US2785511A - Grinding machine and method of operation - Google Patents

Description

March 19, 1957 J. c. WILSON 2,785,511

GRINDING MACHINE AND METHOD OF OPERATION Filed Feb. 12, 1954 2 Sheets-Sheet l r/a-z IN V EN TOR.

March 19, 1957 c WILSON 2,785,511

GRINDING MACHINE AND METHOD OF OPERATION Filed Feb. 12, 1954 2 Sheets-Sheet 2 INVENTOR.

' JOHN 0. WILSON ATTORNEYS United States Patent GRINDING MACHINE AND METHOD OF OPERATION John C. Wilson, Springfield, Ohio,

Thompson Grinder Company, corporation of Ohio assignor to The Springfield, Ohio, a

This invention relates to grinding machines and to a method of operation thereof, and in particular deals with the operation of a grinding machine so as to produce an exceptionally smooth and blemish-free finish on a work member.

Most grinding machines that are in use at the present time for grinding wheels are directly connected with the shaft of an electric drive motor, this being the most economical and convenient manner of supporting and driving the wheel, and an arrangement by which it is quite simple to drive the wheel at the proper peripheral speed.

For most ordinary grinding operations, both rough and finish, such an arrangement is quite satisfactory. It has been found, however, that in certain cases the surface of a work member that is ground to a smooth finish by such a structure is slightly defective in that when the surface is lapped in, as by rubbing against a mating part, irregularities in the surface become discernible which are objectionable, at least from the standpoint of appearance.

The foregoing situation comes about due to the fact that an electric motor, particularly the new types of motors having high permeability alloys in the field and rotor and with the air gap between the field and rotor being held to an absolute minimum, high stresses are developed within the motor which tend to cause the shaft of the motor, on which the grinding wheel is mounted, to vibrate slightly.

Also, particularly with the newer type motors having improved electrical insulation, the electric current density is much higher than formerly, and this likewise produces stresses in the motor that will tend to produce vibrations in the motor shaft.

-In any case, an electric motor that is manufactured by the usual production methods will include certain minor irregularities as to the slot spacing, slot dimensions, the location of the wires within the slots, or similar departures from a perfect product that are a necessary part of production methods of producing articles. These minor irregularities also tend to cause vibratory forces and stresses to be developed within the motor which will produce vibration of the motor shaft on which the wheel is mounted.

The .result of the slight motor defects that have been referred to above are not observed when the workpiece is rough ground, because the load on the wheel at that time is sufiicient to prevent any vibration thereof. However, when a surface is being finish ground, with only a one-half thousandths or so of material being removed, the load on the wheel is lightened enough that the tendency for the motor shaft to vibrate will be transmitted to the wheel, thereby causing slight irregularities in the surface being finished.

These irregularities are not usually visible, but if, for some reason or other, the surface is lapped in and the work member happens to be the way of a machine tool, such as a grinder bed or lathe bed which will lap-in in the irregularities will become visible after the wa i 2,785,511 Patented Mar. 19, 1957 has been used a short time. These irregularities take the form of straight lines arranged in parallel relation and also parallel with the axis of the grinding wheel and spaced apart according to the frequency of the electric power supply to the motor that drives the wheel and the speed at which the work traverses past the wheel.

Having the foregoing in mind, it is a primary object of the present invention to devise a grinding machine in which the difficulties referred to above are eliminated so that a perfectly smooth finished surface can be had entirely free of the defects mentioned above.

A further object of this invention is the provision of a method of operating a motor driven grinding wheel so that a smooth surface free of defects will be obtained.

In brief, the objectives of this invention are attained by reducing the applied voltage on the motor that drives the grinding wheel during a finish grinding operation while using full voltage on the motor during rough grinding operations.

I have found that by reduction of the voltage applied to the motor the several electrical, magnetic and electromagnetic stresses referred to above which tend to vibrate the motor shaft are reduced to the point that the motor shaft does not vibrate suflicient to mark a surface being finished. Similarly, the dimensional irregularities within the motor no longer introduce vibration of the motor shaft when the applied voltage is reduced, because the said irregularities create vibratory forces principally because of the distortion of the magnetic field in the motor caused thereby.

The several objects referred to above, as well as other objects and advantages of this invention, will become more apparent upon reference to the following specifica tion taken in connection with the accompanying drawings, wherein:

Figure l is an elevational view of a typical grinding machine adapted for being operated according to my invention;

Figure 2 is a circuit diagram showing the energizing circuits for the wheel head motor for operating it during rough and finish grinding operations;

Figure 3 is a diagrammatic view showing a modified circuit employing impedances for reducing the voltage applied to the wheel head motor; and

Figure 4 is a view similar to Figure 3 but showing the use of a stepdown transformer for reducing the voltage applied to the wheel head motor.

The grinding machine shown in Figure 1 comprises a bed 10, on which is reciprocably mounted a table 12 that carries a work member 14 having surfaces 16 to be ground. Adjacent bed 10 is a column 18 carrying a saddle 20 on which is mounted a wheel head 22 that comprises an electric motor 24 having its shaft 26 extending to form a spindle on which grinding wheel 28 is mounted.

According to my invention I operate motor 24 at full rated voltage during rough grinding operations, and at which time substantial cuts are being made on the work member up to, say, 15 to 20/ lOOOths of an inch. However, when the workpiece is being finish ground there may be as little as a half-thousandth of an inch or so removed each pass across the workpiece, and this, of course, requires substantially less power. I am thus enabled to drive motor 24 at reduced voltage during finish grinding operations and still obtain substantially the same wheel speed, whereby the grinding operation isadequately carried out, but at the sametime, as previously mentioned, the motor shaft does not vibrate and, thereforefa finish is produced that is superior to one that is produced when finish grinding with the wheel motor operatedat rated voltage.

In Figure 2 there is shown an electrical circuit, by

means of which motor 24, which in this case is a threephase alternating current electric motor, can be operated delta for rough grinding and then converted to Y operation for finish grinding.

In the circuit of Figure 2 the three-phase power sup ply lines are indicated 'at L1, L2 and L3 and a transformer 30 is provided for providing single-phase voltage for operating several control relays of the electric circuit.

In Figure 2 it will be noted that motor 24 is provided with the three field windings 32, 34 and 36 with wires 38, 40 and 42 leading to one end of each of the said coils from the terminals of a contactor 44, the other side of which is connected with the lines L1, L2 and L3.

The opposite ends of coils 32, 34 and 36 are connected by the wires 46, 48 and 50, respectively, with one .side of a relay 52, the other side of which is connected with the wires 38, 4t and 42. It will be evident that closing of relay 52 will serve to connect the coils of the motor winding in delta.

Wires 48 and 50 have branches 48 and 50 leading to one side of a contactor 54, the other side of which is connected to wire 46 by branch 46' thereof. It will be evident that the closing of relay 54 while relay 52 is open will serve to connect the coils of the field winding in Y, whereby with relay 44 closed a selection can be made between delta and Y connection of the field windings of motor 24 by selection of one or the other of contactors 52, 54 to be closed.

Considering first the operation of motor 24 with the field windings connected delta, and in which case full rated voltage will be applied to the motor, there is provided a push button switch 56 which, when closed, will energize the coil of relay 58, the closing of which relay will establish a circuit from one side of the secondary of transformer 30 to Wire 60 through blade 62 of relay 58, wire 64, and the coil of a relay 66 to the other side of the transformer secondary, thus energizing relay 66. Closing of relay 66 will serve to connect a motor 68 to power lines L1, L2 and L3, and which motor '68 .supplies lubrication to the grinding wheel spindle.

When the hydraulic pump driven by motor 68 develops a predetermined pressure it will close pressure switch 70. The closing of pressure switch 70 will complete a circuit from one side of the secondary transformer 30 through wire 72, blade 74 of relay 66, wire 76, blade 78 of relay 58, blade 80 of a relay 82, wire 84, blade 86 of relay 58, and the coil of relay 44 to the opposite side of the secondary of transformer 30 thereby energizing relay 44 and connecting wires 38, 40 and 4 2 to power lines L1, L2 and L3. Simultaneously, there is also a circuit established through the coil of relay 52 which, commencing at blade 80 of relay 82, is established through blade 9% of a relay 92, wire 94, and the said coil of relay 52 back to the secondary of transformer 30.

The circuit now has both relays 44 and 52 energized and motor 24 is operating with its field windings connected delta.

To operate the motor at reduced voltage for finish grinding operations, stop switch 96 is opened, thereby de-energizing all of the energized relays of the control system, and push button 98 is then closed.

Closing of push button 98 will establish a circuit through bladeltlt of relay .92 and the coil of contactor 102 back to the secondary of transformer 31), thereby closing the said relay. When relay 102 is closed a circuit is established from one side of the secondary of transformer 3% through wire 104, blade 106 of relay E102, wire 163, and the coil of relay 66 back to the transformer secondary thereby energizing relay 66 again to supply power to the spindle of motor 68, which will, as before, develop pressure on pressure switch 7 t to close theswiitch.

When switch 7% closes a circuit isestablishedfrom one side of the transformer secondary through wirefl], blade 74 of relay 66, wire 110, blade 1120f relay 1 02,

wblade 114 of relay 92, blade7116 of relay 8 4,wire :118,

and the coil of relay 54 back to the secondary of the transformer, thereby closing relay 54 to place the field windings of motor 24 in Y connection.

Simultaneously with closing of relay 54, there is established an energizing circuit for the coil of relay 44 which, starting from blade 114 of relay 92, is through wire 120, blade 122 of relay 102, wire 124, wire 88, and the coil of relay 44 back to the transformer secondary.

It will be seen from the foregoing that the circuit provides an arrangement for selectively connecting the motor windings either in Y or in delta, and that included in the circuit is the interlocking relay 92 which prevents energization of the low relay 102 when the high relay 58 is closed, and also prevents energization of high relay 58 when low relay 102 is closed.

While Figure 2 shows the preferred and the most practical method so far developed for operating the wheel head motor at either high or low voltage, it is also conceivable that circuit arrangements according to Figures 3 and 4 could be employed, and it is intended that these modified arrangements should come within the purview of my invention.

In Figure 3 I show the motor 24a as being a conventional polyphase electric motor, which may be connected in a suitable manner within the motor frame. Wires 130, 132 and 134 are adapted for being connected directly to power lines L1, L2 and L3 by the closing of relay 136, or can be connected through the impedance means 138 to the said power lines by closing of relay 14%. A selector switch 142 selects between the two relays 136 and 140,

and the motor 24 can thus be connected directly across the line to operate at rated voltage, or can be connected to the line of impedance means 138 to operate at reduced voltage. Impedance means 138 may take the .form of choke coils as illustrated, but conceivably could be resistors or condensers, or combinations thereof, as desired or advisable.

In'Figure 4 the wheel head motor 24b has wires 144, 146 and 1.48 connected therewith which are adapted for being connected directly across power lines L1, L2 and L3 by closing of a relay 156, or can be connected through the step-down transformer 152 to the said power lines by closing of a relay 154. The push buttons 156 and 158 are operable ;for closing relays 15% and 154, respectively, and the relays are provided with the interlock blades 169, 162 to prevent the closing of either when the other thereof is closed.

The Figure 4 arrangement has the advantage of providing a variable voltage so that the voltage at which the motor is to be operated for finish grinding can beadjusted to exactly the amount desired, as opposed to the;fixed ratio between the two voltages that can be had in the Figure2 arrangement. The Figure 4 arrangement is'likewise conserving of power as opposed to the Figure 3 arrangement.

It will-beunderstood that this inventionis susceptible to modification in order to adapt it to difierent usages and conditions, and, accordingly, it is desired to comprehend .suchmodifications within this invention as may fall within the scope of the appended claims.

'Iclaim:

1. In a grinding machine having a three-phase alternating current electric motor with distributed filed windings and an output shaft, said shaft being adapted forsuptactor and said motor energizable for connecting said motor windings in delta for operation of the motor at full rated voltage a second relay between'said contactorand said motor energiz able for connecting said motor windings in Y for operation of said motor at reduced voltage, a

controlcireuit for energizing said contactorand'for selecti e y energi n said' relay i c di an rlo in control relay connected in circuit with said first and second relays for preventing energization of either thereof when the other is energized, said control system comprising means for supplying iubricant under pressure to said shaft, and the energizing circuits for said contactor and first and second relays including a normally open switch adapted for closing in response to a predetermined pressure of said lubricant supply.

2. In a grinding machine having a. alternating current electric drive motor with an output shaft and a grinding wheel mounted on shaft to driven thereby; an alternating current power supply circuit for energizing said motor, voltage reducing means connected between said motor and said power circuit, first contactor means connected around said voltage reducing means whereby energization thereof will apply full rated voltage to said motor, second contactor means connected between said voltage reducing means and said supply circuit whereby energization thereof wiil cause energization of the motor at reduced voltage, and means for selectively energizing said contactor means.

3. In a grinding machine having 'a constant speed alternating current electric drive motor with an output shaft and a grinding wheel mounted on the shaft to be driven thereby; an alternating current power supply circuit for energizing said motor, voltage reducing means connected between said motor and said power circuit, first contactor means connected around said voltage reducing means whereby energization thereof will apply full rated voltage to said motor, second contactor means connected between said voltage reducing means and said supply circuit whereby energization thereof will cause energization of the motor at reduced voltage, said voltage reducing means comprising impedance means, and means for selectively energizing said contactor means.

4. In a grinding machine having a constant speed alternating current electric drive motor with an output shaft and a grinding wheel mounted on the shaft to be driven thereby; an alternating current power supply circuit for energizing said motor, voltage reducing means connected between said motor and said power circuit, first contactor means connected around said voltage reducing means whereby energization thereof will apply full rated voltage to said motor, second contactor means connected between said voltage reducing means 'and said supply circuit whereby energization thereof will cause energization of the motor at reduced voltage, said voltage reducing means comprising a transformer having a secondary connected with said motor and its primary connected with one said second contactor means, and means for selectively energizing said contactor means.

5. A method of operating a grinding machine having a grinding wheel directly supported on the shaft of a constant speed electric drive motor, which comprises the steps of applying the full rated voltage to the drive motor to provide for high speed of operation of the motor when the Wheel is heavily loaded and during rough grinding conditions, and applying reduced voltage to the drive motor to provide for substantially vibration free operation of the motor when the wheel is lightly loaded and during finish grinding conditions whereby the finish grind operation results in the production of a surface free of blemishes which are caused by vibration of the grinding wheel due to electrical and magnetic stresses set up in the drive motor.

6. A method of operating a grinding machine having a grinding wheel directly mounted on the shaft of a constant speed alternating current electric drive motor to provide for full speed operation of the motor when the wheel is heavily loaded as during rough grinding operations and also to provide for substantially vibration free operation of the motor when it is lightly loaded as during constant speed finish grinding operations whereby the finish grind operation will produce a surface free of blemishes caused by vibration of the motor due to electrical and magnetic stress therein which comprises, energizing the motor at full rated voltage thereby to drive the wheel with the full power of said motor during rough grinding, deenergizing the motor and again energizing the motor at a voltage less than the full rated voltage thereof thereby to drive the motor at reduced power for finish grinding.

7. In a grinding machine having a polyphase alternating current electric motor with distributed field windings and a drive shaft, a grinding wheel mounted on said drive shaft and driven thereby, an alternating current electric power circuit connected to said motor for supplying power thereto, a contactor energizable for connecting said motor to said power circuit, and relay means between said contactor and said motor operable for selectively connecting said motor windings in delta for rough grinding and then in Y connection for finish grinding, said motor being selectively operated at full rated voltage or at reduced voltage.

8. In a grinding machine having a three-phase alternating current electric motor with distributed field windings and an output shaft, a grinding wheel mounted on said drive shaft and driven thereby, a three-phase electric power supply circuit for energizing said motor, a contactor energizable for connecting said motor with said power circuit, a first relay between said contactor and said motor energizable for connecting said motor windings in delta for operation of the motor at full rated voltage during heavy grinding load stresses, a second relay between said contactor and said motor energizable for connecting said motor windings in Y for operation of said motor at reduced voltage under light grinding load stresses, and means for selectively energizing said relays.

9. In a grinding machine having a three-phase alternating current electric motor with distributed field windings and an output shaft, a grinding wheel mounted on said drive shaft and driven thereby, a three-phase electric power supply circuit for energizing said motor, a contactor energizable for connecting said motor with said power circuit, a first relay between said contactor and said motor energizable for connecting said motor windings in delta for operation of the motor at full rated voltage during rough grinding, a second relay between said contactor and said motor energizable for connecting said motor windings in Y for operation of said motor at reduced voltage during finish grinding, a control circuit for energizing said contactor and for selectively energizing said relays including an interlocking control relay connected in circuit with said first and second relays for preventing energization of the de-energized relay when the other relay is energized.

References Cited in the file of this patent UNITED STATES PATENTS 1,492,216 Nehlsen Apr. 29, 1924 1,719,889 Kelsey July 9, 1929 1,899,703 Lindner Feb. 28, 1933 1,902,444 Hobart Mar. 21, 1933 2,294,153 Alvord Aug. 25, 1942 2,317,266 Galt Apr. 20, 1943 2,461,076 Necson Feb. 8, 1949 2,641,875 Kirby June 16, 1953 FOREIGN PATENTS 196,657 Great Britain Apr. 12, 1923 469,935 Great Britain Aug. 3, 1937 OTHER REFERENCES Fundamentals of Alternating Machines, by Sah, published by McGraw-Hill, p. 179, 1st ed. 1946.

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