CA1201792A - Feed rate indication for power tool - Google Patents

Abstract

FEED RATE INDICATION FOR POWER TOOL

ABSTRACT

A power tool including means for regulating motor speed to an optimum speed, and means for indicating an overfeed and underfeed condition in which the loading imposed by an operator exceeds or does not come up to a preferred range for operation empirically determined to generate a poor finish or overheat the selected material with the selected size of cutting implement size.

Description

EPS/bt n1?1Pl237 FEED RATE INDICATIO~I FOR POWER TOOL

DESCRIPTION

BACKGROUMD OF THE Ii~VENTION
This invention is in the fleld of power tools; more particularly, it relates to a control for indicating loading based upon loadlng imposed upon the tool by an operator.
In tools that use a moving cutter or cutting edge, an efficient quality cut requires the establishment of three parameters, provided proper tool usage techniyues are followed such as having properly sharpened tools or utilizing cutting lubricants. The three parameters are, speed of the cutting edge, material hardness and relative velocity between the material and the cutter.
In large machine tools, these parameters are taken into account during a setting up operation for the tool.
The operator will look up the appropriate cutting speed and material feed in a book of tables supp:lied by the machine manufacturer, or in a machinist handbook, based upon the type of material to be machined. Generally, as material hardness increases, cutter speed and material feed will decrease. There is a range of volumetric rates of removal for any given material and cutter which results in efficient cu-tting, with a good finish. If the cutter speed is too fast, and/or the feed rate is too slow, the cutter or material may overheat and over an ex-tended period, dull the cutting edge. If the cutter is too slow and/or the feed 5 rate too fast, the material tends to be removed in chunks or pieces, resulting in a poor finish and applying a shock loading on the power tool which may shorten its life.
Although these principles are well accepted and prac-ticed in the industry, there has been little done in this regard in consumer tools, particularly with respect to hand held or bench power tool.s. For many years, portable tools such as drills, sabre saws and polishers, and bench tools such as drill press, band saw, arbor saw, jointer-planer, sander, for example, have more or less incorporated speed controls to vary output speed. Ilowever, outside of general comments about feeding tools or work smoothly, such as described in user manuals, the judgement about feed rate was left up to the consumer since there was no way of having the tool make this decision.
~7hat is required, is an arrangement for a hand held or bench power tool in which the operator may select the cut-ting implement size to be used and a material to be operated upon and have the power tool automatically go to and main-tain an ideal cutting speed. What is further required, is some means of indicating to an operator that an ideal feed rate has been achieved, or alternatively, of accommodating the power tool to a feed rate se]ected by the operator.

SUrl~l~RY OF TEIE INVENTION
The above improvements are obtained in a hand hel.d or bench power kool having capability thereon for operator se-].ection of size of cutting implement such as drill bit, end mil] or grit for paper~ etc.~ and of material hardness, and an arrangement which responds to selected cutting implement size and material hardness to regulate motor speed to an æ
oytimum va]ue determined Erom a table s-tored in a memory.
The cutting implement may be a cut-ter bit for a router or a drill bi-t for a drill or may be grit size for a sander.
The arrangement may regulate motor speed to the predeter-mined optimurm value by monitoring slight changes from theoptimum speed and adjusting effective motor voltage for speed correction. The effective motor voltage may be varied by varying the firing angle triggering on a triac in series with the motor every half line cycle. The longer that the triac is turned on, the greater the effective motor voltage.
Since the firing angle of the triac is a known quantity, as is the total applied line voltage, the effective voltage can be calculated. By dividing the effective voltage by the effective motor impedance at the optimum operating speed stored in the memory, the effective motor current is cal-culated. The effective motor current is directly propor-tional to motor torque or load. As the feed rate changes, the motor load changes, resulting in a corresponding current change which is detected through the triac firing angle change needed to correct for an attempted speed chanae.
Since the arrangement, which may be a microprocessor or microcomputer, controls the firing angle of the triac, it is also able to determine the feed rate for the tool with data stored in the memory. It is not necessary to go through the calculations, appropriate current-speed re-latlonships empirically determined can be converted into relative triac firing angle data and stored in the memory.
Thus, the arrangement may indicate to an operator the sta-tus of the present speed rate, or may control the -tool so as to adjust the cutter speed for the amount of feed rate applied.
A feed rate indication system can be set up utilizing two vertical rows of LE~s which are visible from the front of the hand held or bench power tool. The left row corre-sponds to material hardness selection ranging from soft to ~2~
very hard. The right row indicates typlcal cu-tter size parameters. r~ith the hand l~eld or bencn power tool con-nected to a power source, pressing a momentary switch beneath each row advances -the position of the lit LED from soft to very hard or from the smallest cutter size indl-cated to the larges-t indicated. In this way, appropriate speed selection can be implemented depending on material and cutter size requirements. Feed rate indication may be accommodated by, for example, flashing an underfeed indi-cator LE3 when the power tool is insufficiently loaded, in-dicating an under feed condition, or by flashing an overfeed indication LED whenever the power tool is loaded beyond the rate in which optimum cutting can proceed. ~y keeping both LEDs continuously lit, or continuously off, the feed rate lS is indicated as within the preferred limits or range.
Alternatively, ~,EDS may be located within ~n operatorls line of sight when operating the power tool in a normal fashion.
The over/under feed rate indication system might be used to train an operator on a scrap piece of material until he has acquired a feel which will permit him to operate the power tool with minimum or no reference to the LEDS while wor~ing on the final piece of work material.
In a power tool in which the feed rate information is used as a means of changing cutter speed to correct -for feed rate changes, as the tool is loaded by a feed rate increase, the motor speed wlll increase; or iE the load decreases the motor speed would correspondingly decrease. A
program limit would be set to indicate to an operator when he is at the upper limit of -the speed and Eeed rate range and possib]y shut down the tool if the warning is ignorecl.
A practical approach might be to start out with a slightly low speed which would be changed to a higher, normally regulated speed once a preferred load or feed rate has been achieved. An upper limit control again would be desirable as described. If the load is removed from the tool the speed would, of course, drop to the lower value again.

-DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had to the specification and the attached drawings in which:
FIG. 1 is a front perspective view of a power tool in which the invention has been incorporated;
FIG. 2 is a circuit diagram of the material and cutter size selection, overfeed and underfeed control according to the present invention;
FIG. 3 is a cutter size-material~speed ma-trix stored in the microcomputer shown in FIG. 2;
FIG. 4 is a flow chart outlining the software for the microcomputer utilized in a preferred embodiment;
FIG. 5 is a detailed flow chart for speed correction corresponding to varying feed rate load; and, FTG. 6 is a detailed flow chart for detection and indi-cation of over and under feed rate.
Referring now to FIG. 1, a hand held power tool, speci-fically a router 10 is disclosed. The router 10 is fashioned with substantially cylindrica] housing 12 in the upper end of which is supported a driving motor (not shown) which is normally covered by cap ]3. Projecting from the housing 12 are right handgrip 14 and left handgrip 16, there being visible extending from the right handgrip a trigger switch lock 15 for a trigger switch (not shown) which is located in the right handgrip. ~ cord 17 extends from the right handgrip 1~ to the cap 13 for the motor (not shown), and line cord 18 e~tends Erom a source of power to the cap and to an e]ectronic control which will be described below.
Situated approximately midway of the housing 12 is a depth adjustment ring 20 calibrated as shown to provide for ex-tension of a cutter (not shown) beneath the router base 22 for operation of the cutter upon a work material. Situated above the depth adjustment ring 20 is a panel 24 upon which 3~ are inscribed a material table 26 and cutter size table 2~.

~z~
The material table 26 includes the columnar tabulation "UNDERFEED", "SOFT", "MEDIUM", "H~RD", "VERY HARD"; each adjacent a circle 30 through 34, respectively, which each outlines an LED as will be further explained below. The appellation "SOFT" may apply to white pine; "~DIUM" to yellow pine, "HARD" to oak or cedar, and ';VERY HARD" to composite materials such as particle board. Tlle cutter size table includes a columnar listing "OVERFEFD", "1/4 INCH", "3/8 INCH", "1/2 INCH" and "3/4 INCH"; each also beside a circle 35 through 39, respectively, which a]so each outlines LEDs as will be explained below. The material table 26 is headed by a MATERIAL label which is a pushbutton 27 which may be depressed for a purpose which will be further explained below. Likewise, the cutter size table 28 is lleaded by a CU~'TER SI~ label which is a pushbut-ton 29 for a purpose to be further described below.
Referring now to FIG. 2, there is shown a block dia-gram for a circuit which may be included within the cap 13 of the router ]0, preferably adjacent the panel 24 thereof.
The main component in the circuit is a microcomputer 42, which may be implemen-ted by a PIC1655A manufactured by the General Instrument Corporation. ~ power supply 44 receives power from the 115 volt, 60 hertz mains and provides rec-tified DC voltage for the microcomputer 42, oscillator 46 motor speed detector 64 and firing circuit 68. DC power is also supplied to LEDs 48 through 51 which are located in the circles 36 through 39, respectively, of FIG. l; to LEDs 54 through 57 which are located in circles 31 -through 34, respectively of FIG. l; and to LEDs 52 and 53 which are 30 located in circles 30 and 35 of FIG. 1, respectively. E]ec-tive scanning from the 1/4" cutter size LED 48 to the 3/4' cutter size LED 51 is implemented by a cutter size select switch 60 which is actuated by pushbutton 29 in FIG. 1.
Repeated actuation of the cutter size select switch 60 implements the scanning action through the microcomputer 42 7 9~ 7~:~
, with the SCanninCJ reverting from the 3/~" cu-t~er size LED
51 back to the 1/4;' cutter size ~,ED ~ so that continual scanning of the LEDs are possible by repeated actuar,ion of the cutter size select switch 60. A material select switch 62 actuates through pushbutton 27 of FIG. ]. a similar scan-ning of the material selection from soft through very hard and return to soft by the microcomputer 42 in the identical fashion to the cutter size selection. Selection may be implemented only when the trigger switch is not depressed, for the sake of safety.
Another input to the microcomputer 42 is a motor speed detector 64 which might be implemented by multiple rotating ma~nets with coil, or by an optical interrupter, used in conjunction with a rotating segment disc or other device for feeding back a motor speed to the electronic control.
A preferred method would be to utilize a E~all effect sensor and a multipole magnet. A zero crossing detector 66 receives a signal along line 67 from the low voltage al.ter-nating current in the power supply 44 to detect zero crossing and provide this information to the microcomputer 42 for determination of phase angle for a firing circuit 68 The firing circuit 6~ derives power from the power supply 44 through l.ine 69, and initiates conduction of triac 70 throuyh the gate thereof to provide a ground connection for 25 motor armature 75 and field coils 76 of motor 74. A triy-ger switch 78 llas a sing].e pole to connect the 115 volts, 60 hertz supply to the field coils 76 and motor armature 75. ~ switch detection circuit ~0 detects closing of trig-ger switch 7g and signals the microcomputer 42 to initiate 30 operation of tl1e firing circuit 68. ~ l.amp 77 located ad~acent router base 22 illuminates the work ~rea and is connected across a field coil to a center tap thereof. The microcomputer 42 regulates speed depending on the cutter size selection and material selection selected by means of switches 60, 62, respectively, and accordin~ to a speed ~3 ~ ~ O ~

table shown in FIG. 3 which may be retained in a read only mernory (ROM) in the microcompu-ter. The speed of the arma-ture 75 is detected by the motor speed detector 64, the signal from which is used by the microcomputer 42 to main-tain the motox armature at the optimum speed. The motorspeed detector 64 also receives power through line 69.
Speed regulation is accomplished by the microcomputer 42 monitoring slight changes i,n speed from the speed de-tector 64 and adjusting effective motor voltage for cor-rection. The triac 70 in series with the, motor is trig-gered on every half line cycle. The firing angle (that is the delay time from the start of each half cycle, when the triac 70 is triggered) will determine the eff~ctive motor voltage. Since the firing angle of the triac 70 is a known quantity, as is the total applied line voltage, the effective voltage can be calculated. By dividing the ef-fective voltage by the effective motor impedance at the optimum operating speed, information stored in the ~OM, the effective motor current is calculated. Motor current is di-rectly proportiona] to motor torclue or load. As tlle feedrate changes, motor load changes. This results in a corres-ponding current change and is detected through a change re quïred in the firing angle of the triac 70 to correct for ' an incipient speed change. The microcomputer 42 controls the firing angle of the triac 70 by means of the firing cir-cuit 68. With the da-ta ~hat the microcomputer has stored in its RO~1, the microcomputer is able to determine the opti-mum feed rate for the tool. The microcomputer 42 may or may not ~o through the calculations reEerred to above; however, the appropriate current-speed relationships can be arrived at empirically for typica] type of material and cutter size, and tables oE firing angles for norma] line voltages and given currents may be stored in the RO~. For various materials and cutter sizes, for each comblnation of which there is an optimum speed, power tab],es may also be derived empirically and related to firinq angle. Thus, -the micro-computer 42 has means of either indicating to the operator ~L2~

the relative feed ra-te; or of con-trolling motor speed so as to correct for the feed rate being applied.
When the tool is in use, whichever material and cut~
ter size I,EDS are lit may also be used as feed rate indi-cators. For example, the selected material LED will flashwhenever the router is insufficiently loaded, indicating an under feed condition, such as when the router is up to speed and little or no cutting is taking place. When cutting pro-ceeds, the material LED Will return to its steady on condi-tion as the feed rate moves into a preferred current-speed range. If the feed rate is increased so as to move beyond the upper limit of the desired feed rate range, the cutter si~e LED will commence flashing. By keeping both LEDs steady, the feed rate is within the preferred range. A pre-ferred method, however, is to have separate LED' S 52, 53which are responsive to underfeed and overfeed, respectively.
Separate LED' s for underfeed and overfeed would allow their location to be optimized for operator observance while operat-ing on a work piece. Load tables are established empirical-ly and stored in the RO~ of microcomputer 42 to just avoidunderfeed for each material and cutter size which causes the material to begin to disco]or from excess heat, and to just avoid overfeed for each material and cutter size which causes material to be removed in chunks resulting in poor finish. The use of an underfeed lnd;cator wil] have a bene-ficial effect on the life of -the cutt;ng implement since overheating thereof will be avoided. The overfeed indicator will extend both the life of the power too:L and the cutting implement since by responding to an overfeed indication by re~ucing loading, shock loading to both the cutting imple-ment and the power tool may be avoided.
FIG. 4 ls a flow chart of the software for the micro-computer 92. After insertion of the plug of the power ~ool, an overload timer which serves to disconnect the motor after time out of about 5 seconds on a high current overload o ~
condi.tion to avoi.d destructive motor heating, is reset.
Material and cutter size are se-t to the softest material and smallest size. ~hereafter a system is implemented -to pre-vent operation of the power -tool due to a trig~er switch locked in the on position. With the -tri~ger switch 7~ in the on position when the plug is inserted, LED's 4~ and 54 corresponding to 1/4" cutter size selection and soft mate-rial selection may flash at some set rate to alert an oper-ator that the tool will not further operate until the tri~-ger switch is shut off. If the trigger switch 78 i.s not inthe on position the microcomputer 42 ls ready to receive a material or cutter size selection by scanning from soft to very hard and by scanning from the 1/4 cutter si~e to the 3/4" cutter size upon actua-tion of selector buttons 27, 29, respectively. If the tri~ger switch 78 is actuated, the motor 74 is ramped up to speed, and once at that speed checks the status of feed rate, that is whether an overload condition e~ists which has initiated the overload timer or whether an underfeed or overfeed condition exi.sts. If an overload cond:ition exists, this event may be indicated to an operator by simultaneous flashing of both the underfeed and overfeed LED's 52, 53. If an underfeed condition exis-ts the LED 52 in circle 30 is flashed, and if an overfeed con-dition exists the LED 53 in circle 3S is flashe~. Without ar. underfeed or an overfeed condition, -the LEDS 52, 53 will remain unlit. ~xis-tence of an overload condltion on the motor 74 ;.nitiates a timer wh;.ch wi.]] time out in 5 seconds and stop the motor.
FIG. ~ is a detailed flow chart which may be substi.-tuted between the points A, B in the fl.ow chart ;.n FIG. 4a.in order to obtain a feed rate detection and automatic speed correction therefor. Definiti.ons for the terms used in the flow charts of FIGS. 5 and 6 are found in the ~ppendix to the spec.ification. Feed rate correction operations are shown ln parallel with Feedback correction operations in order to prevent the two operations from interfering with 9~2~79Z
each other by givlng s;mul-taneous orders to control the motor, since on]y one operation can control at a time. The operations are further dlvidlded lnto parallel paths to handle increasing and decreasing speed.
In FIG. 5, lt ls first determined if a load change has occurred which is significant enough to require a feed rate correction. The third decision box sta-tes the necessity for determining if the speed change due to a load change is too large to allow a speed correctlon through a feed rate cor-rection operation. If so, the feedback operation attempts -to correct the speed sufficiently to allow the feed rate operation to take over. This method is used since, for a feed rate speed correction operation, a decreasing load should decrease speed while an increasing load should in-crease the speed. If not careEully controlled by a feed-back operation, the increasing load could allow the speed to run away.
It should be noted that once a speed correction under the feed rate operaticn is achieved, the latest speed be-comes the reference speed to help to keep future speedcorrections at a minimumO This allows the feedback oper-ation to regulate to the new speed-feed locatlon rather than attempting to pull the speed back towards the low load speed, which would be a lower speed.
It should be noted that in all paths of feed rate cor-rection or feedback correction, motor speed changes are achieved through adding or subtracting a small firing angle g 0 F/(A) x (~ILC)] from the present flring angle [ 0 PRES] Thls method allows a great deal of flexibillty with precise contro]. The subscript "F/(~) x (HLC)" refers to firing per multiples of each half line cycle (EILC).
Thus, lf (A) were set at 5, a change in firlng angle would occur every fifth llne cycle until the approprlate speed change was achieved. I-t is likely that the integer ~ulti-plier will be l for feedback, partlcularly for increasingloads, and 1 or a little higher (2 or 3) for feed rate and ~2 program speed chdnges . D;. ~erent mu1tip~iers ~ight be de-sired for increasing and clecreasing loads for -the same oper-ation. Above the resolution limit of the control, the basic change in firing angle which could be used for different amounts of phase angle, and consequently speed, changes within each half line cycle. The various constants such as the range of no load program speeds covering all program settings, constant multip]iers and upper and ]ower speed, firing angle, and current limits as needed are stored in the permanent memory of the microcomputer ~2. These various constants may be determined throu~h motor analysis and/or empirically through tests of prototype products.
In FIG. 6, there is provided a suggested detail flow chart for insertion between points A and B of FIG. ~a indi-cating in detail how overfeed and underfeed indication maybe implemented and how feedback correction may also be imp]emented. Since feed rate cloes not ;nteract wi-th motor operation, it is placed in series with the feedback oper-ation. Since the feedback will regula-te the speed fairly closely with respec-t to the program no load or reference speed, it is only necessary to determine the upper and lower firing angles which are the limits for acceptable feed rate change. Below the minimum firing angle (a fast feed rate) the overfeed is activated and above the maximum firing angle the underfeed is activated. For feedback control an initial decision is made as to whether the speed is be]ow or above the reference speed; and the fir;ng angle for the triac 70 is increased for below speed, and decreased for above speed.
While the preferred embodiment has been disclosed in the above descr;p-tion, as well as certa;n rnoclifications thereto, ;-t wi]l be understood that many more mod;fications may be made wlthout departing from the scope of the inven-t;on. For example, ;t w;]l be apparent that ~or certain power tools which are dedicated to a ]imited use as to mate-rial and cutter s;ze, no such selection would be required ~7~2~
and indication of an optimum feed condition may be provided.Material selection only may be provided based on a limited use as to cutter size, ~nd conversely, cutter size selection only may be provided based on a limited use as to material selection, with an indication of an op-timum feed condition provided in either event. Feed rate may be the rate at which a material is fed through a bench tool, or the rate at which a power tool moves with respect to the material. In other words, it is a relative rate between the power tool and the material. In a sanding power tool, hand held or bench, the pressure exerted between the material and the tool becomes a part of feed rate together with speed of the sanding belt or pad, all of which determines how rast mate-rial will be removed for a given material and grit size and type. All of these factors are referable to power demands made upon a drive motor, and their relationship to power may be empirically determined and stored in a memory in the power tool. The scope of the invention is set forth in the claims below.

,7~2 APPE~DIX

REF Reference speed against which motor speed is compared to determine whether a speed cor-rection is required and what speed correction mode is required.
5 SPRES ~ Current motor speed.
0F Firing angle, angle of sine wave alternating current at which electronic power switch (triac) is activated to control motor vo]tage and thexeby speed.
10 0PR~S Current firing angle 0F/ (L) (HLC) Change in firing angle which occurs at integer mu]tiples (L or ~l) of each half line cycle (~LC).
15 L,~ _ Constant mul-~ipliers Aependent on product function and motor characteristics.
~SL ~ Change in speed from SREF due to load change.
~IN Speed change below which no correction is required. 0 ~SCO~p - Comparison speed change to determine whether feed rate or feedback correction will operate.
COR Change in motor speed relative to reference speed.
~SR~Q - Change in motor speed required to correc-t for current conditions.

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A power tool comprising a frame, a motor supported in said frame, a cutting implement driven by said motor, means supported in said frame for regulating speed of said motor to a desired speed, and means for indicating to an operator the relationship between the actual feed rate condition imposed by an operator and an optimum feed rate condition in which the loading imposed by an operator is in a preferred range for operation of said power tool wherein said cutting implement will provide an optimum finish on a material at said desired speed.

2. A power tool as claimed in Claim 1 further comprising means for selecting a material to be operated upon by said cutting implement, said regulating means further comprising memory means and responsive to a selected material for regulating speed of said motor to an optimum speed for said selected material from a table of speed for various materials stored in said memory.

3. A power tool as claimed in Claim 1 further comprising means for selecting a cutting implement size to be driven by said motor, said regulating means further comprising memory means and responsive to a selected cutting implement size for regulating speed of said motor to an optimum speed for said selected cutting implement size from a table of speed for various cutting implement sizes stored in said memory.

4. A power tool comprising a frame, a motor supported in said frame, a cutting implement driven by said motor, means for selecting a material and a cutting implement size, means including memory means supported in said frame and responsive to a selected material and cutting implement size for regulating speed of said motor to an optimum speed for said selected material and cutting implement size from a table of speed for various materials and cutting implement sizes stored in said memory, and means for indicating to an operator each period of operation of said motor during which a feed rate condition is imposed on the power tool by the operator in a preferred range for operation of said power tool at said optimum speed.

5. A power tool as claimed in Claim 4 further comprising a panel supported by said frame having material and cutting implement size listings thereon; and wherein said indicating means further comprises an overfeed condition indicator and an underfeed condition indicator distinguishable by said operator from said overfeed condition indicator, said indicators being responsive to conditions in which the loading imposed on a power tool by an operator exceeds or does not come up to, respectively, a preferred range for operation of said power tool at said optimum speed, one limit of said preferred range being where a selected cutting implement size removes a selected material at a volumetric rate less than that which results in less than an optimum finish, and the other limit of said preferred range being where a selected cutting implement size operating on a selected material begins to generate heat less than that sufficient to discolor the selected material, said optimum feed condition being indicated when said overfeed and underfeed condition indicators are not activated.

6. A power tool as claimed in Claim 5 wherein said regulating means further comprises a microcomputer and wherein said overfeed indicator and said underfeed indicator further comprises distinctively different LED's activated by said microcomputer according to tables in said memory means of loadings for each material and cutting implement size which might be selected, said tables having empirically determined parameters related to power supplied to said motor to avoid overfeed and underfeed conditions.

7. A power tool as claimed in Claim 6 wherein said motor is an AC motor, wherein said regulating means further comprises a pulse triggered solid state switch, and wherein the loading imposed on the power tool by an operator is determined by the firing angle of said solid state switch implemented by said regulating means in regulating speed of said motor to said optimum speed.

8. A method of indicating an optimum feed condition to an operator of a power tool having a motor, a cutting implement driven by said motor, means including memory means for regulating the speed of said motor to an optimum speed, the method comprising the steps of:
monitoring a value related to power supplied to said motor;
comparing said value to at least one value stored in said memory means which relates to a condition for obtaining a proper finish on a work material;
indicating to an operator the existence of a condition imposed by the operator away from said at least one value.

9. A method of indicating an optimum feed condition to an operator of a power tool having a motor, a cutting implement driven by said motor, means for selecting a material to be operated on by said power tool, means including memory means for regulating the speed of said motor according to an optimum speed for the selected material according to a table of optimum speeds for various materials in said memory means, the method comprising the steps of:
monitoring a value related to power supplied to said motor;
comparing said value to a table of at least one value for each material in said memory means, which relates to a condition for obtaining a proper finish on the selected material;

indicating to an operator the existance of a condition imposed by an operator away from said at least one value.

10. A method of indicating an optimum feed condition to an operator of a power tool having a motor, a cutting implement driven by said motor, means for selecting a cutting implement size to be driven by said motor, means including memory means for regulating the speed of said motor to an optimum speed for the selected cutting implement size according to a table of optimum speeds for various cutting implement sizes in said memory means, the method comprising the steps of:
monitoring a value related to power supplied to said motor;
comparing said value to a table in said memory means of at least one value for each cutting implement size which relates to a condition for obtaining a proper finish with the selected cutting implement size;
indicating to an operator the existence of a condition imposed by the operator away from said at least one value as well as indicating to an operator the direction in which said monitored value differs from said at least one value.

11. A method of indicating an optimum feed condition to an operator of a power tool having a motor, a cutting implement driven by said motor, means for selecting a material and a cutting implement size, means including memory means for regulating the speed of said motor to an optimum speed for the selected material and cutting implement size according to a table of optimum speeds for various materials and cutting implement sizes in said memory means, the method comprising the steps of:
monitoring a value related to power supplied to said motor;

comparing said value to a table in said memory means of at least one value for each combination of material and cutting implement size, which relates to a condition for obtaining a proper finish on the selected material with the selected cutting implement size;
indicating to an operator the existence of a condition imposed by the operator away from said at least one value as well as indicating to an operator the direction in which said monitored value differs from said at least one value.

12. A method of indicating an optimum feed rate condition to an operator of a power tool having a motor, a cutting implement driven by said motor, means for selecting a material and a cutting implement size, means including memory means for regulating the speed of said motor to an optimum speed for the selected material and cutting implement size according to a table of optimum speeds for various materials and cutting implement sizes in said memory means, the method comprising the steps of:
monitoring a value related to power supplied to said motor;
comparing said value to a pair of values in a table of values in said memory means for each combination of material and cutting implement size, said pair of values establishing a preferred range for operation of said power tool, one limit of said preferred range being where a selected cutting implement size removes a selected material at a volumetric rate less than that which results in less than an optimum finish, and the other limit of said preferred range being where a selected cutting implement size operating on a selected material begins to generate heat less than that sufficient to discolor the selected material;
providing selectively one of three discernably different indications to an operator indicating respectively the existance of a condition imposed by an operator where said monitored value exceeds said at least one value, the existance of a condition imposed by an operator where said at least one value exceeds said monitored value, and the existance of a condition imposed by an operator where said monitored and said at least one values are substantially similar.