RIPPLE EFFECT

In the captive die shop of any large forging operation, the pressure's always on to recondition worn dies faster, returning them to duty sooner. Much of the pressure falls on milling operations, which account for the bulk of machining time in this environment.

So when Crosby-Lebus's (C-L) die shop found ways to speed up milling of the dies' deck surfaces by ten to one and cavities by two to one, the improvement in turnaround time actually boosted the entire plant's capacity and speed of delivery. The Longview, TX plant (part of a global company) builds large hoists and cranes able to handle loads up to 300 tons. Its 25-man die shop runs 10/5 – and sometimes 10/6 – to keep up with demand. Diesets measure anywhere from 12 x 12 in to 50 x 60 in, and are typically made of Finkle #2 forging die stock, hardened to Rc40.

"Hot steel erodes and enlarges the cavities over time, so we take material off the mating faces in order to create enough stock in the cavity to re-machine," explains C-L programmer Buddy Walston.

These recent gains stemmed directly from a two-level partnership with Ingersoll Cutting Tools. First, Ingersoll field rep Ernie Schooley walks the plant regularly to bring in fresh machining ideas and problem-solve. "We call it 'Schoole-ing'," quips Walston. "Seems like Ernie takes us to his private productivity school here – and it pays off."  Second, the company's programmers and machinists often attend tooling seminars at Ingersoll's Rockford, IL, headquarters.

Actually, C-L's tenfold improvement in deck-surface reconditioning arose from a problem the company didn't know they had. They were satisfied with the 15 ipm feed rates they were getting with a conventional zero-rake indexable finishing face mill, even though Schooley had given them a Hi-PosDeka face mill, new at the time, for evaluation.

That was back in March 2008. C-L then 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 Rockford seminar, CNC machinist Sidney Maxwell saw that same cutter feeding at 150 ipm on the same type of hardened stock he was running back in Longview.

The day they returned home, 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 just fine before. This was with the same cutter and pitch as before: 2 in diameter, five inserts. The difference was in the cutting-edge geometry.

"Retooling this skin milling process triggered a widespread improvement in the die shop and beyond, since every dieset in for repair undergoes this process," notes Walston. Maxwell and Schooley tweaked the operation to optimize throughput and edge wear.

The new standard settings are now 150 ipm, 850 rpm, 0.035 in DOC, as compared with the previous parameters: 15, 650 and 0.100 respectively. "We traded off some depth of cut for an order-of-magnitude gain in feed. I'll take that any day," adds Walston. Case in point: skin milling a typical mid-size (20 x 20 in) dieset once took 90 minutes. Now it's done in ten minutes.

Here's a closer look at the differences in insert geometry that enabled these higher removal rates. "Although the cutters were the same size and pitch, this new cutter has a positive rake, not zero as before," according to Schooley. "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."

The inserts also provide ten indexable cutting edges (thus the Deka name) vs. four before, reducing insert inventory requirements. Tool life? Three months into production, Walston couldn't say. "We're still running with the original set of inserts."

Faster skin milling also created the potential to redistribute the operations over equipment in the shop to improve economics and turnaround still further. Previously, C-L did the skin milling on the same CNC machine as the subsequent die sinking. Now that skinning runs so fast, and is required on every dieset, Walston plans to move that operation to a separate machine. "This will free up time on the bigger CNC and streamline the entire reconditioning cycle," predicts Walston.

But the "Schoole-ing" story doesn't end here. Another walk-through a couple of weeks later led to retooling the actual die sinking operation, which halved the cycle time for that operation and eliminated all subsequent hand polishing. Originally C-L re-cut the cavity with a standard solid carbide ball mill, followed by an hour of manual polishing.

The change was to a ¾ in Chip-Surfer "high-feed" cutter with replaceable tip. Maxwell, Walston and Schooley ran the trials with it that May and made it standard practice in July. Applications included gutters and flash lands as well as the cavities themselves.

With the previous standard ¾ in ball mill, the usual practice was 30 ipm at 1250 rpm, with a 0.030 in to 0.050 in stepover. With the new cutter, C-L runs at 50 ipm feed, with much lower stepover. And to Walston's pleasant surprise, the as-machined finish proved good enough to completely eliminate the polishing.

"Mind you, we're talking surface finishes on forging dies for hoist and crane parts, not precision plastic molds," he cautions. "As an example, one of our forgings is a 40 in crane hook with a three hundred metric-ton capacity rating."                         

What accounts for the new cutter's higher machining rate? Unlike a conventional ball mill," explains Schooley, "the cutter's silhouette uses an extreme lead angle on the face, which capitalizes on chip thinning. The robust corner blends into a backdraft angle so there are far less radial forces to cause vibration. Finally, the tough alloy shank withstands shock and side deflection better than the more brittle solid carbide cutters. And when it's time to change tips, the machinist does it right in the spindle, saving time and preserving all datum references. This is possible due to the cutter's 0.0005 in repeatability on height and diameter.

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

The Crosby Group, Inc., 2801 Dawson Road, Tulsa, OK 74110, 918-834-4611, www.thecrosbygroup.com.

Ingersoll Cutting Tools, 845 S. Lyford Road, Rockford, IL 61108-2749. Phone 815-387-6600, Fax: 815-387-6968, www.ingersoll-imc.com.