Bill Schwab wrote:
John,

If you are talking about enlarging an existing hole, it will work fine.
You may have to make multiple passes, re-setting the bit after each
pass. You will have to measure very carefully on the last few passes
if you need any real accuracy. All of which will take time.

Accuracy is not a big deal, which makes the discovery of an appropriate
hole saw reasonably likely. Hole saws are getting fairly good reviews
in this thread, which increases my confidence in them.


The boring head, of course, will be a lot faster as you will just crank
out the slide after each pass.

Dumb question: when you say a "pass", are you referring to: (1) set the
bar at a given radius; (2) quill down to contact; (3) slow feed until
breakthrough? Is the bar parallel or perpendicular to the spindle?
Does it matter?


If you don't have the central hole to start with, it's a lot more
difficult. You could grind up a trepanning cutter for a fly cutter
head, but it would be a lot of work to get the edges right for both
inside and outside cutting and it would only work for very thin stuff.
In the boring head you can hold the bit end on to the work, and can
therefore grind it a lot more effectively.

The stock is thin, so I have that going for me. I'll try to do some
reading on trepanning and then take another crack at this and Harold's
answer. Correct me if I'm wrong, but I think you guys are saying pretty
much the same thing.

Thanks,

Bill

The hole saw is the way to go. If you need a better finish or a
different size, finish up with the fly cutter or boring head.

Your interpretation of "one pass" is correct. And the boring bar or
bit can be held in either orientation in the boring head - it all
depends on the size of the head and the size of the boring bar. For
smaller holes it will have to be parallel to the spindle.

Trepanning can be a difficult operation. In a way, it's like parting,
because both sides and the end of the tool are in contact with the
work. Which means no side rake. Since it's not as rigid as a typical
parting setup, back rake can also be a problem. And then, as you'll be
cutting a curve, you need different clearance angles on each side of
the bit.

I suggested that you could grind up a trepanning bit for a flycutter if
you lacked a boring head. That bit would have to have a tit on the
side to do the cutting - sort of like a bit that you might grind up to
cut a small internal thread. Maybe an Acme or square thread.

John Martin