Drill and tap hole in the center of a 1 in aluminum rod - will this work?

[dansawyer]

Background: Construct a gear drive for a telescope. As accurate as it can be made with the tools at hand.

The next step is to make and tap a 1 inch deep hole in the center of a 1 inch aluminum rod. The rod is about three inches long. The hole will be just over an inch deep and tapped to 1/4 by 20 thread.
I have a 9 by 36 South Bend lathe that is fair to good with a 3 jaw chuck. I know the tail stock is a couple of thousands out of alignment. When I mill test holes they end up about 5 thousands over diameter.
My thought is to use that to advantage. If I mount the rod in the lathe chuck and place an under sized mill cutter in the tail stock and slowly mill the hole the result should be a centered hole slightly larger then the cutter. If I use a 4mm cutter the hole should be centered and smaller that .201. I can then use a #7 to finish the hole.
Does this make sense? Am I on the right path or is there a better way?
(If I am very lucky I could test a 5mm cutter and end up with a centered hole close to ready to tap. 5mm is about 5 thou under.)
Thanks in advance. Dan

 

[main_cogg]

I would just center drill and follow that with a #7 drill. The tolerance range for the minor diameter of 1/4-20 is .196 - .207, the #7 drill is .201, right in the middle. If you take your time and don't push the drill too hard the drill will flex enough to stay on center even with the tailstock being a couple thou off.

 

[machPete99]

Sounds like a good excuse to get your tailstock into alignment

 

[brino]

My Take?

If you're using a hardware store grade 1/4-20 bolt then having a tailstock that is a couple thousandths out of alignment doesn't even matter.
I don't have figures in front of me, but I'm pretty sure the thread clearance is bigger than that.

As accurate as it can be made with the tools at hand.

Of course, one should always try to do ones best!

I would:
-put part in 3-jaw chuck, check run-out with indicator
-if not good enough move to 4-jaw chuck and centre it "good enough"
-centre drill (using only the very tip, not the 60 degree section used for lathe centres)
-finish drill to #7
-tap in lathe, turning by hand

-brino

 

[T Bredehoft]

If you want the tapped hole to be "on center", drill it to 3/16 (.187) bore it to .201 or what ever you feel comfortable with. That way the hole will be on center. Don't have a boring bar? Use a broken drill bit, ground to cut, or 1/8 end mill or your imagination.