Bill,
Well it will center it up, but if the tailstock is still offset, and not on
center, the pressure on the side of the hole from the bit will cause the hole
to be larger in diameter than the drill (wallow it out). So, the bit needs to
be true on center. Even at that, a bit is not accurate for a finished hole even
in a lathe. The only way to finish the hole off so it's true, and the correct
diameter is to ream it, or bore it with a boring bar. Gun drills use a reamer
in place of the bit to true up the hole, really they have reaming machines
which look the same as a gun drill. A boring bar is set up on the lathes
carriage and tool holder. It cuts on the side of the hole closest to the
operator although you can do a reverse cut. For large holes, we used a turret
lathe which had a large spade bit chucked up. Then we swung the turret around
to a reamer which took about 0.010 - 0.060" to finish it off. Reaming though is
a lot slower process than boring because of chatter. We did some
with a boring bar on the carriage of the turret lathe and one mounted on the
turret off a slide. For a regular lathe, you only have the tailstock to drill
with as far as its quill will travel. You can bore the holes though using the
carraige. You can also chuck up a reamer on the tailstock and ream the drilled
hole. This is unless you mount a drill chuck on center off the toolpost. The do
make an attachment to do that.
Best,
Will
*********** REPLY SEPARATOR ***********
On 12/23/05 at 2:16 PM Bill Turner wrote:
>ORIGINAL MESSAGE:
>
>At 09:16 AM 12/23/2005, you wrote:
>>Gun drills are similar to lathes except the tailstock is made
>>specially to hold extra long drills. They also have coolant for the
>>bit. The bit has a guide-support at the end closest to the rod to be
>>drilled. The drill has one straight long flute and is stationary. As
>>the rod is turned in the tailstock, the drill is advanced by the
>>tailstock which is geared to the lathe. It advances slowely and
>>takes about 45 minutes to drill a rifle barrell. After drilling, the
>>barrell is reamed, then rifled, and chambered.
>
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>Ok, but that doesn't answer my question: By rotating the rod instead
>of the drill bit, does that automatically keep the bit centered in
>the rod? As I understand the video, if the bit were misaligned a
>small amount, the rotation of the rod would return it to center. True or
>false?
>
>73, Bill W6WRT
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