If you're using normal bi-metal bits then try the TCT Metal Hole Saw bits like these http://www.ebay.com/itm/15-100mm-Steel-Carbide-Tipped-Drill-Bit-TCT-Metal-Wood-Cutter-Hole-Saw-Tool-/281367869418?pt=US_Hand_Tools&var=&hash=item4182d4f7ea. Ive found that just drilling on a slow speed and with...