September 2008 Edition

CUTTING TOOLS

Test Anxiety

Rough milling 15-5 PH stainless gave a shop the fits, so it called in its suppliers to see what solutions they offered. Each offering was tested and, surprisingly, the shop chose the second-fastest cutting tool.

When faced with cutting an increasingly-popular stainless steel stock, a shop brought in a variety of its suppliers to see which could "pull the sword from the stone." After extensive tests, it chose the cutting tool that came in second-best. Here’s why.

The operation at issue was rough milling of mold cavities/cores in 15-5 PH stainless steel mold stock, hardened to R_C 40. The alloy owes its popularity to excellent polishability and corrosion resistance at the finishing stage. But, at the roughing stage, it’s tough to cut. It’s abrasive, gummy, and heats during cavity roughing, which often leads to sudden cratering failures at the cutting edge. For that reason, Minco Tool and Mold Inc., Dayton, OH, couldn’t risk running the operation unattended.

Minco, a mold and diemaker that operates on either a 24/5 or a 24/7 schedule, depending on customer volume, has 85 employees.

Thumb Rules

A familiar rule of thumb says that on the typical machining job, machining and labor time represent about 75 percent of total part cost, with tooling accounting for only about three to five percent.

"That rule may apply for production parts in softer machinable steels and non-ferrous metals," Glen Mullins, Minco procurement manager, said, "but, a key exception is hard-metal machining. In fact, for cavity roughing this hardened 15-5 PH stock, the tooling that proved optimal represented more than a third of total machining cost."

The facemill of choice for this job, an Ingersoll Power-Feed+ mill, wasn’t the fastest tool, although it came within 10 percent of the fastest. Its tooling cost proved to be two-thirds lower than the nearest competitor, which in dollar terms, more than offset the speed gap.

The retooling effort began when Mullins headed up Minco’s effort to clear up this bottleneck operation on such a promising mold cavity/core material. The original tooling was a four-flute indexable face mill, with each insert providing three edges. It worked fine on a variety of other metals that the shop ran, but not on hardened 15-5 PH. Even when process engineers lowered the feed rate and took a shallower cut, the operation still ran too slow and wrecked too many inserts prematurely – and unpredictably – due to edge cratering.

What to Live With

"We could have lived with the lower removal rate, but not the insert replacement costs," Mullins said.

Contributing to the expense was the fact that cratering on a single edge often rendered the entire insert worthless.

"If the failure mode were edge wear, we could index the insert because the other edges would still be good," Mullins said. "Not so with cratering-type failures."

To find a best-practice solution, Mullins gathered a cross-section of the leading tooling supply companies together in the same room, presented the challenge, and invited them to give it their best shots. Challenged were companies already supplying Minco, plus several others.

"Putting them all in the room at once helped produce a better answer sooner," Mullins said.

Its tooling cost proved to be two-thirds lower than the nearest competitor

"We cast a wide net here because we believed that the 15-5 PH material showed great promise over the long term," he said. "Optimizing the roughing operation would be well worth the effort."

All were invited to demo the company’s tooling – once. Then Mullins and Ed Draper, Minco CNC manager, narrowed the field to two finalists, and ran comparative tests of their own design on identical cavity blocks of the hardened stock. The sample blocks measured 6"×12.5"×19.5" with two cavities 4" deep. Test criteria included tool geometry, feed, speed, depth of cut, chip load, stepover, spindle load, volume of material removed, cycle time, and tooling cost. All tests were run on the same vertical CNC machining center.

The company’s original cutter was a zero-rake, four-flute design with 3-sided inserts. One of the finalists used five 4-sided inserts, and the other featured just three flutes with 6-sided inserts.

Shhh! It’s Secret

Times, machine settings, and costs are confidential, but some general findings showed how the exception often proves the rule.

Both finalists finished the blocks in half the time as the original tool, with the 5-flute cutter completing the cavities about four minutes faster than the Ingersoll 3-flute mill. But, because the 3-flute mill used only three inserts, each with six edges, its tooling cost was about 70 percent lower. Further, tooling cost of the 5-flute cutter was 35 percent of total machining cost, more than 10 times higher than the general machining rule of thumb.

Although the three inserts each face a higher chipload than the five on the other, the former lasted longer because of two differences in geometry at the cutting edge: higher positive rake and the helical geometry. The higher rake reduced cutting forces while the helical edge reduced shock and impact as the insert enters the hardened material.

"The harder the workpiece material, the greater the benefit of free-cutting geometries and helical edges," Ken Clack, Ingersoll field engineer who assisted with Minco’s retooling, said. "Positive rake creates a cleaving action at the cutting edge, rather than scraping. With the helical edge, the insert enters the material a little at a time, like scissors shearing paper."

The freer cutting geometry also reduced residual heat in the mold blocks, the cause of the catastrophic cratering failures in the original tooling.

"That geometry also puts more of the machining heat into the chip, leaving less into the workpiece," Clack said.

In fact, now that Minco has standardized on the Power-Feed+ mill for this roughing operation, the process has proven reliable enough to run unattended.

"With edge wear rather than cratering as the edge failure mechanism, we can predict edge life reliably, and schedule indexings accordingly," Draper said. Ingersoll Cutting Tools

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