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Drilling deep in tough
materials If you're generating deep holes in tough-to-machine, high value-added components, you can't afford poor finishes, or worse still, off-spec holes. No one knows that better than Monir Barsoum, tooling manager at Castle Precision Industries, an aircraft landing gear manufacturer. To meet zero defect and higher productivity requirements for deep holes, Castle switched from conventional to tuned boring bars and from the existing indexable drills to more modern deep hole drilling technology. The upgrade led to 50% throughput improvements, 40% lower part cost, 20% tooling inventory savings, and overall straighter, better finished holes.
"Some of our parts easily cost $50,000 to make, and forgings take as long as six months to reorder, so we can't afford mistakes," Mr Barsoum says. "Today's innovations in deep hole drilling make our work easier and faster without compromising hole accuracy, straightness, tolerances, and finish to get the higher productivity," he adds. Castle has recouped its investment in Sandvik Coromant's Single Tube System (STS) deep hole drilling tools and TNS antivib boring bars in less than six months. What's more, deep hole drilling economies are in the order of hundreds of thousands of dollars per year. Castle Precision Industries is an approved supplier for major civil and defense aircraft manufacturers. The company has been making landing gears for commercial and military aircraft since 1973. The Sylmar, CA, facility runs with a staff of 230 employees. Machining operations consist mostly of turning, milling, and drilling. Lot sizes vary from as many as 25 to as few as six components. The plant runs 24 hr, 7 days/week and deep hole drilling runs continuously during that time. Landing gears are assembled from hundreds of components. A representative component, such as a piston, requires short hole drilling on the OD, deep hole drilling on the ID, milling, and pre- and backboring. A typical 300 ksi grade steel piston measures 4´´OD x 3´´ID x 30´´ deep, ±0.010´´-0.015´´ tolerance. After gundrilling, the part is heat treated, raising its hardness from Rc 30 to Rc 52-55, making it tougher to machine. Formerly, deep hole drilling was performed with both brazed carbide and indexable deep hole drills to open and shape holes and conventional boring bars to finish the bore. These deep hole drills were installed on a Laker Craven 100 deep hole drilling machine and the boring bars on three different CNCs: a Mori-Seiki LL7, a VDF 800U, and a Cincinnati Milacron Center 18U, all 50 hp. Maintaining straightness, roundness, diameter, and finish specifications in the deep hole drilling operation was demanding and required multiple tools and setups. First, operators opened the hole with a 21/4´´ dia brazed carbide drill. Then, they turned the ID with a 21/4´´ dia special brazed drill to generate a flat bottom and radius. Next, they counterbored the ID with a special custom-made drilling tool in two progressively larger drill head sizes up to 2.700´´ dia. In addition, to ensure accuracy, operators ran at slower speeds and feeds. Maximum feed
was 1.5 to 2.0 ipr and edges wore out too quickly. Castle operator starts a hole in a landing gear piston with Sandvik's STS deep hole drill. The STS single tube system drills holes up to 100 times the drill diameter. "In deep holes, the deeper you drill, the farther the chip must travel, and the greater the chance of jamming or binding on the way out," explains Mike Shemenski, aerospace product specialist, Sandvik Coromant, Fair Lawn, NJ. "Then there's the problem of getting cutting oil to the drilling face, and the obvious consequences of failure. If you can't get enough coolant to the drill head, you increase the risk of bad chip breaking and chip build-up. The friction between drill, chip, and hole wall can dramatically build up torsional forces and snap the drill." That's when Mr Barsoum approached the Sandvik sales rep for a better way. Mr Shemenski responded by testing Sandvik's STS deep hole drilling system and several different inserts on some axles and other landing gear components. In the first couple of parts, cycle time came down to nearly half. The basic principle in deep hole drilling is to press efficient, balanced carbide cutting edges into the work at high feedrates. The new drill heads have three cemented carbide tips, positioned so that they overlap. Chipbreakers are individually ground or form-sintered in each individual tip to provide uniform chipbreaker width. To index, operators simply twist the head off and replace the bit. No edge measurement or offsetting is necessary for the next edge. Based on the STS success, Castle decided on 14-15 STS drill heads ranging from 3/4´´ up to 21/4´´ dia both brazed-in and T-MAX indexable versions. Edges are brazed-in on the smaller diameters and indexable on the larger ones. For the cutting edges, Castle got the best results with a GC4025 roughing insert for intermittent cutting a GC235 grade, and a CB7020 cubic boron nitride (CBN) grade for finishing the heat treated components. When the STS tooling was in place, operators were trained by Tony Yakamavich, Sandvik
deep hole drilling specialist. Once the tools became operational, operators doubled feed
to 3 to 4 ipr, and they can perform all the operations in one setup with the same tool. As
a result, they cut cycle time for opening and shaping holes to half. And, there is no
problem with tool instability or surface finish. There are hardly any breakages, and
tooling inventory and cost are down by 20% to 30% as a result. Castle Precision Industries maintains zero boring defects in its high value-added pistons for landing gears with tuned carbide boring bars from Sandvik Coromant. Mr Shemenski states that the improved hole finish is the result of rigidity and better chip removal. Since chip evacuation is internal, no groove is required in the tube shank, so the completely round cross section provides excellent stability for depths to diameter ratios of up to 100:1. Internal chip evacuation--through the hollow drill shaft--eliminates all the problems of chip binding and jamming which often occur with conventional deep hole drills. With STS drilling, outside of the tube and the wall of the hole itself. Then the cutting fluid exits through the tubular drill shank, carrying the chips along with it. The boring problem was resolved a few months later, when Mr Shemenski successfully tested Sandvik's newly introduced TNS antivib boring bars. "The improvements were immediately apparent, so we became one of the early converts," says Mr Barsoum. The new pretuned boring bars don't require any shop adjustments. As a result, there are no extra stoppages, and vibration has been eliminated. Bottom line is that the boring operations are now completed in only two to three passes and less than 30 min. "Poor dimensional accuracy is behind us, too, because of the balanced distribution of cutting forces and lower tangential and radial pressures," adds Mr Barsoum. "We hold DOC tolerances of up to 0.0010´´without any effort. The exchangeable cutting heads give us various choices of insert geometries." A full range of tuned boring bar sizes accommodate 99% of the boring operations. Sandvik Coromant, Fair Lawn, NJ, http://www.sandvik.com or circle 393 Originally published in the February 1999 issue |
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