On Mon, 21 May 2012 19:05:03 -0400, Ed Huntress
wrote:

On Mon, 21 May 2012 15:54:37 -0700, "Bob La Londe"
wrote:

"Ed Huntress" wrote in message
. ..
On Mon, 21 May 2012 12:26:00 -0700, "Bob La Londe"
wrote:

Yes. I don't know how to drill gun barrels either. LOL.

Gun barrels are drilled with single-lip bits that self-center by
bearing against the work with the side that's opposite the cutting
lip. In volume gun drilling, both the work and the bit rotate.

I can't figure how you're going to get straight holes in your
application, but I don't see how gun drills would help. They're a
technique unto themselves, and kind of tricky.


Well, the idea was to make a guide for the drill busing to make the hole.
The protrusion is parallel and concentric on 180 degrees to the path of the
hole that needs to be drilled in it (already done by hand with just an
extension). The deep straight drilling was to make the guide. When push
came to shove my hand was accurate enough for this application, but I can
see future applications where it might be necessary to get more precise
angular alignment.

I'm probably missing some of this, but when it comes to drilling long,
straight holes, there are a few things to consider. The straightest
holes are drilled with single-lip bits that are made for the job.
They're balanced around the complete cutting circle in one important
sense: the cutting edge is always presented to the work with the same
radius and with the same angles. That, plus the back-side support, can
produce straight holes.

But you can do Ok with other bits, such as crankshaft bits, used for
drilling oil holes, which have length/diameter ratios exceeding 20:1.
They're production tools used with bushings.

Holes wander because of some complex geometrical factors. I haven't
studied the engineering on this for decades, so I won't venture to
give an explanation. I just remember that it was complex and sometimes
surprising.
I worked for years is a shop that had gundrilling as a major part of
our work. I made lots and lots of bushings for gundrills to get them
started accurately. We looked at machines that rotated the work at the
same time as the drill. Not because of higher production but because
of the increased straightness possible with this technique. We
determined that most of the work we were doing didn't require the
extra straightness. We were already drilling holes that were quite
straight. For example, .094 diameter holes 19 inches deep with less
than .004 deviation. I don't remember the oil pressure used on the
tiny drills but it was quite high. I did design and build some special
rotary unions that had small but constant flow of oil past the seals
in order to keep the seals working. The housings had to catch the oil
to return it to the reservoir and keep it from spraying everywhere.
The rotary unions worked great. Parker seal company helped in the
design by supplying info about their seals and how they worked.
Eric