Double-Disc Grinder Helps Manufacturer Of Hand Tools Strengthen Quality Control

A newly designed vertical double-disc grinding machine is allowing a leading manufacturer of multi-purpose hand tools to maintain control of tight dimensional tolerances, and excellent surface finish quality, in finish-grinding tool components.

Case Study From: 12/1/1996 Modern Machine Shop

A newly designed vertical double-disc grinding machine developed by Gardner Disc Grinders And Abrasives (now Landis Gardner), is allowing a leading manufacturer of multi-purpose hand tools to maintain control of tight dimensional tolerances, and excellent surface finish quality, in finish-grinding tool components. With a new model GV-450 grinder from Gardner, Leatherman Tool Group, Inc. in Portland, Oregon, is able to produce finish-ground components of its top-of-the-line Super Tool product more economically than it previously could by outsourcing, while also gaining more control of surface finish quality.

The Leatherman Super Tool contains 17 different tool features and is designed for full-purpose, or universal, use. According to Leatherman manufacturing engineer, Chris Folkman, the tool has served people on safari in Africa as the only available means to repair a broken-down truck. Dentists in India have used it to pull teeth, and the product has even been used to fix airplanes in the air.

The tool is only one of five multi-purpose hand tools which Leatherman markets through a network of distributors and retailers across the U.S. and in 70 countries worldwide. The manufacturer also faces competition from at least three additional companies entering the multi-purpose tool market. To help grow its business and expand its share of the market, the company relies on a major research and development effort spearheaded by company president Tim Leatherman. "We also get feedback on what customers want both through market research and lots of personal letters," Mr. Folkman says. "Beyond that, we learn what product improvements may be needed from damaged tools that are returned under warranty. These help us design better tools, which in turn helps reduce warranty returns."

Leatherman took delivery of its Gardner GV-450 vertical double-disc grinder in January 1996. "We purchased the machine basically for two reasons," Mr. Folkman said. " First of all, we knew we could save enough dollars by doing our own grinding to pay off the investment in just a couple of years. We also want to expand the overall grinding and polishing we do on our own as much as possible to gain more control of finish quality."

Though one or two competitive grinders would also have allowed Leatherman to meet these objectives, the company found the completely redesigned Gardner "G" Series machine the best overall solution for its needs. As it happens, Leatherman also had a 1984-vintage vertical double-disc grinder, now used for grinding both sides of the Super Tool jaws, that provided a reference for comparing features of the latest Gardner grinder. In making that comparison, it was evident that the new-generation grinder offered a host of advantages. The most obvious was automatic wheel crossing.

"Given our production rate, wheel dressing is important," Mr. Folkman notes. "With the old grinder, you had to dress the wheels manually with an arm. With the GV-450, the entire cycle is automatic."

To perform the dressing cycle, Mr. Folkman explains, the operator simply pushes a button. The wheelheads retract, and the dresser arm advances automatically to dress both grinding wheels simultaneously in a programmed number of passes. The machine compensates automatically for the abrasive material dressed off, and the wheelheads advance back to grindline position.

Mr. Folkman notes that the automatic grindline positioning is an especially important capability, because the correct grindline is essential to obtaining precise part parallelism and flatness. "With the older grinder, you have to reset the grindline manually, which is a tricky business and takes considerable time. With our new grinder, the entire automatic dressing cycle takes only minutes, and you can be sure the grindline setting is on the mark."

Another advantage of the new-generation machine, according to Mr. Folkman, is the availability of statistical process control (SPC) and adaptive gaging to automatically compensate for wheel wear. "To maintain the size with the old grinder," he says, "you have to constantly monitor parts and adjust wheels manually. The manual adjustments are difficult. Do you feed the bottom wheel in, or the top wheel? If you feed the bottom wheel in too much, you change its position relative to the machine table and lose your grindline--which means losing parallelism and flatness.

"With the new machine, the operator doesn't have to worry about these things. SPC makes the adjustments automatically, based on objective measurement data, so you know they'll be exactly right."

Other features of the new machine Leatherman found especially valuable include: servomotor drive and feed control of the parts carrier, which allows more accurate grinding and reduces wheel wear; an access door at the side of the machine that makes it easier to change the grinding wheels when necessary; and much greater machine rigidity, which allows faster grinding speeds and higher production rates.

From the beginning, the Gardner grinder has been used at Leatherman for finish grinding of five Super Tool components--the large screwdriver, medium screwdriver, small screwdriver, container opener and awl. In each case, two parallel surfaces are ground simultaneously to a common specified thickness.

Like all of the Super Tool implements, the five components that go through the Gardner double-disc grinder are supplied from outside as fine blankings from 420 stainless steel that have been tumbled, deburred, and heat-treated. The blanks are produced to Leatherman thickness specifications, with enough grinding stock to permit blade straightening and removal of surface imperfections. During finish grinding, 0.006-inch of material is removed from each side, leaving a finished thickness of 0.105-inch (to a part tolerance of ±0.0005-inch). Subsequent surface polishing and any required edge honing are performed in separate processes.

While the Super Tool components have a nominal thickness tolerance of ±0.0005-inch, the Gardner grinder is in fact, maintaining a much tighter tolerance of ±0.0002-inch. In addition, flatness and parallelism are consistently held to within 0.0005-inch, and part surface finish is maintained to less than 10 Ra, ensuring an imperfection-free surface with secondary polishing.

Maximum output for the Gardner double-disc grinder up to now has been about 3,000 parts per hour--but could be higher with automatic parts loading. "The rate we have been maintaining is consistent with our operator's ability to continuously load the machine by hand," Mr. Folkman notes. "However, we're now in the process of purchasing an automated loading system. Since we're holding very tight tolerances at our current production rate, we really don't know how high our output could go."

"With automatic parts loading," Mr. Folkman points out, "we'll have the option of programming the machine to dress at any time in the process, or after a specified number of parts. Up to now, however, wheel dressing has only been done when a random part inspection indicates a deviation from spec for flatness and parallelism. The machine itself, of course, makes its own adjustments to hold thickness throughout the day."

With adaptive control of its wheel size, Leatherman is trying to maintain a process CpK for part thickness of 1.33--which is equivalent to 8 Sigma. "According to the Gardner data," Mr. Folkman points out, "the 1.33 CpK, or 8 Sigma, performance means that, compared to our part tolerance of ±0.0005 inch, 68 percent of the parts we produce will actually be clustered within a very narrow band of that tolerance range--to within ±0.000125 inch of nominal size. At the same time, about 99.98 percent of our parts will be within our actual part tolerance of ±0.0005 inch."

In practice, Leatherman has obtained production CpK ratings as high as 2.00, or 12 Sigma (usually with new wheels), which means that 68 percent of the parts produced are clustered even closer to the nominal thickness size of 0.105 inch. "The 1.33 rating is an overall average," Mr. Folkman notes.

He also points out the separate importance of the Cp (as opposed to the CpK) rating as an indicator of process control. Currently, Leatherman is maintaining a CP rating of 1.33 or better, which means that the parts it produces are holding an assigned production size tolerance of ±0.0005 inch. This valuation does not, however, measure the "clustering" of parts around nominal part size which is a measurement of CpK.

"What you really want is to maintain a high Cp and CpK together," Mr. Folkman explains. "That means both that you're consistently holding a tight tolerance, and that most parts are holding that tolerance very close to the center of the tolerance band--which is the nominal size specification."

Asked to summarize the major benefit the new generation grinder has provided, Mr. Folkman's response is, "Being able to do our own blade grinding, rather than outsourcing this work has cut our costs per part dramatically. This machine will pay itself off in no more than two years. After that, our cost savings will be the same as added profit to help us grow our business even faster."

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