Coping with Hard Part Milling
Machining hardened stock isn’t for the faint of heart, but many alert shops have figured out how. In fact, some are running steels up to Rc 60 four or five times faster—even in cavity work. Others have even found a way to mill the hard stuff on light-duty machines using conventional end milling.
“Hard materials are less forgiving, so everything matters more when it comes to the milling process,” says Bill Fiorenza, product manager at Ingersoll Cuttings Tools. “Also, tool life is a bigger factor in overall process economics. Minimizing stoppage time for indexing can often save more machining money than ramping up the feedrate.”
Five Times Faster on Mold Blocks
One impressive recent success is at Datum Industries, a die and mold shop in Grand Rapids, MI. Not surprisingly, a mainstay in their diet is cavity milling of hardened molds and mold blocks.
With a simple tooling change, Datum now roughs the blocks five times faster and has completely eliminated a semi finishing operation. Their previous standard tooling was a conventional ½-inch four-flute solid carbide ballmill. Now they do it with a two-flute indexable Ingersoll Chip-Surfer™ high-feed mill with a bull nose tip. Settings follow the modern “feed fast/cut shallow” strategy—400 ipm/0.020”DOC/3855rpm. That’s 10 times faster feed than before. Stoppages due to tool wear are way down as well; just once every 15 hours, done in seconds, right in the spindle. On a material-removal basis that’s a 25 to1 saving in both downtime and tool inventory cost.
And nobody knows this better than John Smith, process engineer at Datum, “The roughed finish is good enough to let us go directly to finishing now. We not only feed 10 times faster during roughing, but also take a much wider swath, cutting chipmaking time two ways.”
The Chip-Surfer tool itself is essentially an alloy steel shank that accommodates a variety of replaceable carbide tips. Datum chose the high feed tip, which has two flutes and a bull nose cross section. The bull nose moves more of the tip’s active cutting surface out near the pitch radius, so that more of the tool is moving at optimum cutting speed. Geometry on a spherical carbide ballmill, by contrast, means that optimum cutting speed can occur only at the midline.
More Secure Milling of A-2 Stock
At moldmaker B&J Specialties (Wawaka, IN), the key problem running hardened A-2 stock was tool failure of solid carbide ball with mills. Whenever time process engineer Scott Sizemore turned up the feedrate, the cutters either wore out or broke off within minutes. So he pretty much settled for a 100 ipm/0.004” DOC/3750 rpm machining rate. With the switch to an Ingersoll Chip-Surfer with a toroidal tip, he could double the feedrate and triple the depth-of-cut (DOC), leading to an overall threefold gain in cavity throughput. Despite the higher settings, edge life also tripled, with in-spindle tip changes taking only 20 seconds. Each tip consistently lasted a full hour.