"Rough-milling that metal 'sandwich' used to create like a lot of thumping and scraping noise. Now it purrs along like a pussycat."
So says Romney D'Antuono, foreman at Precision Mold Base Corp. (PMBC), Tempe, AZ, who faces the problem every day. "It is the most abusive milling operation in the business. The trick was to settle on tooling that handled all aspects of the abuse at once"
The "sandwich" is an integral part of every custom mold base the company produces for moldmakers across the country. D'Antuono supervises a facility that normally runs 24/7, including lights-out. Among the key equipment are eight modern CNC mills and four big surface grinders, each selected for accuracy and ease of duplication of the finished mold bases.
"Our mold base assemblies are built like a sandwich, with mild steel or stainless as the 'bread' and 4140 steel at Rc 33-35 as the 'filling,'" explains D'Antuono. "The operation in question is like trimming off the crust--a very difficult crust."
Originally PMBC did the job using conventional zero-rake button cutters with five inserts. It was slow going, with inserts often breaking off in mid-cut. "We have a shelf full of wrecked and burnt-out button cutters to show for it," says D'Antuono. Then in late 2006, he switched to a positive-rake Ingersoll Form-Master+ button face mill. The improvement was immediate. The material removal rate more than doubled and insert edge life stretched to at least two shifts--reliably. The old thumping and scraping quieted to a purr.
Speeding up the outside milling process is crucial to over-all profitability; that single operation represents a major portion of the total machining cost for each mold base. Every base requires it. Moreover, making it a more secure operation would qualify it for the lights-out shift, which benefited not only efficiency but also delivery speed to customers.
The process involves rough-milling all four sides where the layers of the individual plates in the stack are exposed. During the cut, the milling cutter may encounter several layers at once--and dissimilar metals, with vastly different hardness and machinability--plus interrupted cuts, irregular surfaces, and uneven depths of cut.
It gets worse. All around the 4140 and 420 stainless steel plates, the hardest metal in the package, there are gaps where features have been machined in prior to assembly. In the finished mold, those features will provide...