I am getting closer to doing the job that involves clamping open boxes,
DB 9 holes, etc. There will be some round holes, and my first thought
was a boring head. That would be overkill in precision capability, but
I'd like to learn how to use one, so it seemed reasonable.
The acrylic and facing problems came along, and a flycutter is
officially in transit (3/4 shanks). Can I (ab)use that as a hole saw?
Is it safe? Should I clamp the waste to avoid catching on the bit?
I still plan to get a boring head, but would rather wait a while to let
my wallet cool off a little. Another reason to wait on the boring head
is that I am not convinced I will take to flycutter. If it gives me the
creeps for facing, I might prefer a face mill to a boring head as my
It's very easy to make your own adjustable boring bars. A length of 3/4" or
bigger steel bar, drill a 5/16" or 3/8" cross hole at one end and drill and
tap for a grub screw to hold the cutting tool that goes in the cross hole.
If you drill the cross hole at 45 degrees to the shank instead of 90 degrees
to it then the non cutting end of the tool sticks up out of the way of the
workpiece i.e. you can use longer tools with more diametrical adjustment on
them. Cutting tools can be ground up from old milling cutters, drill bits or
use carbide tipped tools on round shanks which are cheaply available from
Start your hole with the biggest drill bit or milling cutter to hand and
then enlarge it as required with the boring bar.
I've made loads over the years in various sizes to bore from 3/4" up to 4"
or more. Whatever size bar I use for the shank I generally turn a 3/4" end
on it and run it in an R8 collet.
I made my flycutters/boring bars the same as Dave except I drilled and
reamed a 1/4 hole in the bottom end. Ream to a slip fit and use a dowel
slipped into the hole to measure over to the edge of the tool bit.
(Subtract half the dowel dia.)
Drill a 1/4 inche hole in the center of the work piece, install the
flycutter. Index the 1/4 dowel into the drilled hole and you stabilize
the tool to the workpiece. Hole will be dead nuts.
Remove the 1/4 inche dowel for flycutting, of course.
Oh, one more thing, you can slip a small spring over the 1/4 dowel to
hold the scrap down, if you wish.
Depending on the construction of the fly cutter, you may not be able to
install a tool such that it will serve as a hole saw. Fly cutters are
generally intended for tools that somewhat parallel the face being machined.
If you have one that permits the tool to be installed at a right angle,
parallel to the spindle, it might serve your needs, but you may not have the
option to select the diameter you desire. An operation such as that is
called trepanning, where you remove a core piece without reducing it to
chips. It can be accomplished easier with a boring head, but even that can
be challenging---especially when machining tough materials like the majority
of the 300 series stainless alloys or the chrome moly series.
Regards clamping the piece that comes loose---it's usually a good idea to
use a slightly angled tool, so the cut will be quite thin at one edge, the
outer one preferred, so you don't leave a lot in the hole. When it's about
to break through, you stop cutting and remove it manually. By setting a
quill stop, that's easy. Often, when you break through, the core piece will
shift enough to bind and break the tool, so it's not a good way to get the
piece out. If stop cutting at the right point, it's real easy to remove
the core with a light tap of a hammer on one side. Needless to say, you
should be on parallels, not working off the table face. Be mindful of the
tapered edge remaining--it can be quite sharp if you stop at the right
Using a boring head for fly cutting isn't likely to work much better than
using a fly cutter for trepanning. For one, it may not run smoothly if you
are running at reasonable speed. The advantage of fly cutting is the
ability to run fast, so your single point tool will permit a faster feed
rate. As mentioned in a different thread, you're limited by the tool's
ability to cut at the limits of the material, but some materials have
literally no restrictions. Aluminum, for example. If speed isn't a
concern, you might get away with the boring head, but I'd advise keeping the
slide locked to avoid movement sideways, under cutting pressure. Surface
finish would likely suffer otherwise.
If you don't have a boring head now, and you're debating between a face mill
and the boring head, that should be a no-brainer. Buy the boring head.
You can face mill with a fly cutter, which can be shop made with ease, or
even end mills, A face mill won't bore holes, so running without the
boring head would be the greater restriction.
I need to read this more and ask some more questions. However, my
question is not so much about using a boring head for flycutting, as it
is using either a flycutter or boring head to make large round holes in
thin steel and some truly awful gunk passing as metal: when cut, it
reduces to a mix of what seems like cast iron dust and razor sharp oat
meal - terrible stuff.
A hole saw might be the right answer, but I have relatively few such
holes to make, and have more faith in a carbide bit than the hole saws
All of that said, I might see the problem with the flycutter for the
job. I'll think it through some more and post again once I can explain
it. Boring bars can be had cheaply, and given that I don't care about
precision in diameter, I might do well to rig my own holder as suggested
elsewhere. Also, I can afford an import boring set; I was simply hoping
to put it off a while given the additional measuring tools I just
bought. No worries about the flycutter; it was cheap and I will
certainly get use out of it, so it was a great place to start for facing
if nothing else.
I bought a $27 (for about half price it turns out!) flycutter set and
some bits; they should arrive early next week. I reserve the right to
be scared silly of it (always thinking of "Lefty" and the blurry area),
in which case I might opt for the face mill. If the flycutter and I get
along (seems likely), then I agree the boring head would be next.
Are there bits commonly available for trepanning in a boring head? If I
wanted to make one, how would it be shaped? I'd start with round stock so
that it could be mounted in the head, yes?
I was faced with this challenge years ago, when I built a gasketed filter
press for my gold refining operation. I had to cut about two dozen
washers from an unknown (austenitic) stainless material, roughly 3/16"
thick. Finished washers were roughly 3" diameter, with a bore of 1-1/2" as
I recall. The press is long gone now, having been sold along with the
refining business I sold when I retired, so I may be off on the sizes. No
matter, the principle remains unchanged.
I used a 3/8" HSS end mill shank, on which I ground what resembles a parting
tool for a lathe, with a slight cant to the cutting edge, so it would leave
a very thin section on one side of the part being removed (the washer), so
there was little material to machine afterwards. You stop cutting before
breaking through to avoid the tool binding on the part being removed from
the stock, and the thin section makes it easy to get the piece out. The
problems of cutting stainless in this fashion were difficult to overcome. I
ended up buying a BoeLube setup, which solved all the machining problems.
Sulfur oil alone wasn't adequate. That would likely not be true on other
One runs the boring head very slowly, using the finest feed. With a chip
breaker properly ground in the tool, it works great, but would certainly
become a serious challenge if the cut was very deep. There's considerable
chatter, due in part to the rather flimsy setup. You can often eliminate
the chatter with proper speed and feed selection, however.
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?
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
As always, thanks.
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 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.
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.
I had another thought/realization. One ouwld be able to use a higher
spindle speed with a boring/trepanning setup than with a hole saw (chatter
being a gating factor obviously), yes? This would be an advantage to those
of us with a minimum spindle speed of 330RPM... A VFD is looking pretty
good at the moment.
By nature of the design of the trepanning tool for this type of application,
the tool tends to be rather flimsy, which is part of the problem, so you are
likely to be well restricted by chatter. If not, so much the better. In
aluminum you should be able to achieve a good speed, so long as you can keep
the cut well lubed. That, or machine 2024, which is quite forgiving of dry
machining. Remember, my project was one of the worst case scenarios, a
tough grade of stainless, so I was very limited as to RPM. You may have
good fortune and not be so restricted.
Love to hear how it turns out when you give it a go. Can we count on a
Greenlee punches can get expensive but they make clean holes. For round
holes the other option might be one of those multi-step hole cutters. I
don't like the flycutter/boring head idea.
Grant Erwin wrote:
Sorry - large holes, not necessarily non-standard, thin metal, one
steel, one a mix of various metalic substances. If I really get lucky,
I might be able to negotiate the steel out of existence, leaving just
the dusty/grity/disgusting metal that must be penetrated. That stuff
would probably yield to the cheapest of hole saws, assuming I can get
one of a suitable size (likely).
One thing to consider is that hole saws tend to make rather ugly
holes in metal -- at least based on use in a drill press.
What I always like for aluminum or steel panels is a Greenlee
Chassis punch -- but that can be *very* expensive.
And if you're working with the die-cast aluminum zinc boxes, I
think that the stresses of a chassis punch might lead to cracking.
What I've found to do a nice job of making clean holes in sheet
metal on a drill press is something called a "Roto-Bor". I have two
sets, covering a size range from about 1/4" to 1". They have a
spring-loaded pilot which fits into a center punch mark or a center
drill hole to precisely locate the cutter.
The cutter has two flutes for the smaller sizes, or four for the
The major disadvantage of this is that as far as I can
determine, these are no longer made.
Note that using a boring head on the mill will not eliminate the
need for a hole to start with -- and the bigger the hole the better,
because a boring head is rather a rather slow way of removing material.
So, a hole saw to produce an undersized hole is a good thing to use
*with* the boring head.
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Yes -- but that needs greater skill in grinding the tool to go
into the boring head. Sometimes, it may be easier to use the hole saw
followed by the boring head with standard boring bars. After all, the
original poster does not yet have tool grinding skills. (Nor do I
remember whether he has even indicated that he has a bench grinder.)
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