From 12 hrs to 25 mins: giant steps for faster cavity hogging, square-offs follow re-tooling.
Joe Krish, manufacturing engineer at Boehm Pressed Steel, Valley City, OH, recognizes this more than most.
The steps he has taken in the captive die shop of this diversified stamping house have lopped thousands of hours a year out of two mainstay tool and diemaking operations: cavity hogging and squaring-off diestock.
A more agile die shop can punch out a first stamping sooner and deliver the whole lot on a tighter schedule. More importantly, the company can commit to tighter schedules more confidently at bid time, and win more business.
Cavities that once took Boehm 12 hours to rough out are done in 25 minutes. Diestocks for slides, cams, gibs, punch holders, and punch and die bodies are squared off in less than half the time as before.
Annual savings are estimated to top $200,000 a year. Perhaps more crucial, turnaround times for new or modified tooling--and the stampings they produce--are shortened by days or weeks.
Better answers sooner
Krish's solution was to retool the two mainstay milling operations based on the advice of field engineers from Ingersoll Cutting Tools and their distributor, Jergens Inc.
"Reaching out for help early got us up to speed months sooner than going it alone," Krish said. "There are simply too many choices in tooling and machine settings to keep up with ourselves.
"It could have taken us a year of researching and testing," he explained. "Instead, we began to benefit in a matter of weeks."
Krish realized the strategic value of speeding up the front end first.
"In a stamping house, there's more room for speed and cost improvement in the toolroom than in the pressroom, especially if you have a reputation to protect in deep drawing, as we most certainly do," he said.
Boehm prides itself on holding tight tolerances and fine finishes on deep-drawn stampings. On one long-term job for an automaker, Boehm deep-draws bearing pockets to +/- 0.0015".
By nature, deep-drawing dies involve much more development work than piercing and bending. Inevitably, developing deep-drawing tools involves extensive milling.
Boehm's toolroom is a 10/6 operation, supporting a 50-man, 40,000sqft pressroom with presses up to 600 ton. The company also provides welding and assembly services to customers including automotive, aerospace/defense, and medical.
"Sure, the dollar savings stemming from retooling have been nice," Krish said. "What's more important, though, is faster delivery of diesets, quicker, easier modifications, and shorter development times.
"As a domestic company competing against offshore vendors, it's strategically smarter for us to compete on speed and service than on cost," he noted. "And that's what we've done."
The changeover began in the fall of 2007. An analysis in the Boehm die shop quickly revealed that cavity milling and squaring-off accounted for the bulk of machining time, so that's where Krish focused.
Krish called in Craig Novak of Jergens Inc., a full-line supply house, for ideas. Since the problem was milling, Novak quickly brought Ingersoll into the picture because of its track record in milling and application support and recent good results at some other accounts.
Boehm's standard practice for cavity hogging was to run a 3/4" solid carbide ball mill at 2,200rpm/12ipm/0.100" DOC and 1/4" stepover on a Hurco VMC.
Recognizing that the machine had power and rigidity to spare, Ingersoll field engineer Craig Whitaker suggested switching to an Ingersoll PowerFeed+ mill, and adopting the "feed fast, cut shallow" approach that has worked well on today's low power machines.
They set up a test on a 12" x 12" x 6" -inch-deep drawing die in D2 die steel. It provided a suitable comparison. As part of a development project, Boehm earlier hogged out an identical piece with the old tool, and it took eight hours to complete the cavity.
Faster by 40:1
Whitaker worked with CNC operator Doug Sunkel to run the new die using a 1 1/2" PowerFeed+ cutter, and establish parameters. They gradually worked up to these settings: 1,850 rpm/350ipm/0.030 DOC with a step-over of about 75 percent of the tool diameter, or a little more than 1". The piece was completed in 12 minutes flat--about 40 times faster. The inserts showed no signs of wear.
Job No. 2 was to improve throughput for the squaring-off operation--essentially rough and finish face milling--which is the mandatory first machining step on virtually every die component Boehm makes. The same team began this retooling the following April.
Workpiece materials include as-wrought F7, A2, D2, and 4140 steel as well as hardened stock. This is much lighter-duty work and varied, so Boehm generally ran them on a 2hp Bridgeport with a variety of cutters, depending on the application. They included solid carbide end mills and a 1", 2-flute, negative rake indexable mill.
Standard settings with the 2-flute cutter were 1,000rpm/5ipm/0.100" DOC. This produced an average volumetric metal removal rate of 0.5cu. in/min.
"There is much less power and machine rigidity available here and a substantial diet of hard metal machining on die repairs, so we need a very versatile, very flee-cutting tool," explained Whitaker. Nevertheless, for the lion's share of this work, he recommended a 2-inch 5 flute positive rake 45[degrees] MultiCut with integral R8 shank.
"It's twice the diameter as before," Whitaker said, "but the free cutting geometry of the new tool holds down the cutting forces even at higher removal rates. Pulling that many cubes off a Series 1 Bridgeport is impressive."
8-to- 1 gain
The team then tweaked parameters to arrive at this new standard: 900 rpm/20 ipm/0.100" DOC for the wrought stock (0.050" for hardened stock). This was as fast as they could go before the Bridgeport began to stall out.
Even so, the retooling raised the volumetric removal rate for wrought stock to 4 cu.in./min, an 8-to-1 gain, with a proportionate improvement on hardened stock. At these rates, the inserts lasted five times longer than before.
Boehm now handles the bulk of the Bridgeport workload with that single MultiCut cutter, reducing tool inventories. They use an IN 2005 insert for the wrought stock and hardened stock.
In both cases, the "feed fast/cut shallow" approach proved out. A bigger tool was used, but their free-cutting geometries cleave off material rather than scraping it off, actually reducing cutting forces.
"Both operations feed faster but actually run quieter," said Novak. "That's a sure sign of higher cutting efficiency."
Tool geometry makes the difference
Why such a gain in removal rate in the cavity milling operation? "It's all in the cutter geometry," Whitaker explained.
"First, the bigger cutter on a powerful machine simply cuts faster and handles higher cutting loads," he said. "Second, the indexable cutter design is simply stronger and tougher all around than a solid carbide ball mill.
"Because all the cutting takes place at the pitch circle, all the cutting action takes place at optimum surface speed," Whitaker said. This is not so with a ball mill where surface speed varies and only the equator area is cutting optimally.
Whitaker added that the Power Feed+ insert's large nose radius and helical cutting edge combine to create a freer cutting action and put more of the cutting heat into the chip and not into the cutter or workpiece.
"With a helical edge, the cutting action is more like scissors cutting paper, a small section at a time," he explained. "The whole insert doesn't slam into the work at once, as happens with straight cutting edges.
"Finally, large gullets behind the seat pocket provide ample room for chip clearance even at 15-fold higher feed rates," Whitaker said. "But the biggest contributor to higher feed is the large radius that causes a massive chip thinning effect, which is very desirable."
Chip thinning is the base principle for all high feed milling. At a shallow DOC, the large radius causes a massive lead angle which generates the chip thinning.
Hybrid cutter body
The new MultiCut cutter, now the workhorse tool on the Bridgeport, owes its effectiveness to a hybrid cutter body design and free-cutting inserts with a large active face. The cutting edge is unencumbered by clamps, for better chip flow at high feed rates. The result is a rough-and-finish face mill that doubles as a chamfer tool.
"Also, it accommodates several insert grades to match the variety of workpiece materials and machining conditions," Whitaker said.
Ingersoll Cutting Tools, www.rsleads.com/905tp-186
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|Title Annotation:||re-tooling strategies|
|Publication:||Tooling & Production|
|Date:||May 1, 2009|
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