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Using the natural adhesive property of ice to clamp green ceramic parts has many advantages.
Ceramic machining in the unfired state is called green machining. Since the machining of ceramics after firing is very costly, green machining is done whenever possible. This technique is commonly applied to as-pressed parts that are still in a “chalky” condition. Stock removal can be accomplished 15 times faster in the green state than in the sintered state.
Green machining provides parts to tolerances of 0.5-1.0% of their final dimensions. Typical green-machined surface finishes range around 32-64 microinches. The machining centers found in ceramic plants are very similar to those found in standard machine shops and include CNC mills, CNC lathes, drilling equipment, surface grinders, and rotary grinders.
Clamping and Supports
When green machining, it is necessary to choose an adapted clamping system. The primary precaution is to avoid excessive clamping pressure. While the composition of green ceramics can range from fairly strong to very soft, all workpieces must be viewed and treated as easily crushable. Metals (e.g., aluminum, copper, etc.) can take on permanent warpage and deformation, but green ceramic parts—especially those with thin walls—will fracture.
One possible solution may include the reduction of clamping pressure and a reliance on vacuum retention, even if a mechanical stop is required for lateral forces. Mechanical stops and clamps can be cushioned using PVC electrical tape. For turning, a “pot” chuck made of a compressible material may be used to grip the ceramic piece with the pressure of the jaws. Although wax is sometimes used, it may cause contamination if it seeps into the material. To machine small ceramic parts, it is also possible to let connection bars between the part and the ceramic block remain rough. These bars are very difficult to remove later when the piece is fired.
An effective way to clamp parts through a truly green method is to use ice. Frost is an extremely strong adhesive; a glimpse of this strength is seen when an ice cube tray is stuck in the freezer. Using the natural adhesive property of ice to clamp parts has many advantages:
- Extra rigid (214 lb/in2, compared with 170 lb/in2 for magnets or 14 lb/in2 for vacuums)
- Stress-free mechanical clamping
- No solid residues after unblocking parts
- Parts can be held by a single face
- Works for all profiles of parts
- Quick and easy to use
An excellent edge quality can be obtained by machining the ice and ceramic at the same time (see Figure 1), and there is no need to keep the connection bars when machining small parts. In addition, Z-axis repeatability depends only on the surface holding the part because there is no water thickness under the part.
Icing plates were originally developed in 1998 for use in aerospace industry workshops.* Since then, the plates have been used by a number of industries that produce precise parts, including the ceramic industry.
The plates are cooled by a thermal exchanger with double flux; they require only compressed air and a few drops of water to function. Because of their compact monoblock design, the icing plates can be quickly installed. Operators simply connect the apparatus to the compressed air system, spray water onto the icing plate, position the part to be machined and then activate the temperature control button. Within seconds, the water freezes and the part is clamped. Unclamping is achieved just as quickly by reversing the freeze/thaw valve.
“We were able to dry-grind 92% alumina uni-pressed green and post-fired ceramic with no problem,” says Jeff Lassinger of Pittsburgh-based Associated Ceramics and Technology, Inc. “Using a Fantastic 100 grit/100 conc., ½-in.-wide grinding wheel on a Parker 2Z surface grinder running at 1800 rpm, we made multiple, fast .005 in. cuts, then .010 in. cuts and, finally, .020 in. cuts with excellent results. The grinding surface of the wheel stayed much cooler than usual, preserving the wheel’s life and reducing the downtime needed to re-dress. Also, the workpieces’ grinding surface stayed cool, and we had no splintering, burning or cracking.”
*GF series icing plates, developed by AMCC.
AMCC can be reached in the U.S. through Bruno Boucher, Inc., 4118 N. Nashville, Chicago, IL 60634; (773) 850-3100; or email firstname.lastname@example.org.