Tool wear has always been a significant problem in the drilling of abrasive materials such as carbon-fiber-reinforced plastics (CFRP), according to Novator AB. In some cases, only a few holes can be drilled before the cutter must be reground or scrapped. Different types of coating and poly crystalline diamond cutters have been shown to improve tool life, but that process is expensive and time-consuming.
Especially in complex stacks with materials combinations such as CFRP/Ti or CFRP/Al, the process parameters are a large compromise, and cutter life is even shorter.
Novator’s solution to the problem is orbital drilling, which involves machining the material both axially and radially by rotating the cutting tool about its own axis as well as eccentrically about a principal axis while feeding the tool through the material. The tool diameter is smaller than the hole diameter.
One benefit of such machining includes low thrust force requirements during drilling. Low thrust reduces the likelihood of delamination in composites and of burr formation in metals.
Another benefit is the small size of the chips formed in drilling. Small chips, in combination with a hole diameter larger than the tool, allow for efficient chip removal, which in turn prevents heat buildup and eliminates the risk for matrix melting in composite materials (and heat-affected zones in metals).
In orbital drilling, the eccentricity (or offset) is an adjustable parameter, so the same tool can be used for holes of differing size. In more advanced machines, the offset can be adjusted dynamically by 1-2 µm as drilling takes place.
Novator has also developed an algorithm to take into account tool wear as drilling proceeds, based on tests using a three-flute special orbital cutting tool from Kennametal. The testing parameters for drilling in the abrasive CFRP were hole diameter of 14.3 mm, spindle speed of 24,700 rpm, orbital speed of 300 rpm, and feed rate of 100 mm/min. Without the algorithm, the average diameter of the resulting hole in a series of 300 hole-drilling events varied by 60 µm, owing directly to tool wear. With the algorithm in use, the variation was just 7 µm. If the accepted tolerance band is 55 µm, the Cpk value is 2974.
Moreover, Novator says, the tool wear compensation algorithm achieves a predictable and uniform result.
Other trials included different combinations of composite materials, and the results were similar.
This article is based on SAE technical paper 2009-01-3216 by Eskil Larsson, David Eriksson, and Patrik Rydberg, all of Novator AB.