Compression CNC Router bits and mills upcut on the bottom of the
bit and downcut over the rest of the length.
Part Three: More End Mill & Router Bit Details
In my last post, the focus was on the cutting edges, or
flutes, along the sides of an end mill. If you’ve missed the previous segments
on CNC ROUTER mills, here’s the intro and part one. As suggested
earlier, two-flute end mills designed for woodcutting and router bits almost
always work best. We’ve reached the end of this tooling primer but before we
go, I want to pass on a few more details you should know about CNC ROUTER
mills. I’ll be focusing on hobbyist-sized CNC ROUTER machines next month but
I’m certainly not done with this topic. Down the road, there’ll be much more
about CNC ROUTER mills including selecting mills for specific purposes such as
part cutting, carving, specialty and high-performance mills, a suggested basic
mill set for digital woodworkers and more.
CNC or Computer Numerical Control is
growing in importance in schools. This section is an introduction to CNC Router
machining and explains in simple terms the equipment needed and how it can be
used.
Upcut or Downcut?
Many straight end mills or router bits
come in two versions: upcut, where the waste from the cut is pulled upward and
ejected out the top; and, downcut where the waste is actually being pushed down
and back. If you’ve used a hand-held router, chances are you’ve mostly used
upcut bits. If you’re making a mortise that will later house a tenon, for
example, you certainly want to get the chips out of the way as you’re cutting.
The same thinking holds true when using a CNC ROUTER. You want to eject the
chips as fast as you can.
However, there are some trade offs to
be considered when using upcut mills on a CNC ROUTER. For one thing, upcut bits
are more likely to tear the top surface with the cutting edges because they’re
pulling the wood fibers up while cutting. On laminated materials, such as
plywood with a thin top veneer, that can matter a lot. It may not matter if
you’re ultimately going to round over that top edge and/or apply solid wood
edge, but in other situations, it might be a problem.
Another issue that’s different on a CNC ROUTER than with a router is that the upward cutting action pulls the piece of
wood you’re cutting off the table. If you’re not careful, that can cause the
piece to move if it’s not firmly held down. So, in some situations that makes a
downcut bit or mill the better choice. With a downcut mill, the advantage is
that it’s pushing down on the piece you’re working, plus it cleanly cuts the
top surface. The tradeoff is the waste is pushed back into the cut area instead
of being ejected.
So how do you decide on an upcut vs. a
downcut bit on a CNC ROUTER?
If you’re cutting MDF or HDF you
absolutely have to use an upcut bit. Otherwise, between the bit getting
overheated and the fine paper dust-like waste being pushed back into the cut,
there’s a high potential of starting a fire. I know because I’ve made the
mistake of using the wrong bit at a slow speed, and the result was a near-fire
situation with smoke pouring off the MDF board I was cutting – unnerving to say
the least! (And yes, I ran to my dust collector barrel to make sure that I
wasn’t starting a fire there, too.) You can bet that I’m careful about this
detail now. My mantra: MDF = Upcuts.
As you can guess, when cutting solid
wood, most of the time an upcut is the best solution. But not always. On a CNC
ROUTER, I use downcut bits almost exclusively when I’m cutting out solid-wood
parts, because the top surface comes out perfect and the wood is held tighter
the bed because the bit is pushing down as it’s cutting. But I have to run the CNC
ROUTER faster so that the chips and the bit don’t overheat. To do this, I
usually take shallower cuts and multiple passes at higher speeds than I would
use with an upcut bit, where I might take the opposite approach with slightly
deeper cuts and fewer passes.
Compression Bits
Thanks to modern end mill and router
bit design, you can now have your cake and eat it, too. Compression bits do
both upcuts and downcuts at the same time. They have flutes that upcut on the
bottom .250″ or
so, and flutes for rest of the cutting length that are for downcutting. These
are the cutting tool of choice for laminated materials such plywood. But you
have to use them correctly. Your first pass needs to be slightly deeper than
the upcut section of the bit, say .260″, then the remaining cutting area on the bit will push down the
board and cleanly cut that top surface without tearing the thin top veneer
layer. Also, if you cut all the way through the board, you get a bonus: The top
cutters don’t tear out the bottom veneer layers because they are pulling the
fibers inward. A win/win situation.
For solid woods, compression mills or
bits can work just as well. But to get their major benefit you need to keep in
mind the depth of that first pass. For very hard woods, .250″ might be too deep a cut. With other,
softer, woods this may not be an issue. For MDF, which is notoriously hard on
cutting tools, it just seems like such a waste using an expensive bit on such
as abrasive material, so I stick with upcut bits.
Climb vs. Conventional Cuts
With conventional cuts the bit is
biting into the material at it travels. A climb cut, on the other hand, pushes
the bit away from the material. This is why it’s so difficult to control a
hand-held router during a climb cut, and why you really need to pay attention
to the direction that you’re routing. Cutting direction is a different
issue on a CNC ROUTER machine where the stock is tightly held onto its bed.
There, you can take advantage of climb cutting to get the benefits, which are
usually less tear-out and a smoother cut. In fact, on a CNC ROUTER, climb or
mixed cutting are the preferred techniques. Mixed cutting is faster, too.
However, there will be times when cut direction on a CNC ROUTER is important.
I’ve had a few issues with very soft woods with strong grain where I’ve
restricted cutting to conventional cutting, but for the most part, I cut
bi-directional to speed up the milling process.
hough I prefer carbide end mills and router bits for their
longevity, sometimes bargain high-speed still mills have their place. These
heat-treated Onsrud 3/8″ mills are very useful for part cutting.
Cutting-Tool Materials
Router bits and end mills are made out
of a variety of materials. The most common are high-speed steel (HSS) or
carbide. Carbide is harder, lasts longer, resists heat better and can take
faster feed rates. In my work, most of the time I use carbide mills. If you’re
already using a router you likely are using them as well. Carbide lasts a long
time and the better ones are worth resharpening. You need to be very careful
when using carbide cutters; they break with the slightest deflection. Though
carbide is superior in most ways, HSS mills have their place in some situations
and when cost is a consideration. I have occasionally found great deals on quantities
of HSS mills in sizes I like to use for CNC ROUTER part cutting. If the work
isn’t too hard and you’re careful to not overheat the bits, use them. I’ll use
HSS cutters on a few projects; once they’re dull, I toss them.
To further enhance wear resistance, various
coatings, including titanium aluminum nitride (TiAN) and diamond, are sometimes
applied to carbide tools. Though these may add to add to a cutter’s
longevity for specific metals, for woodworking use, an uncoated mill running at
an optimal speed works just fine. Some manufacturers, including Onsrud, use
special coatings on some of their end mills to protect them from higher
temperatures.
wood CNC Router
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