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July 2014  
 
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Facing Hard Facing

Greenleaf Corp. introduced its WG-300 ceramic composite tool material about 25 years ago. The ceramic matrix is reinforced with silicon-carbide “whiskers” that boost toughness. Initially applied in the relatively constant cutting conditions of turning operations, the composite tools provided a significant increase in productivity when roughing aerospace alloys. Shops are now using WG-300 tools in hard milling operations.

Tom Mahusky, shop supervisor at Cleveland Hard Facing Inc., Cleveland, is familiar with machining hard materials. “We’ve dealt with hard materials for the past 20 years, so we’ve learned a lot of tricks,” he said.

A recent job put that knowledge to use. It involved hard facing and machining hammer components used to forge titanium parts. To enable the four roughly 20"×16"×8" L-6 tool steel hammers to survive the high temperatures and pressures of the forging process, hard facing specifications called for application of a 1/8"-thick layer of low-carbon steel followed by a 7/8"-thick layer of nickel-base Hastelloy and finally a ¼"-thick layer of Waspaloy, another nickel-base material. Cleveland Hard Facing applied the layers using the metal-inert-gas arc welding method.

After the welding was completed, Mahusky rough milled the parts. The hammers had pyramid-shaped tapered faces, so the cutting tool had to machine through all of the weld layers as it descended the sloped sides of the hammers. “Hastelloy and Waspaloy don’t like to be cut,” Mahusky said. The alloys are not unreasonably hard; “only going in the low 30s [HRC] as welded,” he said, but added that when machined, the materials “will workharden just like a 300-series stainless.” This increases the hardness by 10 HRC. Without a large-capacity CNC mill in his shop, Mahusky used a 7½-hp manual vertical mill to rough the welded overlays. Running at 1,400 sfm and a feed rate of 18 to 19 ipm, he took DOCs of up to 0.100" with a 4"-dia. Excelerator milling cutter tooled with four round, negative-geometry WG300 ceramic inserts. “A positive insert is not strong enough to withstand roughing in this case,” he said.

Mahusky worked with Denny Carpenter, a Greenleaf sales and service engineer, to set up the operation. Carpenter made sure the machining parameters were “within the capabilities of the machine, the part and the rigidity of the setup,” Mahusky said. “It depends on the configuration of the part. If it’s big and overhanging, you need as rigid a setup as possible. You have to take all that into account when milling with ceramics.”

Machine stiffness is crucial. “Even if you are taking small finishing cuts, you still need a rigid machine,” he said. “If you are going to take bigger cuts, it’s not just a high-speed spindle you need— you must have rigidity and torque at the spindle.”

Mahusky left 0.020" finishing stock on the hammers and subcontracted finish machining to job shop Quality Industries Inc., also in Cleveland.

Quality Industries finished the hammers on an ACRA FVMC-610 CNC vertical machining center using a 1.5"-dia. positive-geometry Excelerator endmill tooled with three RPGN-43 inserts. The tool ran at 3,800 rpm, 80 ipm and a 0.020" DOC. Vice President Jim Kaplan said the light DOC produced a minimal flow of chips. Greenleaf’s Carpenter added that “compensating for the chip thinning effect is 100 percent crucial” in maximizing tool life in the operation.

Mahusky said he would not consider hard milling without ceramics. “Not today, when you know what you know, and you remember how you did it back when. You’d get carpal tunnel syndrome just changing inserts when it was carbide.” He added that according to a sales representative for the weld materials applied to hammers, some shops don’t attempt to mill the hard welds and grind them instead. “That’s gets pretty expensive because you are getting into wheels, too,” Mahusky said.



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