Product miniaturization trend is inevitable. The needs for minimum invasive surgery, smaller sensors for smart machinery, packing more features on a product… require mass production of smaller components from engineering materials. Fabrication of microcomponents requires knowledge of micromachining to avoid costly tool failure and part damage. This research investigates microdrilling of commercially pure titanium, nickel titanium (Nitinol), and 316L stainless steel.
A surface was polished and drilled in rows of ten holes. Through hole drilling at 6:1 aspect ratio was performed on NiTi sheets while blind holes were drilled at 10:1 aspect ratio on Ti or 316L blocks. Microdrills with 100–150 μm diameter and 1.5–3.5 mm flute length were utilized up to 50,000 rpm in minimum quantity lubrication. Finite element models were developed to find upper limits of drilling parameters. Flank wear of 15μm on fine grained WC-Co uncoated tools and peeling of coating layer were used as tool life criteria. Scanning electron microscopy was used to observed tool failure mechanism. Tool life modeling and hole quality were performed to evaluate and compare tool performance.
Although successfully drilling all materials at 10:1 aspect ratio, excessive built-up-edge (BUE) was found on microdrills at all drilling parameters. Such BUE effectively blunted the drill tips and caused drill wandering, degraded hole quality due to rubbing against the drilled wall, work-hardened the drilled surface and accelerated drill wear, and formed burrs at both entrance and exit ends. Wear of a microdrill at the outer corner was more pronounced when drilling CP titanium, but attrition wear at chisel edge was more significant for 316L stainless steel. The classical Taylor equation for macromachining was applicable in microdrilling to rank tool performance and machinability of tested materials. For the same cutting speed of 20 m/min and comparable drilling distance of about 35 mm, CP titanium can be microdrilled 400% faster than 316L stainless steel when applying 0.1 μm/flute chip load for the former and 0.02 μm/flute for the latter. The AlTiN coated drills improved tool life by at least 120%. This coating reduced BUE, drastically improved hole position accuracy by 115%, and decreased hole diameter variation from 0.110% to 0.003% for each mm of drilling distance.