US3571911A - Method of treating a workpiece - Google Patents

Abstract

Magnetic chuck for holding a workpiece, the workpiece often being distorted by the great magnetic force necessary for holding it, and releasing the holding force in steps while reducing the force of the working operation thereon.

Description

United States Patent Inventor Appl. No. Filed Patented Assignee METHOD OF TREATING A WORKPIECE OTHER REFERENCES Rusinoff, Manufacturing Processes, 1962, pp. 613, 616- 618.

Primary Examiner-John F2 Campbell Assistant ExaminerRobert J. Craig Attorney-Paul H. Gallagher 4 Claims, 7 Drawing Figs. U.S.Cl 29/558, ABSTRACT: Magnetic chuck for holding a workpiece, the 29/557, 51/326, 317/453 workpiece often being distorted by the great magnetic force lnt.Cl 823 13/04 necessary for holding it, and releasing the holding force in Field of Search 29/558; steps while reducing the force of the working operation 269/8; 51/281, 326, 327; 317/453 thereon.

A B 2 C flan.) 2101 2 8 DE-ENE/i'a/zzn MAG/vs 17250 GREATLY EXAGEIPRAT z D E 210a. 2/01, 509 .3-

M -NER IZED MAGNET/ZED 11 2/042 2 8 fl L g L W i.- T 4 210e PATEN-TEU 141112 31911 SHEET 2 OF 3 METHOD or TREATING A worumscs CROSS REFERENCES This application is a division of my copending application, Ser. No. 277,253,frled May 1, 1963.

The invention has particular adaptation to such installations as machine tools, in which a workpiece being worked on is held on a chuck by magnetism. In order to remove the workpiece from the chuck it is, of course, necessary to demagnetize the chuck. However, in the overall operation of performing a work operation on a workpiece, it may become necessary, in addition to simply demagrretizing the chuck and workpiece, to partially demagnetize, and again remagnetize them, as when it is desired to remove the workpiece for checking it, or checking the operation thereon, or for other reasons.

A broad object of the invention is to provide apparatus and method for-magnetically holding a workpiece, and relieving the holding force in steps, while also reducing the force of the working operation thereon, whereby the distortion of the workpiece, which is often unavoidable, and the force of the working operation are simultaneously reduced, resulting in a more accurate working operation on the workpiece.

Another object is to provide apparatus and method of the foregoing general character wherein a workpiece may be demagnetized to a selected and controlled degree relative to full magnetization.

Still another object of the invention is to provide apparatus and method of the foregoing general character which includes an arrangement for demagnetizing in individually controlled steps whereby to enable a work operation on a workpiece after each such demagnetizing step, to thereby enable accurate finishing of the workpiece in a manner'to overcome possible distortion thereof due to secure holding of the workpiece on the chuck by full or greater magnetization.

Another object is to provide magnetizing and demagnetizing apparatus, including cycling means operative selectively for magnetizing a chuck to full magnetization level or a lesser controllably adjustable level, for producing steady reverse current to demagnetize the chuck to a predetermined lesser level of magnetization, and for demagnetizing the chuck in a series of steps of successively reverse currents to a level near zero, each according to a predetermined law of operation.

Other objects and advantages of the invention will appear from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is an electrical diagram of the circuit utilized in one form of the invention;

FIG. 2 is an electrical diagram of the circuit utilized in another form of the invention;

FIG. 3 is a detail view of switching mechanism utilized in this apparatus;

FIG. 4 is a detail view of switch means illustrated in FIG. 3;

FIG. 5 is a diagrammatic view illustrating motor and cam means;

FIG. 6 is a fragmentary illustration of certain control switch positions; and

FIG. 7 is a diagrammatic view, greatly exaggerated, showing certain steps in a working operation on a workpiece. Referring now in detail to the drawings, attention is directed first to FIG. 1, illustrating the electrical circuit utilized in one form of invention. In this circuit diagram, an AC source is indicated at 10, with lines leading to a transformer 12 through a tapped terminal strip 14, for providing selective voltage connections to the primary 12p of ,a transformer 12. Preferably the primary 12p includes two separate coils which may be connected in circuit selectively or together. The transformerl2 also includes two secondaries 12d and l2s2. Connected across a po tion of the secondary 12s1 is another transformer. 16, which includes a primary 16p and a secondary 16s.

"The apparatus of FIG. 1 includes portions of the kind of demagnetizing apparatus disclosed and claimed in my prior Pat. No. 2,825,854, dated Mar. 4, 1958, and includes the broad concept of an AC source, converting the AC to DC and transmitting it to the electromagnet, interrupting the AC and while the AC is interrupted, reversing the DC and at each reversal reducing the voltage applied to the electromagnet. In the apparatus disclosed in FIG. 1, the main components of the apparatus just referred to include a motor 18, a rotary tap switch 20, and switching mechanism 22 driven by the motor; the magnetic chuck is indicated at 214 which, as referred to again hereinbelow, is of the permanent magnet type.

Referring to the various components of the circuit of Flg. l individually, the motor 18 is driven from the secondary 12s2, through a rectifier 26 and other elements to be referred to hereinbelow. The motor is associated with and controlled by a plurality of limit switches 27, 29, 31 and 33 (see also FIG. 5) actuated by corresponding cams 35 driven by the motor, in conjunction with the operation of other portions of the circuit, to be described more fully hereinafter. The connection between the motor 18 and the rotary tap switch 20, indicated diagrammatically at 28, may be of any suitable arrangement, such as disclosed in my prior patent referred to above.

The rotary tap switch 20 includes an arm 3 driven by the motor 18, successively engageable with contact terminals 32 arranged in a circular series. The arm 30 is arranged for moving off of each contact terminal to a space between adjacent terminals to an OFF position, and thereafter onto the next contact terminal. The contact terminals 32 are connected with individual tappings of the secondary l2sl through respective conductors 34 for producing different voltage values and applying them to the chuck 24 in a de'magnetizing operation.

Conductors 36 and 38 lead fromthe secondary 12:1 to a rectifier 40 for converting the AC to DC which thereupon is transmitted to the magnet in the chuck; the conductor 36 is connected directly with one end of the secondary'and leads through a fuse 42 to one junction of the rectifier, while the other conductor 38 leads from the arm 30 of the rotary tap switch 20 through an AC interrupting'switch 44 and through conductor 46 to the opposite junction of the rectifier. The switch 44 is incorporated in the switching means 22 referred to above, which also includes reversing switches 48 and 50 connected respectively to conductors 52 and 54 which are connected at opposite center junctions of the rectifier. Conductors 56 and 58 lead from the reversing switches'to the chuck 24.

The AC interrupting switch 44 and the two reversing switches 48 and 50 are operated in unison, as indicated in FIGS. 3 and 4 and as described more-fully in my prior patent mentioned. The three switches are sooperated by switching bars 60 and 62 on opposite sides of the two banks of switches. The contactor bars of the AC operating switch 44 are specially shaped so as to move in a sequence different from those of the reversing switches, so that a make-before-break action is accomplished, i.e., the AC is interrupted, then the DC circuit is established, on both sides of the reversing switches, then the DC is interrupted at the first contacts, and thereafter the AC is reestablished. The switching means 22, as will be understood, is operated in synchronism with the rotary tap switch 20 to accomplish the result just stated, and with the further refinement that the AC interrupting switch 44 is actuated before the switch arm 30 moves off each contact terminal 32, and again reestablished the AC circuit before that am moves onto the next contact terminal. The motor 18 operates through a complete predetermined cycle in a demagnetizing operation, after the operation thereof is initiated by the manual control of the operator, in which the magnetization of the chuck is reduced to zero, or nearly zero.

A relay A has its coil connected in the conductor 58, and adapted to be shunted by a relay switch Bl. Also connected in the conductor 58 is a potentiometer or adjustable resistor 64, adapted to be shunted by relay switch CD1.

The relay A is of a permanent magnet type, of known kind, wherein upon energization thereof, as'by a current in the conductor 58, the switches thereof are thrown to their abnormal position by the energized magnet, and are so retained upon cessation of that current, and are not moved to their normal position until the magnet is demagnetized. Upon energization of the magnetic chuck 24, the relay A is thus actuated, closing switch Al and turning on a signal light as (red) to indicate that fact. Thereafter, the circuit to the chuck is opened, in a manner describwd hereinbelow since it is desired not to retain the circuit closed thereto, because of the permanent magnet characteristics of the chuck. However, in the present instance, even after the circuit to the chuck is opened, the chuck remains magnetized until it is controllably demagnetized, and the signal light so also remains on until the relay A is demagne'dzed, which is done in the same operation in which the chuck is dcmagnetized.

Additional control is exerted on the chuck through a unit indicated in its entirety at 7% and which includes a 292i tube '"2. operation of this unit 763* in the present instance is similar to that disclosed and claimed in my prior Pat. No. 3,045,15 i dated lo]. 17, 1962, to which reference may be had for greater detail thereof. unit ill is utilized for effecting or controlling RESIUUAL, or partial demagnetization of the chuck. The tube 72. is preferably maintained constantly in conducting condition, through an adjustable resistor 74 which reflects the imposition of a positive bias on the tube. The adjustable resistor in the conductor is arranged for impressing a controlling bias on the tube to render it nonconducting upon a predetermined condition, such as a reverse current in the conductor 58, Specifically, the latter control operation is produced by a negative current signal produced in the conductor in a demagnetizing operation. Assuming a positive current indicated by the arrow 7s utilized for energiz ing the chuck (when the switch CD1 is closed), upon a demagnetizing current in the opposite direction indicated by the arrow ill being established (with the switch CD11 then open), the desired negative signal is produced and imposed on the tube 72. This reverse direction current builds up at a certain rate, and upon the magnetization of the chuck decreasing to the desired level according to the setting of the adjustable resistor 64 and consequent development of a signal by that current, the signal effects extinguishing of the tube 72. and opening of the circuit is controlled thereby. This opening of the circult is efiected through deenergization of the l) relay coil resulting in control functions in other portions of the circuit.

As an initial step in the operation of the apparatus, it is first actuated to charge or energize the chuck 2 3 to hold a workpiece thereon. To do so, the VAlRlABLE-FULL switch $4 is set to FULL position, and the CHARGE-RELEASE switch 82 is set to CHARGE position; a circuit is then completed as follows: conductor 84 from one center junction of the rectifier as, the switch 82, conductor as, the switch b0, conductor 88, the upper contacts of a RECllARGE switch 96}, conductor 92, switch E1 of relay E, limit switch 31, conductor 94, through the forward field coil of the motor 13, conductors 98 and w ll: to the opposite center junction of the rectifier 26. The motor is then driven to move the switch Ell from an OFF position indicated in H6. 6, to an ON position and then to a RESlDUAL position. in the latter position, the switch 26? is in circuit closing position, as are all of the components of the switching means 232, and as a consequence charging or energizing current is impressed on the chuck 24.

The circuit to the chuck established by the foregoing step just described is interrupted shortly after its completion and consequent full charging of the chuck through the limit switch 33, the cam controlling this switch being so set that it opens the switch after the desired predetermined interval and at such position of the motor and rotary tap switch 2i) that the arm Bil of the latter is in an OFF position between active contact terminals (F1 8. 43) and with the All interrupter switch 44 open. Althougl the switch interrupts the circuit to the chuck, the relay A coil remains energized because of its permanent magnet nature, as described above.

As indicated above, it is often desired to remove one or more workpieces from the chucir, to check it or to check the operation being performed thereon, or for other reasons. in such case it is desired not to completely demagnetize the chuck, as through a complete demagnetizing cycle, but to reduce the magnetization to a desired degree, usually greater than zero, and this preferably by a quick operation. This can be effectively accomplished in the use of the present invention by the RESEDUAL control, consisting chiefly of the unit 7b which operates to impose a reverse current on the chuck until the magnetization decreases to the desired level.

To perform this operation, the operator actuates the RESlDUAL push button M32, which includes two switches W211 and llllZb. Upon depression of the pushbutton W2, and particularly upon closure of switch 102a thereof in the unit ill, a circuit is completed through the circuit of unit 7b as follows: beginning with the residual switch llllZa, through the switch ltl l, conductor 3%, switch B2, conductor llld, the tube 72, the adjustable resistor 74, conductors llll and M2, and secondary lids, the relay D coil, conductor llll l, switch A2 in return to the residual switch lillla. it will be recalled that at this time the switch A2 is closed due to the energization of the A coil in response to the energization of the chuck. The circuit of the unit 7% is retained through switch D2;

Energization of the D relay coil, and consequent closure of switch Di, energizes the CU relay coil in the following circuit: beginning with switch D ll, conductor lid, switch lll2b, conductors lid and El E9, switch 8h, conductor tilt and conductor $4 to one junction of the rectifier as; the circuit continues from the opposite junction to conductor i212, conductor 12.4 through the CU coil and in return to the switch Di. Energization of the CU coil closes the switch CU2; and imposes a current in the reverse direction on the chuck, the arm Bill of the rotary tap switch Ell being at this time in the RESIDUAL position, indicated at E la in dotted lines, and the reversing switches db and Sill being in such position as to impose a current on the chuck opposite that at which it was charged; the circuit thus set up to the magnetic chuck is as follows; from tapping 22d of the secondary llZSll, the associated conductor 34a, conductor l2ti, the switch CU2, conductor l3ll to the contactor arm 36. thus completing the circuit to the rectifier do as described above, and of course completion of the DC circuit to the chuck. This reverse current as controlled through the adjustable resistor or produces the signal, as described above, which extinguishes the tube 72 and opens the circuit of the unit 79, deenergizating the D relay coil, as a result of which the CU relay coil is deenergized, opening the switch CUZ and interrupting the circuit to the chuck. The reverse current flowing through the A relay coil also demagnetizes this coil and deenergizes this relay, opening switch All and turning off the signal light 68. Deenergization of the A relay coil closes the switch A3 and turns on a signal light 132 (white). The circuit to this light is: beginning with switch A3, conductors i2 3 and 122 to one junction of the rectifier 2b; the circuit continues through the opposite junction of the rectifier through conductor 84, through the switch 32, conductor 129, switch 8t conductors 11% and 134, through the light i332, through the normally closed switch CUZl, and in return to the switch A3.

in this RESHDUAL operation, it is desired that there be at least some magnetization in the chuck in order to retain the workpieces in position thereon, and not risk accidental dis placement of them, since in may if not in most instances, the working operation is to be continued, and it is desired that the workpieces that are not removed remain in their original positions. The control unit 74) thus serves as a means for quickly demagnetizing the chuck and workpieces to the desired degree of magnetization without, however, in all cases, and as desired, reducing that magnetization to zero, and without the necessity for a full cycling operation of the demagnetizing portion of the apparatus which includes the rotary tap switch Ztl, switching means 22, etc.

After the above RESlDUAl, operation is performed, and Wl'lfii'l it is desired to return the apparatus again to full power condition, RECHARGE switch t ll is depressed, closing the contacts at the bottom thereof; this completes the circuit as follows: beginning with the switch itself, conductor the switch 86, conductor 86, switch 82, conductor 84 to one junction of the rectifier 26; in the other direction from the switch @b the circuit continues through conductor lilo, relay E coil, resistor 13%, conductors Mil, E42, M4 and M6 to the reverse field coil M8 of the motor 118', the circuit then continues through conductors 9% and 1100 to the opposite junction of the rectifier 26.

Upon energization of the E coil in the circuit just referred to, the switch E2 is closed, and a retaining circuit is set up as follows: beginning with the switch E2, conductors 150 and $4 to one junction of the rectifier 26; and in the opposite direction through the E coil and the remainder of the circuit as just described, namely, resistor llllil, conductors 14%, M2, 144 and Mt: and M6 to the reverse field 148 of the motor and continuing through conductors 98 and Hill) to the rectifier 26. Also, when the E coil is energized, it completes a circuit through the holding switch E2, as follows: beginning from the switch E2, the circuit in one direction is through conductors llfill and lid to the switch 82, conductor as to the switch 80, conductor 88, the upper contacts of the switch 90, conductors 92 and 4, abnormally closed switch E3, conductors Mil, 142, i144 and 1 th to the reverse field coil led of the motor, and continuing through conductors 9E and lltlll to the opposite junction of the rectifier 26. This circuit operates the motor 18 to the fullon power position, and when that position is reached, the chuck is again charged as previously described, the circuit then being interrupted by movement of the limit switch 29 to an OFF position deenergizing the E relay coil, this control of the motor being through the corresponding cam 35. Thereafter, the motor is then reversed, and run to the residual position through the following circuit: conductor 84 from one junction of the rectifier, the switch 82, conductor 86, the switch 85), conductor 8d, the upper contacts of the switch 90, conductor 92, normally closed switch El and the limit switch 31; the circuit continues through conductors 150, 94 and 152 to the forward field coil 96, and then through conductors 98 and Hill to the opposite junction of the rectifier 26. This runs the motor until it reaches the RESHDUAL position in which the limit switch 3i reaches an intermediate off position.

When the operation on the workpiece is completed, and it is desired to remove it from the chuck, the chuck and workpiece are demagnetized completely, or to a condition near zero magnetization. To demagnetize, the switch 82 is thrown to RELEASE position shown in dotted lines. A circuit is then completed, as follows: from one junction of the rectifier 26 through conductor 84, the switch 82 conductor 156 to the limit switch 27, and continuing through conductors lSh, 94, and 154 to the forward field coil as of the motor and then through conductors 98 and MM) to the opposite junction of the rectifier 26. The motor is then run in forward direction, driving the rotary tap switch through a full demagnetizing cycle as set out in detail in my prior Pat. No. 2,825,854, referred to above, the cycle terminating when the limit switch 27 is opened, under the control of the corresponding cam 35, the switch coming to rest in an intermediate position, deenergizing the motor. in this demagnetizing operation, the motor stops at such position that the arm 30 of the rotary tap switch 20 is in an OFF position (H6. 6) between contact terminals 332, and the AC interrupter switch 44 is open.

The apparatus also provides for a VARIABLE holding power condition, in which the magnetization of the chuck is substantial, but less than full power of which the apparatus is capable of producing. This may be useful in an instance in which, for example, it is desired to hold a workpiece on the chuck with less than full power so as not to distort the piece. instances occur in which workpieces may be distorted by the holding force of the chuck, provided by the full capacity power of the apparatus. This variable, or lesser, holding power is controlled through the use of a variable auto transformer Mil, which is set to the desired position manually, for producing a holding power less than maximum. After setting this auto transformer, the (IliARGE-RELEASE switch 82 is moved to CHARGE position, and the VARIABLE-FULL switch fill moved to VARIABLE position shown in dotted lines. These settings produce the following circuit: from one junction of the rectifier 26 through conductor 84, the switch 82, conductor 86, through the switch 80, conductor 162, relay E coil, conductor 16d, and through limit switch 33 which is then in the position opposite that shown; the circuit then continues through conductors HM and 154 to the forward field coil of the motor 18, and then through conductors 98 and lilll to the rectifier. The relay B coil energized by this circuit closes a holding switch B3; a circuit is also completed from conductor 84 through conductor 166, and through the switch to conductor ids, abnormally closed switch B4, conductor H70, through conductor 88, and the upper contacts of the switch 9%, conductor 92, switch El and through the limit switch 3i; the circuit then continues through conductors 152, 1% and B54 to the forward field coil 96 of the motor and through conductors 98 and 100 to the rectifier. The motor is then run in forward direction to a FULL power position in the manner described above, but producing a value of magnetization as determined by the setting of the variable auto transformer lath. This auto transformer is connected in the AC circuit through the rotary tap switch 20, as follows: from the auto transformer, conductor H72, abnormally closed switch 86, conductor 1174, rotary tap switch 29, conductor 38, AC interrupter switch 44, and conductor is to one junction of the rectifier 40; the circuit continues in the other direction from the auto transformer through conductor 1176, abnormally closed switch B7, conductor Nd, conductor lliil, the transformer secondary i251 and conductor 36 to the opposite junction of the rectifier 49.

The motor continues to run in this VARlABLE setting operation until the circuit is opened by the corresponding limit switch 27, which is controlled by the corresponding cam 35 in the same full power sequence as described above, with the exception that the power applied to the chuck is at a lesser value, and proportionate to the setting of the variable auto transformer 16).

if more power is required, i.e., greater value of magnetization of the chuck, the variable auto transformer 1160 is set at the appropriate higher setting and the switch @d is again actuated; the apparatus will then operate through the sequence just described. If after a first or any subsequent VALE setting it is desired to have full power applied, the switch $0 is thrown to FULL position, and the switch 9il actuated; the chuck will then be charged at full power, as in the case when it is to be charged from a zero magnetization condition. When it is desired to demagnetize the chuck from a VARIABLE condition, the switch 82 is thrown to the RELEASE position, as in the case of FULL power condition, and the demagnetizing cycle is performed.

Attention is now directed to FIG. 2, showing a circuit diagram of a modified form of the invention. The present circuit includes certain of the same components incorporated in the circuit of FIG. I, and identified herein in the same manner, as follows: the AC source ill, transformer 12, motor 18, rotary tap switch 20, switching means 22, rectifier 49 and chuck means 24 which in the present instance includes a plurality of, or two, chucks 24a and 24b. The conductors leading from the switch means 22 to the chuck means include the conductors 56 and 53 identified in the first embodiment, but here having branch portions 56a and 56b and 58a and 5817, respectively, leading to the individual chucks. Associated with the motor B8 are the limit switches 27, 29, Eli and 33 identified above. Other portions of the present circuit correspond to similar portions of the circuit in the first embodiment, as will be observed.

The present form of the control apparatus includes RESlDUAL units 1% and 192, including 2521 tubes li land E96 respectively, These two units are similar operation to the unit 763 in that they interrupt the circuit to the chuck means in response to decrease of the magnetization in the chuck means in a partial demagnetizing or RESlDUAL demagnetization, under controlled conditions, according to a predetermined manual setting.

The RESHDUAL control unit 1190, considered the base unit, includes a relay F coil, and a holding switch Fl, there being an associated switch F2 in the unit W2. The circuit of this unit also includes a switch Gl controlled by a G relay coil in the RECHARGE circuit portion and corresponding to the relay E in the first modification.

in the DC conductor 5% leading to the chuck means is a variable resistor 3953 similar in function to the corresponding component 64 in the first modification, for imposing a controlling bias on the tubes 1% and 11%. Upon reverse current flowing in the conductor 5%, a negative signal is produced and imposed on the tube (1%) to open the circuit of that unit (il ll). Upon the F coil becoming thus deenergized, the switch F2 opens, the KU coil is deencrgized with consequent opening of switch kill, and opening of the circuit to the chuck. This unit may be set by adjusting the adjustable resistor 11% to the desired setting for interrupting the circuit to the chuck means at a predetermined level of magnetization thereof when it is desired, for example, to partially demagnetize the workpieces to remove one or more of them for test purposes, as explained above.

The unit 192 may be referred to as an auxiliary unit which operates within the base range established by the unit 1%. This unit includes a relay coil and a holding switch ill, the li coil controlling a corresponding switch H2. The unit 192 has particular adaptation to a mmhine tool in conjunction with an operation of the following character: it often occurs that a workpiece is distorted when placed on a magnetic chuck and held thereon by a large magnetic force, due to any of various reasons, such, for example, as an irregular shape of the piece. When such a piece is held on the chuck by a large magnetization force, and so distorted, and a working operation performed thereon, such as a grinding operation, the desired surface, such as a flat surface, is apparently produced on the piece, but this is only when considered relative to its condition under full magnetization holding effect and when the piece is released from the chuck by removal of the magnetization force, it assumes, or tends to assume, its original shape, and in doing so, the surface that was regarded as a flat surface when the workpieces was held on the chuck changes to an irregular or nonflat surface. This difficulty is not easily overcome, because such large value of magnetization as may distort the piece is necessary to hold the piece on the chuck sufficiently securely for the working operation, and if it were held with less force, it may be moved by the operating instrumentality, such as the grinding wheel. I have overcome this difficulty by providing the unit 192, by means of which the workpiece can be held in place in successive steps by progressively diminished magnetization force, and the working operation perfonned thereon in successive steps may involve less operating power.

The signal developed in the adjustable resistor 198 is also imposed on the tube 1%, through the conductors 2G0, 202 and 204, but this signal is controlled by a manually settable potentiometer 206. The control we, acting through the circuit of the unit 192, including the secondary 1234, controls the effectiveness of the sigial produced by the adjustable resistor, so that the unit 192 interrupts the circuit according to the setting of the potentiometer sun, which is at a level'of magnetization less than that which controls the unit 190. The signal causes opening of the circuit of the unit, thereby deenergizing the H coil, opening the switch H2, deenergizing the lfiU coil, and opening the switch Kill with consequent opening of the circuit to the chuck. The effectiveness of the unit r92 may be realized from an operation such as the following: a workpiece that may be distorted is placed on the chuck and held thereon by full magnetization force; it is then ground, the circuit is interrupted according to the RESIDUAL operating, which results in lesser magnetization imposed on the workpiece, and it tends to assume its original shape. A second operation is then performed thereon, and after that is done, the circuit is again put through the RESTDUAL cycling, and the chuck magnetized at a still lesser level. Another working operation is performed on the workpiece. At each step the workpiece assumes a condition closer to its original undistorted condition, and the working operation thereon is lighter, e.g., the grinding or cutting operation is shallower. in this series of operations, it is desired not to remove the workpiece from the chuck, so as to eliminate the possibility of foreign matter coming between the workpiece and the chuck, which would disturb its original position thereon.

Fit]. 7 indicates such a condition wherein a workpiece 2% is held on a chuck. This H6. is greatly exaggerated for more clearly bringing out the feature involved. in position A the chuck is deenergized, and the workpiece 2698 thereon is in its normal condition, having an upper convex surface Zltla and an under concave surface Zlllb.

in position B the chuck and the workpiece are magnetized, and the magnetization force draws the workpiece downwardly toward a flat position. it will be understood that the actual change in position in a practical operation may be so small as to be undetectable by the naked eye, but as indicated above, the conditions and p roportions indicated in these FIGS. are many times multiplied relative to a practical operation. A grinding wheel 212 is set to grind a surface on the workpiece represented by the dot-dash line 214, which is flat, in the condition of the workpiece when it is held in its relatively flattened position.

When the chuck and workpiece are thereafter demagnetized, the workpiece tends to reassume its original condition, as indicated in position C, and here it will be observed that the outer surface 210a is again arcuate. However, the surface of the workpiece which was once flat in position B where the workpiece was relatively flattened, is now again curved. Thus, a flat surface in this step is not attained non the workpiece in its normal condition because of the distortion of the workpiece in the holding operation, and the resumption of its original shape when the magnetization is removed.

The central portion of the workpiece after the grinding operation, as indicated at 212a (position C), is thinner than the end portions indicated at 2l2b. This condition is obviated in a succession of grinding operations of the kind just described, but with the workpiece held by progressively lesser forces, i.e., each time the chuck and workpiece are magnetized, they are so magnetized to a lesser degree so that the distortion of the workpiece is progressively less in the successive steps. in this same series of operations, successively shallower cuts are taken, and because of the less force thus applied, it is unnecessary to utilize so great magnetization power to hold the workpiece. Such series of progressively lesser demagnetization and shallower cuts are continued until, in the final step, the distortion is at a minimum, and from a practical standpoint substantially eliminated, whereby the surface that is flat when the final light magnetization is applied, remains substantially flat after the magnetization force is removed.

The same kind of operation can be perfonned on the opposite side of the workpiece, as indicated in positions D and E of Fit]. 7. In position D the same workpiece is placed on the chuck with the convex side down;'in this case also, when the chuck is magnetized, as indicated in position E, the workpiece is flattened. When the grinding operation is performed for producing a flat surface at the line 214, the same kind of result is obtained as described above. In a series of progressively decreased magnetization, and shallower cuts, the concave side of the workpiece can be provided with a flat surface in the same manner as on the convex side, described above. Position F illustrates workpiece 208 in which the convex and concave portions have been ground in the manner above described, these latter portions being indicated by the dotted lines, resulting in a workpiece as indicated by the dotted lines, resulting in a workpiece as indicated in full lines that has fiat surfaces on both sides.

Although in FIG. 2 there are two chucks illustrated, it will be understood that the apparatus is effective in connection with any other number of chucks, including a single chuck, or a greater number thereof. The invention is also effective for individually controlling a plurality of chucks. For example,

switches 11 and .11 are provided in the conductors 56a and 56b, respectively, controlled by respective relaycoils I and J, which in turn are controlled by manually controlled switches 218a and 218b, related to the respective chucks 24a and 24b. A similar arrangement may be provided for accommodating any desired number of chucks.

While I have herein shown and described certain preferred embodiments of the invention, it will be understood that changes may be made in the scope of the appended claims.

lclaim:

1. The method of treating a workpiece which is of rigid structure and normally retains its physical shape in the absence of the application of external pressure thereto, and of which the final shape is to be critical, comprising the steps,

a. holding the workpiece in place by a force of predetermined first value, and of such value that it incidentally but necessarily distorts the workpiece,

b. performing a first operating step on the workpiece involving the application of pressure thereto of a first value approaching that that would dislodge the workpiece if it were not held by a holding force at least as great as said predetermined first value thereof,

c. after the performance of the first operating step, reducing said holding force to a second value less than its said first value thereof but greater than'zero, that also incidentally but necessarily distorts the workpiece but to an extent less than the distortion caused by the holding force of said first value, and I d. performing a successive operating step on the workpiece involving the application of pressure thereto of a value less than said first value thereof and of a value approaching that that would dislodge the workpiece if it were not held by a holding force at least as great as said second value thereof.

2. The method set out in claim 1 wherein the treating operation includes removal of material from the workpiece, and the amount of material removed is reduced in successive steps.

3. The method according to claim 1 and including the additional step of reducing the holding force to a level near zero and thereby releasing the workpiece.

4. The method according to claim 3 wherein the holding force is provided by a magnetic chuck, and the method includes reducing the magnetism thereof in the successive steps.

Claims (4)

1. The method of treating a workpiece which is of rigid structure and normally retains its physical shape in the absence of the application of external pressure thereto, and of which the final shape is to be critical, comprising the steps, a. holding the workpiece in place by a force of predetermined first value, and of such value that it incidentally but necessarily distorts the workpiece, b. performing a first operating step on the workpiece involving the application of pressure thereto of a first value approaching that that would dislodge the workpiece if it were not held by a holding force at least as great as said predetermined first value thereof, c. after the performance of the first operating step, reducing said holding force to a second value less than its said first value thereof but greater than zero, that also incidentally but necessarily distorts the workpiece but to an extent less than the distortion caused by the holding force of said first value, and d. performing a successive operating step on the workpiece involving the application of pressure thereto of a value less than said first value thereof and of a value approaching that that would dislodge the workpiece if it were not held by a holding force at least as great as said second value thereof.

2. The method set out in claim 1 wherein the treating operation includes removal of material from the workpiece, and the amount of material removed is reduced in successive steps.

3. The method according to claim 1 and including the additional step of reducing the holding force to a level near zero and thereby releasing the workpiece.

4. The method according to claim 3 wherein the holding force is provided by a magnetic chuck, and the method includes reducing the magnetism thereof in the successive steps.

Images