Variable edge prep matches cutting-edge hone with cutting conditions.
Reducing hone size at a tool's trailing edge, where just a trace of material is cut and uncut chip thickness is near zero, minimizes the hone-rub zone, so tool cuts efficiently.
Applying the "proper" edge preparation for a given application is no easy task. Until recently, creating edge geometries was more of an art than a science because the qualities desired in cutting tools — resistance to wear and hardness — make them difficult to machine.
However, proper tool cutting-edge preparation has a big effect on performance and service life. Correct edge preparation improves tool life by reducing the common causes of failure, such as chipping, heat-induced failure and builtup edge, and it heavily influences tool reliability. Properly honed tools also improve the repeatability of machining operations, assisting the drive toward lights-out manufacturing.
Edge honing is an erosive process that is performed on a microscopic scale, and it requires complete process control to hold tight tolerances. However, it is difficult to control the metal-removal rate and edge uniformity on cutting tool materials. Often, the honing process is guided by a best-educated guess, and is limited by machine variability and operator expertise.
Conventional honing processes tend to over-work the corners of the tool and can be difficult to control on a tool-to-tool basis because of the variances in incoming parts. Not only is edge honing difficult to control, cutting conditions vary along a single cutting edge so the optimal size of the edge preparation changes along the cutting edge as the work is being done.
"You want the edge radius to be smaller on the corner of the tool because the uncut chip thickness decreases along the corner radius," says Dr. William J. Enders. Enders is an associate professor of Mechanical Engineering-Engineering Mechanics at Michigan Technological University and president of Machining Analysis Technologies, in Houghton, Mich. He has been studying edge preparation for more than a dozen years.
At the leading edge of a cutting tool, the uncut chip thickness is heaviest and the edge requires maximum protection. However, at the tool's trailing edge, uncut chip thickness decreases to near zero and the hone should decrease accordingly. With a constant hone, sized to protect the leading edge, the hone at the trailing edge is larger than the uncut chip thickness so the cutting edge inefficiently removes material and increases friction, cutting force, temperature and wear.
Until recently, the methods of tool-edge preparation did not advance as rapidly as technologies related to the other components of cutting tools, such as material substrates, geometries and coatings. Using its Engineered Micro-Geometry process, Conicity Technologies LLC, Cresco, Pa., (www.conicity.com) produces edge hones of varying sizes on separate surfaces of the same tool. The process uses dense silicon-carbide filament brushes applied with computer numerical control to consistently and precisely shape edges to tolerances of 0.0003 in., an order of magnitude more precise than most conventional honing methods.
"By controlling edge parameters, the engineered micro-geometry process stops removing material when the correct hone size is achieved. Thus, edge-prep size is distributed along the cutting edge, maintaining a specific ratio of uncut chip thickness-to-edge prep size," says Bill Shaffer, executive vice president of Conicity. "For example," he continues, "on an indexible insert or cartridge-type tool the edge prep is blended around the nose radius of the tool with the change in the size of the edge prep mirroring the rate of change in the thickness of the uncut chip. As the uncut chip thickness decreases, the edge prep size is decreased."
Tools that have variable-edge preps cut efficiently because the varying sizes of their cutting edges do not trap cut material between the tool and workpiece. That minimizes tool rubbing and tool pressures and cutting forces decrease, as do tool and workpiece temperatures. The results are long tool life, good surface finishes on parts, flatness and minimal burr formation. Variable edge preparation can be used on almost any form of cutting tool, including drills, endmills and reamers.
The engineered micro-geometry process can also apply the same edge preparation to tool after tool, making the process more consistent than conventional edge-preparation methods. Moreover, the process can apply the optimal micro-geometry to each cutting application. In some situations, this means that a uniform hone is applied along the entire cutting edge; but in others, varying hones are applied for the correct edge preparation.
The most obvious and direct benefit of proper edge preparation is increased tool life. The engineered micro-geometry process, edge-preparation service typically costs 10 to 20 percent of the cost of a replacement tool, and the process can extend the tool's life by 300 to 800 percent.
Longer tool life is importantforanumberof reasons, including cost savings for regrindings and replacements. For example: To take full advantage of its 5-axis machining centers, Oberg Industries, Freeport, Pa., a precision manufacturer, switched from high-speed steel tooling to high performance carbide tools with engineered microgeometry process edge preparation supplied by Seim Tool of Latrobe, Pa.
"With engineered micro-geometry process edge prep, the interval between sharpening typically is extended by about 300 percent, which is a great return on investment for the engineered micro-geometry process. Longer intervals between tool replacement also mean less interruption to production and lower labor costs," says Bob Binner, toolroom manager at Oberg.
More importantly, according to Binner, are the indirect cost savings: "The edge prep practically eliminates tool breakage, which can cause a train wreck in our automated processes. Tool breakage in an unattended machine results in a domino effect that can cause other tools to break downstream. Such failures can be very expensive in terms of tool damage and loss of expensive parts, and subsequent problems can be difficult to correct. With carbide tools having engineered micro-geometry process edge prep, we have been able to operate our high-speed process with confidence, sometimes going weeks without replacing drill bits and reamers."
The economic benefit of increased tool life is especially dramatic in aerospace and medical applications that involve drilling and machining tough materials such as Inconel and titanium.
In an aerospace application drilling 718 Inconel, the addition of an engineered micro-geometry process edge prep extended the life of a tool used on helicopter rotors from 30 holes to 60 with no feed/speed change. The edge preparation strengthened the cutting edge and minimized heat buildup in the tool. With a stronger cutting edge, the end user doubled both feed and speed, increasing penetration rate by a factor of four, while increasing the hole count to 120. The holes also were more precise, with the standard deviation of hole size reduced by two-thirds. The increased feed rate and the reduction of tool changeover time resulted in significant productivity increases.
Anadditional benefit of engineered microgeometry process edge preparation is the reduction of the scrap generated in the tool set-up process. For example: In thread millingholesforartificial hip joints, four or five expensive-parts typicallyhadto be threaded, gauged andscrappedduringthetoolbreakin process to make a suitable thread that could be gauged.
Engineered micro-geometry process edge preparation extended the life of the thread-mill used on these parts from 75 holes to 250, reducing the frequency of the changeover process while improving the thread consistency for the first piece made with new tools. This reduced the amount of scrap pieces produced by more than 75 percent while getting the tool to react properly.