March 2007 Edition

CUTTING TOOLS

Cutting Tool Retooling Cut Out ‘Chernobyl Events’

Machining stainless steel parts could sometimes lead to “meltdowns” that would turn them into scrap until retooling solved the problem

A lot can go wrong when rough milling gummy stainless steel. Unless the cutting edges stay sharp and clean, the part and tool overheat, chips build up on the cutting edge, and the part work-hardens.

The retooling at Millworx Precision Machining increased cutting speed from 1.4 ipm with cutting fluid to 100 ipm without

Since Millworx Precision Machining, Inc., retooled the roughing operation on a challenging ordnance part, a lot has gone right. Feedrates improved to 100 ipm on the 17-4PH steel component, dropping cycle time by two thirds. Edge life quadrupled and tooling inventory costs plummeted.

The machining cost savings totaled $60 per piece, enabling the 20-employee shop, based in Corona, CA, to land a 500-piece per year job and still make a profit.

“You can’t always do that in defense work, but we did,” Steve Wolford, Millworx program specialist, said. Millworx specializes in machining steel aerospace and defense components.

In a five-hour, eight-step process, Millworx machines the part from a saw-cut billet measuring 3"×5.5"×6". A full 2" of stock is removed during the initial hogging step. Originally, rough milling took 90 minutes and wore out two solid high-speed cobalt endmills. Tool wear was the principle bottleneck.

From 1.4 to 100 imp

The retooling, done with the assistance of Steve Cross of Ingersoll Cutting Tools, was with the Ingersoll Form-Master Speed, a hybrid face mill. Wolford and Cross ran several trials to optimize the operation on a Fadal 4020 vertical CNC milling machine. They determined the optimum setting was 100 ipm at 1,910 rpm, cutting dry. By contrast, the best Millworx could get prior to the retooling was 1.4 ipm at 134 rpm with cutting fluid.

“During the trial, we were able to identify a few limitations and obstacles for the new mill,” Wolford said. “It appeared to run into slight difficulty when faced with chip regurgitation or interrupted cuts, but performed well under free machining conditions with full 180° material engagement.”

Wolford and Cross found that the IN2030 insert grade performed the best and lasted longer without coolant. With a few minor programming modifications, they skirted the limitations.

With the changes, rough milling the ordnance part takes 34 minutes instead of 90. Edges last through two parts instead of changing-out halfway through one part, as with the previous cutter. To keep the operation going, Millworx now stocks a small supply of inexpensive disposable inserts instead of a number of $60 endmills that were sent out for reconditioning at the rate of four a day. Savings from all of the changes added up to more than $30,000 a year for this part.

No More ‘Chernobyls’

“From our combined experience, most cobalt roughing endmills present an unacceptably high risk for what we have termed a ‘Chernobyl event’ on stainless steels.” Wolford said. “When the cutter fails, the part work-hardens and raises the possibility of becoming scrap.”

“Meltdowns” went away with retooling. As a result of success on the ordnance part, Millworx stayed with the Ingersoll Form Master Speed cutter for all steel and stainless steel rough milling, with 100 ipm as the standard feedrate.

“None of us here had ever machined steel at the rate this cutter was removing material,” Wolford said. “All told, retooling rough milling this way will save us more than $100,000 a year in machining and tool inventory costs.”

For a hardened 4130 steel part, the new cutter cut roughing time from 20 minutes to five.

Millworx sought other tooling suppliers to match or exceed the Ingersoll tool’s performance.

“The last tool we tried under the same conditions failed halfway through the cut, and the demonstrator left with his tail between his legs,” Wolford said. “Nothing else we have seen so far can even come close to the material removal rates we’re achieving with the Ingersoll tool.”

New Cutter, Close Up

The roughing cutter owes its effectiveness to a hybrid cutter body design and free-cutting inserts with large active faces. The result is a roughing cutter equally effective on the side and bottom of a cut.

“Also, the cutting edge is unencumbered by clamps, for better chip flow at high feed rates,” David Silves, Ingersoll’s Western regional manager, said. Several insert grades are available to match various steel machining conditions.

Moving to indexable tooling from solid endmills also eliminated the reconditioning merry-go-round: the constant shuttling of expensive tools back and forth from the reconditioner. To keep a high-volume operation going with solid tooling, often a shop needs to own three or more tools for each active one. With indexable tooling, all that’s needed are replaceable tips or inserts, an active cutter body, and a spare.

Millworx even manages to squeeze a little more mileage out of its inserts. On more critical or longer-cycle roughing jobs, they change the inserts early, but don’t discard them. They inspect the edges and, if still good, use them again on a less critical job. Ingersoll Cutting Tools

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