I was using an OSG thread forming tap, at 1000 RPM. #4-40 size. After about 90 holes the tap broke. The hole was drill with .0995 drill, and the the drill appears to still be sharp. The hole is a through hole. The tap was held in one of those special tap holders that compensate for spindle/feed synchronization (or lack of). Water based coolant was used, but not sure of the brand. Oh yea, and the material was extruded 6061 aluminum.
My question is what the possible cause of breakage is? It seems like the tap got stuck in the hole, and the fracture pattern looks like it was twisting when it yielded. This is my first time using a forming tool so I'm not sure what the life expectancy should be.
Thanks in advanced, sorry if this is maybe in the wrong forum. Kind of new to posting in this forum.