SPECIAL REPORT: Optimizing Green Machining
February 2, 2006
Following some basic guidelines can help manufacturers optimize cutting quality and productivity in green ceramic machining operations.
Machining unfired aluminum oxide, tungsten carbide, silicon carbide and other green ceramics can be challenging. The abrasive nature of these ceramic materials severely limits the life of carbide tools. While polycrystalline diamond (PCD) tools can handle the material, they are not available in the small, multi-flute configurations required for machining fine detail.
Fortunately, chemical vapor deposition (CVD) diamond tools are ideally suited to these applications. Because they have diamond on the surface, they last much longer (10 to 50 times) and have a lower coefficient of friction than carbide tools. The longer wear life of CVD diamond tools improves the dimensional accuracy and consistency of the machined parts; greatly reduces the number of tool changes needed, thereby increasing productivity; increases machine utilization; allows much longer periods of unattended machining (e.g., overnight); and enables the tools to quickly pay for themselves.
The low friction of CVD diamond tools permits using speeds two to three times higher than carbide-again contributing to productivity-with no degradation of surface quality. Additionally, the tools maintain a consistently sharp edge, which allows delicate, thin-wall sections to be machined quickly and precisely at high speed settings with reduced feed rates.
However, as with all machining practices, some basic guidelines should be followed to fully optimize cutting quality and productivity.
The Green Ceramic Cutting ProcessIn any machining process, a wear-resistant cutting edge separates the material from the work piece because of the velocity of the cutting tool edge relative to the work piece. When cutting metals, intense heat causes plastic deformation, producing chips or a curl of material. Green ceramic is different; it is machined by fracturing the material rather than by plastic deformation. The cutting edge crushes the ceramic just ahead of the tool edge as it moves through the material. This forms small particles, resulting in ceramic powder as an end product rather than chips or a curl, as when cutting metal. For this reason, two of the biggest challenges in green machining are avoiding chipping at the edge of a work piece and preventing the development of internal cracks caused by compressive stresses. Since edge chips and internal cracks typically occur as a result of tool wear, CVD diamond tools can minimize these problems significantly. The process can be further improved by controlling dust and chips during the machining process; using the right clamping, fixturing and supports; and optimizing the feed speed.
Controlling Dust and ChipsBoth compressed air and a vacuum are required to control dust and chipping from the green machining process. It is essential to continually remove ceramic residue from the point of machining; buildup of ceramic dust exacerbates tool wear and increases tool pressure, leading to possible fracturing of the work piece material. Dust and chip removal is best achieved by using compressed air (15 to 20 psi) with two nozzles. One nozzle should be directed at the tool to prevent dust packing, the other in the general vicinity to stir up the dust and thereby aid removal by the vacuum system.
The residue from machining green ceramic can range from small particles (0.001 to 0.005 in.) to fine dust. For this reason, the dust collection system should use high-velocity air to operate efficiently. A minimum air velocity of 500 ft/min is needed to capture the dust at the machining location, and an even higher velocity (2000 ft/min) could be used to prevent dust from settling in the exhaust ducts and part cavities. Screening should be placed in front of the dust filters to catch large particles that might damage the filtration system.
Clamping, Fixturing and SupportsInternal cracks in the green ceramic can typically be prevented by avoiding excessive clamping pressure. While the composition of green ceramics can range from "fairly strong" to "very soft," all work pieces should be treated as easily crushable. Unlike metals (aluminums, coppers, etc.), which can take on permanent warpage and deformation, green ceramic will fracture, especially parts with thin walls, and rods with high length/diameter ratios.
Vacuum chucks and fixtures are the preferred method of holding green ceramic work pieces. Vacuum works well for normal forces, while a mechanical stop is required for lateral forces. Mechanical stops and clamps should be cushioned using PVC electrical tape. Neoprene should not be used as a cushioning or gripping material because it is too soft and will allow the work piece to vibrate.
For turning, "pot" chucks made of compressible material such as Delrin or Nylatron work well. The work piece is inserted in the chuck's recess, and the chuck is then held and compressed in a conventional six-jaw mechanical chuck. The pressure of the jaws compresses the pot chuck so as to grip the ceramic work piece. To prevent slipping, the green ceramic should be wrapped in emery cloth, with the abrasive surface on the outside and double-sided adhesive tape between the emery cloth and the work piece.
Extending Tool LifeWhen cutting metals, the intense heat that is generated during the cutting process causes tool wear to accelerate rapidly with an increase in cutting speed. With green ceramics, most tool wear is caused by the abrasive nature of the ceramic particles, rather than by the material temperature or cutting speed.
This wear is significantly reduced with diamond tools. The tool life is determined by the quality of the cutting edge and the thickness of the diamond layer at the cutting edge, rather than by the cutting speed. Typically, the tool will go through a break-in period that serves to refine the tool surface at the cutting edge, resulting in an improved surface finish on the work piece. This will be followed by a long period of consistent performance, with a very gradual thinning of the diamond due to work piece abrasion. The tool reaches the end of its useful life when the diamond finally wears through, revealing the tungsten carbide substrate, or when the diamond surface develops a discontinuity.
Because small feeds and depths of cut do not lead to increasing the crushing effect on green ceramic, tool wear will advance rapidly with a light feed but will stabilize as the feed is increased. As the cutting action moves toward producing larger cracks and particles of removed material, the flank wear of the cutting tool edge stabilizes. Therefore, increasing the feed can extend tool life while also increasing the volume of material removed.
(Note that the depth of cut should not exceed one-third of the tool diameter. Increasing the depth of cut to one-half of the tool diameter will tend to break the material at the exit of a cut.)
Improving the CutCVD diamond tools are helping numerous ceramic manufacturers improve the speed and accuracy of their green machining operations. By using the right tools and following basic guidelines for dust and chip removal, clamping and fixturing, and feed speed, companies can optimize both their cutting processes and their finished parts.
For more information about green machining with CVD diamond tools, contact:
sp3 Cutting Tools, Inc.
3531 West US Hwy 224
Decatur, IN 46733
Phone: (888) 547-4156
fax (260) 547-4214
e-mail: email@example.com; or visit www.sp3inc.com.
Feed and speed information regarding end milling, drilling, profiling, turning or milling green ceramic can be found at: www.sp3inc.com/grnceram.htm.
About the AuthorsJim Herlinger
President and Chief Executive Officer
sp3 Cutting Tools, Inc., Decatur, Ind.
Vice President and Chief Operating Officer
sp3 Cutting Tools, Inc., Decatur, Ind.
Vice President of Technology
sp3 Cutting Tools, Inc., Decatur, Ind.