"Peter Grey" wrote in message
nk.net...



"Harold and Susan Vordos" wrote in message

One of the things you have to address is considerable side clearance on
the
tool, where it must clear the radius of the cut on the outside edge of
the
circle. The tool looks somewhat odd, but works fine. Should you
decide
to try it, remember, the material you select for your tool will leave
the
tool edge ahead of center unless you reduce it at the point of the cut
to
the centerline of the tool. That the cutting edge is on center, or
not,
has a serious effect on how the tool sees the material, and the relief
angles. Does this make sense to you? If not, lets talk. It's
important
that you understand it well.



I think what you're saying is that if the entire cutting edge is to be
square to the cut, the tool itself may need to be curved, and the inside
and
outside cutting edges will be on the centerline, making the edge look
"slanted" in relation to the rest of the tool (I suppose if the tool were
short enough that it wouldn't need to be curved, then one would get more
chatter...?). Unlike a bit where the cut is straight, although each
relief
may look different, when measured in reference to its own cutting edge
it'll
be the same as the other. Am I understanding this or have a disappeared
over the horizon on a tangent?

Sigh! Trust me to say something in such a way that I use up all the words I
know, but fail to hit on the point at hand.

OK----lets try it this way:

Boring heads (at least the ones I have) are made such that the centerline of
the cutting tool is the centerline of the head. For boring, that's exactly
where you want the tool to fall, due to the position of your boring tool.
For trepanning, that isn't the case. That means that when you make a tool
such as I described, the actual cutting edge will be in front of center
unless you grind away half of the shank you use to grind your tool. Does
that make sense? You'd grind what is a D configuration, with the
straight side of the D the cutting edge, which will parallel the slide of
the boring head. This tool, when installed in the boring head, would have
the cutting edge on the centerline of the head. You then grind away the
sides to establish the width of the tool, grinding greater relief on what
will form the outside of the circle you'll generate, so the tool clears the
circular slot it generates, and much less on the opposite side, for the same
reason. The tool should also have back clearance, so as it goes deeper
into the cut, it doesn't drag on the portion behind the cutting edge. In
simple terms, the point of contact, the cutting edge, will be the broadest
portion of the tool, with clearance in all directions ground on the tool.
Hope that makes it somewhat more clear. It's easy to do, just hard for me
to describe. One more thing to remember. Make the tool cut in a clockwise
direction, so you don't unscrew the boring head from its shank. The tool
you'll grind will be handed because one side has considerably more relief
than the other.


I just finished a job where a setup like this would have made sense. I
was
drilling 2.250" holes in .250 6061 plate and then finishing them with a
boring head. I did 12 of them. The hole saw was pretty sloppy, and
although it wasn't all that important that the holes be of similar size,
the
diameter of the holes made by the hole saw varied quite a bit. The goal
was
to have holes with a decent finish and it would have satisfied my sense of
propriety if the first holes were accurate and finished well enough so
that
passes with the boring bar were unnecessary. It's probable that I'll end
up
performing this operation again, so I'm interested in how to do it quickly
and cleanly.

I fully understand about the hole saw. They're a dreadful tool, although
they are good for opening a large hole. Like you, I've experienced various
sizes from the same saw, and am never pleased with the quality of the hole.

A note to remember. When you open a hole by the means I've described, it
may not come out really pretty. You'll have some trouble with chip flow if
your tool isn't perfect, and the cut well lubricated, so the size could vary
to some degree ( a few thou) from hole to hole, depending on if you have
chip problems, or not. Finish would be a reflection of chip flow and
lubrication. Bottom line-----if you want all the holes to have sharp,
square corners, with a nice finish, treat the trepanning operation in
keeping with what it really is-----a roughing operation. Shoot for a hole
that is somewhat undersized----maybe .020/.040", depending on how well it
goes. That will allow for a finish pass with a boring head after the
fact, providing a uniform hole and finish. That's called good workmanship,
and should be practiced unless the hole just plain doesn't matter.


As always, thanks.

Peter

You're welcome, Peter. Hope some of this helps. All of it has been used
with success, so if you have any particular problems, let me know. Could be
I've been through the same thing and may recall how I got around it.

Harold