those parameters, then it is at risk of breaking and must be replaced. But replacing parts can be a very expensive proposition. In the case of helicopters, masts can cost as much as $25,000 to replace.

Careful visual inspection should, therefore, be standard protocol for any force-absorbing component that exhibits scratches, blemishes or corrosion pits.

Handheld Optical Micrometers
The portable optical micrometer has long been the tool of choice to measure the depth of these defects, from the surface to the bottom of the depression, thereby determining the severity of mechanical damage.

Optical micrometers are portable microscopes specifically designed for measuring depth. They are used for measuring three-dimensional (3D) relief along the z-axis within any x-y field of view, and they accomplish this by measuring the difference in depth between any two focal planes. The instrument is focused on a region of interest and set to zero. Then as a second region is brought into focus, depth is shown on a digital display.

Optical micrometers are particularly good for measuring the depth of corrosion pits, as built-in magnification makes it easy to find the deepest part within a given area. Hours of blend-out labor can be avoided when it is determined ahead of time that a corrosion pit is too deep.Optical micrometers are also used to check dimensional accuracy, like calipers and other workshop and quality control measuring equipment, in manufacturing and production environments. Serving as non-contact height gages, they can confirm the height of a component above a background or substrate on a micro-circuit or assembly, measure the depth of tiny steps, layers and grooves, or the actual depth of small mold cavities, for example.Introduced in the 1950's, optical micrometers hold several NSN's (National Stock Numbers), they are in the supply systems of several corporations and are used on the Space Shuttle and International Space Station. Whether they are used to check dimensional accuracy or to measure the severity of flaws and defects, optical micrometers are compared to N.I.S.T.(National Institute of Standards and Technology) traceable standards and required to be re-certified on a regular basis just like other quality-control measuring equipment.New Generation of Portable Optical Micrometers There have been major product improvements and upgrades with portable micrometers over the years, but none that compare to the completely new and innovative designs of the latest models. The most prominent example being the newly re-designed portable optical micrometer by the J. Chadwick Company (Chadwick), who teamed up with Lincoln Laser Company (Lincoln) to collaborate on the design and manufacturing for a new-generation, electronic version of portable micrometer. Chadwick is an OEM of portable optical micrometers, having manufactured analog and digital versions for the military and the aerospace industry since the early 1990's. Lincoln Laser specializes in the design, engineering and manufacturing of high-precision mechanical, laser-based, and electro-optical systems. Chadwick's latest version of hand-held micrometer, called the Depth Cam, was designed to handle resolutions as low as one micron, and offer a number of electronic capabilities that have not been available in prior portable micrometers. The intent was to build an instrument that was easier and faster to use, produce consistent detailed and accurate results, interface with a PC for real-time control and monitoring, and have the capability to store data for later evaluation and comparison."Portable optical micrometers needed to bring a higher level of precision and integrity to this important judgment process," says John Chadwick, President of the J.Chadwick Company. "Should a defected part be repaired or replaced? An instrument that is giving inaccurate readings could cost a company many thousands of dollars in unnecessary new parts when the defective ones were still within compliance."The system is the first camera- and PC-based portable optical micrometer.Once the micrometer is positioned over a point of interest, the user does not look through a lens as in prior portable micrometers, but instead controls the movement of the objective through a portable PC, by operating a mouse while viewing a monitor, or graphic user interface (GUI).

To find out more about Lincoln Laser, contact Michael S. Adkins, Vice President Sales, Lincoln Laser Company; 234 East Mohave, Phoenix, AZ 85004; Phone 602-257-0407, Fax 602-257-0728; [email protected]; www.lincolnlaser.com

To reach the J.Chadwick Company, contact John Chadwick, President; 1005 South Mountain Avenue, Monrovia, CA 91016; Phone 626-358-9955, Fax 626-358-1448; email [email protected]; www.jchadwickco.com