|Deering Machine rough-mills steel die parts at least five times faster than before using Ingersoll Form-Master high-speed high-feed mills. Feedrates reach 127 ipm on annealed die steels. |
Corkscrew milling involves simultaneously feeding on all three axes: advancing on the Z-axis while simultaneously interpolating on the X and Y axes to enlarge the hole. The process, requiring a machine with interpolating capability, is said to out-perform spade drilling, progressive twist drilling, trepanning and flame cutting. Proponents say it often produces a finish good enough to eliminate finish boring.
“Because there is only a small contact area between tool and workpiece at any instant, cutting forces are lower than in spade drilling,” said Konrad Forman, product manager at Ingersoll Cutting Tools. “And, gone is the friction between drill flutes, chips and the sidewall of the hole."
|Logan Machine Works opens big through-holes from solid stock using corkscrew milling with an Ingersoll Form-Master+ button face mill for a cycle-time savings of 16 to 1. |
“Whereas orbital milling is basically a two-step process, plunge then orbit, corkscrew milling is continuous. The centerline of the mill advances helically, resulting in a faster cut and better finish with no lap lines. Hole size depends on the program, not cutter size. This reduces drill inventory requirements because a drill for every hole size is no longer needed.”
The tool of preference for corkscrew milling at many shops is the Ingersoll Hi-Pos+ indexable mill or the Form-Master+ button face mill. The former is recommended for shallower holes and the latter for deeper ones. Surface quality with the Hi-Pos+ is often good enough to eliminate finish boring, even on guidepin holes.
Both cutters work well for corkscrew milling because the insert’s helical cutting edge geometry, when following an interpolated corkscrew path, leads to straight sidewalls and a 90-degree bottom. “By contrast, conventional square inserts following the same path can’t create a square corner at the bottom of a blind hole, and can’t avoid leaving lap lines in the sidewalls,” noted Forman.
The insert’s helical cutting edge also creates a gentler entry into the work, much like scissors shearing paper. The result is the kind of finish obtained from a solid carbide endmill.
“The main caution in any corkscrew milling operation is to provide an independent source for chip clearance in vertical holes opened from the solid,” Forman advised. “Because there is no provision for chip evacuation in the insert geometry itself, gravity often can do it in horizontal holemaking, but you’ll probably need compressed air on vertical holes.”
Field experience at scores of mold and die shops has shown corkscrew milling also eliminates a lot of extra setups and machine-to-machine transfers, which are especially expensive on large workpieces. “With conventional drilling, we’ve seen a lot of mold and die makers forced to shuttle heavy workpieces between heavy-duty drill presses and jig borers because of the horsepower required for the drilling operation. With corkscrew milling, the whole friction issue is eliminated and you can open a bigger hole on a lower horsepower machine. And, if finish boring is needed, you can grab the part once and do both operations on the same machining center,” said Forman.
Reports from the field
Here’s a look at how some shops have benefitted from the technique of corkscrew milling.
Logan Machine Works, Phenix City, AL, makes about sixty trunnion blocks a year, each with several through holes of different sizes. Previously, Logan opened the larger holes with a pilot drill, flame-cut it to leave … in. of stock, then bored it to final size. Now, the shop opens four-inch diameter holes in large machine components in 30 minutes of machining, down from eight hours of drilling or occasional flame cutting. Moreover, it mills and finish-bores the hole on the same machine, eliminating two setups and two part transfers.
Logan mills workpieces from solids with the Form Master+ at 800 sfpm and 85 ipm. For smaller holes and thinner stock (up to … in.), the previous cycle was pilot drill, laser cut and bore. Now, it corkscrew mills with an Ingersoll Quad Drill+ at 650 sfpm and 8 ipm, reducing a one-minute cycle to ten seconds and eliminating all boring. In addition to speeding up the operation, the Quad Drill+ nearly doubled insert edge life. The shop had toyed with another end mill in an orbital milling process, but without much success.
Deering Machine, a one-man shop in rural Georgia, is saving 15 minutes per hole in a variety of work involving 1- to 1.5-in.-diameter holes. “Corkscrew milling also requires less horsepower than progressive twist drilling, our previous standard process,” said president Steve Deering. The shop’s previous standard practice was to twist-drill, using progressively larger diameters. Ingersoll’s John Spray recommended corkscrew milling, relatively new at the time, and demonstrated it penetrating solid stock. As a result of the demo, he projected a 15-minute cycle time saving per hole.
“I saw the demo but didn’t believe my eyes,” said Deering. “Who in their right mind plunges a face mill into solid stock, let alone runs it in like a corkscrew?”
Now corkscrew milling is standard practice for all larger holes. Deering uses 0.625-in. Hi-Pos+ for holes up to 1.125 in. and a 0.75-in. mill for larger holes. Feedrates in both cases are 61 ipm.
A large Texas forging plant corkscrew mills all of its guidepin holes, cutting an eight-hour job per dieset to just 1.5 hour. After spade drilling, its previous standard process, the holes needed finish boring; however, by corkscrew milling the finish boring operation was eliminated, resulting in an expected annual savings of more than $35,000 on guidepin holes alone.
|In corkscrew milling, a cutter advances into the work as it orbits, opening the hole much faster than drilling, and reduces friction as well. |
To compensate for thermal expansion of the bottom die, which can cause misalignment in the diesets over the course of a day, the company created a slot rather than a perfectly round hole. The slot is about 0.005- to 0.010-in. longer than it is wide. “Corkscrew milling enabled us to improve the dieset design as well as save time,” said the manager. “We couldn’t do that with a spade drill.”
Vaughn Manufacturing Co. in Nashville estimates it is saving $100 per dieset on stamping dies for appliances and patio products. Holes average 2- to 3-in.-diameter by 2- to 3-in. deep. Previously, the company relied on spade drilling. Vaughn plant manager Thomas Austin said, “The bigger the hole, the greater the saving. First, we don’t have to open up the hole in diameter steps as we did with twist drills. Second, material removal rate isn’t hobbled 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 essentially one heavy chip all at once,” explained Ingersoll’s Konrad Forman. “The new process takes a lot of the friction out of large holemaking. By contrast, spade drilling a large hole can easily stall many modern lowpower CNC machining centers.”
As a rule for hole-size-to-cutter diameter, Vaughn standardizes on 2 to 1. For instance, for a 2.5-in. hole, the choice is a 1.5-in. Hi-Pos+ cutter. According to Forman, “This relationship is a matter of shop preference, but 2 to 1 leaves plenty of room for chip evacuation. Also, for straight sides and square corners, be sure the inserts have positive rake geometry axially and radially.”