New Fixture Multiplies Productivity Of Horizontal Machining Center

L.H. Thomson Company (Macon, Georgia) is an aircraft sub-contractor and maker of after-market components for high performance mountain and road bikes. One particular bike component is a line of aluminum handlebar stems. Sixty-four versions of the stem are made, combining different lengths, rise angles ranging from –10 and +15 degrees and two different handlebar diameters. All versions require machining on all six sides. Each end of the stem has features that must be parallel with or perpendicular to its centerline. Traditional processing of the component required two operations. The setup for batch processing of each stem version and time expended in load/unload for the second operation created a production bottleneck that became more critical as market demand for the part took off.

Company president Ronnie Thomson and vice president Randy Tolbert attended a major machine tool show to search for a solution to their bottleneck. No commercially available package they could find could combine their operations into a single setup.

After their fruitless search, Mr. Thomson and Mr. Tolbert conceived of a new fixturing approach that can eliminate work handlings for second operations. It lets a standard A-55E horizontal machining center from Makino (Mason, Ohio) tackle five-sided machining on eight work parts in a single setup.

Development of the fixture details was the work of L. H. Thomson design and process engineers. Mr. Thomson ultimately called on sales engineer George "Bud" Cowan of Beckman Precision Inc. (Greer, South Carolina) to help integrate the new fixture into a horizontal machining center system. Mr. Cowan coordinated the efforts of the Beckman Precision applications support team, a custom hydraulic systems supplier and Makino with the A-55E horizontal machining center.

The company's patent-pending fixture is based on a high precision, four-sided tombstone design. It is mounted on the A-55E's standard rotary contouring table. Each side of the tombstone incorporates a rotary indexing fixture that holds two work parts. This arrangement provides full CNC contouring when rotating around the vertical axis and hydraulically actuated indexing around each horizontal axis. With these combined motions, all sides of each workpiece are accessed in one operation, even the sixth side (the back of the work part) for true "done in one" machining.

The A-55E control provides master program and sub-program functionality that lets one program address all 64 stem versions. The operator enters three parameters to program the machine for each specific stem.

Repeatability of the fixture is within 0.0001 inch at 8 inches from the indexing fixture's centerline. The hydraulic system that produces fast fixture indexing also provides a positive in-position verification using a pneumatic sensing circuit. Integrated design places all the actuation hardware inside the 10-inch by 10-inch tombstone, with power supplied through a rotary coupling mounted at the top. Open access for loading the fixtures is from the front of the machine. An after-market light curtain disables automatic functions during load/unload. Dual palm buttons provide the safety interlock to enable fixture motions.

Machining operations include drilling of blind holes as large as 1.113 inches in diameter and up to 2 inches deep. Integral shank drills from Kennametal turn at 14,000 rpm and feed at 168 ipm, finishing the deepest holes in under 0.7 second. To keep chips clear and temperatures low, the A-55E is equipped with an optional 1,000 psi coolant system. Deburring of the complex cross-holes, threaded sections and exterior is performed in the fixture using a carbide ball deburring tool in the A-55E spindle.

High speed milling on the stems uses a ¾-inch end mill at 14,000 rpm. It feeds at 400 ipm, cuts at 2,748 sfpm and is completely buried when cutting around the ends of the stem, actually cutting off a complete piece of stock, not making chips, and creating a finish that requires no polishing.