AUTOMATIC MONITORING OF TOOL STATUS

Abstract: Automatic monitoring system and method for providing an instantaneous tool status indication for a stock removal cutter when cutting 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 R0. The system includes a torque monitor (7) for measuring a main drive cutting torque M during the cutting of an ith successive workpiece. Coupled to the torque monitor (7), is an instantaneous tool wear coefficient processor (13) for calculating a plurality of instantaneous tool wear coefficients r(j) <during the cutting of the ith successive workpiece. Coupled to the instantaneous tool wear coefficient processor (13) is an average tool wear coefficient processor (14) for calculating an average tool wear coefficient R>(i)< from the plurality of instantaneous tool wear coefficent r>(j)<. And finally, coupled to the average tool wear coefficient processor (14) is a tool wear coefficient comparator (15) for comparing each average tool 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 for each ith successive workpiece.

Description: AUTOMATIC MONITORING OF TOOL STATUS 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, US Patent 4,208,718 to Chung and US Patent 4,802,095 to Jeppesson. US Patent 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. US Patent 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. 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 RQ, the system comprising: (a) a torque monitor for measuring 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 for the stock removal cutter during the cutting of said ith workpiece; (c) an average tool wear coefficient processor for calculating an average tool wear coefficient for the stock removal cutter from said plurality of instantaneous tool wear coefficients and (d) a tool wear coefficient comparator for comparing said average tool wear coefficient to the reference average tool wear coefficient RQ to provide an instantaneous tool status indication for the stock removal cutter after cutting each ith successive workpiece. In Applicant's W094/ 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 p γ 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 γ 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 a R . 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 AQ, α and β are coefficients dependent on a particular stock removal cutter cum workpiece material combination. In addition, a coefficient triplet (AQ, α, β) is determined such that the ratio where R is the coefficient indicative of the tool wear of a stock removal cutter after cutting an ith successive workpiece and RQ is a reference average tool wear coefficient for a substantially new stock removal cutter executing the same machining operation equals a predetermined upper critical threshold e u corresponding to when the stock removal cutter is substantially dulled. A stock removal cutter is replaced when the ratio either exceeds the upper critical threshold e u or suddenly drops below a lower critical threshold e j indicative of a broken stock removal cutter. For the sake of convenience, both the upper and lower critical thresholds e u and e j are machining operation invariant and respectively equal 1.6 and 0.9. The lower critical threshold e j preferably equals 0.9 rather than 1, thereby indicating that a stock removal cutter is truly broken rather than the ratio dropping below 1 for some other reason, i.e. breakage of a workpiece. Against this, when the ratio 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 e u = 1.6, the tool wear percentage is calculated from the following relationship: 1.6 - Rt Ro x 100% 0.6 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 Q F J? P is adapted to: M =A Q F R ^ 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 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 RQ, the method comprising the steps of: (a) measuring a main drive cutting torque M during the cutting of an ith successive workpiece; (b) calculating a plurality of instantaneous tool wear coefficients for the stock removal cutter during the cutting of the ith successive workpiece; (c) calculating an average tool wear coefficient R for the stock removal cutter from said plurality of instantaneous tool wear coefficients r , , and (d) comparing said average tool wear coefficient R to the reference average tool wear coefficient RQ to provide an instantaneous tool status indication for the stock removal cutter after cutting each ith successive workpiece BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention and to show how the same can be carried out m practice, reference will now be made, by way of non-limiting example only, to the accompanying drawings in which Fig. 1 is a schematic view of an automatic monitoring system (AMS) for providing an instantaneous tool status indication for a milling cutter; Fig. 2 is a schematic block diagram of the AMS of Figure 1; Fig. 3 is a flow diagram of the operation of the AMS of Figure 1; and Fig. 4 is a graph showing a typical curve of the ratio RJ/RQ against ith successive workpieces for a given machining operation. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Figure 1 shows an automatic momtonng system (AMS) 1 for providing an instantaneous tool status indication for a milling cutter 2 rotatably driven by a machine tool 3 under the control of a CNC umt 4 as modified by an adaptive control system (ACS) 5 for adaptively controlling at least the milling cutter's feed rate in response to the torque developed by the machine tool 3 as measured by a torque monitor 7 connected thereto. Such an ACS 5 is, for example, described in Applicant's W094/ 14569 which can also adaptively control a milling cutter's spindle speed S. The instantaneous tool status indication can be displayed as a tool wear percentage on a tool status indication momtor 8 for appropriate use by a machine operator i.e. the change of a worn milling cutter. Alternatively, tool status can be indicated on the monitor 8 as a readout "operative" or, alternatively, "inoperative" when either the milling cutter 2 is worn or broken. In addition, when the milling cutter 2 is worn or broken, the AMS 1 can issue a tool change interruption signal (TC) to the CNC unit 4. Also coupled to the AMS 1 is an operator interface 9 via which an operator inputs a milling cutter (MC) code indicating a milling cutter type, a workpiece material (WM) code indicating a workpiece material and a machining operation (MO) code indicating a machining operation. Figure 2 shows that the AMS 1 includes a memory 10 storing coefficient triplets (AQ, α, β) for different milling cutter cum workpiece material combinations and the reference average tool wear coefficients RQ for different machining operations. Typical coefficient triplets (AQ, α, β) for a solid end milling cutter having four flutes are as follows: (A = 148, α =0.75, β = 0.83) for soft steel, (A 0 = 116, α =0.68, β =0.75) for cast iron and (AQ = 79, α =0.72, β =0.8) for aluminum. The coefficient triplets (AQ, α, β) for different milling cutter cum workpiece material combinations are determined empirically on the basis of a large number of machining operations. A new coefficient RQ for a new machining operation is determined during a trial run with a new milling cutter by a calibration unit 11 which acquires and is provided with a coefficient triplet (AQ, α, β) for a particular milling cutter cum workpiece material combination as indicated by a machine operator. Coupled to the ACS 5, the torque monitor 7 and the memory 10 is an instantaneous tool wear coefficient processor 13 for calculating instantaneous tool wear coefficients Γ Λ in accordance with the relationship M,-. = ς y/ft Ϊ" ( Λ where is the instantaneous feed per tooth of the milling cutter 2 at different positions along its cutting path while cutting an ith successive workpiece. If the instantaneous feed per tooth ^ z( \ ) is unavailable directly, it can be readily calculated according to the relationship Fy / ft - F',p./(S',pZ) where is an instantaneous feed rate of the milling cutter 2 relative to a workpiece, S'/ :\ is its instantaneous spindle speed and z is the number of its flutes. Coupled to the processor 13 is an average tool wear coefficient processor 14 for calculating an average tool wear coefficient for the milling cutter 2 cutting an ith successive workpiece in accordance with the general relationship: Coupled to the memory 10 and the processor 14 is a tool wear coefficient comparator 15 for comparing an average tool wear coefficient to a reference average tool wear coefficient RQ to provide the tool status indication on the monitor 8 and to issue a tool change interruption signal TC, if appropriate. The operation of the AMS 1 is now described with reference to Figure 3 which illustrates that after determining an unknown RQ for a new milling cutter for a new machining operation, for each ith successive workpiece, the processor 13 calculates instantaneous tool wear coefficients τ/ -\ typically at a sampling rate of 10 samples per sec. from which the processor 14 calculates a non-weighted average tool wear coefficient RΓΛ for the milling cutter which is then compared to the reference average tool wear coefficient RQ by the comparator 15 to determine the ratio being indicative of the instantaneous tool status of the milling cutter. As mentioned above, when the ratio lies in the operative range [0.9,1.6], the instantaneous tool status is shown as a tool wear percentage in accordance with the relationship: 1.6 - R, R Q x 100% 0.6 As shown in Figure 4, as a milling cutter wears, the ratio for each successive ith workpiece asymptotically increases towards the predetermined upper critical threshold 1.6 corresponding to when it is substantially dulled and requires replacement. In the example shown, the milling cutter is worn after about 45 workpieces. In an automatic mode of operation of the AMS 1, when the ratio falls outside the operative range [0.9,1.6] when it is either broken or worn, the AMS 1 issues a tool change interruption signal TC to the CNC unit 4. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention can be made by those ordinarily skilled in the art. For example, the automatic monitoring system can be installed as an original OEM feature or retrofitted to existing CNC machines.

Claims: CLAIMS 1. 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 RQ, the system comprising: (a) a torque monitor for measuring 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/ Λ for the stock removal cutter during the cutting of said ith workpiece; (c) an average tool wear coefficient processor for calculating an average tool wear coefficient for the stock removal cutter from said plurality of instantaneous tool wear coefficients and (d) a tool wear coefficient comparator for comparing said average tool wear coefficient R J to the reference average tool wear coefficient RQ to provide an instantaneous tool status indication for the stock removal cutter after cutting each ith successive workpiece. 2. The system according to Claim 1 wherein said instantaneous tool wear coefficient processor calculates an instantaneous tool wear coefficient r :\ in accordance with the general relationship: M ( = A Q F 'T,^ . 3. The system according to Claim 1 wherein said instantaneous tool wear coefficient processor calculates an instantaneous tool wear coefficient r ;\ in accordance with the general relationship: M, -, - A^F ^ ^T,^ forwhen the stock removal cutter is a milling cutter. 4. A method 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 RQ, the method comprising the steps of: (a) measuring a main drive cutting torque M during the cutting of an ith successive workpiece; (b) calculating a plurality of instantaneous tool wear coefficients for the stock removal cutter during the cutting of the ith successive workpiece; (c) calculating an average tool wear coefficient for the stock removal cutter from said plurality of instantaneous tool wear coefficients τ - ; and (d) comparing said average tool wear coefficient RΓΛ to the reference average tool wear coefficient RQ to provide an instantaneous tool status indication for the stock removal cutter after cutting each ith successive workpiece. 5. The method according to Claim 4 wherein step (b) includes calculating an instantaneous tool wear coefficient in accordance with the general relationship: M,-. = A Q F,-, r,- . 6. The method according to Claim 4 wherein step (b) includes calculating an instantaneous tool wear coefficient r Λ in accordance with the general relationship: M,-. - A Q F^-^Γ, ^ for when the stock removal cutter is a milling cutter.

Inventors: FAINSTEIN BORIS, RUBASHKIN IGOR, TABACHNIK EDUARD, ZUCKERMAN MARK, FAINSTEIN, BORIS, RUBASHKIN, IGOR, TABACHNIK, EDUARD, ZUCKERMAN, MARK
Applicants: FAINSTEIN BORIS, OMAT LTD, RUBASHKIN IGOR, TABACHNIK EDUARD, ZUCKERMAN MARK, FAINSTEIN, BORIS, OMAT LTD., RUBASHKIN, IGOR, TABACHNIK, EDUARD, ZUCKERMAN, MARK
Application year: 1997
Main classification: G05B19
IPC codes: G05B19