We have a job that requires precision drilling and reaming in a turning center- on center and about 10x diameter deep. Runout needs to be less then .001". The material sucks, it is a nickle bronze that work hardens really easily...
Anyways, after playing with it a lot, the devil seems to be in the details of spotting it. The bar we are drilling is only 1/2" diameter and we pull it out after the last piece is parted off, then machine it down until it has a fresh, flat face on the front. Stopping and checking at that point, that seems to leave a very small tit in the center- from the tool pushing the bar around. So, I think the center drill is hitting that right on the point, this is pushing the bar slightly off center, and this causes the center drill itself to be off center before we even get anywhere... I grabbed a little carbide end mill and just bored a small pocket that wasn't even the right size, and this seemed 100x better- it dropped the runout on the far side by a factor of about 10- but depending on how large I bore the hole on the front side, the drill wants to chatter a little until it's really in the cut.
What do you guys do when you need to get something started DEAD on center on a turning center, with the bar stock hanging out about 5x diameter already? We might drill and bore a little "guide" pocket with either a drill and little bar, or a 2 flute end mill. Seems like we might have to figure out the right diameter to make the drill happy though.
Anyways, after playing with it a lot, the devil seems to be in the details of spotting it. The bar we are drilling is only 1/2" diameter and we pull it out after the last piece is parted off, then machine it down until it has a fresh, flat face on the front. Stopping and checking at that point, that seems to leave a very small tit in the center- from the tool pushing the bar around. So, I think the center drill is hitting that right on the point, this is pushing the bar slightly off center, and this causes the center drill itself to be off center before we even get anywhere... I grabbed a little carbide end mill and just bored a small pocket that wasn't even the right size, and this seemed 100x better- it dropped the runout on the far side by a factor of about 10- but depending on how large I bore the hole on the front side, the drill wants to chatter a little until it's really in the cut.
What do you guys do when you need to get something started DEAD on center on a turning center, with the bar stock hanging out about 5x diameter already? We might drill and bore a little "guide" pocket with either a drill and little bar, or a 2 flute end mill. Seems like we might have to figure out the right diameter to make the drill happy though.