US3410028A - Control of automatic abrading machines - Google Patents

Description

Nov. 12, 1958 M. R. ESTABROOK 3,410,028

CONTROL OF AUTOMATIC ABRADING MACHINES Filed March 30, 1965 2 Sheets-Sheet l III! I IIIH IHIN IIIIIIIiIZLSIIIIIIIIL +1 11c. SOURCE Q- I E J 1-. Z

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EXPANDER ROD FORCE 8 0 no 20 so 40 so so 70 so so MOTOR VOLTAGE vou's United States Patent 3,410,028 CONTROL OF AUTOMATIC ABRADING MACHINES Mark R. Estahrook, Rockford, Ill., assignor to Barnes Drill Co., Rockford, Ill., a corporation of Illinois Filed Mar. 30, 1965, Ser. No. 443,875 12 Claims. (Cl. 51-165) ABSTRACT OF THE DISCLOSURE An automatic honing machine having a reciprocating rotary tool of conventional form that is progressively expanded within a work bore to feed the honing stones into the bore wall, thereby removing surface roughness and enlarging the bore to a predetermined size. The expansion mechanism is driven by a variable speed and torque electric motor having an armature that draws current through a tungsten lamp bulb filament so that the resistance value of the filament increases with the current level to modulate the motor torque and honing force in accordance with the load on the tool. The force range of the motor is varied either by a rheostat in parallel with the lamp or by a voltage divider for adjusting the field strength of the motor.

This invention relates to the control of automatic abrading machines :and is particularly adapted for use in a honing machine for enlarging and finishing work bores by reciprocating and rotating an expandable honing tool within the bore while gradually expanding the tool to press the abrasive elements thereon against the bore wall. More particularly, the invention relates to the control of an electric motor driving the mechanism for feeding abrasive elements into a rough work surface at a controlled rate that determines the rate of removal of stock from the surface.

In the honing operation, efficient action of the tool is dependent upon the pressure with which the abrasive stones of the tool are forced against the bore wall by the hone expansion mechanism. Optimum hone performance depends upon the proper correlation of motor torque and feed rate with the resistance to expansion offered by the bore wall. If the tool is expanded too rapidly, the abrasive stones are subjected to excessive pressure and destructive strains which tear the grit out of or crack the stones, thereby materially reducing the useful life of the stones. If, on the other hand, the rate of expansion is too slow, the length of the honing cycle is increased and, in addition, the efiiciency of the stones may be impaired by glazing, that is, either loading of the exposed surfaces t of the stones with worn-away material or dulling of the exposed abrasive particles at the surfaces of the stones. Thus, both too slow and too rapid expansion rates materially increase production costs.

Control of hone expansion to produce optimum hone performance is complicated by the nature of the work surfaces to be honed. At the beginning of a honing operation, the bore wall is comparatively rough and irregular and the hone operates primarily on ridges and high spots left on the wall by the previous operation. These can be worn away at a relatively rapid rate. As honing progresses, however, the stones encounter a progressively increasing surface area and correspondingly increasing resistance to expansion of the tool.

The general object of the present invention is to reduce the length of the honing cycle and increase the life of abrasive stones in service use by maintaining optimum honing pressure throughout the honing cycle.

Another object is to sense the changing resistance offered by the work surface throughout the cycle and vary the feed rate automatically and progressively in response to changes in such resistance.

A more detailed object is to sense the resistance by measuring the current drawn by the electric motor driving the feed mechanism, :and to regulate the speed and torque of the motor automatically in response to variations in the motor current to obtain an expansion rate and motor torque for producing optimum honing pressure for the prevailing condition of the work bore, that is, a high rate of initial expansion and progressively slower expansion with controlled torque as the work resistance increases.

A further object is to provide flexibility of adjustment of the range of expansion rates, motor torque, and the resulting honing pressures obtained during the honing cycle to adapt the cycle for different anticipated honing conditions.

Still another object is to obtain optimum honing effectiveness throughout the honing cycle for increased stone life, reduced cycle time, and generally improved hone performance.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which FIGURE 1 is a fragmentary front elevational view of an automatic honing machine embodying the novel features of the present invention.

FIG. 2 is a schematic view showing the feed mechanism.

FIG. 3 is a diagram of the important parts of the feed control.

FIG. 4 is a chart showing the changes in the resistance of the filament of a lamp bulb, rated at 50 watts at 250 volts, with changes in the current through the filament.

FIG. 5 is a diagrammatic view similar to FIG. 3 but showing a modified form of the invention.

FIG. 6 is a diagrammatic view showing still another form of the invention.

FIG. 7 is a chart illustrating the effect of changes in the rheostat resistance on the feed motor voltage and expander rod force ranges.

As shown in the drawings for purposes of illustration, the invention is incorporated in a power actuated mechanism for expanding a tool 10 (FIG. 1) for honing the bore wall 11 of :a workpiece 12 by reciprocation of the tool along the bore and simultaneous rotation of the tool with abrasive elements 13 thereon pressed against the bore wall. In this instance, the tool is carried on a shank 14 on the lower end of a spindle 15 journaled on a head 17 slidable back and forth on vertical guide rods 18 supported on the machine frame. The spindle :and the shank are connected by a universal joint 19 and are rotated by an electric motor (not shown) connected to the spindle by gearing 20. Raising and lowering of the head along the guide rods 18 to reciprocate the tool within the bore is effected by a suitable actuator (not shown) in a well known manner.

The honing tool 10 is of a conventional type comprising a series of elongated abrasive sticks or stones 13 of generally rectangular cross-section disposed in and projecting outwardly through elongated slots angularly spaced around a hollow cylindrical body 21 on the lower end of the shank 14 with followers 22 on the inner sides of the stones engaging a conical cam 23 within the body. The cam shown herein is fast on an axially movable rod 24 which extends upwardly through the tool shank and the spidnle 15 and is part of the mechanism for expanding the tool within the bore, the rod being formed in two separate sections to permit pivotal motion at the joint 19. As the rod is moved downwardly relative to the shank and the stones, the latter are ca-mmed radially outwardly :at a rate determined by the slope of the cam and the rate f endwise movement of the rod, and are pressed against the bore wall 11 with a force proportional to the force exerted on the rod.

Feeding of the stones 13 outwardly into the bore wall 11 during honing is effected by an electric motor 25 driving the expansion mechanism and operable to feed the expander rod 24 downwardly at a controlled rate relative to the tool. While the connection between the feed motor and the expander rod may take various forms, herein a worm 26 (FIG. 2) on the motor shaft meshes with a worm wheel 27 on one end of an intermediate shaft 28 driving another shaft 29 through worm gearing 30. During the honing cycle, this shaft is coupled by a clutch 31 to a drive shaft 32 whose rotary motion is converted to endwise motion of the expander rod 24 by a worm 33 meshing with a nut 34 threaded onto the upper end of the expander rod and mounted in the head for rotation in the position shown in FIG. 2. Thus, rotation of the nut in one direction moves the expander rod downwardly through the spindle and the shank to expand the tool, while rotation of the nut in the opposite direction draws the rod upwardly to permit contraction of the tool.

For rapid expansion and contraction of the tool at the beginning and end of the honing cycle, a reversible electric motor 35 is geared at 37 directly to the drive shaft 32 to rotate the latter at a rate many times faster than that obtained with the feed motor. During operation of the traverse motor 35, the clutch 31 is deactivated to disconnect the feed motor and the drive shaft. When the traverse motor has expanded the tool sufficiently to bring the stones 13 close to or into engagement with the bore wall 11, it is deenergized and the feed clutch is engaged to initiate slower feeding of the stones. Control of termination of rapid hone expansion may be accomplished in various ways, for example, by a device of the type disclosed in Johnson Patent No. 2,819,566. During honing, the deenergized traverse motor turns idly with the drive shaft as the latter is rotated at a controlled and relatively slow rate in direction to move the expander rod downwardly through the tool shank and feed the stones radially outwardly at a rate which determines the rate of expansion of the tool.

In accordance with the present invention, the speed and torque of the electric feed motor 25 are controlled in an extremely simple and effective manner throughout the honing cycle to vary the feed rate and stone pressure against the work automatically in response to variations in the resistance encountered by the stones 13 thereby to produce the optimum feed rate and honing pressure throughout the cycle. In this way, stone life is materially increased and the length of the honing cycle is reduced.

To these ends, the motor armature 38 is energized from a voltage source 39 through a series resistor 40 having an impedance or resistance that increases and decreases sharply, that is, to a high degree, with increases and decreases in the current flowing through the resistor. With this arrangement, the progressively increasing resistance to expansion of the tool 10 during the honing cycle automatically and progressively reduces the tool feed rate and modulates the torque exerted by the feed motor, thus regulating the honing pressure throughout the cycle.

To facilitate an understanding of the operation of the invention, it should be pointed out that the amount of cur rent drawn by the armature 38 in operation is dependent upon the difference between the terminal voltage impressed on the armature and the back voltage it generates. Back voltage, in turn, is determined by the speed of rotation of the armature. Accordingly, when a constant voltage is impressed on the motor armature and field 41, the current drawn increases and decreases as the motor speed decreases and increases. Moreover, the motor speed varies from a maximum speed with no load on the motor to zero speed (a stalled condition) when the load is greater than the motor can move. Thus, the current drawn by the motor varies from a relatively small value when the motor is running under no-load conditions and the back voltage is nearly equal to the terminal voltage, to a relatively large value when the motor is running very slowly, the back voltage then being near zero in value. In addition, it should be remembered that the torque exerted by the motor is directly proportional to and increases with the current drawn by the motor.

With these fundamentals in mind, it will be seen that the current drawn by the feed motor 25 through the resistor 40 increases gradually as the honing tool 10 wears away the rough surface metal of the bore wall 11 and encounters progressively increasing resistance to expansion of the tool. Since the current through the resistor is the same as that in the motor, and the resistance value of the resistor increases sharply with increases in current, the resistance in series with the armature 38 rises as the work load increases. Thus, the resistor senses the increases in the work resistance by measuring the current drawn by the feed motor.

The immediate effect of the increasing resistance is to reduce the terminal voltage of the motor armature 38 by an amount proportional to the resistance increase. It is fundamental, of course, that a reduction in the armature voltage correspondingly reduces the speed of the motor, so the resistance increase further reduces the motor speed and the tool feed rate. In addition, it will be evident that the reduction in the terminal voltage reduces the difference between the terminal voltage and back voltage generated by the armature at the prevailing motor speed, and thereby reduces the current drawn and the torque exerted at that speed. Thus, as the motor speed is gradually reduced by the progressively increasing work load, the resistor 40 reduces the terminal voltage of the armature and thereby further reduces the motor speed while simultaneously controlling and modulating the increase in current flow and the resulting torque exerted. Accordingly, the resistor constitutes a load-responsive speed and torque controller for the motor.

In putting the invention into practice, advantage is taken of the Well-known fact that the tungsten filaments of standard incandescent lamp bulbs display the desired resistance characteristics. Such filaments have positive temperature coetficients of resistance, that is, a resistance value that increases with the temperature of the filament. The temperature of the filament, in turn, varies over a wide range in accordance with changes in the current through the filament due to the heating of the filament by energy conversion according to the well-known relationship W=I R, where W is the rate of heat energy dissipation in watts, I is the current in amperes, and R is the resistance in ohms. The line 36 in FIG. 4 illustrates the rate of rise of filament resistance with changes in filament current witth a lamp bulb rated at 50 watts at 250 volts.

In this instance, the feed motor is a 1/ 150 horsepower shunt-wound DC. motor with its armature 38 and field 41 connected across power lines L1 and L3 having input terminals connected to a volt direct-current voltage source at 39, and with an incandescent lamp 42 in series with the armature on the positive side of the latter as shown in FIG. 3. To begin a honing cycle, the tool 10 first is expanded by the traverse motor 35 to bring the stones 13 close to the bore wall 11, the traverse motor is deenergized, and the clutch 31 is engaged to couple the feed motor 25 to the expander rod drive shaft 32. The feed motor is started by closing a switch 43 (FIG. 3) to complete the circuits to the motor. Thus, current flows through the lamp filament 40 to the feed motor armature 38 and begins heating the filament as the motor starts. Since the load on the motor is relatively small, the motor almost immediately attains a relatively high speed and feeds the stones outwardly at a coarse feed rate until the stones engage the bore wall.

With the feed motor running rapidly, the back voltage generated by the armature 38 is nearly equal to the terminal voltage of the armature, and the current drawn by the motor through the filament 40 is small. At the same time, the current flowing through the filament rapidly heats the latter to a temperature correlated with the motor current. It will be appreciated that the initial incerase in the filament temperature and resistance reduces the terminal voltage of the armature. If the work load on the motor remained constant, however, the motor speed, motor current, filament temperature and filament resistance would attain constant and interrelated equilibrium values.

As the stones 13 engage the bore wall 11, they are fed outwardly at a relatively rapid rate to remove the surface roughness from the wall. At the same time, the resistance to feeding of the stones begins to increase and the increasing work load begins to reduce the speed of the feed motor 25 so that the back voltage generated by the motor begins to decrease and the motor current begins to increase. This increase in current increases the heat produced by the filament 40, correspondingly raising its temperature and resistance after a very slight thermal delay. The increase in filament resistance increases the voltage drop across the fialment and, therefore, correspondingly reduces the terminal voltage of the armature, further reducing the speed of the motor and the rate of expansion of the tool.

As the honing cycle progresses and the stones encounter a progressively increasing surface area, the resistance to expansion gradually increases and the motor draws more and more current thereby gradually increasing the filament temperature and resistance, and the terminal voltage of the motor gradually is reduced. This voltage reduction further reduces the speed of the motor and controls the rate of increase in the motor current and the resulting torque exerted on the push rod 24 and the expanding force exerted on the stones 13.

When all the ridges, high spots and other surface roughness have been removed and the stones 13 encounter base metal, that is, the metal of a true and substantially smooth cylindrical wall, the remainder of the honing operation is devoted to the enlargement of the bore to the desired size. During this portion of the operation, the resistance to feeding of the stones into the work remains substantially constant. Thus, the load on the feed motor 25 remains constant and the system becomes stabilized with the motor operating relatively slowly and drawing current at a relatively high but constant rate that produces a constant filament temperature and resistance value. Accordingly, the stones continue to be fed relatively slowly and at a fine feed rate into the bore wall 11 to enlarge the bore until it attains the desired size. The equilibrium values of the motor speed and torque, of course, determine the final expansion rate and honing pressure exerted by the stones.

Optimum honing pressure is that pressure which will wear away the metal of the bore wall 11 at the fastest rate without excessive wear on the abrasive stones 13. This pressure will vary in service use with the type of abrasive stones being used and the hardness of the metal being removed, and the motor torque and push rod force required to produce the desired pressure will vary with the active area of the stones and the diameter of the bore being honed. Since the torque of the motor 25 is proportional to the current drawn by the armature 38 when the field strength is constant, the torque exerted by the motor is limited by the value of the resistance in series with the armature.

While the selected starting voltage and the range of voltage and resistance values obtained during a honing cycle may be changed by using lamp bulbs 42 of different ratings, more convenient and flexible adjustment of the motor torque and the feed rate is accomplished by means of a rheostat 44 (FIG. 5) connected in parallel with the lamp filament to increase and decrease the equivalent resistance of the filament-rheostat as the rheostat resistance is adjusted upwardly or downwardly. In the illustrative control, the rheostat resistance preferably is adjustable up to 5000 ohms and is used with a lamp bulb having a SO-Watt rating at 250 volts.

The setting of the rheostat 44 determines the equivalent resistance of the parallel combination of the filamentrheostat at any given filament temperature and thus determines the armature voltage and the motor current and torque at any given point in the honing cycle. Higher rehostat settings result in lower torque values both initially and as honing progresses, while lower settings produce higher motor speeds and torques with correspondingly greater honing pressures. The effect of different representative settings on the relative values of motor voltage and expander rod force with the 50 watt bulb is illustrated in FIG. 7. It will be seen that the addition of a rheostat in parallel with the automatically adjustable resistor 42 provides a family of curves from which the operator may select the honing rate most favorable for a particular workpiece.

With the rehostat setting determined by experience to be best suited for the workpiece 12 and the type of stones 13, the motor torque developed in the equilibrium condition will maintain optimum honing pressure to Wear away the base metal of the bore wall 1 without excessive stone wear. A voltmeter 45 preferably is connected across the motor armature 38 to indicate the varying armature voltage during the cycle. Since this voltage is a direct indication of the prevailing feed rate and motor torque, the tool performance with a selected rheostat setting is indicated by the voltmeter. If the motor voltage does not decline at a rapid enough rate toward the minimum voltage expected for the final portion of the cycle (to a range of 5-10 volts with the illustrative control), this is an indication that the selected rheostat setting is not high enough and the feed rate is too rapid and the torque is too high. If the voltage drops too fast, this is an indication that the rheostat resistance is too high and the torque developed is not sufficiently high for the particular Workpiece. Experience with the control makes it possible for the machine operator to evaluate the honing performance and adjust the rheostat for optimum performance with various types of workpieces.

Under production honing conditions, it has been established that the foregoing control can increase stone life by as much as three times while reducing the length of the honing cycle to one-third, as compared to stone life and cycle time on the same machine equipped with a conventional, constant-speed feed mechanism. The reduction in cycle time is attributed to two things. First, the faster initial expansion rate is better suited to the condition of bores at the beginning of the cycle and removes ridges and high spots from the bore walls more rapidly than in prior machines. In addition, it is believed that the automatically variable feed rate dictated by the condition of the bore walls substantially reduces or eliminates glazing of the stones 13 and accordingly increases their cutting efficiency. The increased stone life probably is related to the increased cutting efficiency and reduction in heating of the stones in service use, as well as to the precise control of the honong pressure and avoidance of excessive pressure and wear near the end of the cycle when an extremely slow feed rate maintains relatively high honing pressure.

In machines that are to be used to hone relatively large bores, it may be desirable to provide a larger feed motor, for example, a horsepower motor as shown at 47 in FIG. 6. If such a machine also is to be used to hone relatively small bores, starting difficulties can develop when the armature rheostat is adjusted for high resistance to lower the speed and torque range of the larger motor. The modified control illustrated in FIG. 6 is designed for such machines. Instead of controlling the armature voltage range, a voltage divider 48 is provided for adjusting the field strength. In this way, the motor torque may be reduced while maintaining reasonably high starting speeds. In all other important respects, the alternate control is the same as the preferred form.

From the foregoing, it will be seen that the present invention controls the feed rate of the abrasive elements 13 and the torque exerted by the feed motor to maintain optimum working pressure throughout the abrading operation. The motor first feeds the stones into the work surface at a relatively rapid rate for rapid abrading of the rough surface metal thereby avoiding prolonged cycle time. As the work resistance increases, however, the feed rate is reduced automatically and progressively at a rate dictated by the condition of the work surface itself, and the rate of increase in motor torque is controlled to prevent the application of excessive pressure. Moreover, the range of expansion rates and motor torques throughout the cycle may be adjusted for different anticipated honing conditions. The result i an automatic control that materially increases stone life and reduces the length of the honing cycle required.

Amplified theoretical analysis From an inspection of the chart shown in FIG. 4, it will be seen that the filament is a resistor having a current-versus-resistance characteristic with a positive slope less than 90 degrees, showing that the resistance increases with increases in the current fiow rate. In the lamp filament, of course, this change is due to the positive temperature coefiicient of the filament and the heating of the filament to materially different temperatures by different current rates.

From another viewpoint, it will be evident that the lamp filament is a so-called non-linear resistor (in terms of current-versus-voltage characteristics), producing a voltage drop in the armature circuit that increases at a progressively increasing rate, whereas the increase in voltage drop in linear resistors is at a fixed rate. This characteristic of the resistor is believed to accomplish the desired motor control in accordance with the progressive increase in workload encountered by the tool, controlling the rate of torque increase to suit the changing condition of the work.

It should also be noted that the reduction gearing of the expansion mechanism shown in FIG. 2 elfects a very substantial reduction between the feed motor 25 and the push rod 24, and the result of this is a smoothing out of the response of the control by the resistor 40, as well as enabling the use of standard lamp bulb resistors to control the application of substantial working forces without relatively complicated control modifications. Through this reduction gearing, a change of several pounds of push rod force is reflected in the armature current as a small change on the order of one milliampere which changes the filament voltage drop only a slight amount.

I claim as my invention:

1. In a honing machine, the combination of, an expandable honing tool having radially movable honing elements, feed mechanism for expanding said tool by feeding said element outwardly into engagement with a bore wall to be honed, a shunt-wound direct-current motor driving said feed mechanism to expand said tool at different rates determined by the speed of operation and torque of said motor, and a control for said motor including input terminals adapted for connection to a direct current voltage source, an incandescent lamp bulb hav ing a tungsten filament connected in series with the armature of the motor between said terminals to reduce the armature voltage progressively with increases in the resistance to tool expansion, and a rheostat connected in parallel with said filament for selectively adjusting the equivalent resistance of the filament-rheostat combination at any given temperature of the filament thereby to select the maximum honing pressure produced by said motor.

2. In a honing machine, the combination of, an expandable honing tool having radially movable honing elements, feed mechanism including reduction gearing for expanding said tool by feeding said elements outwardly into engagement with a bore wall to be honed, a shunt-wound dried-current motor driving said feed mechanism to expand said tool through said gearing at different rates determined by the speed of operation and torque of said motor, and a control for said motor including a direct current voltage source and an incandescent lamp bulb having a tungsten filament connected in series with the armature of the motor to reduce the armature voltage progressively with increases in the resistance to tool expansion and the resulting increases in the current drawn by the motor thereby to control the feed rate and torque of said motor.

3. The combination defined in claim 2 further including means for selectively adjusting the field strength of said motor and thereby adjusting the honing pressure produced by the motor.

4. In an abrading machine, the combination of, a tool having a movable abrading element, mechanism for feeding said element into engagement with a work surface, a variable speed and torque electric motor driving said feed ing mechanism to feed said element at different rates determined by the speed of operation of said motor, and a control for said motor including an incandescent lamp bulb having a tungsten filament connected in series with the armature of said motor whereby the terminal voltage of said motor is reduced progressively as the resistance to feeding of said element increases, and a rheostat connected in parallel with said filament for selectively adjusting the equivalent resistance of the filament rheostat combination at any given temperature of the filament thereby to select a range of motor speed and torque developed as a surface is abraded.

5. In an abrading machine, the combination of, a tool having a movable abrading element, mechanism for feeding said element into engagement with a work surface, a variable speed and torque electric motor driving said feeding mechanism to feed said element at different rates determined by the speed of operation of said motor, and a control for said motor including an incandescent lamp bulb having a tungsten filament connected in series with the armature of the motor thereby to control the speed and torque of said motor in response to progressive increases in the resistance to feeding of said element.

6. The method of feeding an abrasive element into a rough work surface with a variable speed and torque elec tric motor and feed mechanism driven .by said motor, said method comprising the steps of, initially energizing said motor to run at a relatively rapid speed with a selected armature 'voltage thereby to feed the element at a coarse feed rate, continuously sensing the current drawn by the motor armature as the element is fed into the work surface, and reducing the armature voltage progressively as the armature current increases and directly in response to each sensed increase in such current thereby progressively reducing the motor speed and controlling the motor torque throughout the feeding of said element.

7. The method of feeding a machine tool element into a workpiece with a variable speed and torque feed mechanism, said method comprising the steps of, initially operating said feed mechanism to run at a first preselected no-load rate to feed said element into the workpiece at a relatively rapid rate, sensing the increase in the resistance to feeding of said element as the latter engages the workpiece and continuously sensing variations in such resistance while the element is being fed through the workpiece, reducing the feed rate and increasing the torque output of said feed mechanism in response to all sensed increases in the resistance, and increasing the feed rate and reducing the torque output of said feed mechanism in response to all sensed decreases in the resistance there by constantly controlling the feeding of said element in accordance with the changing condition of the workpiece.

8. In an abrading machine, the combination of, a tool having an abrading element, mechanism for feeding said element relative to a workpiece in a'brading engagement therewith, a variable speed and torque electric motor for driving said feeding mechanism at different rates determined by the speed of operation of said motor, said feeding mechanism including reduction gearing between said motor and said element, and a control for said motor including a resistor connected in series with the armature of the motor, said resistor having a current-versus-raistance characteristic that varies non-linearly with the resistance, increasing and decreasing with, and at a greater rate than increases and decreases in the current through the resistor, thereby to adjust the terminal voltage of said armature automatically in response to and in accordance with variations in the resistance to feeding of said element relative to the workpiece.

9. The combination as defined in claim 8 in which said resistor has a positive temperature coefficient and generates heat to change its own temperature in proportion to changes in the current through the resistor.

10. The combination defined in claim 9 in which said resistor is an incandescent lamp filament in a protective atmosphere.

11. In a machine tool, the combination of, a tool element for performing a metal removing operation, mechanism for feeding said element into engagement with a workpiece, a variable speed and torque electric motor for driving said feeding mechanism at ditferent rates determined by the speed of operation of said motor, and means sensing changes in the resistance to feeding of said element into the workpiece and operable automatically in response to increases in the resistance to reduce the speed of feed and control the torque output of said motor in accordance with the changed resistance, said sensing means also operating in response to decreases in the resistance to increase the speed of feed of said motor thereby to regulate the speed and torque in relation to the load encountered.

12. In an abrading machine, the combination of, a tool having a movable abrading element, mechanism for feeding said element into engagement with a Work surface, a variable speed and torque electric motor having an armature and driving said feeding mechanism to feed said element into said surface at diiferent rates determined by the speed of operation of said motor, and a control for said motor including means responsive automatically to increases in the current drawn by said armature and operating to reduce the terminal voltage of said motor progressively as said current increases and at a rate greater than the rate of increase in the current, said means including a non-linear resistor connected in series with said armature.

References Cited UNITED STATES PATENTS LESTER M. SWINGLE, Primary Examiner.

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