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Research into the cutting mechanisms that occur between high
speed cutting tools and a workpiece is yielding better understanding of the
machining process. A result of this work is helping make selection of the
right combination of cutting tool material, coatings, geometry, shank, and
toolholder more an exercise in science than art.
To get some insight on current research into high speed
machining, from a cutting tool perspective, we visited the Engineering
Research Center (ERC) at The Ohio State University (Columbus, Ohio). They
are looking closely at the mechanics of the cutting process.
Among the numerous research projects being conducted by
the ERC lab is a high speed machining project that ultimately will result in
a database of feeds, speeds, tool geometry, and tool material that will be
matched to the material and cutting requirements of a shop.
ERC's lab uses a strong simulation package that allows
researchers to graphically illustrate what's happening in the cut. Using
"what if" scenarios, different parameters and permutations can be tested.
In the lab's attached machine shop, a
Makino horizontal machining center is used to confirm the high speed
milling simulation. This process of confirming by cutting has helped ERC
continuously refine their simulation, enabling it to get close to reality.
In The Chips
Integral to high speed machining is chip formation.
Material is removed in a controlled way thereby modifying the existing
geometry by creating a new shape. The fundamentals of what happens when a
cutter meets metal, at conventional speeds and feeds, has been understood
for many years.
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illustration above shows the action that occurs in the cutting zone.
Plastic deformation allows the chip to shear from the parent material in
the primary zone. |
Chips are formed by deformation of the parent material. As
a cutting edge moves into steel, cast iron, aluminum, or other materials, it
generates high temperatures and stress in the material at the point of
intersection and slightly ahead of the cutting tool edge.
These stresses and temperatures are sufficient that
plastic deformation of the workpiece material takes place. The metal deforms
easily within an area called the primary shear zone. As the material reaches
its yield point, a chip breaks away from the parent material and slides
along the primary shear plane, pushing material ahead of the cutting tool.
A secondary shear zone occurs along the face of the
cutting tool. As the chip slides up the face of the cutter, friction raises
temperatures in this zone. Studies reveal that the secondary zone
temperature is as high as 1200 degrees Celsius when machining tool steel.
As the cutting edge moves through the material, deforming the material to
shear a chip, a third shear zone occurs under and behind the leading edge.
This zone is a result of material springback.
Key to successful matching of cutter and workpiece is
predicting when the deformation process will occur, and as a result of that
prediction, determine the force necessary to drive the cutter. It is also
important to understand how heat will be dissipated in the cut.
At High Speed
Much of the chip formation mechanics at high speed involve
heat. At high speed, which is relative to the material being cut, higher
heat is generated. More energy is going into the workpiece and that energy
becomes heat.
Higher temperature at the primary shear zone helps speed
up the plastic deformation process that results in a chip being formed.
Because of the increased rate of plastic flow, high speed cutting
experiences a decrease in the cutting force needed to remove a chip.
Researchers estimate the heat input distribution this way:
- About 80 percent of heat is generated by the mechanical
deformation that creates the chip,
- 18 percent is created at the chip/tool interface or
secondary shear zone, and
- 2 percent is created on the tool tip.
What goes in must come out. In the case of high speed
machining, heat generated in the cut is dissipated three ways:
- About 75 percent is taken by the chip,
- 5 percent by the workpiece, and
- 20 percent is conducted through the tool.
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High speed tests run at ERC in hardened H13 steel
demonstrated tool life performance of different insert geometries and
coatings. Test parameters are listed to the right of the chart. Inset
photos show examples of different coatings. |
Some of ERC's experiments involve manipulating tool
parameters to influence the heat input and output percentages. For example,
by using a cutter with a coating that insulates, the researchers can reduce
the amount of heat conducted through the tool and thereby redirect more of
the heat into the chip and workpiece. It's a balancing act.
Cutters
For high speed applications, centrifugal force becomes
significant. In Germany, new standards for safe operation of high speed
cutters have been proposed.
These proposals are based on cutter diameter. For cutters
6 to 8 mm in diameter, a recommended speed of 45,000 to 50,000 rpm would be
the operational high end for those tools. For 12 mm diameter cutters,
recommended speeds would be 15,000 to 20,000 rpm.
Solid body cutters handle the centrifugal forces better
than indexable inserts, but most cutting tool makers have high speed
configurations that can handle the forces. Moreover, most of the high speed
research being done at ERC involves indexable insert cutters.
They are cutting primarily in tool steel. Cutter geometry
is generally a neutral to positive cutting edge. They do not recommend a
chipbreaker for material hardness above 45 Rc. The breaker tends to weaken
the cutting edge. ERC's tests show a simple straight geometry has the best
results cutting hardened tool steel.
Holding On
Regardless of the cutter body used for high speed
machining, the toolholder is an important component to successful machining.
Debate continues between basically two toolholder standards—steep taper
V-flange and HSK (the German translation means "hollow shank taper").
In reality, the debate is not between competing toolholder
standards but rather centers around two methods of securing the holder in
the spindle. So what's the big deal about HSK versus V-taper? Fundamentally,
the difference between these systems is how they seat in the machine tool
spindle bore.
We'll look at V-flange first. Regardless of the taper
size, 30, 40, 50 or other, the V-taper or steep taper toolholder is pulled
into the spindle bore by a drawbar or similar mechanism. The toolholder
seats on a mating taper in the spindle. This system serves shops well, and
probably a majority of machining center tools are of this design.
The HSK toolholder is designed to provide simultaneous fit
on both the spindle face and the spindle taper. There are other hollow shank
designs in addition to HSK. Common to all is the idea of two-plane fit
between holder and machine spindle.
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Equivalent toolholder sizes of HSK and V-taper. The
difference in shank length is dramatic between the toolholder standards
of 4 inches for the V-taper and 1.259 inches for the HSK. This
illustration is courtesy of Dapra. |
Simultaneous fit is not exclusive to HSK. There are other
designs, including one that is a steep taper, one that provides simultaneous
spindle taper and one for face contact. In this case, two-plane contact is
achieved by adding stock to the toolholder flange and spindle nose,
effectively closing the 3 mm gap between the two surfaces and delivering a
taper and face fit for V-taper tools.
High Speed Toolholders
At any speed, a machine tool spindle is subjected to
centrifugal force. Centrifugal force increases geometrically as speed
increases. This means if you double spindle speed, centrifugal force goes up
by a factor of four.
At high speed, centrifugal force is strong enough to make
the spindle bore grow slightly. Because V-flange tools contact the spindle
bore only in the axial plane, spindle growth can cause the tool to be drawn
up into the spindle by the constant pull of the drawbar. This can lead to a
stuck tool or dimensional inaccuracy in the Z axis.
Simultaneous fit tooling has contact on the spindle face
and taper. When this spindle begins to grow, the face contact prevents the
tool from moving up the bore. The hollow shank design is also susceptible to
centrifugal force but it is designed to grow with the spindle bore at very
high speeds.
Rigidity is another touted advantage for simultaneous fit
tooling. Supporting the cutting tool and holder in both the axial and radial
planes makes a significantly more rigid connection between the tool and
spindle.
Spindle face contact lowers the bending moment for the
tool from inside the spindle bore to outside the bore on the face. It
increases contact area giving better rigidity, which enables users to cut
with longer tools and to use side-cutting milling tools more aggressively.
For high speed machining applications, the trend
definitely points toward simultaneous fit, spindle tool interface as a
requirement.
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High speed means high centrifugal force. Reinforced
cutter bodies, such as this
Iscar face mill, all shops to safely machine with indexable inserts
as well as solid body cutters. This illustration is courtesy of Iscar
Metals. |
Squeeze That Tool
How the cutter body is gripped in the toolholder is
another tooling consideration for high speed machining. Some toolholders are
not symmetrical. For example, some have drive notches that are of unequal
size. Others use side-locking setscrews or other asymmetrical features. This
creates less impact at conventional machining speeds.
A shop planning to do HSM should try to build tools using
a holder and cutter combination that is symmetrical. Shrink fit, which uses
a heated bore that expands for the cutter and then clamps as it cools is a
popular HSM choice. It's simple, reliable, and economical.
Hydraulic toolholders work the same as shrink fit in that
they support the tool shank completely. These holders use hydraulic pressure
to actuate a membrane or other device to grip the tool. Regardless of the
toolholder cutter combination, symmetry is a key consideration.
It's Now
Many technologies have coalesced to make high speed
machining a practical process. Machine tool actuation, high speed/high
frequency spindles, cutting tools, high accuracy balancing, and software to
run powerful CNCs all combine to make HSM a defining trend in metalworking
manufacturing.
At day's end, success is measured by how many good parts
ship and how profitably they were produced. High speed machining is a means
to this goal by reducing cutting forces, increasing metal removal rates,
improving heat dissipation, and making better surface finishes.
But high speed machining is a system, with its many
components interlinked. Understanding the interplay between these various
components may help you take better advantage of HSM in your shop. MMS
Recommendations
The research at ERC, which is on-going, has a laundry list
of recommendations for high speed cutting. These are based on their
simulation and verification experiments, which were conducted using H13
steel (46 Rc).
- Select symmetrical tool designs. Sticking a
whistle-flat cutter into the holder and tightening a side-mounted set
screw won't do for HSM. It's best to start with toolholders that are
symmetrical (HSK style E and F for example) and use a wraparound
connector such as shrink fit or hydraulic holders. This will help
eliminate vibration before it can occur. Likewise cutter shanks should be
round—no hold-down flats.
- Keep chip thickness constant. Maintaining a constant
chip thickness is key to stable cutting. A constant chip thickness equates
to constant chip load. The maintenance of which keeps the high speed
cutting bugaboo of chatter reasonably at bay. Calculate tool path with
constant load in mind.
- Use multi-layer coatings. For cutting steel, use TiN,
TiCN, TiN physical vapor deposition (PVD) over chemical vapor deposition (CVD).
Performance of these coating combinations and application methods gives
longer tool life with better heat transfer.
- In hardened steel, use inserts with a flat rake face.
At the cutting speeds ERC is using, they have found a chip breaker
unnecessary. A breaker used on a cutter at high speed in tool steel can
weaken the cutting edge and increase wear on the cutter face.
- Use high pressure air to clear chips.
ERC's research, along with the experience of other high speed machining
practitioners, recommends cutting without coolant when possible. At the
surface speeds that the cutting tools are traveling, coolant cannot get
into the cut zone. Coolant's primary values for HSM are chip clearance and
lubrication. An air-and-oil mist combination is considered a better medium
for chip evacuation than coolant because it introduces much less thermal
shock to the cutter.
While ERC's research is targeted at high speed machining
of molds and dies, many of their recommendations carry over to other
materials as well. |
