First off, I'm a machinist and usually work in metal and both dislike and
don't have much experience with woodworking...no flames please, just a
I have a CNC table that I use extensively for metalworking. I got dragged
into a project and volunteered to cut out some plywood countertops, no big
deal I thought.
I got one sheet of plywood and cut out a few pieces, no problem, using a
1/4" x 1" wood bit, cut through 3/4" plywood at 32 inches per minute running
at 18,000 rpm (5 hp motor). Must have made cuts totalling about 16 linear
feet. Cuts looked beautiful.
Then I went back to home depot and got another sheet of what appears to be
identical plywood (finished one side exterior) and immediately broke my bit.
I put in another bit, slowed down the feed rate to 22 ipm and almost
immediately broke that bit. Then I set up the machine to take 3x 1/4" depth
cuts, got about 4 feet cut and broke a fourth bit.
Can somebody tell me what the heck is going one? I don't see any difference
between the two sheets of plywood.
I don't have a clue why you are having problems, but a couple of
1) Don't know what you are using, but you will get better results with
carbide rather than HSS bits.
2) 1/4" dia is a bit skinny for this job IMHO. I'd probably use at least
1/2" dia, probably 3/4" dia bit for this job.
S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
No flames, just some heartfelt sympathy from us generous and
considerate woodworkers for your unfortunate serious affliction. :)
Don't know what could be wrong, but the tool of choice for cutting
through plywood is a saw, not a router bit. What surprises me is that
you managed to cut 16' without breaking the bit or burning the edge.
Routers are used for edging or for short plunge cuts such as mortises.
Remember that wood has different properties depending along which axis
you're cutting, and different pieces of wood have different machining
properties, even when they come from the same tree. Most of us have no
idea at which speed we feed materials, we work by feel.
Maybe cut it out slightly oversized (1/16" or so) with any kind of saw
you might have access to (hand saw, jigsaw, circular saw) and then use
your CNC to clean up the cuts.
Replace "no" with "yk" for real email address
Wood isn't metal. You're used to working with materials where everything is
consistent, and has very predictable characteristics.
Programming a depth of cut and a feed rate into a CNC machine is a pretty
alien concept to the average woodworker. You're trying to bring digital
precision to an analog world, my friend. You need to *feel* the wood.
Plywood is remarkably consistent as wood goes, but it's still wood. Those
two sheets could be made from completely different trees with different
growth rates, and different densities. The moisture content might vary.
They might even be made of different species entirely, perhaps only on the
inner plies, and you'd not likely notice the difference. Then there's all
the stuff on the inside that you can only imagine. Knots, voids, wood
filler, pockets of glue. It has to look good where people can see it, but
utilitarian grades of plywood are not meant for making fine furniture, and
they're not manufactured to the same standards.
The bottom line is if you're breaking that many bits, you're pushing them
Consider that routers normally run at 25,000 RPM or higher. Consider that
you're using 1/4" bits, which are intended for the low end of the router
spectrum, where 1.5 hp is a big motor.
I suspect you're turning the bit too slowly, with way too much motor behind
it, and feeding too fast. Your first piece of plywood was relatively
uniform, and your second one has a bunch of knots or other hidden variables
buried in the plies. Maybe they're from different lots, which would
increase the chance of variability between them considerably.
Maybe try punching in a much slower feed rate and/or higher RPMs the next
time you program your Big Trak, depending on what you can control with that
Michael McIntyre ---- Silvan < firstname.lastname@example.org>
Linux fanatic, and certified Geek; registered Linux user #243621
Good thoughts and comments Silvan... As someone else mentioned, a 1/4"
diameter bit is pretty fragile - a 1/2" or 3/4" diameter would be
stiffer and likely longer lasting. Also a 1/2" shank over a 1/4" would
be advised. His 22 inch per min feed rate seems pretty slow and a
non-issue, just by pretend routing a 22" edge on my desk. I agree on
speeding up the rpm's - 25K, which, with a larger diameter bit would
yield a faster cutting edge speed and may help power through knots or
other tough stuff in the plies.
Q for the OP: Did you notice if the breaks were happening at any
consistent point with respect to area on the sheet or direction in
relation to the length/width? Did you examine the sheet where the
breakage occurred to see if there was any sort of internal anomoly with
the material? What is the source of the bits and their configuration -
one, two, four flute - solid spiral, brazed carbide - etc.?
Because you had success with the first sheet, and lacking personal
experience with CNC routing of plywood, I'm hesitant to offer up
commentary, but... plywood is tough on cutting edges. The glues will
dull an edge much faster than solid wood as well as cause greater heat
buildup of the cutter. Light passes at 1/4" max per pass at 25K rpm with
a larger bit may help.
Owen Lowe and his Fly-by-Night Copper Company
Offering a shim for the Porter-Cable 557 type 2 fence design.
Try a jigsaw for the curved cuts and a table saw or circular saw with
a staight-edge guide for the straight cuts.
As with most things, using the right tool for the job makes all the
Cross-cutting or ripping with with a router is tough on the bit. The
only time I've done it was while making a dado jig.
Most likely the bit is breaking because it is loading up. Increase bit
speed, decrease feed rate and keep the cut area free of sawdust with good
airflow. 1/4" bit is not too small for what you are doing. There can
different cut characteristic from sheet to sheet in plywood.
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I have a ShopBot CNC router in my garage that gets used on both plywood and
HDPE plastic. I also use it to engrave equipment labels (text down to 14
point) and mill printed circuit boards, but that's another story.
I'm cutting at 21,000 RPM (nominal) with a Porter Cable 3-1/2 HP (yea ...
another story there) router using both 1/4" and 1/2" inch bits. Cut speed is
3 inches per second (180 inches per minute) ... but I don't make the entire
cut in one pass. Rule of thumb is to cut no more than 1.5 times bit
diameter, but I seldom go over .66 times bit diameter (0.166 inch depth of
cut on a 1/4" diameter bit). Yes, it takes a few more passes, but the only
bit I've ever broken was the one I ran sideways into the material with the
router off (oops!). I get a nice clean edge, and since the repeatability on
the ShopBot is on the order of 0.005 over the entire 4' x 8' bed, I don't
worry about the multiple passes. In fact, once you make the first pass, you
can walk away knowing the rest will miss the clampdown bolts, parts of the
I get the smoothest cuts with a two-flutie, O-flute with no spiral. Up
spiral really keeps the chips clear, but lifts the work off the table, and a
down spiral packs chips into the cut so tightly that you're recutting the
same path again and again (loading the spindle down in the process).
Sounds like you have a heavy duty spindle motor (Columbo?). Listen to
determine if you're loading the spindle down when you start cutting. If you
have any means of monitoring motor current, that would be more accurate than
by ear. You don't want to push the bit to the point where it can't cut fast
enough. Rule of thumb I was given when I started out was to achieve chip
thickness of around 0.007" to 0.009"; any thicker and you're going too fast.
Thinner is ok, but it just takes up more machine time.
I also have a dust collector pickup that surrounds the bit and pulls most of
the waste away from the working surface. Chip clearance is important ...
lacking a pickup, you could use compressed air to keep the path clear.
The ONLY burning I've ever experienced was when I tried using a "high speed
steel" bit to countersink some holes in the waste board. Switched back to
Carbide bits and have not had any burning. I found that Onsrud bits are
pretty good ... they design the bits around the material being cut ...
getting a bit specifically ground for CNC plywood cutting will help.
Hope this helps,
"Tom Kreyche" wrote
From a former machinist:
It's not the material you're trying to cut, it's the junk bit you're
Don't use "wood" bits. Use a quality HSS 2-flute center-cutting spiral
endmill, 4-flute mills tend to clog up with sawdust. Use all the
spindle speed you've got (18000RPM is way too slow) and feed at about
0.0007 per flute per rev. That would be 56 IPM @ 40000 RPM or 140 IPM
@ 100000 RPM.
18000 rpms is fine, your feed rate may be too fast and the depth of
cut should be no more than 3/16" deep. Use carbide tips ,or put an end
mill in the machine instead of a router bit. The glue in the
laminations wears out highspeed steel and carbide quickly.I am not
familiar with Cnc routing although I did see it once. Is it possible
that you are routing in the wrong direction? I imagine with metal it
makes no difference ,but wood the cutter should chop into the wood.If
you go the wrong direction ( climb cutting ) the bit will heat up
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