US3798846A

US3798846A - Method of grinding

Info

Publication number: US3798846A
Application number: US00238569A
Authority: US
Inventors: R Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-05-23
Filing date: 1972-03-27
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26

Abstract

The invention provides for controlling the after grinding size of a work piece and the production rate while maintaining surface integrity and finish of the work piece substantially uniform. The method includes the step of controlling the surface velocity of a grinding element to maintain the specific grinding energy substantially constant.

Description

United States Patent 1191 Smith Mar. 26, 1974 METHODOF GRINDING 3,178,861 4/1965 MiliaS 51/1345 R [76] Inventor: Roderick L. Smith, 2012 Greenfield 51:32: 5 5 Rockford 61107 3:560:826 2/1971 tona l' 'r'IIIIIIIIIIIIIIIIIIQH1345 R [22] Filed: Mar. 27, 1972 I Y ExaminerDnald Attorney, Agent, or FirmMorsbach, Pillote & Muir Related US. Application Data [63] Continuation-impart of Ser. No. 834,928, May 23.

1969, Par. NO. 3,653,855. [57] ABSTRACT [52] U 8 Cl 51/2811 R 51/134 R The invention provides for controlling the after grind- [51] B24b 'l/oo ing size of a work piece and the production rate while [58] Fie'ld 134 5 R maintaining surface integrity and finish of the work "5 87 1 6 piece substantially uniform. The method includes the l step of controlling the surface velocity of a grinding [56] References Cited element to maintain the specific grinding energy sub- UNITED STATES PATENTS Manually Constant 3,264,788 8/1966 Coes 5l/l6 5 R 10 Claims, 10 Drawing Figures 'l I GRIND/N6 use ELEMENT I COMPENSATOR l i X l F w w CONTROL MEANS 54 56 4 I INPUT COMPARATOR I 4 /aA FEED T1 MEA rvs CONTROL 5I\ D5O I 3 7- FEED l POWER A CONTROL MEANS l sE/vsl/ve I 56 I MEANS I 47 I 39 I I 34$ l COMPARATOR /Y COMPARATOR I I MEAN8 1 MEAN8 444/ l 38 l Liji28 2 45 LEVEL 261 LEVEL SENSING SENSING I l 46 MEANS MEANS sIs I l 0 I l l REFERENCE REFERENCE I SIGNAL j SIGNAL 1 GENERATOR GENERATOR I L l PATENIED MARZB I974 sum 1 nr 3 NOTOR cONTROL MEANS \Is MEANs 2O 27 v T I I 22 Fi .1 /O

39" 1:; 1 38 a I INPUT 1 A FEED MOTOR cONTROL Fij .3

"1 GRIND/N6 I I0 was ELEMENT l cONPENsATOR} /I/6O I sREEO I a; I cONTR L MEANS l" I 54\r56 I 64 INPUT COMPARATOR 18A FEED 1 NEANs 48 CONTROL- 5x550 s1 /49 49 52 I FEED POwER /l I SENSING I cONTROL IMEANS MEANS s5 s6 4 I COMPARATO /r 5 I2 COMPARATOR I MEANS |3I MEANS- 44; l4 58 4/2a LEVEL 26 I LEvEL 2 I SENSING SENSING I MEA Ns I NEANs is I REFERENCE REFERENCE SIGNAL I SIGNAL GENERATOR I GENERATOR L 1 J METAL SPECIFIC PATENTEDMARZB m4 3798.846 7 sum 2 or 3 N Lu E M 1 I 4 L. Q: O: F A Lu a I o 2 I m Q 2 l 3 Lu 5 Q: 1 00 X! +1000 I DOWNf-E 0- VELOCITY Q: {5 u 3 3 O o q 0.

j VELOCITY U) 0: Lu 2 H u 6 N M R L DOWNFEED METHOD OF GRINDING CROSSREFERENCE:

This application is a continuation-in-part of application Ser. No. 834,928, filed May 23, 1969, now Pat. No. 3,653,855.

Background This invention relates generally to grinding machines, and more particularly, relates to a grinding system comprising automatic adjustable means for controlling grinding operations.

Grinding machines may be characterized either as controlled force or controlled feed grinding machines. The subject invention is particularly suitable to provide controlled feed grinding.

In order to accomplish removal of material, prior controlled feed grinding systems were generally designed in accordance with the concept that the material removed from the work piece depends upon the wear of the grinding element and the feed or relative forced interference of the work piece with the grinding element.

A typical controlled feed grinding system comprises a grinding wheel powered by a motor driving a belt sheave arrangement. The work piece is mounted on a carrier which is moved toward the grinding wheel at a controlled rate of speed, and the carrier may move under the wheel in continuous backward and forward cycles. Means are provided to move the rotating grinding wheel into the work piece at a controlled downward movement. A cross-feed mechanism may be providedto move the work piece transversely relative to the grinding wheel.

The great majority of the conventional industrial general purpose and special purpose machines use grinding wheels in their grinding systems. Many grinding systems may, however, use elongated, tapered, scaled down or various other shapes of grinding elements.

The main objectives of controlled feed grinding machines are (1) to remove material from a work piece to a predetermined desired size (2) in a substantially pree time (3) while maintaining the surface integrity of the work piece and (4) developing the desired surface finish. Surface integrity andsu'rfacefinish are referred to hereinafter as surface quality.

Generally with present-day general purpose grinding machines, the time required to attain a desired work piece size is variable and relatively unpredictable. This is primarily due to the unpredictability and instability of grinding element wear which causes variation in the amount of material removed.

With present-day grinding machines, the initial satisfactory surface quality of the work piece generally cannot be maintained after repetitive grinding operations. In time, symptoms of poor surface quality develops, such as chatter marks, grinding burn and grinding cracks.

Prior to an initial grinding operation, the abrasive cutting grains of a grinding element are in a sharp condition. With continued grinding, they become dull. Therefore,'external sharpening of the grinding element called dressing is usually required from time to time to prevent poor surface quality.

The proper selection of the correct grinding element for a specific grinding job is a highly skilled art and is generally achieved by a trial and error process. The most important variable of the grinding element specification is hardness, which indicates the relative wear resistance of the grinding element. Grinding elements are manufactured in a range of grades, with soft to hard grades designated by the latters Athrough Z respectively. The general practice is to make fine adjustments of the grinding performance of the grinding system by varying the volume rate of interference of the work piece with the grinding element, and thereby alter the performance of the particular grade of hardness of the grinding element. In this manner, a particular grinding element grade may actually perform in the system as a softer or harder grade grinding element. However, although this method affords some control over grinding performance it is unableto provide a desired work piece size in a given predictable time, nor is it able to provide freedom from poor work piece surface quality.

In order to obtain the desired 'work piece size repetitively on a high production basis more advanced grinding machines used inprocess workpiece gaging'apparatus to signal when the work piece was at desired size. However, grinding time was still variable as before because of unpredictable and variable grinding element wear. With these machines and in order to minimize grinding element wear and provide more predictable grinding time relatively hard grade grinding elements were used. These harder grade grinding elements were more unstable in their wear rates and tended to become dull very quickly and lead to surface quality problems. In order to avoid these quality problems, such machines were equipped with diamond dressers which automatically sharpened the grinding element before each cycle. The greater expense incurred for more frequent grinding element replacements and the substantial cost for replacement of diamond dressers contributed appreciably to cost of production when these'machines were used.

Some machines in contrast to the above used inprocess work piece gaging after each grinding head of multiple head systems, the last head removing very little material and functioning primarily to provide desired work piece size and finish, with the work piece only going through the system once. In order to be able to provide continuous production without interruption for diamond dressing and to avoid surface quality problems' such systems used very soft grinding element grades. Again the undesirable feature was the greater expense incurred for frequent replacement of the grinding element.

The subject invention overcomes these various disadvantages by providing desired work piece size in a predictable time and predictable satisfactory surface quality with a minimum of diamond dresser and grinding element replacement cost. In addition the subject invention enables one grinding element grade to be used in achieving the above results over a broader range of work piece materials and configurations.

As used herein the following definitions apply:

1. The relative interference of the grinding element and the work piece, commonly called grinding feed, is defined as the linear difference in position of the cutting face of the grinding element and the surface of the work piece when this difference will result in an interference of the two surfaces.

2. The volume rate of interference is defined as the linear interference accompanied by relative movement of the cutting face of the grinding element and the surface of the work piece.

3'. Specific energy or specific grinding energy is de' fined as the energy used by the grinding element drive motor when the grinding element removes a unit volume of material from the work piece. In more common terms it is the power used by the drive motor divided by material removal rate from the work piece.

SUMMARY The present invention relates generally to grinding methods and more particularly to a method of grinding in which the specific grinding energy is maintained substantially constant. The subject invention also controls the work piece size and grinding time, adjusts effective grinding grade action of the grinding element, and/or maintains the desired work piece quality.

There is disclosed a grinding machine in which a control system is provided for varying the surface velocity of the grinding element to maintain the specific energy substantially constant at a predetermined amount. The control system includes a speed-control apparatus responsive to a signal proportional to the deviation of specific energy from the desired constant. The control system also includes a converter responsive to a first signal correlative to material removal, and a second signal representing grinding element torque or power. The converter provides a resultant signal to the speed control apparatus to vary the speed accordingly and maintain the specific energy and work piece quality substantially uniform. I

In one embodiment, the grinding system has an input feed control which includes compensation means responsive to variations in after-grinding work piece size from a referenc work piece size. When the compensator means responds to a sensing signal corresponding to a variation inafter-grinding work piece size, the relative position of the grinding element is automatically adjusted to maintain after-grinding work piece size substantially constant. Within the limits of variation in work piece size before grinding, the compensator means also serves to maintain the material removed from the work piece substantially constant.

Another feature of the invention includes controlling grinding time by varying the speed of movement of the work piece or grinding element through the grinding zone. By controlling the amount of material removed from the work piece and controlling grinding time, control of material removal rate is thereby achieved. Thus, with material removal per unit time held constant, any changes in grinding element motor torque or power are caused by changes occurring in the specific grinding energy, caused by changes in, the grinding fluid, the physical properties of the work piece material, and other such grinding system variables.

Another featureof the invention includes automatically adjusting the effective grinding grade action of the grinding element by varying the surface velocity of the grinding element to achieve a desired specific grinding energy level. Thus, if the horsepower or torque required varies from a predetermined normal level, the surface velocity of the grinding element varies in response thereto to bring the grinding performance back to the normal level. Therefore, an initial setting of the operating point with respect to a horsepower or work level per unit time is maintained without sacrificing work piece surface quality.

A primary object of the invention is to provide a method of grinding in which the specific grinding energy is maintained substantially constant.

Another object is to provide a method of grinding that maintains the final size or level of material removal with respect to a reference substantially constant.

Another object is to provide a grinding method which maintains the rate of producing finished parts substantially constant. 7

Still another object is to provide a grinding method which is substantially independent of variations in the abrasive surface of the grinding element within a particular range of abrasive grade variations.

Another object is to provide a controlled feed type, grinding method that automatically maintains operation at substantially a particular specific energy point regardless of variation in the surface of the grinding element, variations in the properties of the work piece material, variations in the amount of material to be removed from the work piece, and variations in the volume rate of interference.

Yet another object of the present invention is to provide a method of grinding which varies the surface velocity of the grinding element to maintain the specific grinding energy substantially constant.

Another object is to reduce the problem of multiple and complex grinding element selection and stocking problems by providing a grinding method that may be used to obtain a plurality of grinding grade actions from one particular grinding element grade.

It is another object of the present invention to provide a grinding method which can operate at a minimum specific grinding energy so as to maximize removal rate and productivity from a given power machine.

These, and other objects and advantages of the invention, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings which illustrate the manner of performing the method.

DRAWINGS:

FIG. 1 is a block diagram of a grinding system for performing the method of the invention;

FIG. 2 is a detailed schematic block diagram of the control means of the grinding system in FIG. 1;

FIG. 3 illustrates another preferred embodiment of the invention which utilizes a particular motor loadspeed characteristic for controlling the operating point of the system;

FIG. 4 illustrates the relationship of metal removal rate and downfeed for various grinding wheel hardness grades;

FIG. 5 illustrates the power used to obtain the metal removal rate in FIG. 4;

FIG. 6 illustrates the relationship of specific energy and downfeed for the same wheel hardness grades as FIG. 4;

FIG. 7 illustrates horsepower versus surface velocity for an M grade grinding element;

FIG. 8 illustrates horsepower versus surface velocity curves respectively for a motor and a grinding element driven by said motor of FIG. 3 system;

FIG. 9 illustrates another preferred embodiment for performing the method of the invention which maintains the specific grinding energy of the grinding operation substantially constant; and

FIG. illustrates a more detailed block diagram of the embodiment of FIG. 9.

DESCRlPTlON:

Reference is now made more particularly to the drawings which illustrate the best presently known mode for performing the method of the present invention, and wherein similar reference characters indicate the same parts throughout the several views.

As shown in FIG. 1, a grinding system 10 includes a grinding means indicated generally by the numeral 12 comprising a grinding element or wheel 14 driven by a motor 16. A control means indicated generally by the numeral 18 automatically controls the operation of system 10. Control means 18 comprises an input feed con trol 18A and a grinding element compensator 18B more fully described below.

A work piece 20 is supported on a carrier 22 which feeds the work piece to the grinding wheel 14. In the illustrative embodiment it is contemplated that the relative interference of the grinding element 14 and work piece 20 is controlled primarily by varying or changing downfeed, infeed or transverse feed of the grinding element 14, and/or longitudinal transverse feed of the carrier 22.

Turning now particularly to FIG. 2, the control means 18 will be described. The input feed control 18A controls the position of the grinding element 14 and the grinding element compensator 18B varies the speed of rotation of the grinding element in response to variations in the specific grinding energy. Specific grinding energy depends on the grinding action of the grinding element, the grinding fluid, the physical properties of the work piece material and the volume rate of interference between the grinding element and the work piece and other such grinding system variables.

The input feed control 18A comprises an aftergrinding sensor 26 having an outer pressure sensitive pick-up needle 27-to contact work piece 20 and sense 'the size or level of the work-piece after a grinding operation, with respect to the top of the carrier or other fixed reference level. The sensor 26 converts variations in level of the work piece into electrical signals to provide a sensing signal at output 28. The'needle 27 is sta tionary as the work piece is fed to the grinding element 14.

Sensor 26 may comprise a suitable transformer means such as a transducer for converting level variations occurring at the surface contacting pick-up needle 27 into electrical signals at output 28. Alternatively, instead of the pick-up needle, an air or fluid gauge, or inductive pick-up, may be used to provide sensing signals corresponding to level variations.

A comparator means 30 receives the sensing signal from, the after-grinding sensor 26 at input 31 and compares it with a preset reference signal impressed at input 32 from a reference signal generator 33. A variation or difference between the relative magnitudes of the sensing signal and reference signal is translated into an error signal at. output 34 of comparator means 30.

A feed control means 35 receives the error signal at input 36 from output 34 of comparator means 30 and converts it into a drive signal at output 37. A positioning means 38 receives the drive signal at input 39 and in response provides a mechanical force for varying the interference position of the grinding element 14 from a normal preset interference position. Thus, if the size or level of a ground work piece is too high after grinding, the error signal at output 34 causes the grinding element 14 to move to a lower position. If the level of the finished work is too low, the error signal at output 34 causes the grinding element to move to a higher position.

The grinding element compensator 188 comprises a pregrinding sensor 40 having an outer pick-up needle 41 to contact work piece 20 and sense the level or size of the work piece before removal of material. Sensor 40 converts variations in level into electrical signals to provide a sensing signal at output 42. Sensor 40 may, similar to after-grinding sensor 26, comprise a transducer for converting level variations into electrical signals.

A first comparator means or converter 43 receives the sensing signal at input 44 from output 42 of level sensing means 40 and a reference signal at input 45 from reference signal generator 46. The signal from reference signal means 46 is preset to a predetermined level. A variation or difference between the relative magnitudes of the sensing signal and reference signal is translatedinto an error signal at output 47 of first comparator means 43.

A second comparator means 48 receives the error signal at input 49 from output 47 of first comparator means 43 and a load sensing signal at input 50 from output 51 of a power sensing means 52. The power sensing means 52 may comprise a transducer for converting variations in motor torque to electrical signals at output 51. Alternatively, power sensing means 52 may monitor input electrical power of the motor means 16.

The second comparator means 48 provides a drive signal at output 54 when the torque of the motor means 16 increases from a normal point and the size or level of the work piece just prior to grinding does not exceed a predetermined normal level, and the input feed control 18A is functioning to maintain the level of the surface of the workpiece after grinding substantially constant (which in effect maintains the grinding surface of the'grinding element at the same relative preset position). The predetermined normal level of the work piece may be provided by a signal from a generator 149 which may be built into the second comparator means 48. Under these conditions, variations in torque is due essentially to changes in frictional losses caused by variations in the grinding surface of the grinding element or variations in the material of the work piece.

On the other hand, if torque increased and the specific grinding energy (frictional losses mentioned above) has not increased, the increased torque would be due primarily to anoversize of the work piece and a motor drive signal would not be generated at output 54. This would be due to the sensing signal at input 50 indicating increased torque being cancelled by the effect of the increased error signal at input 49; and any change in torque or power not attributed to frictional losses caused from variations in the grinding surface and work piece surface or variation in the position of the grinding element, is causedessentially from oversize of the work piece before grinding. Thus, the error signal at input 49 due to an oversize work piece cancels the sensing signal appearing at input 50 for increased torque.

The output 54 of second comparator 48 is connected to the input 56 of speed control means 60. The drive signal at input 56 is converted into a speed control signal at output 64. The speed control signal varies with the magnitude of the drive signal at output 54. Thus, for example, if the normal specific grinding energy increases due to variation in the grinding action, a drive signal is developed causing the speed control means 60 to generate a speed control signal for decreasing the speed of the grinding element, and if the normal specific grinding energy decreases the speed control means causes the speed of the grinding element to increase. This automatic adjustment maintains specific energy substantially constant irrespective of normal variations in the grinding. element, grinding material and other grinding operation variables.

Referring now specifically to FIGS. 3, 7, and 8, another preferred embodiment of the invention will be described. Similar parts to those in FIGS. 1 and 2 are designated by the same numeral and a prime suffix. The control systemin FIG. 3 is indicated generally by a reference numeral An input feed control 18A controls the position of the grinding element 14, and is similar to the input feed control 18A in FIG. 2. Motor 16' is preselected to provide a specific load-speed characteristic curve such as curve W shown in FIG. 8. The load-speed characteristic curve W may be obtained from many types of motors, such as the conventional D.C. series, A.C. induction motor, A.C. wound rotor motor with resistance, or DC. shunt motor with resistance in the armature. Curve Z in FIGS. 7 and 8 illustrates the relationship of power to surface velocity for a grinding element driven at various speeds. The desired operation for drive system 10' is to automatically cause the surface velocity of the grinding element to decrease when a change in load occurs due to an increased change in grinding grade action as indicated by increase in specific energy; and the surface velocity to increase upon a decreased change in the grinding grade action as indicated by a decrease in specific energy. The change in grinding grade action may be due to a change in the work piece material, the grinding element, etc. I 1

Turning now specifically to FIG. 8, when the load increases, the motor operating point tends to move from an operating point T of the system 10' towards point V, and the surface velocity of the grinding element automatically decreases because of the operating curve W of the motor. Due to the decreased surface velocity, the grinding element decreases its required power in accordance with curve Z and returns the system operation back to operating point T. Similarly, if the load decreases due to a change in grinding grade action and the motor operating point tends to move from a point T toward a point Y, the surface velocity increases in accordance with curve W and system performance returns to point T, due to the action of the grinding element according to curve Z.

Depending upon the disturbance causing the change in grinding grade, the system may or may not return to point T. For instance, if the grinding grade action when the system was previously operating at point T cannot be repeated by a change in the surface velocity of the grinding element, a slightly new operating point would be established. This would be the case when the change in grinding grade action was caused by the grinding element grade varying with wear or the physical characteristic of the work piece had changed. However, if a change in the grinding grade action occurred due to dulling of the grinding element, the system operation would tend to be maintained at point T (FIG. 8). Dulling of the wheel increases removal rate of material which increases the load toward the point V of curve W. Increased load along curve W decreases the surface velocity of the grinding element and the motor operation returns to point T.

The input feed control 18A of system 10 (FIG. 3) functions identically to the input feed control 18A in FIG. 2 to provide substantially constant surface level after grinding with respect to a reference, byvarying the position or downfeed of the grinding element into the work piece. Hence, by controlling the downfeed, the grinding element wear is compensated for and thereby maintains after grinding level substantially constant. Thus, since the level after grinding is held constant, any change in horsepower is attributed to variation in specific energy caused by a change in the grinding element material, the work piece material, or other frictional variable provided the size or level of the work prior to grinding remains unchanged. Without the grinding element compensator 188, the operating point of the system may shift if the level of the work piece prior to grinding varies from a normal level which would thereby vary horsepower.

In setting the proper parameters for the operation of the grinding system 10, grinding data charts from conventional grinding systems, such as those illustrated in FIGS. 4 through 7 may be utilized. FIG. 4 shows the relationship of metal removal rate and downfeed for grinding grade hardness values L, M. and N for the grinding element.

Points E, A, and F indicate metal removal rates respectively for grinding grades N, M, and L" at a downfeed of R. For greater downfeeds, a noticeable increase in the metal removal rate is shown for higher hardness grades.

FIG. 5 shows that a greater horsepower is used for the harder grades of the grinding element at the same level of downfeed. Power levels at points C, B, and D in FIG. 5 provide respectively the material removal rates at points E, A, and F for grinding element's N, M", and L at an R downfeed.

FIG. 6 indicates the specific energy curves for grinding grades N, M, and L. Specific energy is the horsepower minutes per cubic inch of grinding. Point G designates the specific energy for an M grade grinding element at a downfeed of R. Point H designates the specific energy when the M grade grinding element acts as an N grade at a power level of V (FIGS. 5 and 8) and point I designates the specific energy when the M grade grinding element acts as an L" grade at a power level Y (FIGS. 5 and 8).

The curve Z in FIGS. 7 and 8 shows the horsepower increasing as the surface velocity of the grinding element increases and the horsepower decreasing as the surface velocity decreases.

Thus, if the surface velocity of an M grade wheel providing a removal rate R at point A in FIG. 4 is decreased approximately lOOO sfpm, the grinding grade action shifts to effectively an L grade and the removal rate decreases to the level at point F. If, on the other hand, the surface velocity of the M grade providing the removal rate R at point A is increased approximately 1,000 sfpm, the grinding grade action w shifts to effectively an N grade and the removal rate of the material increases to the level at point E (FIG. 4).

Curve Z in FIGS. 7 and 8 illustrates power variation of an M grinding element for corresponding variation in velocity. The power value for an M" grade grinding wheel at 1,000 sfpm higher than a predetermined normal (X1) is substantially equivalent to the power value of an N grade wheel at said normal velocity and is designated as the power point C in FIGS. 4, 7 and 8. The power value for an M grade wheel at 1,000 sfpm lower than normal velocity is equivalent to the power value of an L grade wheel at normal velocity and is designated point D in FIGS. 5, 7 and 8.

Grinding grade action is a result of the material of the work piece, the material of the grinding wheel, and the operation of the grinding system which controls downfeed and the surface velocity of the grinding element. The grinding grade action is most clearly represented by the specific energy. Poor work piece quality is characterized by high values of specific energy, which gradually become higher and higher as the wheel dulls. As may be seen from FIGS. 4, and 6, the condition of harder grinding element grades and lower downfeed, leads to higher specific energy values. Metals exhibiting high specific energy values are difficult to grind and easy to injure. Therefore, by controlling the surface velocity of the grinding element with the grinding element compensator 188, a desired specific energy point may be maintained and thereby provide a consistently high work piece quality.

For maintaining the work piece size constant, the level of the surface after grinding is sensed by the sensor 27 in FIG. 2, or sensor 27' in FIG. 3. The deviation FIGS. 1 and 2 and carrier 22 of FIG. 3 through thegrinding zone of the level after grinding are maintained relatively constant, the amount of material removed from the work piece per unit time or material removal rate is substantially constant provided there is no appreciable variation in the work piece size or level prior to grinding.

In FIG. 4, point A provides, for purposes of example, a typical desired constant metal removal rate fora grinding element ofM grade. Point B in FIG. 5 is the associated power level point for the A removal rate and point G in FIG. 6 is the associated specific energy point.

If the grinding element grade increases from M to N and the material removal rate increases from point A toward E (FIG.4), sensor-27 senses the increases in the material removal. The input feed control 18A, responsive to the sensing signal from sensor 27, causes the grinding element 14 to be moved away from the work piece and hence decreasing the downfeed with respect to the work piece. This action causes the material removal ra'te to decrease back to point A.

In FIG. 6, H designates the specific energy point for an N grade grinding element to provide an A metal removal rate (FIG. 4). As may be seen, the effective downfeed for the N" grade element at a specific energy of H is less than the effective downfeed rate for the M grade at G (FIG. 6). This is due to the N grade wearing less than the M grade. If on the other hand, the grinding grade decreases from M grade to L grade and the material removal rate decreases from point A toward point F in FIG. 4, sensor 27 senses the decrease in material removal. The input feed control 18A, responsive to the sensing signal fromsensor 27, causes the grinding element 14 to be moved inward toward the work piece and hence increasing the downfeed; thus compensating for grinding element wear. This action causes the material removal rate to increase back to point'A.

Referring to FIGS.- 5 and 8, power levels at Y and V of motor curve W of the preselected motor means 16' are sufficient to enable respectively a harder or softer grade change in the grinding action by causing a plus or minus 1,000 sfpm change to occur in the grinding wheel velocity. Thus, if the grinding action changes from an M to effectively an N grade, the increased load of power exhibited at point V decreases the velocity of the grinding element 1,000 sfpm. The increase in power from point T in FIG. 8 to point V of the motor curve W causing a drop in velocity of 1,000 sfpm is less than the decrease in powerrequired by the grinding element at 1,000 sfpm, lower velocity, as the velocity decreases from points B to D along curve Z in FIGS. 7 and 8. If the grinding action changes from an M to an L, the power level exhibited at point Y increases the velocity of the grinding element. The decrease in motor power from point T in FIG. 8 to point Y of motor curve W causing an increase in velocity of 1,000 sfpm is less than the increase in power required by the grinding element at 1,000 sfpm higher velocity, as the velocity increases from points B to C of curve Z in-FIGS. 7 and 8. With such a relationship between the motor and grinding element, the operation is stable and capable of returning to point T (FIG. 8) in the event of an increase or decrease in velocity of the grinding element from the desired point T.

From FIG. 6, it can be seen that the grinding energy points H, G, and J for power levels V, T and Y (FIG. 8) are substantially the same. Therefore, the compen sation of the system to increase and decrease the velocity of the grinding element for correspondingincreases and decreases of the grinding grade action has very little, if any, effect upon grinding quality, since there is very little change in the grinding energy.

The work piece size is maintained substantially constant by the input feed control 18A of system 10 and 18A of system 10. Surface integrity and finish, herein defined as work piece quality, is maintained relatively constant by the action of the grinding element compensator 18B which senses variations of grinding energy as reflected by increased motor torque or power.

Alternatively, as pointed out previously, the desired work piece quality may be maintained by the preselec tion of the motor means 16' whose load-speed characteristic curve enables it to selfcompensate for variations in grinding grade energy. Thus, the preselected motor means 16' cooperating with the grinding element 14' maintains the operation of the grinding system at a desired load-speed operating point. However, it should be noted that system reacts to torque or power variations caused by variations in size of the work piece prior to grinding. Therefore, system 10 (having the grinding element compensator 183 which reacts only to variations in specific energy of the grinding operation and not to load changes due to variation in work piece size prior to grinding) more precisely controls work piece quality than system 10.

Referring now specifically to FIGS. 9 and 10, another preferred system of the subject invention indicated generally by reference numbral 66 will be described. Similar parts to those in the other figures are designated by the same numeral and a double prime suffix The system 66 in FIG. 9 and FIG. 10 provides control of grinding grade action and work piece quality. System 66 is particualrly suitable when a high degree of preciseness of size control is not necessary and manual adjustment of the downfeed of the grinding element is acceptable for compensating for wheel wear.

System 66 comprises a specific energy detector means 65 which detects variation of the specific energy, from a predetermined normal specific energy to produce an error signal. The error signal is fed to the grinding element compensator 18C which varies the surface velocity of the grinding element 14" in response to said error signal; thereby maintaining the specific energy rate and work piece quality substantially constant.

Turning now to FIG. 10, system 66 will be described with greater detail. A pick-up needle 27- sensing the level or size of the work piece after grinding is connected to an after grinding sensor 26" and the pick-up needle 41" sensing the size or level of the work piece prior to grinding is connected to a pre-grinding sensor 40. Sensors 26" and 40" may be transducers which convert size or level variations into electrical signals. The outputs from sensors 26" and 40" are connected respectively to inputs 70 and 73 of a subtractor 72.

The difference between the signals at inputs 70 and 73 generates a difference signal at the output 74 of subtractor 72 which in turn is impressed at input 75 of a multiplier 76. The difference signal is multiplied by a preset signal proportional to the work piece area. The signal appearing at the output 77 is substantially proportional to the volume of the material ground off the work piece 20". Output 77 is connected to the input 78 of a divider means or converter'83. An output 86 from an energy sensing means 85 is connected to a second input 69 to the divider means 83.

The energy sensing means 85 monitors electrical energy used by the motor and converts it to an electrical drive signal at the output 86. The divider means 83 includes means for dividing the energy signal from input 69 by the volume signal at input 78 to provide a signal proportional to specific energy or horsepower minutes per unit volume of material removed at its output 84. The output 84 as shown, is connected to an input 67 of comparator means 48" through a point 87.

A preset signal generated from the reference signal generator 46" is connected to input 50". If a variation exits between the signals at input 50" and 67, a drive signal appears at output 54" which is connected to speed control means 60". The output 64" of the speed control means 60 is connected through point 87A to the motor means 16". An increase in specific energy indicated by the signal at output 54" causes a decrease in speed of the grinding element, and a decrease in specific energy causes an increase in speed of the grinding element. By this action, the specific energy is maintained substantially constant, and thereby providing control of the quality of the surface being grounded. The level of the ground surface of the work piece, however, may not remain constant in system 66, as is the case with systems 10 and 10' which include the input feed control 18A and 18A.

As shown in FIG. 10, output 84 for the divider means 83 is connected to point 87 (FIG. 10). Alternatively, output 84 may be disconnected from point 87 and the compensator 18C, and connected instead to an input point 88 for a monitor means 90 which monitors specific energy. In such an arrangement, the motor means 16" must be disconnected from point 87A and connected instead to point 88A of a manual control means 92. Control means 92 enables the operator to vary motor speed in accordance with the information provided by the monitor means 90, which in turn varies the surface velocity of the grinding element.

It is now deemed obvious that a work piece is ground by supporting it on the grinding machine and controlling the surface velocity of the grinding element to maintain the specific grinding energy substantially constant. By sensing the level of the work piece after grinding, it is possible to adjust the relative position of the work piece and grinding element to maintain the aftergrinding level of the work piece substantially constant. If the after-grinding level is thus maintained andif the before-grinding level is substantially constant, then the material removal rate is held substantially constant. Under this condition, any change in power used is caused only by changes in frictional power losses such as caused by dull versus sharp wheels or harder or softer spots in material being ground, etc.; in which case one may sense the power utilized in the grinding operation and thereupon adjust the speed of the motor, and thereby surface velocity of the grinding element,,to

maintain the specific grinding energy substantially coni stant. The before-grinding level is deemed substantially constant if it is within $0.010" representative of normal tolerances from machining operations such as milling, turning, etc.

When the materail removal rate is constant, changes in power required in the grinding operation are proportional to changes in separating force; tha'tis, the force tending to separate the grinding element and the work piece causing spring back in the grinding machine structure. Such changes in power may be measured directly or by measuring the separating force, and the term measuring changes in power, as used herein, should be understood to also include measuring changes in separating force.

The method is also performed by sensing the level of the work piece both before and after grinding and dctermining the volume of material being removed. The speed of the motor is then adjusted to maintain the specific grinding energy substantially constant.

It is also deemed obvious that an improved grinding method is provided by driving a grinding element by means of a motor having a load-speed characteristic curve which is substantially the inverse of the power to surface velocity curve of the grinding element.

In one preferred method of the present invention, the after-grinding level is maintained substantially constant and the before-grinding level is sensed to determine any variations outside of preselected limits. Any changes in power required in the grinding operation which are attributable to the variations outside said limits, are ignored. Any changes in power which are not attributable to the variations outside said limits, are attributable to a change in friction power losses. Whereupon the surface velocity of the grinding element is adjusted to return the specific grinding energy to a preselected amount.

The invention in its braoder aspects is not limited to the specific steps and mechanisms shown and described but departures therefrom may be made within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

1 claim:

l. A method of grinding a work piece to maintain a substantially constant specific grinding energy, i.e., power consumption per unit volume, including the steps of: supporting the work piece on a grinding machine having a movable grinding element driven by a motor; moving the grinding element and work piece relative to each other and removing a substantially constant volume of material from the work piece per unit of time during the grinding operation; and selectively varying the surface velocity of the grinding element during the grinding operation to maintain a substantially constant power level; whereby the specific grinding energy is substantially constant.

2. Amethod as set forth in claim 1 wherein the grinding element has a known power to surface velocity curve; and the motor has a load-speed characteristic curve which is substantially the inverse of the power to surface velocity curve of the grinding element.

3. A method as set forth in claim 1 including the steps of: sensing the power utilized in the grinding operation; and adjusting the speed of the motor to vary the surface velocity of the grinding element and maintain the power level substantially constant.

4. A method as set forth in claim 1 including the steps of: during the grinding operation, sensing the aftergrinding level of the work piece adjacent the grinding element; and adjusting the relative position of the work piece and grinding element to maintain the aftergrinding level of the work piece substantially uniform.

5. A method of grinding a work piece to maintain a substantially constant specific grinding energy, including the steps of: supporting the work piece on a grinding machine having a movable grinding element driven by a motor; moving the grinding element and work piece relative to each other while sensing the level of the work piece both before and after the grinding element to determine the volume of material being removed; and adjusting the speed of the motor to vary the surface velocity of the grinding element in a manner to maintain the specific grinding energy substantially constant.

6. A method of grinding a work piece using a grinding element having a known power to surface velocity curve, including the steps of: supporting the work piece on a grinding machine; during the grinding operation sensing the after-grinding level of the work piece adjacent the grinding element; adjusting the relative position of the work piece and grinding element to maintain the after-grinding level ofthe work piece substantially constant; and driving the grinding element with a motor having a load-speed characteristic curve which is substantially the inverse of the power to surface velocity curve of the grinding element.

7. A method as set forth in claim 6 including the step of: feeding the work piece relative to the grinding element at a rate commensurate with the type of material of the work piece and the grinding element being used, which. has said known power to surface velocity curve, so that the motor operates near its maximum rated capacity.

8.. A method as set forth in claim 7 wherein the motor drives the grinding element within the range of a pres e lected surface speed plus or minus about l,000 sfpm.

9. A method ofgrinding a work piece including the steps of: supporting the work piece on a grinding machine having a movable grinding element driven by a motor; sensing the after-grinding level of the work piece and adjusting the relative position of the work piece and grinding element to maintain the aftergrinding level of the'work piece substantially uniform; sensing the pre-grinding level of the work piece to determine any variations in the pre-grinding level outside of preselected limits; and monitoring the power consumed in the grinding operation and selectively adjusting the surface velocity of the grinding element to return the specific grinding energy to a preselected amount when there are changes in the power not attributable to said variations in the pre-grindirig level.

10. A method as set forth in claim 9 including removing the material at a substantially constant rate except for said variations in the pregrinding level outside the preselected limits; and adjusting the speed of the motor to adjust the surface velocity of the grinding element. k

Claims

1. A method of grinding a work piece to maintain a substantially constant specific grinding energy, i.e., power consumption per unit volume, including the steps of: supporting the work piece on a grinding machine having a movable grinding element driven by a motor; moving the grinding element and work piece relative to each other and removing a substantially constant volume of material from the work piece per unit of time during the grinding operation; and selectively varying the surface velocity of the grinding element during the grinding operation to maintain a substantially constant power level; whereby the specific grinding energy is substantially constant.

2. A method as set forth in claim 1 wherein the grinding element has a known power to surface velocity curve; and the motor has a load-speed characteristic curve which is substantially the inverse of the power to surface velocity curve of the griNding element.

4. A method as set forth in claim 1 including the steps of: during the grinding operation, sensing the after-grinding level of the work piece adjacent the grinding element; and adjusting the relative position of the work piece and grinding element to maintain the after-grinding level of the work piece substantially uniform.

6. A method of grinding a work piece using a grinding element having a known power to surface velocity curve, including the steps of: supporting the work piece on a grinding machine; during the grinding operation sensing the after-grinding level of the work piece adjacent the grinding element; adjusting the relative position of the work piece and grinding element to maintain the after-grinding level of the work piece substantially constant; and driving the grinding element with a motor having a load-speed characteristic curve which is substantially the inverse of the power to surface velocity curve of the grinding element.

7. A method as set forth in claim 6 including the step of: feeding the work piece relative to the grinding element at a rate commensurate with the type of material of the work piece and the grinding element being used, which has said known power to surface velocity curve, so that the motor operates near its maximum rated capacity.

8. A method as set forth in claim 7 wherein the motor drives the grinding element within the range of a preselected surface speed plus or minus about 1,000 sfpm.

9. A method of grinding a work piece including the steps of: supporting the work piece on a grinding machine having a movable grinding element driven by a motor; sensing the after-grinding level of the work piece and adjusting the relative position of the work piece and grinding element to maintain the after-grinding level of the work piece substantially uniform; sensing the pre-grinding level of the work piece to determine any variations in the pre-grinding level outside of preselected limits; and monitoring the power consumed in the grinding operation and selectively adjusting the surface velocity of the grinding element to return the specific grinding energy to a preselected amount when there are changes in the power not attributable to said variations in the pre-grinding level.

10. A method as set forth in claim 9 including removing the material at a substantially constant rate except for said variations in the pregrinding level outside the preselected limits; and adjusting the speed of the motor to adjust the surface velocity of the grinding element.