Slow cutting speeds for large holemaking in ferrous metals
Switch to corkscrew milling with new indexable face mills
Corkscrew milling, new milling tools, speed up holemaking for AC/DC motor manufactuer
Large holemaking in ferrous metals can make for a pretty slow cutting process. Big twist drills take a lot of power and can get expensive and hard to find in odd sizes. Flame cutting is cumbersome, leaves a lot for finishing, and works only on flat plate. Trepanning, with its expensive dedicated tools, is limited to repetitive high volume work. Orbital milling, while faster and more versatile, raises the risk of snapping off the brittle solid carbide endmills like toothpicks.
Recently, Baldor Electric Co.’s Gainesville, GA, plant improved the large holemaking process on one part so successfully that it adapted it for ten other jobs at the same site and is considering it at several other plants. A member of the ABB Group, Baldor-Gainesville has 400 employees and runs 24/6 to produce AC and DC motors ranging from 1 to 1500 HP.
Big holes three times faster
By switching to corkscrew milling with an indexable Ingersoll Hi-Feed Deka face mill for holes over 1.500 in. (38.1 mm), Baldor has cut cycle time by three to one on average and improved tool life by ten to one. Previously, the standard process was orbital milling.
The holes are then finished by boring, as before. The annualized saving from the process change at the plant alone tops $40,000 a year.
It all began in early 2010 when Darimus Glasper, manufacturing engineering technician at Gainesville, started looking around for a faster way to open three-in. (76.2 mm) holes in a ductile iron casting with two-in. (50.8 mm) thick walls. The part is an explosion-proof cover, which encloses the electrical leads in large motors. Annual volume is 100 pieces, done intermittently in 10-piece lots.
“Because of the workpiece geometry, the holes must be drilled from solid in a long-reach setup,” says Glasper. “Not only was it slow going with the orbiting method and the solid carbide tool, we also went through a lot of tools.” Moreover, operators were concerned because the spindle load meter read 90 even at that slow rate.
At the time, Baldor’s standard practice to rough out the holes was orbital milling with a ¾ in. (19.05 mm) four-flute solid carbide end mill. In orbital milling, you plunge about 0.100 in. (2.54 mm) then orbit the tool at a careful feedrate (so it doesn’t snap off as brittle solid carbide endmills are prone to do). Then you repeat the process until you break through, hoping that the resulting slug doesn’t pose a safety hazard or damage the tool. It’s basically start-and-stop. For each hole in the two-in. (50.8 mm) thick walls, it took about 20 steps.
Glasper chose the explosion proof cover for starters because, as one of the plant’s higher volume items requiring large holes, the company looks closely at it in sourcing because of price. “Solve it on the highest volume part, and we get the biggest immediate payback,” he explains.
Looking for Ideas
For ideas he asked Ingersoll’s Chris Pope, who suggested the corkscrew technique using a larger-diameter indexable mill with high feed inserts.
“In corkscrew milling, the cutter advances continuously as it orbits,” explains Pope. “And the high feed insert geometry means you can advance faster with shallower cuts, getting through the material faster while reducing cutting forces and spindle loads.”
Corkscrew milling has been around for a number of years, but until recently was in limited use. In corkscrew milling, the toolpath is helical and continuous, not stepwise as in orbital milling. Principal applications included already-opened holes and thin materials, and horizontal setups where gravity took care of chip clearance.
The idea of plunging vertically into thick solid metal can require a leap of faith.
In another small job shop, the owner saw a demo of the process on guidepin holes in wrought tool steel diesets. He bought the tool on the spot and jotted down the field engineer’s recommended parameters.
Then he ran it in scrap wood for weeks and weeks before working up enough courage to run it on a real workpiece. Once he finally got running, holemaking time dropped by eight to one.
Field trial confirms corkscrew milling benefits
In Chris Pope’s first trial run at Baldor, a three-flute Ingersoll Hi-Feed Deka face mill running in the corkscrew mode completed each hole in five minutes, versu 17 minutes with the orbiting method. For the trial, the cutter helically advanced 0.031 in. per “orbit,” opening the hole in six revolutions, and orbited at 214 ipm verus 12 ipm. Surface speed was 800 sfpm versus 335.
The demo was so convincing Glasper standardized on the corkscrew process within two weeks. “I’ve never seen a hole roughed out from solid that fast in my life,” he says.
Re-engineering the process wasn’t a simple drop-in tool replacement. It involved reprogramming toolpaths and machine parameters as well as re-plumbing the machine to provide through-spindle flood coolant for chip clearance. Pope and Ingersoll southeast manager Phillip Johnson assisted with the parameter setting.
The change paid off immediately. Cycle time to rough out each hole in the cover dropped from 17 minutes to five, exactly as in the test, and edge life improved by 10 to 1. The previous solid carbide roughing cutter lasted through just one hole, while a set of edges on the indexable cutter lasts through ten. “The new method is not only much faster, it’s also much lower maintenance,” says Glasper.
Here’s the math: Each insert has ten edges, yielding about 100 pieces apiece. A new set of three inserts, good for the next 100 pieces, costs much less than a single solid carbide mill that goes dull after only one.
The production gains stem from a total method change based on completely different tool designs, according to Ingersoll’s milling product manager Mike Dieken. “First, the cut is continuous down the helical toolpath, not start-and-stop. Second, the indexable tool is twice the diameter as the solid carbide mill and has a steel cutter body,” he explains. “This makes it much better able to withstand the lateral forces involved inherent in the process at those high MRRs. Moreover, the 70° lead angle helps mitigate the impact and lateral forces at entry.”
He adds that the combination of high feed insert geometry and fewer flutes enables much higher chip loads.
“The 1.5 in. (38.1 mm) indexable Hi-Feed Deka face mill can withstand feed rates of 0.015- 0.040 in. per tooth versus 0.002-0.006 for a .75 in. (19.05 mm) solid carbide mill,” adds Dieken. “And with fewer flutes, there is more room for the chips that come off so much faster.”
The change also removed a wild card, adds Chris Pope. “Since most holes at measure less than three in. (76.2 mm) and the 1.5 in. tool (38.1 mm) covers more than half the diameter, there is no slug to upset things. All the metal exits as chips.”
Over the next several months as orders came in, Glasper applied the same process to other, smaller-volume workpieces.
Today Baldor-Gainesville uses it on about nine different jobs with holes 1.500 in. (38.1 mm) and larger. Many involve odd diameters, for which standard twist drills are hard to find. Soon after, Glasper and commodity manager Doug Bushey shared their experience via Baldor’s weekly idea-sharing conference calls among all plants. As a result, ten other Baldor plants are in various stages of implementing the corkscrew milling method. “The process is tough to believe,” says Glasper, “but the results speak for themselves.” SMT