Faster Milling in a busy forging die shop

Faster Milling in a busy forging die shop

Milling cutter technology cuts time in skin milling and cavity milling

A Hi Pos Deka tool in process and making chips.
A Hi Pos Deka tool in process and making chips.

In the captive die shop of any large forging operation, the pressure is always on to recondition worn dies faster to return them to duty sooner.

Much of the pressure falls on milling operations that account for the bulk of machining time in this environment.

So, when the Crosby-Lebus die shop found ways to speed up milling of the deck surfaces on dies by 10 to one, and to speed up the milling of die cavities by two to one, the improvement in turnaround time boosted the entire plant’s capacity and speed of delivery.

The Longview, Texas, plant builds large hoists and cranes that are designed to handle loads to 300 metric tons (330.69 tons).

The company’s 25-man die shop runs 10 hours a day, 5 days a week — and sometimes 10 hours a day, 6 days a week — to keep up with demand. Die sets measure anywhere from 12 in. by 12 in. to 50 in. by 60 in., and typically are made of Finkle #2 forging die stock, hardened to Rc 40.

“Hot steel erodes and enlarges the cavities over time, so we take material off the mating faces to create enough stock in the cavity to re-machine,” Buddy Walston, programmer for Crosby-Lebus, said.

Fresh Ideas
The gains the shop recently saw came directly from a two-level partnership with Ingersoll Cutting Tools. First, Ingersoll’s field rep Ernie Schooley walks the plant regularly to bring in fresh machining ideas and problem-solve. “We call it ‘Schoole-ing,” Walston said. “It seems like Ernie takes us to his private productivity school here, and it pays off.”

Comparison diagram: silhouettes of ball nose and Chip-Surfer bull-nose cutter.
Comparison diagram: silhouettes of ball nose and Chip-Surfer bull-nose cutter.

Second, the company’s programmers and machinists often attend tooling seminars at Ingersoll’s headquarters in Rockford, Ill.

Not Broke, but Fix It Anyway
Actually, Crosby-Lebus’s tenfold improvement in deck-surface reconditioning arose from a problem the company didn’t know it had. The shop was satisfied with the 15-ipm feedrates it was getting with a conventional zero-rake indexable finishing face mill, even though Schooley had given it a Hi-PosDeka face mill, new at the time, for evaluation in March, 2008.

Crosby-Lebus began a general evaluation to see what the new cutter could do, but never pushed the feed or targeted an application. Then, six weeks later at a seminar in Rockford, CNC machinist Sidney Maxwell saw the same cutter feeding at 150 ipm on the same type of hardened stock that he was running in Longview.

The day they returned from the seminar, Maxwell and Schooley set up a trial on a live repair job and got the same results — a tenfold gain on an operation they thought was running fine before.

That was with the same cutter and pitch as before: 2 in. diameter, five inserts.

The difference was in the cutting-edge geometry.

A bull-nosed Ingersoll Chip-Surfer doubled the metal-removal rate for forging die cavities at Crosby-Lebus. Previous tool was a solid carbide ball mill.
A bull-nosed Ingersoll Chip-Surfer doubled the metal-removal rate for forging die cavities at Crosby-Lebus. Previous tool was a solid carbide ball mill.

Ripple Effect
“Retooling this skin milling process triggered a widespread improvement in the die shop and beyond, since every die set in for repair undergoes this process,” Walston said.

Maxwell and Schooley tweaked the operation to optimize throughput and edge wear. The new standard settings are 150 ipm, 850 rpm, 0.035 in. depth of cut, compared with the previous parameters of 15 ipm, 650 rpm and 0.100 in. depth of cut.

“We traded some depth of cut for an order-of-magnitude gain in feed: I’ll take that any day,” Walston said.

For example: Skin milling a typical mid-size (20 in. by 20 in.) die set used to take 90 minutes. Now it’s done in 10 minutes.

Geometry Makes the Difference
Here are the differences in insert geometry that enabled the higher removal rates.

“Although the cutters were the same size and pitch, the Hi-PosDeka cutter has a positive rake, not zero as before,” Schooley said.

“Moreover, a large wiper flat behind the cutting edge ‘burnishes’ the newly machined area to maintain good surface finish despite the higher feed. And, the inserts are thicker than the ones used before, and therefore stronger,” he added.

The inserts also provide 10 indexable cutting edges, which contributes to their “Deka” name, versus four cutting edges on previous inserts. That also helps to reduce insert inventory requirements.

As for tool life, Walston could not be specific. As of July, he said the shop was continuing to run with the original set of inserts Schooley provided.

Faster skin milling also created the potential to redistribute the operations over equipment in the shop to improve economics and turnaround still further. Previously, Crosby-Lebus did skin milling on the same CNC machine that did subsequent operations.

Now that skinning runs so fast, and is required on every die set, Walston moved that operation to a separate machine.

“This will free up time on the bigger CNC, and streamline the entire reconditioning cycle,” Walston said.

Second Step: Faster Cavity Milling
However, the story doesn’t end there.

Another walk-through a couple of weeks later led to retooling the other operations. That halved the cycle time and eliminated subsequent hand polishing.

Originally, Crosby-Lebus re-cut the cavity with a standard solid carbide ball mill, followed by an hour of manual polishing.

It changed to a -in. Ingersoll Chip-Surfer “high-feed” cutter that has a replaceable tip. That tool also was demonstrated at the March seminar.

Maxwell, Waltson and Schooley ran the trials with it in May, and made it standard tooling in July. Applications included gutters and flash lands as well as the cavities themselves.

With the previous -in. ball mill the shop used, it ran at 30 ipm at 1,240 rpm, with a 0.030 in. to 0.050 in. stepover.

With the new -in. cutter, Crosby-Lebus runs at 50 ipm feed, with a lower stepover.

And, to Walston’s surprise, the as-machined finish proved good enough to eliminate the polishing.

“Mind you, we’re talking surface finishes on forging dies for hoist and crane parts, not precision plastic molds,” Walston said.

“As an example, one of our forgings is a 40-in. crane hook with 300 metric ton capacity rating,” he said.

It’s All in the Silhouette
What accounts for the Chip-Surfer’s higher machining rate is that it uses an extreme lead angle on the face that capitalizes on chip thinning, Schooley said.

The robust corner blends into a backdraft angle so there are less radial forces to cause vibration. Finally, the tough alloy shank withstands shock and side deflection better than the more brittle solid carbide cutters, he said.

And, when it’s time to change tips, the machinist does it in the spindle. That saves time and preserves datum references. Schooley said that is made possible by the Chip-Surfer’s 0.0005-in. repeatability on height and diameter.

“The tool design also recognizes that efficient cavity milling is essentially side milling, which involves higher lateral forces. The tough alloy shank handles lateral forces that would snap off a solid carbide cutter,” Schooley said.

Additional information on these cutting tools can be found at www.ingersoll-imc.com, or by contacting Ingersoll Cutting Tools, 845 S. Lyford Road, Rockford, IL 61108-2749. Telephone 815 387 6600.

This article was prepared by Ardrey Inc. for Ingersoll Cutting Tools.

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