Conicity Technologies says correct edge prep for tool inserts (bottom) can provide better cutting than chamfered tool inserts (top).
Chip flow from chamfered tools (Figure B) display an extreme change in chip direction, causing the cut chip to dig into the surface of the chamfer.
Conicity says radial style edge prep provides a 90°-rake angle for the tools, and clears cut chips. (Figure C)
Conicity Technologies (www.conicity.com) of Cresco, Pa., a subsidiary of Weiler Corp., says its precision edgepreparation technology can be used as an alternative to chamfering for cubic boron nitride (CBN) and polycrystalline diamond (PCD) cutting tool inserts.
Chamfers were developed more than 20 years ago for these superhard tools to mitigate chipping. Typically these inserts are made with 20-degree T-lands, that produce 110-degree cutting edges on tools and bands along edges that are between 0.004 in. to 0.008-in. wide.
Bill Shaffer, executive vice president of Conicity, says these chamfers are a sub-optimal solution for tools because they limit tool life and diminish cutting performance and were developed before grinding wheel and effective edge-preparation technologies.
While he acknowledges that a chamfered tool has less of a propensity to chip, and thus, lasts longer, Shaffer says chamfering also introduces unintended consequences. Those tend to decrease tool performance and limit tool life.
"The negative cutting surface created by the chamfer limits the natural chip flow, creating a pinching of the chips between the tool and workpiece. That action, coupled with the blunt cutting edge, significantly increases tool pressure and heat," Shaffer says.
Chamfered tools present a blunter edge angle — greater than 90 degrees — to the workpiece, so cutting is done at a negative rake angle. A 20-degree chamfer results in a "super" negative cutting angle, with a rake of only 70 degrees, Shaffer says.
The negative rake angle makes the tool plow through the workpiece, and either pinches or traps the chips it makes. "This occurs because the feedrate of a tool when cutting hardened materials typically is less than the width of the T-Land chamfer. It has also been found that in most hard turning applications, the feedrate normally does not exceed one half the width of the chamfer. This condition can be referred to as under-feeding," Shaffer says.
The result is that chips are not cleared from the tool and tend to dig into the surface of the chamfer, increasing pressure and producing the common cratering and, eventually, causing the tool to fracture in a horizontal plane.
He says his company's Engineered Micro-Geometry edge-preparation technology provides some of the same functions as chamfering but eliminates the chip-pinching problems caused by the negative rake angle of a chamfer.
The technology applies a specific geometrically shaped rounded edge — radius, oval or waterfall-shaped — that makes the cutting edge stronger.
The specific geometrically shaped rounded edge gives the edge of the cutting tool its best condition to attack the workpiece material, Shaffer says.
The chip flow on the tool with the Engineered Micro-Geometry radius edge prep allows chips to escape the cutting zone and reduces the angle of incidence with the cutting tool, Shaffer says. That reduces the tool pressure and tool temperature and increases the life of the tool, he adds.
When using radial-style edge prep, the tool feedrate will always be greater than the size of the edge prep. Therefore, the 90-degree rake angle of the tool effectively clears the cut chips without trapping material between the cutting tool and the workpiece (see drawings below).