Method for indicating end mill wear

A system is provided for measuring tool wear of a rotating end mill. The system measures wear by monitoring the side-loading forces on the tool during cutting. The resultant side-loading forces FRES have tangential FT and radial FR components. The radial component FR increases correspondingly with tool wear. FRES and FT are measured directly and on the basis of these measurements FR is determined mathematically to provide an indication of tool wear.

DESCRIPTION 
1. Technical Field 
The subject matter of this patent application relates generally to milling 
machines, and more particularly, to inventions which are concerned with 
the detection of wear of a milling machine's cutting tool. 
This patent application is related to another application filed by the same 
inventor, such other application having a filing date of Jan. 31, 1986 and 
Ser. No. 825,134, now U.S. Pat. No. 4,698,773. 
2. Background Art 
A milling operation typically involves the use of a sharp-edged tool that 
cuts a workpiece or part in a desired manner. The cutting tool gradually 
becomes dull after a period of time and a tool that is too dull can 
undesirably affect the final dimensions and finish of the cut part, and 
can have other undesirable effects as well. 
In the past, the most common method of detecting tool wear has been to rely 
on the subjective judgement of the milling machine operator. Generally, 
increases in tool noise and heat indicate tool wear. The operator can 
therefore use his senses to decide whether a tool is worn by monitoring 
these kinds of parameters. The usual response of an operator to higher 
than normal levels of noise and heat is to initially reduce the tool's 
feed rate, which thereby reduces loading forces on the tool as it cuts. It 
is also a usual response to stop machine operation for the purpose of 
inspecting both the tool and the finish of the part. 
One problem associated with the above procedures is that they tend to make 
milling operations less efficient. Since the operator makes a somewhat 
subjective determination of tool wear based on his senses, and since the 
operator does not wish to cause damage to the part because of a worn tool, 
the operator tends to err on the conservative side when determining wear. 
What this usually means is that when the operator slows feed rate, he 
tends to slow it to a rate lower than necessary; or machine operation is 
stopped an unecessary amount of time for the purpose of inspecting the 
tool or part; or when a tool is replaced, it is done too early and before 
the tool is used to its fullest extent. 
To avoid the above occurrences and to maximize tool life, in certain 
industries statistical data is developed on how long a particular tool can 
be used when cutting a particular part. In this situation, after the tool 
is used for a statistically predetermined period of time it is replaced 
without inspection and regardless of its actual wear condition. This is 
typical practice in the automotive industry, for example, where milling is 
often done in high volume and large lot sizes, and it is relatively easy 
to keep records of the accumulated time of tool use. Although this method 
is not particularly inefficient in this kind of production environment, it 
is difficult to apply in other industries where milled parts are made only 
in small lot sizes. There has been a long felt need therefore to develop 
systems that can accurately detect tool wear, and preferably, detect tool 
wear automatically thus eliminating reliance on a machine operator's 
judgment. 
At least two different kinds of automatic tool wear monitoring systems are 
known to presently exist in the art. The first kind determines tool wear 
on the basis of a tool's acoustic and/or mechanical vibration 
characteristics during milling. The second kind determines tool wear by 
monitoring variations in the amount of horse power used to drive the tool. 
Monitoring acoustic or mechanical vibration requires appropriate sensors, 
usually in the form of piezo-electric sensors suitably mounted to a 
milling machine in a manner to sense tool vibration. As the tool wears, 
the vibration power spectra generated by the sensors change and thus can 
be used to measure wear. A great amount of research effort has been 
expanded on developing wear indicating systems of this type. In addition, 
many patents have issued using this or similar approaches including: U.S. 
Pat. No. 3,548,648 issued to Weichbrodt et al.; U.S. Pat. No. 3,694,637 
issued to Edwin et al.; U.S. Pat. No. 3,710,082 issued to Sloane et al.; 
and others. 
The implementation of vibration monitoring systems in the working 
environment, however, has been slow, mostly because these systems require 
complex data processing capabilities. Further, these types of systems have 
reliability problems because it is known the response of vibration power 
spectra to tool wear is not consistent for different types of milling 
operations where different tools are used. It is known, for example, that 
spectra response varies as a function of (a) tool type, (b) material 
making up the part, (c) size and shape of the part, and (d) cutting fluid 
and other parameters. It is therefore difficult to develop automatic 
systems of this type which are commonly adaptible to a variety of 
different milling machines and/or milling conditions. 
The drawbacks associated with the second kind of system mentioned above, 
i.e. one which senses tool wear on the basis of horsepower, were discussed 
at great length in the above-identified copending application Ser. No. 
825,134. Briefly, the loading force on an end mill type cutting tool 
typically has a tangential component and a radial component. The radial 
component has little influence on the amount of horsepower required to 
drive the tool. The tangential component does, however. Therefore, when 
horsepower is sensed it primarily indicates only the tangential component. 
When an end mill wears, however, it is the radial component that changes 
and increases, and the tangential component remains relatively constant. 
Therefore, for this reason and other discussed in application Ser. No. 
825,134 sensed changes in horsepower do not give a good indication of tool 
wear. 
It is an object of the present invention to provide a system that can 
accurately indicate tool wear, and which can be easily implemented on most 
of the currently existing microprocessor controlled milling machine 
systems. 
DISCLOSURE OF THE INVENTION 
The present invention provides a system for indicating cutting tool wear or 
dullness that is particularly applicable to a rotating cutting tool such 
as an end mill. 
A system in accordance with this invention can detect end mill wear as it 
occurs during milling. This is accomplished by first sensing the total or 
resultant side-loading force or forces acting on the tool. The resultant 
force has a radial component and a tangential component which are 
mathematically related to the resultant force by the following equation: 
##EQU1## 
where FRES is the resultant force; FR is the radial force; and FT is the 
tangential force. 
In addition to sensing the resultant force, the average torque driving the 
tool during milling is also sensed. An end mill has a cross-section 
generally as shown in FIG. 4 and its radius is known. Knowing the torque 
driving the end mill permits calculation of the resultant force's 
tangential component by merely dividing the value of the sensed torque by 
the tool's radius. Once the tangential force component is calculated, and 
since the resultant side-loading force is known from sensing it, the 
radial force component can be calculated algebraically from the above 
equation. 
It is known that the radial force component increases with increased tool 
wear but the tangential component remains relatively constant. The above 
steps are performed continuously during milling and when the radial 
component exceeds a certain level or magnitude, then the tool is known to 
be dull. Preferably, a ratio of the radial component to the tangential 
componet is continuously calculated during milling and when this ratio 
exceeds a certain amount the tool is deemed dull. 
Typically, an end mill is mounted to a spindle which is received within a 
spindle housing mounted to a milling machine. Preferably, resultant 
side-loading forces are continually sensed by means of strain gauges 
suitably mounted to the spindle housing. These sensors can detect 
compressive and tensile forces in the housing which indicate the total 
resultant side-loading force, but not its radial and tangential 
components. The tangential component is calculated from the torque driving 
the end mill which can be easily determined by measuring the electrical 
power required to drive the spindle motor. As per the above, knowing the 
resultant force and its tangential component permits calculation of the 
resultant component. 
The invention as disclosed above will be more fully described and better 
understood when reading the following description of the best mode for 
carrying out the invention in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
I. INTRODUCTION 
In the following description of the best mode for carrying out the 
invention, the contents of the subject matter disclosed in copending 
patent application Ser. No. 825,134, now U.S. Pat. No. 4,698,773, are 
incorporated herein by reference. The following description is directed 
specifically, but not in the limiting sense, to rotating cutting tools 
such as end mills, which are well-known in the art. Any mention of tool 
wear therefore, is directed to wear of an end mill. 
The final dimensions of a milled part or workpiece can be significantly 
affected by tool wear because the diameter of the tool gradually decreases 
as it wears. Another phenomenon which more substantially, albeit 
indirectly, affects final part dimensions is dynamic tool bending which 
occurs during milling, and which increases in correspondence with 
increased loading forces on the tool caused by wear. For example, and 
referring now to FIGS. 5 and 6, therein is shown a sharp cutting edge 10 
of an end mill shearing a chip 12 from a part 14. The direction of motion 
of both the edge 10 and the chip 12 are indicated, respectively, by arrows 
16 and 18. What has hereinbefore been mentioned as side-loading forces on 
a tool is actually the force acting on the edge 10 as it cuts. This force 
has a radial component FR indicated by vector 20, and a tangential 
component FT indicated by vector 22. Referring now to FIG. 7, there is 
schematically shown how the total or resultant side-loading force 24 is 
the sum of the radial 20 and tangential 22 components. 
As the edge 10 wears or becomes dull a wear-land area 26 grows from the 
edge's tip 28 in the manner shown in FIG. 6. As this happens, the 
magnitude of the radial component 20 increases significantly but the 
tangential component 22 remains relatively constant. This has been proven 
by test results which are presented in FIG. 4. 
Referring now to FIG. 4 therein is shown actual measurements of loading 
forces acting on a particular end mill when the end mill is both sharp and 
dull. These measurements were taken in conjunction with an end mill having 
two cutting flutes and a diameter of two inches. The end mill was turning 
at a rate of 1000 rpm during the tests and had a feed rate of 60 inches 
per minute while cutting aluminum. The end mill was cutting at an axial 
depth of 0.5 inches and a radial depth of 0.05 inches. The measurements 
were made in a stationary coordinate system and therefore the force "FX" 
is not a true measurement of tangential force but only an indicator 
thereof. Similarly, the force FY only provides an indicator of the true 
measurement of the radial force. As can be seen by comparing the results 
on the left-hand side of FIG. 4, which indicates tangential and radial 
forces for a sharp end mill, with the results on the right-hand side, 
which indicates the same forces for a dull end mill, the tangential forces 
tend to remain relatively constant with perhaps a slight increase when the 
end mill becomes dull. However, the radial forces tend to increase 
significantly, perhaps as much as a factor of 3. The peak forces shown in 
FIG. 4 result from the fact that the end mill actually cuts during only a 
fractional period of each total revolution. The rest of the time the end 
mill is "air cutting." 
The side-loading forces on a cutting tool act on the tool in a plane normal 
to the tool's axis of rotation. As discussed above, these side-loading 
forces can be thought of as having two components: a tangential component 
FT and a radial component FR which are normal to each other. The total or 
resultant side-loading force "FRES" caused by these components can be 
directly measured relatively easily. Additionally, the tangential 
component FT is relatively easy to directly measure. However, the radial 
component FR, whose value represents tool wear is difficult to measure 
directly. 
Referring now to FIG. 1, there is shown schematically at 26 a typical 
milling machine system, such as a Sundstrand OM-2 (trademark) milling 
machine. The system 26 includes an end mill 28 connected to a spindle 29 
(see FIG. 8) which is received in a housing 30. The housing 30 has a base 
portion 32 connected to the milling machine and four buttresses 34, 36, 
38, 40 interconnecting the base portion 32 and the housing 30. The spindle 
is driven in rotation by a motor which is schematically indicated at 42. 
Each rotation of the spindle is indicated by a spindle index pulse sensor 
schematically indicated at 44. The system 26 as thus described above would 
be familiar to a person skilled in the art and thus need not be described 
in further detail. 
As the end mill 28 cuts a part, the side-loading forces acting on the mill 
cause compressive and tensile forces in the spindle housing 30. These 
forces can be sensed by suitable strain gauges 46, 48, 50, 52 connected to 
the buttresses 34, 36, 38, 40. The strain gauges 46, 48, 50, 52 output two 
force component signals, FX and FY, similar to those shown in FIG. 4. 
These signals are suitably filtered as indicated at 54 and transmitted as 
shown at 56, 58 for processing by a microprocessor 60. 
The microprocessor 60 calculates the resultant side-loading force from the 
force components FX, FY. As mentioned above, these components are not the 
same as the radial component FR and the tangential component FT of the 
side-loading force since FX and FY are measured in a stationary coordinate 
sytem, and not a rotating one. However, the vector sum of FX and FY is 
equal to the resultant side-loading force FRES and since the resultant 
force is related to the radial and tangential forces by the Eq. (1) above, 
the radial force can be mathematically extracted therefrom if the 
tangential force component is known. 
The tangential component FT is equal to the torque on the end mill 28 
divided by the end mill's radius. This torque can be sensed by measuring 
the amounts of the armature 62 and field currents 64 of the spindle drive 
motor 42. The armature and field currents 62, 64 can be sensed as voltages 
across two resistors 66, 68, for example. These voltages are passed 
through analog opto-isolation circuitry 70 as shown at 72, 74, 76, 78, and 
transmitted onwardly as shown at 80, 82 for processing by the 
microprocessor. As a person skilled in the art would know, the analog 
opto-isolation circuitry 70 eliminates interference from noise which is 
typically generated in the high power level circuits of the spindle drive 
motor 42. The product of the two signals 80, 82 provides a good measure of 
the torque acting on the end mill 28. 
The force signals FX, FY must be adjusted to compensate for spindle defects 
in the manner disclosed in copending application Ser. No. 825,134. 
Similarly, the signals indicating the levels of armature and field 
currents 80, 82 must also be adjusted to compensate for the extra torque, 
or tare torque, required by the spindle drive motor 42 to overcome 
friction in the motor 42 and any gear mechanisms connecting the motor to 
the spindle. This is a relatively constant value and can be obtained by 
running the motor 42 during a noncutting condition. 
The microprocessor 60 is suitably programmed to calculate the resultant 
side-loading force FRES, and the radial and tangential components thereof 
from the signals received at 56, 58, 47, 80, and 82. The programming 
requirements for the microprocessor 60 are set forth below. 
II. SYSTEM PROGRAMMING REQUIREMENTS 
The microprocessor 60 may be any one of a wide variety of suitable 
microprocessors. The strain gauge signals 56, 58 should be processed 
approximately once every millisecond. This is necessary because the force 
acting on the end mill 28 is cyclical corresponding to the periods during 
each revolution when a cutting edge is either cutting material or air 
cutting. The relatively high frequency of processing or sampling is also 
necessary in order to compensate for fairly large cyclic fluctuations in 
the strain gauge signals 56, 58 which are caused by spindle defects, a 
phenomenon discussed in application Ser. No. 825,134. Preferably, 
therefore, if the end mill 28 is rotating at a rate of 1000 revolutions 
per minute, which is typical, the microprocessor 60 is programmed to 
process the signals 56, 58 from the strain gauges 46, 48, 50, 52 
approximately 60 times per revolution. 
The microprocessor 60 adjusts the strain gauge signals 56, 58 to compensate 
for spindle defects in the same manner disclosed in application Ser. No. 
825,134. The spindle rotation indicator 44 provides a pulse as indicated 
at 45 for every revolution of the spindle. This pulse is processed by 
proximity sensor circuitry 47 which transmits a pulse signal to the 
microprocessor 60 as indicated at 49. The pulse signal is therefore used 
by the microprocessor 60 to determine spindle position for each 
revolution. 
Torque on the spindle and end mill 28 changes much more slowly than the 
side-loading forces. For this reason, preferably the microprocessor 60 
could be programmed to calculate end mill torque once every four to eight 
milliseconds. 
The microprocessor 60 calculates the value for end mill torque which 
represents an average of the true torque on the end mill. The reason this 
is an average value is because of the large moments of inertia in the 
rotating members that make up the spindle drive system such as, for 
example, the spindle drive motor 42 and any gear mechanism connecting the 
motor to the spindle. The instantaneous torque on the end mill 28 in 
actuality pulsates widely as each cutter flute enters and leaves the part. 
To better understand this, if a cut is being made that has a shallow 
radial depth, then a flute will actually be cutting only during a small 
fraction of each revolution. During this time the torque is relatively 
high. However, during the rest of the time the tool is air cutting and the 
torque is zero. 
Since the torque calculated by the microprocessor 60 is an average value, 
it is therefore a requirement that the resultant side loading force FRES 
also be calculated as an average value. The resultant force FRES is 
calculated from the strain gauge signals 56, 58, after such signals have 
been corrected for spindle defects, by the below equation: 
##EQU2## 
where, as mentioned before, FX and FY are stationary components of the 
resultant force provided by the strain gages 46, 48, 50, 52. 
FRES is calculated once per millisecond and is accumulated in memory for a 
period of eight spindle revolutions. At the end of this time the total 
accumulated FRES is divided by the number of samples taken during the 
eight revolutions (approximately 8.times.60 for the example referred to 
above) to get an average resultant side-loading force AVGRF. During the 
same eight revolutions, torque data is accumulated and the sum is then 
divided by the total number of samples taken during the same eight 
revolutions. After correction for the tare torque caused by spindle motor 
and gear mechanism frictional losses, which was previously described, the 
average torque AVGTRQ is yielded. 
The tangential component FT of the resultant force is equal to the value of 
the average torque AVGTRQ divided by the radius of the end mill 28. Thus 
knowing FT, FR can be calculated from Eq. (1) by substituting AVGRF for 
FRES. Alternatively, the average resultant force AVGRF may be multiplied 
by the tool radius "R" and related to the quotient Q by the below 
equation: 
##EQU3## 
The average value of FR/FT is calculated simply by taking the closest value 
from the table of the function: 
##EQU4## 
where this value may be scaled for an analog output through a digital 
analog channel from the microprocessor 60 if so desired. 
The above described system requires only input data for the diameter of the 
end mill or tool from either an operator keyboard or from statements added 
to a preexisting numerical control program as indicated at 84 in FIG. 1. 
The microprocessor 60 could indicate tool wear when Eq. 4 achieves a 
certain value if so desired. The system described herein may be 
implemented independently or as part of a feed rate override system as 
described in copending application Ser. No. 825,134. 
The description of the best mode for carrying out the invention as 
presented above is not meant to limit the scope of obtainable patent 
protection. Rather, the outer boundary of patent protection is to be 
limited by the subjoined claims which follow, wherein such claims are to 
be interpreted in accordance with statutory and judicially established 
doctrines of patent claim interpretation.