I'd like to bore a hole approx 2.75", I have a boring head from APT which will do it, but cranking the slide over far enough to use either of the vertical bar mounts leads to significant vibration on my Sieg X3.
Can I use a horizontally mounted boring bar in order to reduce the rotating offset mass? .500 inch round shanks in a 2.5" head.
I recently cut some panel meter holes that way, after roughing them out with the nearest undersized hole saw. The feed outward was 0.010 per pass, more than that caused trouble. Speed was 180 RPM, the slowest pulley setting, and still somewhat too fast.
Not sure I understand the question. Your boring head looks like the Harbor Freight model 33562-0VGA L? With two holes that are vertical and one hole that is horizontal? And you are asking if you can use the horizontal hole?
I have not read anything about how to use a boring head, but I would assume the horizontal hole is put there to use.
Have you calculated what the RPM ought to be when boring a 2.75 inch hole? I have not, but 500 rpm sounds pretty high to me.
I happen to have a calculator handy. At 500 rpm the SFM is 360. Okay for brass or aluminum.
Second, when using a boring bar you want the cutting edge slightly above center. That is so any deflection of the cutting edge reduces the cut. The same principle as having the cutting edge slightly below center when making a cut on the OD.