Drilling tube on lathe is offcentered

[horst007]

I am drilling a .312x.063 wall stainless tubing on a lathe. The front of it is nearly perfectly concentric whereas the back side is almost .012 off centered. The part is 1.269 long and the hole I am drilling in it is .193. This is on a cnc lathe I am facing, then drilling, then pulling the part out of the chuck, and then parting off. Some parts are fine others are scrap. Please Help!

[ALLtra Mach]

Theres alot than can cause this.
With such a small drill, and ar over 6 1/2 times diameter, it's going to lead off easily.

I break up problems into 5 areas;

1.Programming.
Should go without saying, but surprising how many times little things in the code get missed.

2. Tooling
First, is the drill sharp, and is it sharpened correctly?
Speeds & feeds, tto much/too little tool pressure? Whats the finish like?

3.Fixturing
Chuck pressure, are you deforming the tube?
Is your tool holder holding the tool parrallel to the bore?

4.Process
I would be tempted to ream rather than drill.
I don't know if core drills come that small, but that would be better than a 2 flute.

5. Machine
Is the turret true to the chuck? Centered and square.
Is your Axis straight, meaning does it feed straight in Z? Do you normally make taper edits when turning or boring?

Not always in that order, but generally a good start.

[HuFlungDung]

If enlarging an existing hole, I've also seen problems making the drill or reamer follow the original bore. The best method I found was to make a piloted drill. Of course, the tubing that you are working with needs to have a tight tolerance on the ID, in order that the pilot not bind in the hole.

Grind the pilot a couple of thousandths under the ID size, the pilot only needs to be 1/4" long. Then, regrind cutting flutes further up the drill where the pilot ends. This requires some tool and cutter grinder capability, because of the restricted access.

I would not guarantee that you won't see some spiralling waviness to the hole. It helps to feed coolant from the back end of the tube stock, to keep chips from getting pinched on retraction of the drill during its peck cycle.

Maybe someone could advise on the feasibility of getting some kind of a gun drill modified for this type of work. I never tried that.