Ultimate Guide for Ceramic Machining

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Machining precision glass and ceramics

Silicon carbide's CTE is an order of magnitude better than stainless steel, but isn't as low as Invar, while silicon carbide's specific stiffness is much higher than Invar. Higher specific stiffness, calculated as Young's modulus over density, is another key advantage of these materials over metals. Specific stiffness is really a comparative value: the higher the number, the stiffer the material. The thermal expansion of ceramics isn't as good as that of glass ceramics, but for silicon carbide, one of the stiffer ceramics, its stiffness is a huge advantage.

For lapped and polished parts, glass or ceramics can be processed more easily than metals using optical fabrication techniques&#;thereby incorporating optical features, such as a mirror surface that could be used for positional feedback systems.

Glass, glass ceramic, or ceramic?

Having narrowed down material choice to nonmetallics because of their relative advantages, designers choose between glass, glass ceramic, or pure ceramic options. Generally speaking, it's a balance between cost and performance. Glass ceramics often have better thermal-expansion performance, so if that's the key performance requirement, then they might be the best choice, despite higher costs.

In an application where heat is generated within the system and where that heat must be transmitted out of the system, know that glass ceramics tend to be less thermally conductive. If the need is to conduct heat out of your system, ceramics have a much higher thermal conductivity than glass ceramics or glass.

If stiffness is what's important, because of high acceleration or dynamic loading, then the increased stiffness of ceramic materials is an advantage over glass. If cost is primary, then glass and glass ceramics processing tends to be a little bit more cost effective compared to ceramics, because of the nature of the material. Ceramics are tougher materials, and can be harder to process taking more time than glass and glass ceramics.

To reduce ceramic-processing costs, some rough shaping, called green machining, may be possible. Many ceramics, like silicon carbide, can be molded or formed into a rough shape prior to sintering. The sintering process is what ultimately gives them their ceramasized characteristics. Shrinkage occurs during sintering, and variations of 10% to 20% before and after sintering are common, lowering dimensional control. So, while parts with loosely toleranced features are suitable, tightly toleranced features on a component must be machined and polished post-sintering, though the hardness makes the process more time-consuming.

Designing for the materials

Compared to metal materials, there are some important differences to account for during the design phase to ensure manufacturability of nonmetallics. The most important point is to respect the brittle nature of the materials.

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While metals are malleable, glass, glass ceramics, and ceramics are not, so their fabrication techniques are significantly different. Think about glass as a hardened steel-type material: once hardened, it can no longer be milled in the same way. Typically, these materials are processed with fixed-diamond grinding tooling (see Fig. 1). They are similar in function and appearance to milling tools, but the method by which they work is quite a bit different.

Glass and ceramics can be ground, similar to an inside-diameter/outside-diameter (ID/OD) grinding process, but can't be turned on a lathe with a pointed tool or a single-point tool like a metal. So, in designing, bear in mind that features often are going to be 3D machined or profiled, not turned.

Another difference to manage is that, while a metal component will first go into an elastic state causing it to deform or bend before its fails catastrophically, glass and ceramics are highly brittle, inelastic materials, failing catastrophically when the yield strength is exceeded. While a metal may dent from impact, these materials are instead susceptible to chipping and fracturing. Ordinarily, a failed component in these types of materials cannot be repaired.

Refrain from having corners and edges that are sharp. Sharp edges are prone to damage from minor impacts. Inside corners can lead to stress concentrations and be a seed location for a failure. Threading&#;possessing both inside corners and outside edges&#;isn't done in these materials, and certainly not threading under load. Glass or ceramic threads lack strength when it comes to holding force. Instead, a metal insert of some sort will usually be designed and bonded into the component.

Lightweighting limits

The technique of removing a portion of the material from a component while maintaining adequate structural strength and rigidity is called lightweighting. It is easy to lightweight components, and the technique has a long history. Fifty-percent lightweighting is commonplace. Lightweighting 80% or more is also achievable, but the designer should take care to design not only for function, but also for manufacturing. Any dynamic or static loads that the component is going to experience should be modeled, thus ensuring the strength is adequate.

Deep pockets, up to 75 mm deep, and 3 mm thin walls are common in glass or glass-ceramic materials. In specific cases, deeper and thinner is possible, and certainly so in ceramic materials. Every application is different and the range of implementation available is wide, but success comes down to doing adequate design up front while the component is being conceived.

Figure 2 shows an optical component that was lightweighted for an airborne application. All of the pockets show sharp inside corners. With no beveling, one would expect that if this part was FEA modeled with some type of dynamic or static loading, there would be stress concentrations between the pockets' floors and the walls. That would be an area of concern in manufacturing, and would be best addressed by adding radii to the bottom of the component's pockets to relieve that stress and create an appropriate safety factor. Keep those loads safely below the yield strength of the material.

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