Slicing blanks from centrifugal cast gray iron and ductile iron tubing dulls even the best bandsaw blades. That's what a major automotive parts manufacturer found when it sought to improve production rates. A critical bottleneck was the cut-off operation to make a key part for the company's product line. The answer was a carbide-tipped blade with shock-resistant construction and patented "Triple-Set" tooth design. Compared with earlier blades cutting abrasive tube stock, the Series 3869 Triple Set blade from Sandvik Saws and Tools Co., (Scranton, Pennsylvania) increased output per blade 15-fold. It also cut individual part cycle time by more than 50 percent.
The sawing operation cuts centrifugal cast gray or ductile iron tube stock, depending on the type of part being produced. Both metals are specially formulated for the application, and both are especially difficult to cut. Their abrasiveness slowed production and shortened the life of conventional bandsaw blades. Blade wear quickly compromised the quality of cut pieces. The company tried just about every blade on the market with disappointing results.
The cutting problems didn't end there. Since the stock is tubular, sawblade teeth are subject to twice the interrupted cutting forces as when cutting a solid. The blade is also bathed intermittently in coolant. That means both mechanical and thermal shock as each tooth enters the cut, crosses the void, re-enters the cut and exits. Under such conditions, conventional bimetal and carbide-tipped blades dulled quickly and cause wavy cuts.
The long cutting time for each piece and frequent downtime to change blades had adversely impacted production. "We just couldn't tolerate a three minute, 45-second cycle time just to cut each blank," explains a production engineer. "And getting only 100 or so blanks before the blade dulled was unacceptable." He added, "That rate of wear and tooth breakout would mean shutting down to change blades every 5.75 hours. Besides, the surface quality wasn't there."
The engineer sought a better way to produce the parts. He arranged some trial runs with a sister plant using several types of blades, including bi-metal. Results were not much better, although the bandsaw in the other plant was meeting the quality requirements.
"I was almost at the point where I was considering outsourcing the cut-off operation," the engineer recalls. The sawmaker, HEM, suggested it could help him, pointing out past successes with a carbide tipped blade. A HEM Model H105A bandsaw, fitted with Sandvik Series 3869 Carbide Triple Set blade, corrected the production problems. The 3869 blade used here is 15 feet 8 inches long and 1 1/4 inches by 0.042 with three teeth per inch. "We reduced our cycle time to our goal of between one and one-and-a half minutes. Our parts-per-blade count climbed to 1500," so says the production engineer. Key to the success on such abrasive materials, is the Sandvik Triple-Set configuration—one tooth set left, one tooth unset and one tooth set right. This geometry, combined with carbide blade tips helps account for better performance. The Triple-Set design helps clear chips while leaving a clean kerf about 1/8 -inch wide.
The hard, abrasion-resistant carbide gives the blade its long life. Special bonding keeps the carbide attached to the blade. The blade tooth gullet helps evacuate chips quickly avoiding chip build-up in the center of the tube, where debris could damage the surface.
Now, the company is consistently getting the higher production rate and high quality cuts without premature blade failure. Selecting the right bandsaw and identifying the right blade was key to this critical production step.
The engineer explains, "Despite the operating conditions, we never break blade teeth, and we consistently get straight, smooth-surface cuts. Nor have we experienced metal breakout. Our scrap rate for this operation has dropped to nil."
Stock cutoff is the first machining step in the production of this part. "Since this is where it all starts, we need a reliable flow of stock to keep the rest of the production sequence on schedule," he adds. From here, the blanks go to CNC machines for finishing.
The company's own foundry casts the tubes in 2- to 4-foot lengths with diameters from 3 to 9 inches. Wall thickness ranges from 1/2 to 3/4 inch. The raw cast tubes go directly to the bandsaw after cooling. They find the ideal feed system for this HEM is hydraulic over air, where air provides the cushioning and the two-way hydraulic system the feedstock flow control. Blade speed is 270 sfpm. This combination is designed for easy feeds and high speeds.
This plant runs the cut-off operation three shifts per day, six days per week. At 1500 pieces per blade, the Sandvik 3869 blade lasts about 5.3 shifts, compared with less than a shift before.
"We persisted," says the engineer. "Once we identified a blade that would work, we kept on the case until we arrived at the right combination; blade and machine. Speed, feed and tooth set were all optimized, and our target goal was met."