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7/1/1997 | 6 MINUTE READ

Controlling Quality on Numerous Finishes

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Hohman Plating's coating diversity is key to aerospace, military and commercial customer's satisfaction...


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It has been said that a company can be judged by the company it keeps. In Dayton, Ohio, there is a plating company considered one of the most versatile finishers in a tri-state area. The finisher is Hohman Plating & Manufacturing, Inc.

Its customers read like a who's who in the aerospace, military and commercial markets. This customer base is a result of Hohman's high standard of quality control, delivery and efficient testing to government and industrial specifications. Some of its customers include Boeing, Bell Helicopter, General Dynamics Space Systems, General Electric Engine Group, General Motors, Lockheed, Martin Marietta, McDonnell Douglas Helicopters, Sunstrand, Textron Lycoming, Space Systems/Loral, Storagetek, NWL Control Systems, United Technology Pratt & Whitney, Williams International, B.F. Goodrich Aerospace, Allied Signal/Bendix Engine Controls Division and Lord Corporation, among others.

Hohman says it keeps its customers because of its diversity of coatings. "By offering the diversity of coatings that we do, we can offer the best coating to the customer for his application, rather than trying to sell a less desirable coating because it is all we offer," stated Art Hattan, sales manager.

The company has been in business for 80 years, founded in 1917. It is privately held, with all of the owners employed by the corporation.

Hohman Plating offers processes so varied that a partial list alone includes such finishes as tungsten/nickel/boron codeposit; vacuum deposition; plasma spray; and organic coatings; all types of aluminum finishing; cadmium, chromium, copper, electroless nickel, lead, nickel, zinc-cobalt, zinc-nickel, zinc, tin, brass and precious metal plating. Also, Hohman Plating does phosphating, solid film lubricant coating, stainless steel finishing, metal conditioning, thermal spray coatings, low-temperature titanium nitride, physical vapor deposition, sputtering and ion plating.

Supporting these processes are well-located testing facilities and instruments for atomic absorption analysis; microscopic cross-sectioning measurement; salt spray testing; humidity testing, solderability testing, taber abrasion tests; adhesion tests; and friction and wear testing.

Test instruments used include: AC Hypot testers, interferometer, Coulometric thickness tester, micrometers, profilometer, hand-held as well as bench-type magnetic/eddy current testers and x-ray fluorescence systems.

There are three x-ray fluorescence systems used in the Hohman coating/finishing facilities. One is a deep well XRX system for measuring the coating thickness of large finished parts or parts that require a longer than normal focal length to measure. The other two are "mini-well" systems for checking smaller parts.

The deep-well x-ray fluorescence system is used primarily to determine the thickness of electrodeposited tin on aluminum in deep recesses. Hohman developed a template to hold 48 of these parts on the programmable sample table. As a result, each template represents a savings of several hours in testing labor.

According to Hohman, the parts are cylindrical in shape with an outside diameter of one-and-one-half inches and a height of 9/16-inch with a1/2-inch-wide and1/2-inch-deep blind bore. The parts are for the automotive industry. Hohman Plating processes tens of thousands of these parts daily.

"The coating thickness at the bottom of the blind bore is significant and must be measured. Because of the location, a long focal length x-ray machine was desired," Todd Boyer, production supervisor, reported.

The deep well system is used mainly by inspectors in determining the plating thickness of the tin in the blind bore, while the built in software calculates the CPK and prints it along with the statistics and histogram.

Mr. Boyer stated that prior to development of the longer focal length of the x-ray fluorescence thickness tester, Hohman had no non-destructive method to test the thickness of the tin deposit due to the parts geometry. The production requirements and established quality control procedures required that 42 pieces be tested for every hour of production. The mini-well x-ray fluorescence system was able to test the thickness, but due to the geometry of the part and the location of focus, the parts tested had to be destroyed by cutting them prior to testing. Testing the thickness of six pieces required approximately 30 minutes from start to finish.

As mentioned, Hohman designed a template to hold 48 parts on the deep well unit's programmable table. The table of the x-ray system was programmed to test the 48 parts automatically.

With the need for cutting the parts eliminated and going to the use of the programmable table, the labor used for testing the 48 samples was reduced to six minutes total. This is a reduction from five minutes per part to approximately 7.5 seconds per part. According to Mr. Boyer, "Using x-ray fluorescence gets results much more rapidly, and the area to be tested is controlled by the programmed table which reduces the chance for errors."

The Vacuum Coating Division of Hohman Plating has coated more than one million parts for the aerospace industry alone, pioneering many coatings that have become industry standards such as MoS2. The main focus of coatings is tribological in nature. The coating chambers are capable of processing parts by sputtering, electron beam ion plating and cathodic arc plasma deposition. All engineers, technicians and operators at Hohman are highly skilled and undergo extensive training programs in the field of vacuum coatings. Modern testing facilities are available for insuring compliance of coatings to specifications. Vacuum coatings are deposited according to stringent specifications, with thickness requirements common. The x-ray fluorescence system is a convenient method for measuring coating thickness on actual parts rather than on flat coupons.

The x-ray fluorescence system can be used for testing the thickness of vacuum deposited titanium nitride and other hard-wear coatings, which have been successfully used to extend the life of various tools and machined parts. Titanium nitride and other hard-wear coatings are applied to a variety of mechanical components such as gears, splines, shafts, spindles, ways, bearings or any device where wear resistance is important. There are many environments in which surfaces are subjected to heavy abrasion by foreign particles. Perhaps the most severe of these is in gas turbine engines. Turbine engines, particularly those on helicopters, continually ingest large quantities of sand, dust, and other debris that bombard the compressor blades and impellers in the engine and abrade their surfaces.

Other test instruments used in the plant for quality control include an EMX thickness testing tabletop instrument. It is used for high inspection productivity with high accuracy for measuring cadmium, copper, zinc and electroplated nickel over ferrous metals.

Four hand-held CGX-A and CGX-B coating thickness gages are used throughout the plant for coating thickness verification. The A gage is used to check non-ferrous coatings over ferrous substrates and the B gage uses the eddy-current principle for checking anodized coatings on aluminum.

The EM is located in the plating department for easy operator access. Two CGX units are assigned to quality control, and two CGX units are in the quality control department for general plant sign-out.

Training operators on the many test instruments is vital and essential to quality control. The length of time it takes to orient the operators in the use of the test instruments varies with operator, machine and application. "Usually 15 to 20 minutes enables the operator to go it alone," according to David Wall, quality assurance manager.

"Our emphasis is on the importance of quality in meeting customers' specifications. This demonstrates Hohman's commitment to high quality work," Mr. Wall further asserted.

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