Laser-Based Tool Maps Surface Roughness

May 19, 2003
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The ranks of the devices that measure and map surfaces such as brake rotors and camshafts have grown by one. To the list of measurement devices such as the stylus and the focused-laser beam, add a new noncontact alternative called Lasercheck.

The Lasercheck is a surface roughness gage from Optical Dimensions LLC (Lake Forest, CA) that uses a laser beam to capture hundreds of data points in seconds in a handheld application or in an automated production process. Laser-check works by positioning the 1-pound measurement head above the surface of the part to be measured. The Lasercheck head rides one-tenth of an inch from the surface of the part, separated from the surface by a removable "shim" plate in a handheld operation and by the part fixturing in an automated line.

Inside the head is a laser diode that bounces a laser beam off the surface of the part. Surface imperfections cause the light to scatter. The scattered light is "seen" by an array of 35 detectors, or small sensors, that are arranged inside the 3- by 1.4- by 1.75-inch head. The reflection is converted to a series of voltages that are recalculated by an onboard computer to determine a raw surface roughness number. The roughness figure is factored into a calibration equation that determines the surface roughness (Ra) of the part.

"The laser beam itself is small," said J. Glenn Valliant, co-owner of Optical Dimensions. "On a smooth surface, only one or two sensors would detect a reflection. As surfaces become rougher and rougher, the reflection is no longer a well-preserved laser beam. It is scattered light and more of the sensors illuminate."

The laser, which Valliant said can work on any material or color, inspects a 1- by 5-millimeter area at a time and returns an average of the area. The system measures a range of roughness between 1 microinch and 120 microinches. If the surface is farther out of specification than that, the changes in scattered light will lose resolution, which is an indication of the surface Ra.

Optical Dimensions developed the Lasercheck for K.J. Law Engineers Inc. (Novi, MI), a supplier of automatic gage and nondestructive test systems. A K.J. Law customer wanted a tool to check the finish on a disc brake rotor, said Ken Law Sr., founder and chairman of K.J. Law. In an automated measuring station, the Lasercheck can be linked to other measuring devices into what K.J. Law calls a surface finish profiling system.

In a mass production environment, fast measurements are required and lasers can take up to 70 readings per second, with the potential for 100 readings per second, said Valliant. Speed is important because, "if a company makes 30,000 to 40,000 parts a day and it takes 10 to 20 seconds to make one measurement, then you can't inspect every part," he said. "They have to take a part off the line every few hours to inspect it and if the process has drifted out of specification in between those Arial, you've got a bin full of bad parts," said Valliant.

The Lasercheck is made for the factory floor, said Valliant. It has no moving parts and can be dropped without breaking. It is vibration resistant because it does not make direct contact with the part. In tests, pounding the table on which the part rested did not affect the measurements that the Lasercheck took. Also, because the laser takes nearly instantaneous measurements and averages the data, small movements of the part caused by vibration have little or no effect on the overall data.

In an automated version, the Lasercheck's measurement heads are moved in and out of the part on a slide mechanism or by an air or hydraulic cylinder. "The part has to be staged, held in an accurate and positive position," said Law. "For a brake rotor, two heads measure the top and bottom and move in and out to scan different diameters around the surface of the part."

The part does need to be cleaned to enhance accuracy. In hand applications, wiping it with a cloth is usually sufficient. "In an automatic application, we use pressurized air to remove the grime and fluids that build up," said Valliant.

The $9,600 instrument can take static- or encoder-based measurements on multiple radii while either or both the instrument and part are in motion. The Lasercheck features an embedded computer and digital input and output serial ports. Measurements can be saved in the control unit and output to a computer, to be saved to any location on a computer network. Data formats include graphical Lasercheck file format and ASCII file format for direct input to commercial statistical process control and spreadsheet software packages.

The unit can be interfaced to existing K.J. Law equipment via RS232 serial communication, RS485 serial communication, 4-20 milliamp current loop and a 0 to 10 volt analog output.

The Lasercheck also features a read, hold and return on request program. A number of readings can be taken and held until requested by the gage. Therefore, one or more lasers can measure simultaneously with other devices, such as eddy current testers, without slowing cycle rates.

"We do automatic gaging of all dimensions on these parts, such as disc brakes, rotors, crankshafts and bearings," said Law. "The Lasercheck integrates to the machine and our gages. It also integrates to our gaging electronic controllers that compute the gaging dimensions and sorts the parts according to the tolerances."

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