Feature Article
The Role Of Balance In High Speed Finish Boring
Precision boring is particularly vulnerable to unbalance, but not
every high speed boring application demands a balanceable tool.
By
Peter Zelinski
A single-point
boring tool with adjustable cutting diameter may be used for the
tightest-tolerance cutting that a given machining center is likely
to perform. At the same time, this tool is probably the most inherently
unbalanced object the machine will ever use to take a cut.
This irony
has real consequence as spindle speed increases, because the centrifugal
force resulting from unbalance increases as the square of rotational
speed. Raising the speed from 4,000 to 8,000 rpm, without any change
in tooling, causes centrifugal force to quadruple. Raising the speed
of the same process to 12,000 rpm raises the force from unbalance
to 9 times what it was at the original speed. Because of the exponential
increase, an effect that was once negligible may be amplified to
the point that tight tolerances can no longer be held.
One company
providing variable-diameter precision boring tools is KPT
Kaiser of Elk Grove Village, Illinois. During the past decade,
as maximum spindle speeds for all classes of machining centers were
increasing significantly, this company introduced updated versions
of its tools better suited to boring at high rpm. Among the newer
versions are adjustable-balance models that use moving counterweights
to compensate for the change in unbalance that comes from changing
the radial position of the cutting insert.
But not every
boring application—not even every high speed application—is a candidate
for one of these balanceable tools. Vice president of engineering
Jack Burley says that tools with adjustable balance account for
only 10 percent of the company's sales of the variable-diameter
tooling. Tools that are not balanceable by means of a moving counterweight
are nevertheless engineered to be balanced at the middle of the
tool's adjustable diameter range. And for most finish boring applications,
just this level of balance is enough.
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On this finish boring tool, the gold-colored
dial is used to change the cutting diameter. Compensating for
the resulting unbalance is the purpose of the silver dial that
follows the tool's circumference. Moving this dial moves a counterweight
mechanism located inside the body of the tool.
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Understanding
unbalance as it applies to single-point boring is useful for shops
that want to machine precision holes at faster cutting speeds. In
addition, the same understanding is also useful in a larger context.
Shops moving to higher spindle speeds for various applications are
often concerned about the potential effects of unbalance. The example
of high speed finish boring, which combines a highly asymmetric
tool with a very light depth of cut, can provide a useful benchmark
for these shops. The extreme set of conditions shows the extent
of the impact that unbalance can have on the performance of the
process.
Mr. Burley
says a common mistake in high speed machining is to give too little
consideration to balance. Another common mistake, he says, is to
give too much consideration to balance. Higher speeds do make it
worthwhile to use high-quality tools and toolholders manufactured
to tight balance requirements, but where end mills and other inherently
symmetrical tools are concerned, just choosing quality tooling is
probably enough to ensure that balance is sufficient. Trying to
improve balance further through some off-line adjustment to the
tool is likely to be overkill, resulting in a change in centrifugal
force that's tiny compared to the force from the cut. The reason
is that single-point boring tools are more prone to require balance
adjustment precisely is that the light depth of cut makes a small
unbalance force more significant by comparison.
And even
at that, the effect of unbalance would not be considered significant
to everyone. Comparison testing at 10,000 rpm between balanceable
and non-balanceable single-point tools showed a difference in hole
roundness error of about 5 microns (see Figure
1). Many machining operations could tolerate error of a comparable
magnitude. But in precision boring, error of this magnitude has
to be addressed.
Boring And
Speed
An example
of an adjustable-diameter finish boring tool is shown in Figure
2, below left. The adjustability allows this one tool to machine
a variety of different hole diameters.And the fineness of the adjustability
lets the tool zero in on a precise diameter by compensating for
differences in deflection from machine to machine or from setup
to setup. Toperform this compensation, the tool is generally run
through two passes when it's used within a particular process for
the first time. The first pass is taken at a slightly undersize
diameter. The bore is measured after this cut. The tool's diameter
is then increased by the amount necessary to remove the stock envelope
that remains.
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| Fig. 2—The finish
boring tool shown here is balanced at a diameter setting near
the middle of its adjustable range. |
This tool
finishes holes for which an acceptable location accuracy, roundness
and straightness have already been established. A rough boring tool
used to establish these parameters is shown in Figure 3, below right.
The diameter of this tool is also adjustable, but not with the same
fine control. This tool machines pre-existing holes such as the
cored holes in castings and forgings. Hole roundness and straightness
result from the tool's design, which positions the cutting inserts
so that their cutting forces directly oppose one another. Other
tools used to rough out holes don't feature the same complementary
forces, and the lack of equilibrium can affect hole geometry. A
drill, for example, tends to walk. An end mill used for circular
milling has a tendency to deflect away from the cut that increases
as the tool gets longer. The rough boring tool's balance of forces
overcomes these problems.
With this
tool, the benefit of using a higher speed is simple productivity.
Unbalance is no cause for concern because the tool doesn't take
precision cuts, and because the symmetry of the tool tends to limit
unbalance to a level small enough that it doesn't affect performance.
With the
finish boring tool, on the other hand, the benefits of using a higher
speed are more numerous. And the challenge posed by unbalance is
greater.
High speed
in finish boring increases productivity, but it can also improve
bore quality. The higher-end cutting tool materials such as PCD,
CBN and ceramic (see Figure
4) can generate a smoother surface as cutting speed increases.
In addition, faster cutting can translate directly to reduced tooling
costs, because these same tools achieve longer life at higher speeds.
The potential
obstacle to realizing all of these benefits is the centrifugal force
from unbalance.
Balancing
Action
The balanceable
version of the adjustable-diameter tool shown in Figure 2 makes
its balance correction automatically. Inside the body of the tool,
mobile counterweights move in unison with any change to the cutting
diameter.
More complex
is the balance correction required for the finish boring tool shown
in the photo at the beginning of this article. With this extended-reach
tool, the diameter adjustment is not the only variable affecting
unbalance. Also significant is the user's choice of extension length.
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| Fig. 3—This
rough boring tool uses opposing inserts to balance cutting forces,
minimizing deflection during cutting. |
To compensate
for this mix of variables, the tool uses a manual balancing system.
A numbered dial moves the counterweight mechanism inside the tool's
body. The dial's numbers refer to a chart from the tool manufacturer.
On this chart, the user looks up the intended bore diameter and
bore depth, along with the nose radius of the cutting insert, to
determine the correct setting for the counterweight dial.
There are
many other factors than these that also contribute to unbalance.
How tightly the extension is screwed on will also affect the tool's
balance condition. So too will the choice of insert, because a cermet
insert weighs one half as much as the same insert made of carbide.
With the adjustable-balance tool, influences such as these are not
compensated.
But then,
the objective in balancing this tool is not to meet some narrow
balance target. The goal instead is to reduce unbalance to the point
where a stable cut is possible because the cutting force is much
larger than the centrifugal force. Beyond this point, further improvement
would not noticeably affect the amount of force that the workpiece
or the machine tool sees while the tool is cutting. The most important
"balance," in other words, is the balance struck between the amount
of process improvement that is possible and the amount of improvement
that is sufficient for the needs of the application at hand.
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How
Much Force Are We Talking About?
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Only
about 10 percent of the adjustable-diameter single-point boring
tools sold by KPT Kaiser feature moving counterweights to
compensate for unbalance as the diameter changes. All of the
other tools are engineered to be balanced only at some point
near the middle of the adjustable diameter range. The reason
there aren't more "balanceable" tools sold is that, even at
high speeds, the unbalance does not necessarily generate enough
centrifugal force to have a significant impact when compared
to the cutting force.
The
force from unbalance can be approximated using this formula:
where
F = force in pounds and u = unbalance in gram-millimeters,
the typical units for this characteristic.
KPT
Kaiser offers one single-point boring tool that is adjustable
to diameters ranging from 0.984 to 1.299 inch. Within this
range, the tool's most extreme out-of-balance condition occurs
at 0.984 inch, where the unbalance is 30 g-mm. According to
the formula above, using this tool at 10,000 rpm would result
in centrifugal force of about 7.5 pounds. This level of variation
added to the total force in the cut would not affect the outcome
of most work performed on a machining center, but it might
affect the success of a finish boring operation.
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| Changing the insert holder
attachment increases the cutting diameter and also increases
unbalance. |
A change
of insert holder (see photo at left) can extend the reach
of this tool out to 1.850 inch. Unbalance at this diameter
is 70 g-mm. Now the centrifugal force at 10,000 rpm becomes
17.5 pounds.
However,
this would be an aggressive application. That combination
of spindle speed and diameter would result in a cutting speed
of 4,800 sfm—well above the cutting speeds that most shops
routinely employ. A shop committed to running at this speed
to cut this diameter could move up to a larger finish boring
tool from the same manufacturer—a tool that places the required
diameter somewhere closer to the tool's balanced condition.
The
conclusion is this: It takes a fair amount of unbalance, even
at 10,000 rpm, to generate enough force to have a noticeable
impact on the process. Finish boring, with its low cutting
forces and narrow tolerances, may be particularly susceptible
to the effects of unbalance. However, not even in finish boring
does a high spindle speed automatically create the need for
some mechanical balancing adjustment to the tool. Instead,
the "balanceable" tooling is appropriate either for a speed
significantly higher than 10,000 rpm, or else for applications
that combine high rpm with an extreme level of precision and/or
the need to run at some inherently off-balance condition of
the tool.
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