Gun drilling - ID bore concentric to OD - methods?

[pmurdock]

Looking at doing some deep hole drilling. Hole length is 10 inches. Bar OD is 0.940".

I'm looking at starting with 1" OD stock, then gun drilling the ID hole which promises to provide a very straight hole (using Sterling Gun Drills on a Haas SL-30 lathe).

A question in my mind arises about the concentricity of that hole with the OD. If it's not concentric I'll need to turn the OD down to make it concentric.

How is this accomplished?

It seems like one would want to remount the piece so that the ID bore is inline with the centerline of the lathe so that the OD can be turned concentric to the ID bore, but how does one do that? Do I need to machine a piece that will fit inside the bore and mount that between centers and somehow fix my tube to that so I can turn the OD down to .940".

curious as to what other have done in this regard.


cheers!
Paul

[PixMan]

Om Swiss-turn (sliding headstock) lathes, I've had the best results by making a short pilot hole first, using a rigid tool that's the same size as the gun drill. Then introduce the gun drill to the hole, mounted on a live tool station, but with "dead spindle." Only once the gun drill is in the starter hole do I start it counter-rotating with the spindle. This was small-diameter (1mm to 1.7mm) through holes in cannulated bone screws, and I found it best to rotate the gun drill at about 6x higher RPM than the workpiece. The holes always came out with less then .1mm of runout in lengths of up to 5".

Alignment of the two counter-rotating spindles is critical for good results.

[HuFlungDung]

Paul,
Turning between centers would be the method I would use. I would run a center directly in a small chamfer cut or bored in each end of the gun drilled hole.

A face driver might be something you could look at: it has dogs which bite into the end face of the part, while it is supported on a center. Those things are expensive.

However, if you can allow for a bit of overlength on the stock when drilling, then you could chuck on that and run a tailcenter in the outboard end.

There are quite a few pains necessary to get accurate results between a chuck and a tailcenter. For best chucking effect in an accurate chuck, you should chuck the end of the part where the gundrill entered, because this is where the runout will be minimal. If possible, even skim the OD for a short distance while it is setup for the drilling, if that is possible.

Then turn the part end for end, and only grab maybe 1/2" of the fresh turned part length in the jaws. This makes it fairly flexible in the chuck. Then, the tailstock will have a chance to pull the part on center, without introducing a warp.

For deadly accurate work, I always check the tailstock quill for flex after I think the part is set up. If you roll the chuck over and see some movement of the indicator needle while it is resting against the quill, then you'll know that your chuck is applying enough force to the part that the tailstock is unable to bring it into true. I usually just note the high side, back the tailstock off a bit, tap the part with a soft hammer to try to correct the runout error and retry until everything stays steady according to the indicator. At that point, it is ready to machine.