Compression Bits. CNC Tooling Basics for Woodworkers: Part Three

Compression Bits

Compression router bits and mills upcut on the underside of the
bit and downcut over the remainder of the size.

Half Three: Extra Finish Mill & Router Bit Particulars

my last post
, the main focus was on the reducing edges, or
flutes, alongside the perimeters of an finish mill. In the event you’ve missed the
earlier segments on CNC mills, right here’s the
part one
. As advised earlier, two-flute finish mills
designed for woodcutting and router bits virtually all the time work
finest. We’ve reached the top of this tooling primer however earlier than
we go, I wish to cross on a number of extra particulars it’s best to know
about CNC mills. I’ll be specializing in hobbyist-sized CNC
machines subsequent month however I’m definitely not performed with this subject.
Down the highway, there’ll be way more about CNC mills together with
deciding on mills for particular functions reminiscent of half reducing,
carving, specialty and high-performance mills, a advised
primary mill set for digital woodworkers and extra.

Upcut or Downcut?

Many straight finish mills or router bits are available in two variations:
upcut, the place the waste from the lower is pulled upward and
ejected out the highest; and, downcut the place the waste is definitely
being pushed down and again. In the event you’ve used a hand-held router,
chances are high you’ve principally used upcut bits. In the event you’re making a
mortise that can later home a tenon, for instance, you
definitely wish to get the chips out of the best way as you’re
reducing. The identical pondering holds true when utilizing a CNC. You
wish to eject the chips as quick as you possibly can.

Nevertheless, there are some commerce offs to be thought-about when utilizing
upcut mills on a CNC. For one factor, upcut bits are extra doubtless
to tear the highest floor with the reducing edges as a result of they’re
pulling the wooden fibers up whereas reducing. On laminated
supplies, reminiscent of plywood with a skinny high veneer, that may
matter quite a bit. It might not matter in the event you’re finally going to
spherical over that high edge and/or apply stable wooden edge, however in
different conditions, it is perhaps an issue.

One other situation that’s completely different on a CNC than with a router is
that the upward reducing motion pulls the piece of wooden you’re
reducing off the desk. In the event you’re not cautious, that may trigger
the piece to maneuver if it’s not firmly held down. So, in some
conditions that makes a downcut bit or mill the higher alternative.
With a downcut mill, the benefit is that it’s pushing down on
the piece you’re working, plus it cleanly cuts the highest floor.
The tradeoff is the waste is pushed again into the lower space
as an alternative of being ejected.

So how do you resolve on an upcut vs. a downcut bit on a CNC?

In the event you’re reducing MDF or HDF you completely have to make use of an
upcut bit. In any other case, between the bit getting overheated and
the superb paper dust-like waste being pushed again into the lower,
there’s a excessive potential of beginning a fireplace. I do know as a result of
I’ve made the error of utilizing the improper bit at a sluggish pace,
and the end result was a near-fire state of affairs with smoke pouring off
the MDF board I used to be reducing – unnerving to say the least! (And
sure, I ran to my mud collector barrel to ensure that I
wasn’t beginning a fireplace there, too.) You’ll be able to guess that I’m
cautious about this element now. My mantra: MDF = Upcuts.

As you possibly can guess, when reducing stable wooden, more often than not an
upcut is the most effective answer. However not all the time. On a CNC, I take advantage of
downcut bits virtually solely once I’m reducing out solid-wood
components, as a result of the highest floor comes out good and the wooden
is held tighter the mattress as a result of the bit is pushing down because it’s
reducing. However I’ve to run the CNC sooner in order that the chips and
the bit don’t overheat. To do that, I often take shallower
cuts and a number of passes at larger speeds than I’d use with
an upcut bit, the place I would take the alternative strategy with
barely deeper cuts and fewer passes.

Compression Bits

Due to fashionable finish mill and router bit design, now you can
have your cake and eat it, too. Compression bits do each upcuts
and downcuts on the identical time. They’ve flutes that upcut on
the underside .250″ or so, and flutes for remainder of the reducing
size which can be for downcutting. These are the reducing software of
alternative for laminated supplies such plywood. However you must
use them accurately. Your first cross must be barely deeper
than the upcut part of the bit, say .260″, then the
remaining reducing space on the bit will push down the board and
cleanly lower that high floor with out tearing the skinny high
veneer layer. Additionally, in the event you lower all over the board,
you get a bonus: The highest cutters don’t tear out the underside
veneer layers as a result of they’re pulling the fibers inward. A
win/win state of affairs.

For stable woods, compression mills or bits can work simply as
nicely. However to get their main profit you want to remember
the depth of that first cross. For very arduous woods, .250″ may
be too deep a lower. With different, softer, woods this might not be an
situation. For MDF, which is notoriously arduous on reducing instruments, it
simply looks like such a waste utilizing an costly bit on reminiscent of
abrasive materials, so I keep on with upcut bits.

Climb vs. Standard Cuts

With standard cuts the bit is biting into the fabric at
it travels. A climb lower, however, pushes the bit away
from the fabric. Because of this it’s so troublesome to manage a
hand-held router throughout a climb lower, and why you actually need to
take note of the path that you just’re routing. Slicing
path is a completely different situation on a CNC machine the place the
inventory is tightly held onto its mattress. There, you possibly can take
benefit of climb reducing to get the advantages, that are
often much less tear-out and a smoother lower. In reality, on a CNC,
climb or combined reducing are the popular methods. Blended
reducing is quicker, too. Nevertheless, there will likely be instances when lower
path on a CNC is vital. I’ve had a number of points with
very comfortable woods with sturdy grain the place I’ve restricted reducing
to traditional reducing, however for essentially the most half, I lower
bi-directional to hurry up the milling course of.

Although I want carbide finish mills and router bits for his or her
longevity, generally discount high-speed nonetheless mills have
their place. These heat-treated Onsrud three/eight″ mills are very
helpful for half reducing.

Slicing-Software Supplies

Router bits and finish mills are made out of a number of
supplies. The most typical are high-speed metal (HSS) or
carbide. Carbide is more durable, lasts longer, resists warmth higher
and may take sooner feed charges. In my work, more often than not I
use carbide mills. In the event you’re already utilizing a router you doubtless
are utilizing them as nicely. Carbide lasts a very long time and the
higher ones are price resharpening. It’s essential be very cautious
when utilizing carbide cutters; they break with the slightest
deflection. Although carbide is superior in most methods, HSS mills
have their place in some conditions and when value is a
consideration. I’ve often discovered nice offers on
portions of HSS mills in sizes I like to make use of for CNC half
reducing. If the work isn’t too arduous and also you’re cautious to not
overheat the bits, use them. I’ll use HSS cutters on a number of
tasks; as soon as they’re boring, I toss them.

To additional improve put on resistance, varied coatings, together with
titanium aluminum nitride (TiAN) and diamond, are generally
utilized to carbide instruments. Although these could add so as to add
to a cutter’s longevity for particular metals, for woodworking
use, an uncoated mill operating at an optimum pace works simply
superb. Some producers, together with Onsrud, use particular
coatings on a few of their finish mills to guard them from larger

— Tim

P.S. Subsequent month, I’ll introduce a category of CNCs which can be
well-suited for hobbyist woodworkers and I’ll observe that up
with a evaluation of considered one of them.

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