Automatic monitoring of tool status

Details Automatic monitoring of tool status Automatic monitoring of tool status

1998-09-02

2001-03-13

Gordon, Paul P. (Department: 2786)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C702S044000, C318S568100, C700S052000

Reexamination Certificate

active

06202002

ABSTRACT:

FIELD OF THE INVENTION
The invention is generally in the field of automatic monitoring of tool status during a stock removing operation. In particular, the invention relates to the automatic monitoring of the tool status of a stock removal cutter as a function of the torque developed by a machine tool during a machining operation.
BACKGROUND OF THE INVENTION
A CNC program instructs a milling cutter rotatably driven by a machine tool to cut a workpiece along a cutting path. Without adaptive control of at least the milling cutter's feed rate relative to a workpiece, the machine tool is required to develop an ever increasing torque for the milling cutter to cut successive workpieces in accordance with the CNC program due to its progressively worsening tool wear.
Automatic torque responsive monitoring of the tool status of milling cutters is known in the art, for example, U.S. Pat. No. 4,208,718 to Chung and U.S. Pat. No. 4,802,095 to Jeppesson.
U.S. Pat. No. 4,208,718 to Chung describes an automatic monitoring method in which a tool status indication of a milling cutter is determined as a function of the length of cutting time under increased target horsepower as a percentage of the total cutting time for a machining operation.
U.S. Pat. No. 4,802,095 to Jeppesson describes an automatic monitoring method in which a resultant side loading force FRES, having tangential FT and radial FR components. during a milling operation is measured directly along with its FT component and, on the basis of which, its FR component is determined mathematically for providing an indication of tool wear.
In U.S. Pat. No. 4,547,847 there is described an adaptive control for machine tools which provides tool monitoring functions under a subroutine TLfiNTR illustrated in FIG.
13
H and described in col. 35, line 64 to col. 38, line 49. The subroutine TLMNTR assumes a basically linear dulling process and requires several cutting parameters from the operating NC program, such as feed rate and spindle speed. In addition, the subroutine requires either a known or constant depth of cut which, in the case of the former, is compensated for in a linear fashion. As such, the subroutine is suitable for predicting tool life in relatively straightforward turning applications.
SUMMARY OF THE INVENTION
A primary object of the invention is to provide for automatic monitoring of the tool status of a stock removal cutter, and in particular a milling cutter, during a machining operation under the control of an adaptive control system adapted to maintain a substantially uniform torque as developed by a machine tool during the machining operation.
In accordance with a first aspect of the present invention, there is provided a system for automatically monitoring tool status so as to provide an instantaneous tool status indication for a stock removal cutter cutting identical workpieces in accordance with a given machining operation for which a substantially new stock removal cutter of the same type has a reference average tool wear coefficient R
0
, the system comprising:
(a) means for determining a main drive cutting torque M during the cutting of an ith successive workpiece;
(b) an instantaneous tool wear coefficient processor for calculating a plurality of instantaneous tool wear coefficients r
1
(j) for the stock removal cutter during the cutting of said ith workpiece;
(c) an average tool wear coefficient processor for calculating an average tool coefficient R
(i)
for the stock removal cutter from said plurality of instantaneous tool wear coefficients r
(j)
; and
(d) a tool wear coefficient comparator for comparing said average toot wear coefficient R
(i)
to the reference average tool wear coefficient R
0
to provide an instantaneous tool status indication for the stock removal cutter after cutting each ith successive workpiece;
characterized in that
said instantaneous tool wear coefficient processor calculates an instantaneous tool wear coefficient r
(j)
in accordance with the general relationship: M
(j)
=A
0
F
(j)
&agr;
r
(j)
&bgr;
wherein F is the instantaneous feed rate of the relative movement between the stock removal cutter and a workpiece and A
0
, &agr; and &bgr; are coefficients dependent on the stock removal cutter cum workpiece material combination.
In Applicant's WO94/14569, there is described an adaptive control system for adaptively controlling at least a milling cutter's feed rate in response to the torque developed by a machine tool rotatably driving the milling cutter in accordance with the general relationship: M=AF
Y&rgr;&ggr;
where M is the measured torque developed by a machine tool rotatably driving a milling cutter, F is the instantaneous feed rate of a milling cutter, p is the cross-sectional area of a workpiece instantaneously worked on by a milling cutter and A, y and &ggr; are coefficients dependent on a milling cutter and a workpiece material.
It has now been found that the tool status of a stock removal cutter for a given machining operation defining all factors pertinent thereto including, inter alia, a machine tool, a workpiece including its material, a cutting path, a feed rate, a spindle speed and the use of a particular type of coolant can be deduced from the same type of general relationship: M=A
0
F
60
R
&bgr;
.
In this relationship, M is the measured torque developed by a machine tool, F is the instantaneous feed rate of the relative movement between a stock removal cutter and-a workpiece and A
0
, &agr; and &bgr; are coefficients dependent on a particular stock removal cutter cum workpiece 20 material combination. In addition, a coefficient triplet (A
0
, &agr;, &bgr;) is determined such that the ratio R
(i)
/R
0
where R
(i)
is the coefficient indicative of the tool wear of a stock removal cutter after cutting an ith successive workpiece and R
0
is a reference average tool wear coefficient for a substantially new stock removal cutter executing the same machining 25 operation equals a predetermined upper critical threshold &egr;
u
corresponding to when the stock removal cutter is substantially dulled.
A stock removal cutter is replaced when tne ratio R
(i)
/R
0
either exceeds the upper critical threshold &egr;
u
or suddenly drops below a lower critical threshold &egr;
l
indicative of a broken stock removal cutter. For the sake 30 of convenience, both the upper and lower critical thresholds &egr;
u
and &egr;
l
are machining operation invariant and respectively equal 1.6 and 0.9. The lower critical threshold &egr;
l
preferably equals 0.9 rather than 1, thereby indicating that a stock removal cutter is truly broken rather than the ratio R
(i)
/R
0
dropping below 1 for some other reason, i.e. breakage of a workpiece.
Against this, when the ratio R
(i)
/R
0
falls within the operative range [0.9,1.6], the tool wear of a stock removal cutter is preferably indicated to a machine operator as a tool wear percentage where 100% indicates that a stock removal cutter is new and 0% indicates that it is worn and should be replaced. Based on the nominal upper critical threshold &egr;
u
=1.6, the tool wear percentage is calculated from the following relationship:
[
1.6
-
R
(
i
)
/
R
0
0.6
]
×
100
⁢
%
The machine tool is dependent on the stock removal operation and, consequently, in a milling operation, the torque M is the torque developed by a machine tool rotatably driving a milling cutter whilst, in a turning operation, the torque M is the torque developed by a machine tool rotatably driving a workpiece. In the case of providing the instantaneous tool status indication of a milling cutter, it has now been found that a more accurate indication is obtained when the relationship M=A
0
F
&agr;
R
&bgr;
is adapted to: M=A
0
F
z
&agr;
R
&bgr;
where F
z
is the feed per tooth of a milling cutter to take into consideration that a milling cutter is a multi-edge cutting tool.
In accordance with a second aspect of the present invention, there is provided a method for automatically monitoring tool status so as to provid

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