Cermet Inserts Indexable Ceramic Insert
Lathe Machine Cemented Carbide Inserts Good Rigidity and High Wear-resisting
Description:
Among carbide indexable inserts there is a huge selection of shapes, substrates, coatings and geometries from which to choose. Likewise there are different kinds of ceramics. In this article we will concentrate on ceramic inserts that use silicon carbide crystals to reinforce the base ceramic compound. The silicon carbide reinforcements of these ceramic inserts are often referred to as whiskers because they resemble small hairs when examined under a microscope.
To find out more about ceramic inserts and especially how their application in metalworking differs from carbide inserts, we talked to Keith Smith, manager marketing and international sales .It introduced whisker reinforced ceramic inserts to the United States and has extensive experience in their successful application in metalworking shops.
Chemically, the main ingredient in ceramic insert cutters is alumina (Aluminum Oxide). It's a very hard substance that has for many years been used in grinding wheels. Basically, a ceramic insert is a very fine-grained aluminum oxide grinding wheel. The biggest difference between the two cutting tools is that the insert is manufactured without the porosity necessary for grinding.
During the manufacturing process, reinforced ceramic is pressed into insert blanks which are subsequently cut to the desired shape. They are subjected to a much more extreme bonding process than the sintering used for carbide. Usually whiskered ceramic undergoes a hot pressing process that uses external heat and high pressure, evenly distributed, to compress the insert blank and eliminate porosity.
Many of the inherent cutting advantages of the grinding process—high heat tolerance, excellent surface finish, long tool life—transfer to milling and turning operations with the application of ceramic inserts.
Probably the major down side of the ceramic insert is its brittleness. Generally ceramic cutters don't hold up well to interrupted cuts or thermal shock. To help alleviate this performance constraint, small crystals of silicon carbide are put into the aluminum oxide compound.
The ratio is about 50/50 alumina to silicon carbide whiskers. These crystals are like fibers and act as reinforcement to the brittle ceramic. They act in much the same way as glass threads reinforce resin in fiberglass.
"Whiskered ceramic, like all cutting tool materials, has specific areas of application that are best suited to its capabilities," says Mr. Smith. "In general, it works best on hard ferrous materials and nickel base alloys. It does not work on ferrous materials below 42 Rc because some reaction problems occur. We find about 80 percent of reinforced ceramic usage is on nickel alloys and aerospace alloys such as Inconel, Waspoloy, Hastelloy and others."
Application:
It is within the metalcutting operation that ceramic cutters earn their stripes. While much research has been conducted to describe the dynamics of chip formation, one common factor found in all of the theories is that much heat and stress gets generated as a cutting tool displaces the metal.
The winners in this nasty environment must be very hard materials that are resistant to heat and that are geometrically as well as chemically stable. In general, the operational variable needed to be successful is speed—faster cutting generates more heat.
Every insert has a melting point, which reflects the temperature at which it is made. Ceramic's melting point (3,700° F) is higher than sintered carbide, which means it can be driven through the cut faster.
Turning is an almost ideal operation for ceramics. In general, it is a continuous machining process that allows a single insert to be engaged in the cut for relatively long periods of time. This is an excellent vehicle to generate the high temperatures that make ceramic inserts perform optimally.
Milling, on the other hand can be compared to interrupted machining in turning. Each insert on the tool body is in and out of the cut during each revolution of the cutter. Compared to turning, hard milling demands much higher spindle speeds to achieve the same surface speed.
To match the surface speed of a turning operation on a three inch diameter workpiece, a three inch diameter milling cutter with four teeth must run four times the turning speed. With ceramics, the object is to generate a threshold of heat per insert. Therefore in milling operations, each insert must travel faster to generate the heat equivalent of a single point turning tool.
Ceramic cutters in many cases demand a new set of application skills from the shop looking to implement them. Often the process knowledge acquired from sintered carbide insert use is 180 degrees opposite the experience needed to use ceramics.
"It's very important for shops to understand that the way whiskered ceramic inserts work is completely different from other cutting materials," says Mr. Smith. A major difference is the way ceramics use heat generated in the cut.
In most traditional metalcutting, heat is the enemy. It's bad for the tool and generally bad for the workpiece (work hardening). The heat dissipation objective for most carbide cutting inserts is to get heat into the chip and quickly out of the cut zone.
Not so for ceramics, says Mr. Smith. "We set out to generate high temperature ahead of the cutting tool in order to soften or plasticize the workpiece material to facilitate its removal. The ideal cutting temperature in nickel alloy, for example, is in the area of 2,200° F. This cutting temperature is beyond the tolerance range for sintered carbide because 2,200° F is close to the melting point of cobalt, which is what cements the carbide insert together. At these temperatures, carbide will soften, deform and fail."
To achieve the elevated operating temperatures for ceramic inserts to prove their mettle, much higher cutting speeds are suggested. "Beyond all question," emphasizes Mr. Smith, "successful cutting with ceramic inserts demands high surface speed coupled with balanced feed rates. High speed cutting is necessary to generate the high heat within the cutting zone and to assure that the heat propagates into the workpiece immediately ahead of the cutter."
In general, a conservative approach to cutting speeds and feeds is the single largest contributor to process failure when trying to implement ceramic cutters. When cutting speeds are too slow, insufficient heat is generated. Because heat cannot be transferred ahead of the cutter, in effect, to anneal the already hardened workpiece, cutting forces become too high and insert failure occurs.
In most cases, insert failure is a sign to the machinist to back off speeds and feeds even more. In the application of reinforced ceramic insert cutters, that is the wrong thing to do and only exacerbates the situation.
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