October 2007 Edition

CUTTING TOOLS

Taking a Bite Out of the 'Sandwich-Cutting' Costs

Milling a layered mold base of different metals and hardness was causing problems until the company found the tooling solution it needed

Precision Mold Base Corp. — PMBC — wasn't getting the results it needed when cutting stacked layers of different metals for molds.

"Rough-milling that metal sandwich used to create a lot of thumping and scraping noise," using conventional zero-rake button cutters with five inserts, Romney D'Antuono said.

The sandwich is an integral part of the custom mold base the company makes for moldmakers across the country. D'Antuono, foreman at PMBC, Tempe, AZ, supervises a shop of 30 employees that runs 24/7, including lights-out operations.

"Our mold base assemblies are built like a sandwich, with mild or stainless steel as the bread and 4140 steel at 33 to 35 RC as the filling," he said. "The operation is like trimming off the bread — a very difficult crust."

PMBC has eight CNC mills and four surface grinders, each selected for accuracy and ease of duplication of the finished mold bases.

"It is the most abusive milling operation in the shop. The trick was to choose tooling that handled all aspects of the abuse at once," he said.

It was slow going, using the button cutters, and inserts often broke in mid-cut.

Not Much to Show

"We have a shelf full of wrecked and burnt-out button cutters to show for it," D'Antuono said.

"With 10 edges per insert, we're getting 20 shifts of life per insert — compared with perhaps half a shift per insert before."

In late 2006, he switched to a positive-rake Ingersoll Form-Master+ button face mill. D'Antuono said he saw immediate improvement: material removal rate more than doubled and insert edge life stretched to at least two shifts. The thumping and scraping quieted to a purr.

Increasing the speed of the outside milling process was crucial to over-all profitability; that single operation represented a major portion of the machining cost of each mold base, and was required in every job. Making the operation more secure would qualify it for the lights-out shift. Such a switch would increase efficiency and improve customer delivery speed.

The process involves rough-milling four sides of the exposed stack of individual plates. During the cut, the milling cutter first encounters several layers of dissimilar metals at once, with different hardness and machinability—plus interrupted cuts, irregular surfaces, and uneven depths of cut—DOC.

But, the challenges didn't end there. All around the 4140 and 420 stainless steel plates the hardest metal in the package — are gaps in which features have been machined prior to asembly. In the finished mold, those features provide pathways for hot runners, coolant, heaters, sprues, bolts, release pins, aligning pins, and other features. Each of those features was a problem that could wreck the tool or trigger a chatter cycle that could wear out an expensive machine long before it reached its return on investment.

"I call it a hyper-interrupted cut," he said.

The last problems in making the cuts were the plates' size variations, which varied the DOC from a nominal 0.060" to 0.250" in an instant.

With the 4" diameter button face mill that PMBC used, the best the company could get was 25 ipm at 550 rpm, at a 0.060" DOC, with a 3.7" radial engagement. Even at those speeds and feeds, inserts failed so often, and so unpredictably, the term "average edge life" became meaningless. The operation was so unstable, an operator had to check it regularly and shuttle back and forth to the tool crib for indexing. Lights-out operation was out of the question.

Too Slow for Payback

"If we did try it lights-out, we'd have to slow down so much that it wouldn't pay," D'Antuono said.

Thinking there must be a better way, D'Antuono began investigating. His research paid off. Now, the operation runs 2-1/2 times faster, inserts last four to five times longer than the previous average, and catastrophic insert failure is just a memory. With the increased depth of cut, the volumetric removal rate is actually up by seven to one. PMBC runs at 55 ipm and a 0.190" DOC.

Based on experience, PMBC projects an annual saving of more than $120,000

D'Antuono replaced the button cutters with Ingersoll Form-Master+ face mills. The change in the cutters' rake angle made the difference. The Form-Master+ face mill presents the cutting edge at a positive angle, which cleaves the metal rather than scraping it off.

"You can tell the difference by listening," D'Antuono said. "The old operation sounded like pounding and scraping. It purrs because of the difference in cutting action."

"Inserts now wear gradually, over a couple of shifts, to a predictable end-point," D'Antuono said.

Reliability increased to the point where it could be done as a lights-out operation without slowing down more than 10 percent.

Material removal rate more than doubled and insert edge life stretched to at least two shifts

The Ingersoll 4" cutter has seven inserts, each with up to eight cutting edges. The cutter settings are now 55 ipm, 440 rpm at a 0.190" DOC. Although the chip load is much higher, the cutting action is smoother.

 You Get What You Pay For

 D'Antuono had to swallow his concern over sticker-shock once he saw how the Form-Master+ could perform.

 "We took our problem to George O'Gar, an Ingersoll application specialist," D'Antuono said. "George suggested the Form-Master+, but I balked at the price. The cutter cost about 30 percent more than the one we were using, and the inserts ran 10 percent more. Allowing for the additional edges on every insert, I figured I'd have to run 30 percent faster and get 30 percent better edge life," what D'Antuono called his "30/30" break-even point. "Given the gruesome cutting conditions, it didn't seem possible."

 O'Gar brought a test cutter to PMBC. D'Antuono chose a low-risk job on which to experiment. He selected a mold base 18"×12" and 12" thick, and set it up on a 35 hp Mazak Ultra 650 horizontal CNC machine with a 50-taper spindle, one of the four of the company's robotized FMS systems.

 The 420 stainless steel center was between 4140 and A36 steel plates 2-1/2" thick. To deal with machining three different metals at once and the severe interrupted cuts, O'Gar used an IN2015 insert grade.

 With conservative settings — around 40 ipm — material removal ran about twice as fast as before. There was no insert failure. Results surpassed D'Antuono's 30/30 break-even point.

Tweaking the System

 Further adjustments increased feedrates up to 55 ipm at 440 rpm, the PMBC standard. DOC depended on the size variations of the layers, to avoid going deeper than 0.250".

 "If the edges mate well, we'd ramp up the depth of cut," D'Antuono said.

 "The new Form-Master+ mill is freer cutting," O'Gar said, "and has a radius that cuts a wider chip and takes advantage of chip thinning. There's optimum cutting action all the way to the bottom of the tip."

 He said the longer-lasting edges stem from the combination of reduced cutting forces plus the absence of chip breakers on the top face.

 "Get rid of the chip breaking bumps or grooves, and you get rid of a lot of stress-raisers that cause the edges to break," O'Gar said.

Now, operation runs 2-1/2 times faster, inserts last four to five times longer

 PMBC completed the retooling in late 2006 and, despite almost daily use, hasn't had a catastrophic edge failure.

 "Edge life is now purely a function of wear," D'Antuono said. "We index the cutter about once every other shift. With 10 edges per insert, we're getting 20 shifts of life per insert — compared with perhaps half a shift per insert before."

 Based on experience, PMBC projects an annual savings of more than $120,000. Ingersoll Cutting Tools

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