As researchers build ever smaller machines and devices, demand increases to create miniaturized equipment for the 'micro-machining' of tiny components on a large scale. Xin Ding at the Singapore Institute of Manufacturing Technology (SIMTech) of A*STAR and co-workers* have improved our understanding of some of the main technical challenges in micro-machining through experiments with a micrometer-sized diamond cutting tool.
One difficulty in using micro-machining equipment is that the cutting tools are on the same scale as crystal grains or defects in the materials. This means that the cutting tools may experience many changes in phase as they progress through the material, like a knife encountering gristle or bone when carving a joint of meat.
“When the cutting tool moves from one metallurgical phase to another, the cutting conditions change, causing machining errors,” explains Ding. These include a degraded machined surface, changes in cutting force or direction, and rapid wearing-down of the tool.
To investigate the physics of this process, Ding and co-workers used a focused ion beam to fabricate a single-crystalline diamond tool with a cutting edge less than 31 micrometers long. They mounted their tool on an ultra-precision machine capable of monitoring tiny forces during micromachining. Then, they tested the tool by cutting four different materials: two aluminum alloys, some very pure oxygen-free copper, and a nickel–phosphorous plate—three polycrystalline metals of various grain sizes and one grain-free amorphous metal plating material.
The researchers found that the oxygen-free copper left the smoothest surface when it was cut, despite having the largest average grain size. The nickel–phosphorous surface also provided a smooth cut surface, but required quite high cutting forces because of its very high hardness. In contrast, the aluminum alloy samples required relatively low cutting forces, but ended up with very rough surfaces with several holes, probably left behind by hard grains that get knocked out during cutting.
Meeting with the team’s expectations, the study has revealed that, owing to the size effect, the microstructure of the work piece can have a significant influence on the cutting force, chip formation and surface quality. However, Ding notes that they could prevent or minimize these negative effects by changing machining strategies. She believes that the research effort should now focus on developing new materials to be used in micro-manufacturing, such as high-purity materials or materials with refined or removed grains.
She also hopes to eventually develop tools and structures on an even smaller scale.
* Ding, X., Butler, D.L., Lim, G.C., Cheng, C.K., Shaw, K.C., Liu, K., Fong, W.S. & Zheng, H.Y. Machining with micro-size single crystalline diamond tools fabricated by a focused ion beam. Journal of Micromechanics and Microengineering 19, 025005 (2009).