CVD Diamond Coated End Mill Keeps Its Cool in Ceramic Machining Test
A recent ceramic machining test compared the performance of a Norton Diamond Film DF1000 CVD diamond coated end mill with that of an uncoated tunsten carbide end mill and a TiN-coated carbide end mill. Results proved the DF1000 CVD diamond coated tool to be a cool winner, capable of resisting excessive wear and frictional heating.
Machining Parameters
The test was performed on a SiC-containing bisque ceramic, which was pre-sintered at temperatures greater than 1200°C. The workpiece material and machining conditions were selected for product testing purposes with a goal of accelerating the abrasive wear mechanism common to applications such as green ceramic and graphite machining.
The performance of Norton DF1000, 3/8 in., four-flute diamond-coated end mills was compared to uncoated tungsten carbide and TiN-coated carbide tools of the same geometry. The spindle speed was 10,000 rpm, with a table speed of 20 in. per minute corresponding to a cutting speed near 1000 sfpm and a chip load of 0.0005 in. per tooth. The pre-sintered ceramic workpiece was machined in alternating, 3-in.-long climb and conventional passes using a 3/16-in. radial depth of cut and a ½-in. axial depth of cut. Cutting forces on the workpiece and spindle power were measured using a three-axis dynamometer and a computerized data collection system.
Results
The results of the machining tests showed the dramatic performance advantage of thin-film CVD diamond-coated end mills at the selected machining conditions. The uncoated tungsten carbide and TiN-coated tungsten carbide tools failed within 6 in. of machining (two passes). The failure was highlighted by excessive abrasive wear, which was enhanced by frictional heating of the tools. Both the TiN and uncoated carbide tooling showed a strong sensitivity to density variations in the pre-sintered ceramic workpiece. In the case of one uncoated carbide tool, the excessive wear and overheating reduced the 0.375 in. cutting diameter of a new tool by over 0.100 in. in less than 9 in. of machining! The TiN coating offered no clear advantages under these machining conditions. Optical pyrometry of the carbide and TiN-coated carbide tools indicated cutting temperatures in excess of 900°C. In contrast, a diamond tool that had already run 80 previous passes was barely warm to the touch after eight sequential passes.
High Lubricity of CVD Diamond Reduces Friction and Wear
The observed temperature differential between diamond and the alternative tools is primarily attributed to the high lubricity of the diamond coating in the ceramic workpiece material. In the first 3-in. machining pass, the spindle power consumption for the TiN-coated carbide and uncoated carbide tools was over 10 times higher than the power consumption for the diamond tool after 80 passes.
The lower cutting forces seen with CVD diamond coated tools offer the potential for reduced yield losses due to cracking in green or pre-sintered ceramic machining. The low cutting temperatures seen with the CVD diamond tool are viewed as an indication of the tremendous potential for CVD diamond tooling in high-speed machining of ceramics, as well as other difficult-to-machine materials.
Headquartered in Northboro, Mass., Norton Diamond Film is the world's only full-line supplier of CVD diamond for cutting tools and wear parts. For information about Norton CVD diamond products, call 800-393-8670, (508) 351-7842, or (508) 336-6115 Fax. Or, visit our web site at www.nortondiamondfilm.com.