Why endmills have
gotten
D
L
WEIR
ike almost everything in manufacturing, endmill
development chases ever faster performance
plus the highest possible part quality and
the least possible downtime. Meanwhile, the parts
themselves are becoming harder to cut as other factors
drive the use of tougher alloys, composites, and
sometimes both in combination. So tool manufacturers
have improved tool materials, like better grades of
carbide, synthetic diamond, and sophisticated coatings.
They have also created some strange new endmill
geometries. That’s the focus of this article.
Consistency leads to chatter
For years, the main goal in producing endmills was to
achieve perfect accuracy and consistency. Naturally,
every cutting edge should fall along the same
circumference so that all teeth cut as the tool spins. In
fact, in most applications, several cutting edges should
be in use simultaneously.
This illustration (from
George Schneider’s book
Cutting Tool Applications)
shows the basic cutting
action of an endmill on
a workpiece, which in
the case of steel looks
much like plowing. Rake
angle largely determines
the size and shape of the
chip.
Users also needed predictable performance to plan their
tool changes efficiently. Too frequent tool changes or
unexpected tool breakages would both be disastrous for
productivity. The advent of CNC tool grinders in the
late 70s made it possible to consistently create nearly
perfect and predictable endmills.
On the other hand, a perfectly balanced tool will
resonate at some operating speeds, creating chatter
that can damage the tool, the workpiece, and even
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Journal
7
Cutting Edge
Primary Clearance
Secondary Clearance
Flute
Helix Angle
Equal indexing (left) versus unequal indexing. Spacing the teeth unequally
is one technique for reducing chatter caused by harmonics.
Relief
Peripheral
cutting edge
Primary clearance
Secondary clearance
Radial rake
angle
Basic endmill features like helix will determine the angle at which the
cutting edge enters the workpiece, while the clearance angles will
contribute to chip evacuation
the milling machine if the vibration is severe enough.
Unfortunately, damaging harmonics become more likely
as you push the endmill to take deeper cuts. And as we
said at the outset – the goal is both faster performance
and better surface finishes. So tool manufacturers
have looked for ways to “break up” the harmonics by
deliberately creating inconsistencies in the tool geometry.
the timing and angle at which the cutting edge contacts
the workpiece. Another way to look at this is to say that
once you vary the helix, you necessarily create variable
unequal indexing along the length of the tool. The key
is to balance how the two factors interact. For example,
if the tool is equally indexed at the end and the helices
vary, the flutes may interfere with each other farther
down the cut. So most variable helix tools use unequal
indexing at the end and then varying flutes that produce
equal indexing at a given point along the tool.
Some designs feature a different helix angle on each
flute. This would properly be called a multi-helix tool
and it’s a common approach to solving the chatter
problem. Other designs vary the helix angle along
individual flutes, creating a class of variable helix tools.
20° Helix
angle
40° Helix
angle
Stop the music!
Creating inconsistencies to combat vibrations
One relatively simple approach to breaking up the
harmonics has been to change the timing with which
each tooth strikes the part by spacing the flutes
unevenly (unequal indexing). Although this idea works,
it is better to combine this idea with other techniques.
Some tool manufacturers have found that varying the
helix angle along with the indexing produces better
results. By varying both features, you are changing both
Fall/Winter 2008
In this endmill, each flute starts with a 20° helix and then quickly
transitions to a 40° helix. Walter calls this a “blended helix.” In other
variable helix endmills, the helix may change continuously from one
extreme to the other along the entire cutting edge, or according to a
certain function (see next page).
Like helix angle, rake can be varied from flute to flute
or along a flute. Mixing and matching these factors is
part science, part art. Walter’s Tool Studio software can
decrease the R & D time by allowing the technician
to simulate different geometry combinations without
designing special wheels.
The endmill on
the top transitions
gradually from a 10°
to a 30° helix from
front to back. The
endmill on the right
transitions from 20°
to 40° according to a
specific table:
Odd geometries can address material
challenges too
% (along cutting
length)
Helix
0
20
20
20
40
30
60
30
80
100
40
40
Each section of the
cutting edge, (0-20%,
20-40%, etc.) is treated
as a linear helix, so
the helix is constant at
20° for the first 20% of
the flute length, then
changes linearly from
20° to 30° for the next
20%, etc. Walter calls
this a tabular helix
Juggling yet another factor – rake
Some tool manufacturers have determined that varying
the rake angle along with the other factors offers even
better chatter suppression. Changing the rake changes
the way the cutting edge creates a chip – altering its
shape and size. These differences may be small, but they
are enough to fight chatter.
5° rake at a point where the
endmill has a 20° helix
15° rake at a point where the
endmill has a 40° helix
Besides the need to combat chatter, manufacturers
are also calling on strange tool geometries to help deal
with other material challenges. For example, as Ludwig
Preinesberger of Sonic Tools, Inc. (Ashland, VA)
explains, “aluminum needs to be cut, not pushed like
steel. So you want a very aggressive helix. A 45° helix
at the end would be aggressive, but fragile. One option
is to grind a 20° helix with an aggressive 20° rake at
the end and transition to a 35 or 40° helix toward the
shank. The edge will last longer and the design will help
chip evacuation, as chips will be thrown up instead of
into the cut.”
The three-flute endmill from Sonic Tools
starts with a very low helix and then almost
immediately transitions to an aggressive high
helix, while the rake is an aggressive 20°
throughout (see below). Without that variation
in the helix, the cutting edge would come to a
fragile point at the end.
Another interesting technique is to create a flute within
a flute, typically called a “ski-flute.” First introduced
by Weldon Tool (part of Dauphin Precision Tool,
Grinding Journal
9
The yellow
area indicates
a “ski-flute”
within the
main flute,
creating an
aggressive
rake at the
cutting edge
without
compromising
core strength
Millersburg, PA), this approach allows you to combine an
aggressive, high rake at the cutting edge with a stronger,
lower rake at the core. Like the previous example, this
approach also helps throw the chip clear, and like the
previous example, it’s used primarily for aluminum.
Cautionary remarks
Picking the perfect tool requires a thorough analysis of
the application, including the material, required cuts,
and the particular milling machine planned for the job.
It pays to use suppliers who understand these factors
and can offer advice.
If you are contemplating grinding your own complex
endmills, you should know that geometries like variable
helices are beyond the realm of manual tool grinding.
It takes the latest machines and grinding software (like
Walter’s Helitronic Tool Studio) to make these ideas
even possible, let alone efficient. This is especially true
for re-sharpening these tools.
Finally, some of the wizardry you see on the market
today is patented. The gurus deserve their due!
INFO
grinding.com
Ed Sinkora • 540.710.2408
[email protected]