Stephen A. Diehl
TrueMill is a winner of Industry Week’s Technology of the Year award for
its innovative tool path that dramatically increases manufacturing efficiency. TrueMill controls
the tool’s engagement with the material, allowing for material removal rates that are two or more
times higher than conventional high speed machining for roughing operations. Most CAM software
can control the spindle speed, feedrate and depth of cut, but not the tool’s engagement.
TrueMill’s control of the tool engagement (and spindle speed, feedrate and depth of cut) allows
the tool to be consistently used at its optimal cutting conditions. This results in high material
removal rates throughout the entire toolpath. Cutting velocities (SFM or m/min) at two to four
times the tool manufacturer’s recommendation can be achieved while cutting at a depth equal to two
times the diameter of the tool on materials such as titanium, hardened steel (40RC to 50+RC) and
other difficult to machine materials. Controlling the tool engagement also allows for controlling
the chip thickness and greatly reduces chatter. The maximum tool temperature is reduced, reducing
part warpage and workpiece hardening. All of this is achieved with an increase in tool life by a
factor of two or more. TrueMill is patented technology offered by Surfware.
The goal in roughing is to cut with the highest possible material removal rate (MRR) while
maintaining a good cut everywhere and having a long tool life. It is a high and consistent MRR that
causes short roughing time.
Traditional toolpaths follow contours of the pocket geometry using a constant stepover. When the
tool encounters an inside corner, the tool’s engagement with the material radically and rapidly increases.
This greatly increases the load on the tool and the temperature of the tool. Chatter and other
undesirable cutting conditions may result. This does not happen with TrueMill. With TrueMill the
stepover is varied, if necessary, along the toolpath, depending upon the toolpath radius and in-process
material boundary, such that the tool engagement is kept nearly constant. This greatly reduces the
variation in tool load, tool temperature and chip thickness allowing for a high and consistent material
The TrueMill toolpath consists of “racetracking” and “D-Slotting”. Throughout the entire toolpath,
the tool engagement never exceeds the user’s specified value. Throughout the section of the path that
is racetracking, the tool engagement is kept nearly constant. This is the largest part of the toolpath.
Throughout the section of the path that is D-Slotting, the tool engagement varies more, but the variation
is much less than with a traditional toolpath.
With a conventional toolpath, the problems caused by the spike in the tool load, tool temperature and
chip thickness, are mitigated by a variety of methods, each of which has only limited success. In
conventional machining, the MRR is lowered over most of the tool path by making a shallow depth of cut
or lowering the feedrate. Lowering the MRR reduces the load on the tool (for a given surface speed).
Then when the MRR spikes due to the radical and rapid increase in tool engagement, the load on the tool is
not excessive at that point. The obvious disadvantage of this technique is that having a low MRR almost
everywhere in order to handle brief, but large spikes, results in a longer roughing time than if a high
and consistent MRR were maintained everywhere.
A method that is sometimes used in an attempt to achieve a more consistent MRR, is to use a feedrate
optimizer. The idea is to vary the feedrate such that some parameter, such as MRR, is kept constant.
There are several problems with this technique that limit its effectiveness. First and foremost, it is
simply not possible to vary the feedrate as quickly as the tool engagement varies. The momentum of the
machine tool table is simply too great even for high end NC machines to overcome. This means that the
optimal target feedrate is not achieved in practice. Furthermore, it is not practical to adjust the
spindle speed. Adjusting the feedrate without adjusting the spindle speed leads to a variation in chip
thickness. In all cases, varying the geometry of the chips to higher temperature and more load on the tool.
In practice, a feedrate optimizer only improves roughing time by about 15% for most parts.
TrueMill does not have spikes in tool load, tool temperature and chip thickness. There is no need to
lower the MRR to account for spikes that do not exist. There is no need to apply a feedrate optimizer to
attempt to undo the negative affect of lowering the MRR below its optimal value. TrueMill, by consistently
operating at a high MRR almost everywhere can reduce roughing time by a factor of two or more while improving
the quality of the cut and increasing tool life by a factor of two or more.
This paper will show how benefits such as increased MRR, increased tool life, and better quality of cut
are all a direct result of being able to control the tool engagement. This paper will provide hints as to
how the patented technology controls the tool engagement. This paper will also address a few of the
misconceptions about TrueMill.
2. TrueMill Cutting Environment
There are four parameters that uniquely and completely define the cutting environment (for a given material,
tool and setup). These are: spindle speed, depth of cut, feedrate and tool engagement. TrueMill is the only
CAM system that controls all of these parameters. This allows for complete control over the cutting environment
which includes tool load, tool temperature, chip thickness and MRR. More precisely, by limiting the maximum tool
engagement and keeping the spindle speed, feedrate and depth of cut constant, the maximum tool load, maximum
tool temperature and maximum chip thickness are all limited. Many of the benefits of TrueMill are a direct
result of limiting these maximum values. Furthermore, for the racetracking section of the toolpath, there is
very small variation in tool load, tool temperature and chip thickness allowing for an extremely high consistent MRR.
Rephrasing the above, TrueMill maintains a more consistent rate of volume removed (MRR) than any
other CAM system, including those that attempt to control the volume removal rate. Futhermore, TrueMill accomplishes this
while maintaining a consistent chip geometry everywhere which allows for a much higher MRR and longer tool life.
The optimal cutting environment is one that produces a great cut (no chatter, consistent tool deflection,
acceptable level of heat) and a very high MRR. TrueMill maintains this optimal cutting environment almost
everywhere resulting in a very short roughing time.
An equivalent way of understanding the cutting environment is to consider the shape of the uncut (or
undeformed) chip. The chip’s geometry is defined by its height (depth of cut), length (tool engagement) and
thickness (determined by the feedrate, spindle speed and tool engagement). If the shape of the chip is
constant, regardless of whether the tool is cutting along a straight line or a curve, then the tool load and
chip thickness must also be constant. If the same chip is cut everywhere with a high (and constant) spindle
speed, then a high MRR is achieved. This is the secret to TrueMill’s effectiveness. This applies to the
racetracking section of the TrueMill toolpath. The D-Slotting portion, which is shorter, is a little bit
3. How TrueMill Works
This section explains why controlling the tool engagement (as well as the other three cutting parameters.)
solve many of the problems that exist in conventional milling. Solving these problems allows TrueMill to
reach a high and consistent MRR throughout the entire toolpath while cutting well with long tool life. What
once were conventional milling limitations, are now TrueMill benefits. The version of TrueMill that ships
today uses a constant spindle speed, feedrate and depth of cut. This is the easiest way to
achieve and to understand the benefits described in this section. Although small improvements to the overall
MRR are possible by making slight variations to the feedrate, this section will assume constant spindle
speed, feedrate and depth of cut.
TrueMill limits the maximum chip thickness produced anywhere along the toolpath. The TrueMill software
allows the machinist to enter the maximum tool engagement, feedrate and spindle speed. The machinist also
chooses a tool with a certain number of flutes. These four factors define the maximum chip thickness
encountered anywhere along the toolpath regardless of part geometry. Limiting the maximum chip thickness is
key to several of the benefits described below. The maximum chip thickness equals: feedrate / (spindle speed *
numberFlutes) * sin(engagement angle).
For a given tool, material and setup, the tool load depends only upon the tool engagement and chip thickness.
TrueMill limits both the tool engagement and the chip thickness thereby limiting the maximum tool load and
maximum tool deflection, regardless of part geometry. It is by eliminating the spikes in tool load that a
high and consistent MRR can be maintained resulting in a very short roughing time.
For a conventional toolpath, the great majority of the tool wear occurs during the corners where the tool
temperature briefly but significantly spikes. This spike in tool temperature causes the tool to lose a
significant amount of its hot hardness resulting in extremely rapid tool wear. The tool temperature depends
upon the chip thickness and surface speed. TrueMill limits both the chip thickness and the surface speed.
This allows the tool to be used below the critical high-wear temperature point everywhere along the toolpath
regardless of part geometry. Initial reports by users of TrueMill are that tool life is extended by a factor
of two to four relative to conventional machining. Studies have shown that for the same MRR, the tool temperature
is lower using a smaller tool engagement.
Spikes in tool temperature increase the part temperature, causing part warpage and workpiece hardening.
Removing the spikes in tool temperate, as TrueMill does, eliminates part warpage and workpiece hardening.
Chatter is one of the greatest deterrents to achieving a high MRR in a production environment. Chatter
does not occur unless the tool engagement exceeds some critical value. TrueMill eliminates chatter by
limiting the maximum tool engagement. With no chatter to worry about, the user can push the other cutting
parameters to extremely aggressive values to obtain a very high MRR.
Traditional toolpaths have sharp changes in direction requiring the NC machine to come to a complete stop.
This dwelling (and its associated problems) do not occur with TrueMill. TrueMill toolpaths are very smooth
everywhere (TrueMill toolpaths are C1 continuous, everywhere.) In addition to eliminating dwelling, TrueMill’s smooth toolpath also allows the actual feedrate
of the NC machine to be maintained much closer to the programmed feedrate because less acceleration is required.
TrueMill is exceptionally effective on hard to machine materials. Hard to machine materials have a narrow
range of operating conditions under which a good cut can be made. For example, the range of acceptable chip
thickness is less with titanium than it is with aluminum. As TrueMill controls the chip thickness far more
precisely than any other CAM system, TrueMill is far more effective at cutting difficult to machine materials.
Tests have been run with steel hardened to 40+ RC, using a depth of cut that was equal to two times the
diameter of the tool, in arbitrarily shaped pockets (with and without islands, convex and concave), with a
standard half inch coated carbide tool that resulted in material removal rates that were several times that
which could be achieved with a conventional toolpath. Spectacular results have also been shown to exist for
titanium. With difficult to machine materials, TrueMill’s effectiveness is well in excess of a doubling of
the MRR relative to a conventional toolpath.
4. TrueMill Patented Technology
It is easy to validate that the TrueMill toolpath does indeed limit the maximum tool engagement to the value
specified by the machinist and that the tool engagement varies much less than that for a conventional toolpath.
One way to validate this is to use a tool engagement calculation utility and apply it to the TrueMill toolpath.
TrueMill creates a toolpath that precisely adjusts the stepover, dependent upon the radius of the toolpath
and the in-process material boundary, such that the tool engagement is bounded and nearly constant.
5. TrueMill Misconceptions
Some have said that the TrueMill toolpath is longer than a conventional toolpath and have therefore wondered
if it might take more time to rough a part, not less time. For a given pass, the TrueMill toolpath is longer,
however there are far fewer passes required to reach the total depth of cut. Whereas a conventional toolpath
may cut at a depth less than half the diameter of the tool, the TrueMill toolpath cuts very well at a depth
of twice the diameter of the tool. A better way to think of this is to consider the MRR. The higher the MRR,
the faster the material is being removed and the shorter the cycle time. TrueMill achieves a high MRR along
its entire toolpath, thereby reducing cycle time. The length of the toolpath for a single pass really doesn’t
TrueMill (in its current release) uses a constant feedrate for the entire toolpath. The question arises
if applying a feedrate optimizer to TrueMill would improve its performance. For those places where the actual
tool engagement is less than the target tool engagement, the feedrate can be increased until the chip thickness
reaches the target chip thickness (The machinist specified a spindle speed, feedrate and maximum tool engagement
angle along with a tool having a certain number of flutes. This determines the maximum or “target” chip thickness.)
As the maximum tool load depends upon the maximum tool engagement and the
maximum chip thickness, the maximum tool load will still be limited as before. As the maximum tool temperature
depends upon the maximum surface speed (a constant with constant spindle speed) and the chip thickness, the
maximum tool temperature will still be limited as before. Therefore it is fine to optimize the feedrate by
chip thickness. Tests using a feedrate optimizer to optimize by chip thickness showed a 5% to 15% reduction
in roughing time.
It appears that machinists using TrueMill for the first time often stop pushing the milling parameters
(spindle speed and feedrate in particular) as soon as they achieve a roughing cycle time that is just a bit
faster than anything they have ever seen before. In most cases, this is severely underutilizing the technology.
In the following section, a brief overview will be presented to ensure that the extraordinary MRR rates
possible with TrueMill are actually achieved.
6. How to Best Use TrueMill
This section will present a high level overview for how to get the most out of TrueMill. This
will also help to explain more about how TrueMill works.
At the turn of the 20th century, Frederick Taylor produced the first report on recommended feeds and speeds.
Over a period of 26 years, Taylor created an estimated 800,000 pounds of chips in order to arrive at his feeds
and speeds recommendations. One hundred years later, similar feeds and speeds are still in use. This is
because the same limitations exist due to the use of constant stepover for a contoured toolpath. TrueMill
does not have these limitations. Therefore an entirely new set of feeds and speeds needs to be used. These
feeds and speeds are completely outside the experience of most machinists. The problem then is how to
determine the best feeds and speeds for TrueMill.
As of the time of this writing, a comprehensive set of feeds and speeds recommendations, per tool type, per
tool diameter, per material, does not exist. A TrueMill calculator has been created to provide a general
guideline for feeds and speeds and this can be used as a starting point. However, since the TrueMill toolpath
is designed so that the most difficult (highest tool engagement) corner is exactly the same as making a
straight line side cut at the equivalent width of cut, the best approach to arrive to the optimal set of
cutting parameters values is to perform a set of simple straight line cuts along the side of a block of
TrueMill will not exceed these optimal cutting conditions, anywhere, including the corners. For example,
if a machinist makes a straight line cut on titanium with a depth of cut of .930, a feedrate of 75 inches per
minute, a straight line step over of .0893 inches (50 degree tool engagement), a surface speed of 600 feet per
second with a 5 flute coated solid carbide 0.5 inch diameter tool resulting in a chip thickness of .0025
inches and decides they like the quality of the cut and the MRR (6.2 cubic inches per minute in this example),
the machinist can tell the TrueMill software to never exceed these cutting conditions. TrueMill will then
match this set of optimally determined cutting conditions as close as possible.
For a more comprehensive method of how to perform the straight line test cuts, please refer to the document
“How to Best Use TrueMill” by the same author. Here are a few hints. The goal is to have the highest MRR with
a good cut. Start with a chip thickness equal to the middle of the range of chip loads recommended by the
tool manufacturer. Start with a surface speed of 1.5 (aluminum) to 2.2 (titanium) times the maximum recommended by the tool manufacturer.
Tool load drops with increasing surface speed. MRR increases with increasing surface speed. So use very high
surface speeds. TrueMill can handle it. Measure the tool deflection to ensure that the tool load is
For most production shops, the single greatest cost is the time spent roughing out the material. With
TrueMill, this cost can be reduced by a factor of two or more.
TrueMill is a completely new way to cut metal, the first and only CAM software system which allows the
user to control the entire cutting environment. With TrueMill, the four cutting parameters are controlled
such that the highest MRR is obtained while making use of the optimal tool load, tool temperature and chip
Although it is not possible for any toolpath to have a constant tool engagement everywhere, TrueMill
significantly reduces the variation in tool engagement which is the key to having a high and consistent MRR.
TrueMill guarantees that the user specified tool engagement is never exceeded anywhere along the toolpath.
The machinist never has to worry about the tool encountering bad cutting conditions anywhere, for any part
Tool Engagement (TE): the portion of the periphery of the tool in contact with the material. It is
usually measured in degrees.
Step Over: the distance cut in the direction perpendicular to the in-process part boundary.
True or Effective Step Over: the width of material being removed by the tool regardless of whether
it is moving in a radius or a straight line. Controlling the Effective Step Over is identical to controlling the tool engagement.
Chip Thickness: maximum chip thickness as measured along a line to the tool center. This is how
deep the flute is cutting into the material.
Tool load: the force on the tool that causes tool deflection. If the tool load is too high, the tool
deflection is too high, greatly increasing the likelihood of tool failure and breakage. This evidenced by a
noticeable deterioration in surface finish.
Tool Temperature: although the actual temperature is difficult to measure, the negative affect of
having too high a tool temperature is easy to observe. The tool temperature is observed to be too high for
steel if the chips turn deep blue and for other materials if the chips begin welding to the tool. If the
tool temperature is too high, the tool loses its hot hardness and extremely rapid tool wear occurs, chips
may weld to the tool and part warpage and workpiece hardening can occur.
Good Cut: to create a good cut you need to have tool load not too high, tool temperature not too
high and chip thickness not too light and not too heavy. These conditions will result in a lack of chatter,
consistent chip thickness, no part warpage, no workpiece hardening and a consistent finish.
The author gratefully acknowledges Alan Diehl and Robert Patterson for inventing TrueMill and Larry
Diehl for TrueMill technical contributions.
Hongcheng Wang and James A. Stori, 2002, “A Metric-Based approach to 2D Tool-Path Optimization for
High-Speed Machining”, ASME international Mechanical Engineering Congress & Exposition.
J. A. Stori, P.K Wright, 2000, “Constant Engagement Tool Path Generation for Convex
Geometries”, journal of Manufacturing Systems, Vol. 19, No. 3.
David A Stephenson, John S. Agapiou, “Metal Cutting Theory and Practice”, Second Edition,
CRC Press, Boca Raton FL, 2006.
TrueMill™ is only available in the new SURFCAM V5.
Contact your local sales partner for more information.