April 2011 / Volume 63 / Issue 4|
By Ed Woksa, Ingersoll Cutting Tools
Courtesy of Ingersoll Cutting Tools Advancements enable ceramic inserts to tackle difficult machining applications.
If you still think ceramics are “too delicate” for anything but continuous cuts and finish passes, think anew. Advanced ceramic inserts—even grades that are not reinforced with microscopic fibers, called “whiskers”—are now capable of rough turning as well as finishing. Milling and boring are also suitable applications.
And unreinforced ceramic grades are also applicable in difficult-to-cut materials such as Inconel, outproducing reinforced styles by a factor of two or more in some applications. This is achieved because the silicon-nitride-based material is similar in density but tougher than reinforced grades. These characteristics provide more consistent results because cutting edges wear rather than fracture. Many of the tougher grades of unreinforced ceramic inserts can handle feeds of 0.024 ipr and higher when roughing.
For example, a valve manufacturer in Texas turns Inconel 718 valve balls four times faster with new unreinforced Si3N4 ceramic inserts from Ingersoll Cutting Tools than with carbide inserts and twice as fast as with any other ceramic insert the valve manufacturer tried. (Ingersoll also offers a new whisker-reinforced ceramic grade.) The valve manufacturer also improved the material-removal rate fivefold when rough milling compared to carbide and doubled the mrr vs. other ceramic grades. Moreover, the manufacturer is successfully boring Inconel—an application usually considered off limits for ceramics because of the high risk of recutting chips.
The inserts have a special edge preparation, including a 0.2mm land width and a 25° lead angle, which helps them run cooler by putting the heat into the chip and not into the insert or workpiece.
Advanced ceramic inserts have also boosted productivity when cutting cast iron. A recently developed unreinforced silicon-aluminum-oxynitride (Sialon) ceramic insert increased the cutting speed by 40 percent and extended edge life when rough and finish turning gray cast iron brake shoes with interrupted cuts vs. an existing Si3N4-based insert.
For gray cast iron, optimal machining parameters are cutting speed of 2,400 sfm and 0.024-ipr feed rate for roughing, and speeds up to 3,000 sfm and 0.008-ipr feed for finishing. For nodular cast iron, the parameters are 1,200 sfm and 0.020 ipr for roughing, and 1,700 sfm and 0.008 ipr for finishing.
Achieving these parameters stems from advances not only in substrate composition but also from coating, insert geometry, edge preparation and clamping system enhancements.Substrate Choice Continuum
The range of available ceramic substrate materials has broadened significantly to cover a wide application range. Where once there was only hard, brittle alumina good for only fine finish turning (and requiring a very rigid machine), today there are more than 10 grades built around three basic ceramic chemistries: Al2O3, Si3N4 and, more recently, SiC. Some are still whisker-reinforced, but many of the newer grades are not, and productivity isn’t sacrificed. The performance of the unreinforced grades matches or exceeds whisker-reinforced ceramics because their blended substrates are more impact resistant than the alumina used in reinforced grades.
For example, zirconium-dioxide and Ti(C,N) “alloying agents” add toughness to some of the harder grades. Grade AW20 is a blend of Al2O3 and ZrO2; AB2010 is Al2O3, TiC and TiN; AB20 is Al2O3 and TiN; AB30 is Al2O3 and TiC; TC430 is whiskered SiC; AS500 is Sialon; SC10 is Si3N4 and TiC-coated; AS10 is Si3N4; and AS20 is Si3N4 and TiN (Table 1 below).
Eliminating reinforcing whiskers dramatically reduces the cost to produce inserts. There will, however, always be a place for whiskered ceramics, particularly for high-speed milling and turning of Rene, Waspaloy and other nickel-base superalloys.
As a result, end users can find an effective ceramic grade for high-feed and medium- to high-speed rough turning and mild milling, as well as high-speed and low-feed finish turning. The inserts’ improved reliability qualifies them—even unreinforced grades—for more lights-out machining applications.
Increasingly, ceramic inserts are available with the same PVD and CVD coatings that improve the cutting performance and tool life of carbide grades. For example, a CVD TiN coating helps alumina inserts run longer and cooler, and PVD TiN coatings improve the performance of ceramic inserts when cutting gray cast iron, which is used to make brake parts.Stronger, Thicker Inserts
Newer insert shapes also help users get more mileage out of each insert. In the past, choices were limited to zero-lead triangles, squares, rhombi and rounds. The sharp-cornered triangle and rhombus geometries offered multiple cutting edges per insert at the expense of strength, while round inserts were stronger and reduced potential stress-raising sites but offered an uncertain number of edges. Although users can safely index a round to get six edges per side, most settle for two or three because they lose track of orientation on a featureless round. This is unnecessarily wasteful.
As a solution, thick, hexagonal ceramic inserts are being developed to enhance strength and provide clear references for indexing. The idea is simple: be closer to an inherently strong round shape than a square or triangle. Because the hex shape provides discrete orientations in the clamp, users don’t throw away an insert that still has plenty of life left in it. In addition, the insert’s thicker cross section provides greater process security, and the 45° OD and facing lead angle facilitates high-feed machining.
Newer ceramic inserts are also available with edge preparations that assist when interrupted cutting and reduce cutting forces during roughing. Special edge preps can improve performance in specific applications. For example, a sharper edge reduces heat when cutting nickel-base superalloys, and a honed edge enhances toughness in interrupted cuts.Dimple Clamping
Improved clamping systems have also reduced the risk of insert fracture, qualifying ceramics for more roughing and interrupted-cut milling. Dimple-type clamping systems eliminate the center hole through the insert, leaving a stronger tool with reduced stress-raising geometry. Instead of a hole, a precise bump in the clamp engages a matching dimple in the insert top face.
A recent refinement is the round dimple, rather than a square or elongated dimple. A round dimple ensures more uniform distribution of clamping stresses and full tangential contact regardless of indexing position.
These improvements are solving problems when machining difficult-to-cut materials during lights-out production.
Ceramic is orders of magnitude harder and more temperature-resistant than carbide, and the developments outlined in this article allow users to take full advantage of those attributes while minimizing the risk of fracture or catastrophic failure. As long as users follow manufacturer’s recommendations, they can count on gradual, predictable wear to determine edge life and not a sudden tool wreck.
Today’s ceramics perform well in difficult as well as ideal conditions. There’s no need to baby them anymore or limit their applications. And once they overcome a particular bottleneck, payback is quick. CTE
About the Author: Ed Woksa is marketing manager for turning and holemaking products for Ingersoll Cutting Tools, Rockford, Ill. For more information about the toolmaker’s product line, call (866) 690-1859, visit www.ingersoll-imc.com or enter #350 on the I.S. Form.
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