January 2008 Edition
SPOTLIGHT: CUTTING TOOLS
Not Such a Screwy Idea
Corkscrew milling is both a time and energy saver for a Southern die maker
Corkscrew milling with the Ingersoll Hi-Pos+ indexable endmill cuts holemaking time in half at Vaughn Manufacturing Co.
There's a lot of talk about "thinking outside of the box," but sometimes thinking becomes so canalized that people might not realize that there's even such a thing as a box. For Nashville diemaker, Vaughn Manufacturing Co., taking a look at holemaking alternatives without thinking about drills saved the company money and energy.
Vaughn sinks 2" and 3" guidepin holes in its diesets in half the time as before, saving more than $100 per dieset in the process. Vaughn stamping diesets are used to make appliances, HVAC equipment, and patio products. Every dieset has at least four pairs of guidepin holes 2" to 3" in diameter and 2" to 3" deep.
Corkscrew milling rather than spade drilling was the outside-of-the-box solution. Vaughn uses an Ingersoll Hi-Pos+ indexable endmill smaller than the hole diameter, and programs the machine to plunge the cutter into the material as it orbits.
Power Miser
"Corkscrewing uses less power, too," Thomas Austin, Vaughn plant manager who spearheaded the process change, said.
The changeover began when Austin started looking for ways to speed up the guidepin hole drilling operation, which is required on every dieset. He noticed a trade magazine article about how a Texas forging company solved a similar problem by corkscrew milling with an Ingersoll endmill. So he called Ingersoll's Paul McCown to arrange a test in the Vaughn shop.
In corkscrew milling, the cutter orbits into the work as it rotates, opening the hole much faster than drilling. The hole size depends on the program, not the cutter size. This reduces drill inventory requirements because a drill for every hole size isn't needed.
"All you need is a low-horsepower CNC mill with orbiting capability and the Hi-Pos+ indexable endmill," McCown said.
A test was run on Vaughn's Fadal vertical CNC mill programmed for an orbiting cut, opening a 2-1/2" diameter hole in 2-1/2" mild steel. The endmill was 1.25", about half the required hole size, providing clearance for chip evacuation.
Settings for the roughing pass were 800 sfm and 66 ipm, creating about a 0.080" advance per orbit. For finishing, the orbit was expanded 0.010" radially to enlarge the hole to final size. Total time for roughing and finishing: less than half as much as before.
"Hole roundness and finish were well within spec as well," Austin said.
Based on that success, Vaughn standardized the corkscrew milling method and Ingersoll tooling for all guide pin holes as well as all holes larger than 2-1/2" in the die patterns themselves.
Bigger is Thriftier
"The bigger the hole, the greater the savings," Austin said. "First, we don't have to open up the hole in diameter steps as we did with drilling big holes. Second, material removal speed isn't limited by machine horsepower, as often happens when drilling big holes. With corkscrew milling, the cutter nibbles off a lot of tiny chips in rapid succession rather than hogging out one heavy chip all at once."
Vaughn's conventional practice involves corkscrew milling the rough pass only, then either boring or corkscrew milling the finish pass.
Vaughn sinks 2" and 3" guidepin holes in its diesets in half the time as before, saving more than $100 per dieset
"When we have more experience with the corkscrew practice, we may use it for both passes," he said, "but for now we still lean toward boring the finish cuts."
Austin figures he saves about $20,000 a year in guidepin hole production alone by the process switch. Corkscrew milling the other holes in the stamping tools themselves saves Vaughn about as much again.
Payback
"For sure, the tooling paid for itself by the third dieset we did the new way," Austin said.
Corkscrew milling gets its name because the cutting face generates a corkscrew toolpath. It involves simultaneously feeding on all three axes: advance on the Z-axis while interpolating on the X- and Y-axes to enlarge the hole. It's a step beyond orbital milling, where the Z-feed is done separately from the X-Y interpolation.
The cutter's centerline follows a helix. It's a programming step up from orbital milling, where the machine plunges to depth on the Z-axis and then interpolates.
Because there is only a small contact area between tool and workpiece at any instant, cutting forces are much lower than in spade drilling. And friction between drill flutes, chips, and sidewall of the hole is sidestepped.
The Ingersoll indexable mill works because the edge of the inserts are manufactured to trace a true helix with respect to the cutter's centerline, leading to straight sidewalls and a 90° bottom.
Insert geometry also promotes free cutting with low cutting forces. Results are similar to that of a solid carbide endmill. By contrast, kinematically, conventional square inserts following a helical path cannot create a square corner at the bottom of a blind hole, and can't avoid leaving lap lines in the sidewalls. Ingersoll Cutting Tools
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