Counting Scales Improve Accuracy

At M&M Aerospace Hardware Inc., Miami, quality counts. Literally.

The company is a distributor of nuts, bolts, washers, rivets and other fasteners to aerospace manufacturers like Boeing/McDonnell Douglas, Lockheed and Airbus. Founded in 1974, it has more than 300 workers and eight locations in the U.S., Canada and Europe.

Customers judge vendor quality based on the accuracy and speed with which the company fills orders, especially when counting some 1,000,000 rivets, for example. Previous scales were only 94 percent to 95 percent accurate, according to vice president of quality, Terry Bond, responsible for overseeing the warehouse operation from shipping to receiving. To obtain a more accurate count with the older scales, workers often had to weigh parts several times, which ultimately resulted in additional time to fill orders.

To improve the accuracy of its scales and reduce the time required to pick parts, the company chose Super II counting scales.

This scale uses patented variable-capacitance technology, which differs from conventional counting scale strain-gauge load-cell technology. Variable capacitance employs a device consisting of two parallel gold-plated electrodes fused onto a ceramic load cell. The electrodes, which have a gap between them of just a few thousandths of an inch, are wired into an LC oscillating circuit. When a load is applied to the scale pan, the ceramic beams bend, changing the gap across the two electrodes, which changes the capacitance in the electronic circuit.

Changes in capacitance result in changes in the frequency of oscillation. As weight is added to the scale, that frequency may vary by as much as two million cycles per second. A microprocessor in the scale detects changes as small as one cycle per second, providing the scale with extremely high internal counting resolution. The Super II can measure a weight change of as little as one part per million. Because the concept is simple, there are few moving parts to fail.

The results have been dramatic. Since purchasing 86 Setra Super II scales, M&M Aerospace has improved accuracy more than tenfold, and fills orders up to 30 percent faster, according to Ray McCormick, director of warehousing.

The scales not only are used when filling an order, but also when hardware arrives from suppliers. When an order placed by M&M arrives, workers make a dimensional inspection of the hardware, check to ensure that all paperwork is in order, then forward the hardware to the receiving department, where all parts are counted using the scales. Today, part count accuracy is at 99.5 percent, Bond says.

Another important consideration for Bond was speed. That measure was improved because fewer weights are taken per order. Employees weigh a sample, hit ‘Count’ and get a reading that is already accurate. Orders are filled faster, increasing productivity and improving inventory control.

Super II counting scales by Setra Systems Inc.; www.setra.com.

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