US3774348A - Horizontal double disc grinder with anti-vacuum control - Google Patents

Abstract

A double disc abrading machine, such as a double disc grinder, which is particularly adapted for grinding opposite surfaces of thin workpieces (W), includes the injection of a metered amount of air into the coolant supplied through the spindles (13L and 13R) to eliminate a partial vacuum formed between the abrasive discs (14L and 14R). It is believed that hydrodynamic forces create a partial vacuum between the closely spaced abrasive discs (14L and 14R) which are rotated in the same or opposite directions. There may also be cohesive and/or adhesive forces acting to draw the abrasive discs (14L and 14R) too close together. It is believed that this combination of forces and the ambient air pressure exerted on the outside of the abrasive discs (14L and 14R) causes the workpieces (W) to be ground undersize. The metered air is injected into the coolant during the grinding process to eliminate the vacuum effect. In some instances it is desirable to add supplementary un-metered air at the completion of the grinding process so that the discs may be retracted in unison at a rapid rate.

Description

United States Patent [191 Dunn [ Nov. 27, 1973 HORIZONTAL DOUBLE DISC GRINDER WITH ANTI-VACUUM CONTROL [75] Inventor:

[73] Assignee: Litton Industries, Inc., Beverly Hills,

Calif.

[22] Filed: Apr. 30, 1971 [21] Appl. No.: 138,871

Elman R. Dunn, Roscoe, Ill.

Primary ExaminerHarold D. Whitehead Attorney-Joseph R. Spalla [57 ABSTRACT A double disc abrading machine, such as a double disc grinder, which is particularly adapted for grinding opposite surfaces of thin workpieces (W), includes the injection of a metered amount of air into the coolant supplied through the spindles (13L and 13R) to eliminate a partial vacuum formed between the abrasive discs (14L and 14R). It is believed that hydrodynamic forces create a partial vacuum between the closely spaced abrasive discs (14L and 14R) which are rotated in the same or opposite directions. There may also be cohesive and/0r adhesive forces acting to draw the abrasive discs (14L and 14R) too close together. It is believed that this combination of forces and the ambient air pressure exerted on the outside of the abrasive discs (14L and MR) causes the workpieces (W) to be ground undersize. The metered air is injected into the coolant during the grinding process to eliminate the vacuum effect' In some instances it is desirable to add supplementary un-metered air at the completion of the grinding process so that the discs may be retracted in unison at a rapid rate.

3 Claims, 4 Drawing Figures lOL Patented Nov. 27, 1973 3,774,348

2 Sheets-Sheet 1 INVENTOR $(LF 43 T T M ELMAN R. DUNN AIR SUPPLY 47 BY M ATTORNEY Patented Nov. 27, 1973 2 Sheets-Sheet ILSA N 40R CARRIER IN LIMIT 4TR3 I CYCLE START I I l IPB I SPARKOUT CLUTCH iE CYCLE sTART I am I I3 CA A fila LRRIER DVANCE 5CRl V IL II 4cRI 4TR2 FACECUT ADVANCE I% 7CR2 AL 4TRl 3L8 4L5 I SPARKOUT MOTOR N N LH FACECUT RH FACECUT W m AIR INJECTOR GTRI m AIR INJECTOR 50L 0 cARRIER ADVANCE II b Y sou) FACECUT ADVANCE H L Z sou AIR INJECTOR II L 4TR SPARKOUT CLUTCH REsET TIME 4TRI O X 0 4TR2 X X 0 4 4TR3 x x 0 MR4 O O X INVENTOR ELMAN R. DUNN ATTORNEY HORIZONTAL DOUBLE DISC GRINDER WITH ANTI-VACUUM CONTROL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates in general to new and useful improvements in double disc grinders, and more particularly, to a double disc grinder which is particularly adapted for grinding the opposite surface portions of extremely thin workpieces. A metered supply of air is directed into the coolant volume during the grinding operation to eliminate the hydrodynamic forces which create a partial vacuum which interferes with the controlled movement of the disc supporting heads.

2. Description of the Prior Art An apparatus for advancing the opposed discs of a horizontal disc grinder is disclosed in U.S. Pat. No. 3,001,337, issued Sept. 26, 1961. An improved advancing apparatus is disclosed in the inventors pending application, Ser. No. 51,860, filed July 2, 1970 and now US. Pat. No. 3,721,046, dated Mar. 20, 1973. The feeding movement of the abrasive discs is synchronized to advance at an equal rate controlled by various hydraulic and mechanical arrangements. The power to effect the feeding movement is applied in equal amounts to each of the disc supporting members by means of identical feed mechanisms. In the prior art, workpieces have been ground to a predetermined size with parallel sides, after which the abrasive discs are retracted. However, when the surface portions of extremely thin workpieces are ground, a vacuum-like phenomenon occurs, and it is believed a partial vacuum is created between the abrasive discs. This condition seems to be caused when the desired amount of coolant is directed through hollow spindles into the grinding zone, and the centrifugal or rotational forces acting on the coolant prevent penetration of atmospheric air therebetween when the abrasive discs are positioned very close together. Therefore, the positive movement of the abrasive discs toward one another cannot be effectively controlled until the effect of static pressure of the atmosphere acting on the exterior'surfaces of the abrasive discs is overcome or satisfactorily counterbalanced. Therefore, thin workpieces are 'sometimes ground undersize because the abrasive discs are ad vanced closer together than intended by the feed mecha certain amount of entrained air, in practice, it has not However, these air nozzles are for an entirely different purpose and could only be used where the workpieces are relatively large in size. In such cases, there probably is no partial vacuum problem due to the wide spacing of the discs. Furthermore,application of the air near the periphery of the abrasive discs, as taught in U. S. Pat. No. 3,151,422, is considered ineffective in solving the problem of a partial vacuum since the outward rotational movement of the coolant would prevent inward penetration of the air.

SUMMARY OF THE INVENTION In accordance with the invention, there is provided a double disc abrading machine for abrading the parallel sides of a thin workpiece to a precise dimension. The machine includes a pair of abrasive discs, which are supported and rotated by respective spindles. Facilities are provided to advance and retract the discs, and for supporting the workpiece. Coolant is supplied through at least one of the spindles to the area between the discs. In order to eliminate the partial vacuum that forms between the discs, a volume of air is directed into one of the spindles during the abrading operation.

' In the preferred embodiment, the air which is injected during the abrading operation is metered. Furanism, due to the assisting effect of normal atmospheric forces acting on the exterior of the abrasive discs. Also, the tooling arm or work holder maybe damaged when the abrasive discs are not retracted in unison, due to the vacuum or suction between the discs. The work holder is susceptive to damage because of its thin size and the fact that the clearance between the work holder and the surface of the abrasive discs must be kept to a minimum to provide adequate support for the workpiece. This latter problem has been previously solved by the inventor through the injection of a quantity of air into the coolant system after the grinding cycle was finished, but before the abrasive discs were retracted.

In U. S. Pat. No. 2,089,040, issued Aug. 3, 1937, there is disclosed an apparatus for supplying coolant fluid to the central portion of the rotating grinding discs. However, there is no suggestion that metered quantities of air could be added to the coolant to overcome a partial vacuum created between the abrasive discs. While the coolant may have inherently contained thermore, at the completion of the abrading operation, but prior to the retraction of the discs for workpiece removal, an additional amount of air is injected into one of thespindles to assist in separating the closely spaced discs.

A primary object of the instant invention is to provide a pneumatic system of metered air for reducing or eliminating a partial vacuum formed between narrowly spaced abrasive discs to enable specific workpiece size tolerances to be maintained.

Another object is to prolong the life of a thin tooling member which houses a workpiece in the grinding station, by avoiding overheating and buckling.

Another objectis to prevent either of the grinding heads from drifting inwardly in response to the cohesive and adhesive forces of the coolant which forms a vacuum'between the abrasive discs, which later resists retraction of the abrasive discs.

The above and other features and advantages of the present invention will become better understood from the detailed description of the invention that follows, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a horizontal double disc grinding machine embodying the principles of the invention, showing the workpiece, workpiece fixture, grinding discs, and other supporting structure;

FIG. 2 is a diagrammatic pneumatic and coolant circuit showing means for eliminating the hydrodynamic forces which create a vacuum-like phenomenon;

FIG. 3 is a wiring diagram showing the control circuit for operating the retraction of the grinding heads in unison; and

FIG. 4 is a diagrammatic schedule of the contact and status of the sparkout control device 4TR of FIG. 3.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is illustrated a longitudinal feed mechanism L and 10R, which advances an opposing pair of grinding heads 11L and 11R toward a thin workpiece W. EAch of the grinding heads 1 1L and 11R is slidably mounted on a machine bed 12. The grinding heads 11L and 11R include spindles 13L and 13R, which carry abrasive discs 14L and 14R. The abrasive discs 14L and 14R are driven through suitable connections by motors 161. and 16R.

The means for advancing the abrasive discs 14L and 14R toward one another consists of feed crews 17L and 17R mounted in the bed 12. The feed screws 17L and 17R are in operative engagement with feed nuts 18L and 18R, which are mounted in bed 12, and in alignment with the grinding heads 11L and 11R respectively.

The details of a preferred feed mechanism are disclosed in applicants co-pending application Ser. No. 5l,860, filed July 2, 1970, which is expressly incorporated herein. Accordingly, there will be described here, only the anti-vacuum control apparatus which cooperates with the feed mechanism to enable the grinding heads 11L and 11R to be easily retracted in unison, following the simultaneous removal of a predetermined amount of stock from the parallel faces or sides ,21 and 22 of the workpiece W. It is to be understood that the invention has utility with other feed mechanisms, and is not limited to the specific mechanism disclosed herein or in the above referenced pending patent application.

The workpiece W is held by a gun-type work carrier assembly 23 which is the subject of applicants U. S. Pat. No. 3,503,155, issued Mar. 31, 1970, which is expressly incorporated herein. The work carrier assembly 23 is secured to the bed 12 and includes a work fixture or carrier 24 having an opening 26, which retains the workpiece W in an accurate position within the carrier 24. The carrier 24 advances the workpiece W between the abrasive discs 14L and 14R by means of a fluid motor or cylinder 27. The cylinder 27 is mounted on top of a track unit 28 and includes a piston rod 29 which is secured to the carrier 24. Movement of the piston rod 29 advances and retracts the carrier 24 and the workpiece W into the grinding position 30, as shown in FIG. 2, and the abrasive discs 14L and 14R are advanced in unison. The carrier 24 is reciprocated during the grinding operation by movement of the piston rod 29 to effect the desired finish of the workpiece W and to promote uniform disc wear in a conventional manner.

The desired amount of liquid coolant from a supply source is delivered to the hollow spindles 13L and 13R by lines 31 and 32, respectively, for grinding purposes only. Also, coolant is supplied through a line 33 which is directed into the fixed hood 34, surrounding the abrasive discs 14L and 14R, for flushing swarf out of the hood. The volume of coolant directed through the hollow spindles 13L and 13R to the workpiece W must be sufficient to provide a cooling effect during the grinding operation and to remove metal and abrasive particles. When grinding thin workpieces W, too little coolant permits overheating and distortion of the workpiece W, and too much coolant results in hydrodynamicdisturbance of the thin workholding or guiding tooling, such as the carrier 24.

When the suitable amount of coolant is supplied to the workpiece W, a vacuum-like phenomenon occurs between the abrasive discs 14L and 14R, and apparently a partial vacuum is created between the abrasive disc 14L and 14R. This condition is believed to be caused by the volume of coolant and by the centrifugal force of the coolant flowing from the spindles 13L and 13R, which displaces the air between the cutting surfaces of the abrasive discs 14L and 14R, and possibly by the cohesive and adhesive forces of the coolant. The discs 14L and 14R appear to be drawn together by suction as a result of the above forces.

It is important that each of the abrasive discs 14L and 14R be retracted in unison at completion of the grinding cycle after the parallel sides of thin workpieces W have been ground, in order to maintain size, and because of the minimum clearance which exists between the sides of the carrier 24 and the cutting surfaces of the abrasive discs 14L and 14R;

Actual tests have been conducted that demonstrate the magnitude of the problem. Using 30 inch diameter discs 14L and 14R that were spaced 0.066 of an inch apart, tests were performed both with and without the carrier 24 and workpieces w positioned between the discs 14L and 14R. When the carrier 24 and workpieces w were in position, and with the discs 14L and R rotating in opposite directions the grinding heads 11L and 11R moved together as'much as 0.0017 of an inch (left) and 0.0025 of an inch (right) when full coolant flow began. This represents a possible error of 0.0042 of an inch in the thickness of a workpiece W which is significant when grinding workpieces W to to]- erances, such as 1*: 0.005 of an inch or less. When the discs 14L and 14R were rotated in the same direction there was slightly less movement.

Other tests were performed without the carrier 24 in place, but with the discs 14L and 14R again spaced 0.066 of an inch apart. With the discs 14L and 14R rotating in opposite directions, the grinding heads 11L and 11R moved together as much as 0.0032 of an inch (left) and 0.0035 of an inch (right) when full coolant flow began. This represents a possible error of 0.0067 of an inch in the thickness of a workpiece W. If the coolant flow was reduced, to say one quarter of normal full flow, there was also less movement of the grinding heads 11L and 11R. When the coolant was shut off entirely and with the discs 14L and 14R rotating, there was only a negligible amount of movement, such as 0.0001 of an inch on each of the grinding heads 11L and 11R, which could be attributable to indicator error.

When the grinding heads 11L and 11R are thus actually shifted toward each other, it should be apparent that there is substantial interference with the precise control by the feed system of the relative positions of the discs 14L and 14R. Precisely controlled relative positioning of the faces of the abrasive discs 14L and 14R becomes exceedingly critical when the prescribed tolerance range of a workpiece W is within close limits.

It should be understood that the width of the carrier 24 mustbe just slightly less than the width of the finish ground dimension of the workpiece W, because the carrier 24 remains between the cutting surfaces of the abrasive discs 14L and 14R during the entire grinding operation, as shown in FIG. 2. As an example, the

width of the carrier 24 may be 0.048 of an inch when a workpiece W. to be ground has a finished dimension of 0.054 of an inch. This provides a clearance of 0.003 of an inch between each side of the carrier 24 and the respective abrasive discs 14L and 14R. Also, the height of the carrier 24 is sufficient to withstand the torque which is applied through the workpiece W by each of the abrasive discs 14L and 14R, and to resist sidewise deflection, as much as is possible.

Deflection occurs when the hydrodynamic coolant pressure upon one side of the carrier 24 exceeds the corresponding pressure on the opposite side. The carrier 24 may then be deflected to contact one of the respective abrasive discs 14L or 14R, and overheating, buckling, and ultimate destruction may result.

The carrier 24 can also be damaged if one of the abrasive discs 14L or 14R drifts inwardly, during completion of the grinding cycle. This can occur if the grinding heads 11L or 11R are urged toward each other, as the ambient air pressure against the rear side of the abrasive discs 14L or 14R acts upon them in the manner of huge pistons to cause inward movement.

It should be understood that this condition exists only when thin workpieces are being ground. The abrasive discs 14L and 14R are normally rotated'in opposite directions to reduce the torque load on the carrier 24 when thin workpieces are being ground. However, the vacuum problem between the abrasive discs 14L and 14R still exists when the abrasive discs 14L and 14R are rotated in the same direction.

While tests have not been made to measure any movement of the grinding heads 1 1L and 1 1R when the instant invention is employed, actual tests have shown that the above problems are eliminated by employing this invention during a grinding operation.

The anti-vacuum control system shown in FIG. 2 includes a line 36 which is connected from a shop air supply 37 through a filter 43 to a line 46 having a metering valve 47 and a check valve 44. The metering valve 47 provides a continuous metered amount of air to the spindle 13R through a connection 41 of a rotary union 42. The check valve 44 prevents coolant from backflowing into the air control system. A control valve 38 is mounted in a line 39 which is connected between the lines 36 and 46. The valve 38 is used to supply an additional, unmetered amount of air to the spindle 13R before retraction at the completion of a grinding operation in order to facilitate the separation of the abrasive discs 14L and 14R.

It should be understood that in the preferred embodiment, the anti-vacuum system operates to inject a metered supply of air through the valve 47 throughout the entire grinding oepration to reduce or eliminate the partial vacuum formed in the area between the abrasive discs 14L and 14R. This is particularly important during the final portion of a grinding cycle, known as the sparkout" or dwell interval, when the abrasive discs 14L and 14R are neither being advanced nor retracted by the feed mechanism L and 10R. The partial vacuum which normally occurs between the closely spaced abrasive discs 14L and 14R is thus prevented from forming as the metered inflow of air remains continuous whenever coolant is being directed through the lines 31 and 32 to the spindles 13L and 13R. Inward movement of the abrasive discs 14L and 14R can then be more precisely controlled by the feed mechanisms 10L and 10R, and the abrasive discs 14L and 14R are more precisely advanced and stopped in unison without the undesirableforces resulting from the vacuum which interferes with such movement at completion of the sparkout operation positions.

As previously described, it is desirable that both of the abrasive discs 14L and 14R be retracted immediately and simultaneously at the conclusion of the sparkout operation. Therefore, just prior to the moment of retraction, the additional unmetered volume of air is injected into the fluid body between the abrasive discs 14L and 14R through the valve 38 which is at that moment, shifted to the right (FIG. 2) for a brief time interval. The sum effect of the two systems of air injection occurring during this interval prevents less-thanambient pressure frornbieng generated between the closely spaced discs 14L and 14R by the sudden separation of the discs. Also, the injected air appears to reduce the tenacious quality of the fluid coolant body in rotative circulation between the discs 14L and 14R.

OPERATION motors 16L and 16R. Coolant is being directed from a supply source to the hollow spindles 13L and 13R through the lines 31 and 32 respectively, in a conventional manner, as shown in FIG. 2. Check valves 48 and 49 are provided in lines 31 and 32 respectively to'prevent backflow of coolant. A constant volume of air is directed into the spindle 13R through the valve 47 in the line 46 to prevent the grinding heads 11L and 11R from drifting inwardly.

An unground workpiece W is placed into the opening 26 of the work carrier 24 and a -cycle startpushbutton IPB (FIG. 3)- is depressed. A circuit is completed through a -cycle stoppushbutton 2PB and a normally closed contact 4TR3, to energize a -cycle startrelay SCR, A sparkout clutch 4TR is energized which closed a contact 4TR1, and a -carrier advancerelay 6CR is energized through a limit switch contact lLSB which is normally closed.

The energization of the relay SCR closes a contact 5CR'1 which provides a holding circuit around the pushbutton IPB, after its release, to maintain energization of the relay SCR, the sparkout clutch 4TR, and the relay 6CR. A contact 6CR1 is closed which energizes a solenoid SOL C and the piston rod 29 advances the work carrier 24 and the workpiece W into the grinding position 30. A limit switch contact lLSA is closed which energizes a -carrier in limitrelay 4CR when the workpiece W is in its innermost position 30 and opens the limit switch contact lLSB which deenergizes the 'relay 6CR. A contact 4CR1 is closed to complete a cir- 6CR1 opens to deenergize the solenoid SOL C and the piston rod 29 retracts the work carrier 24.

The work carrier 24 and the workpiece W are reciprocated within a predetermined range as determined by reversal dogs (not shown) on the carrier 24 by actuation of the limit switch contacts lLSA and lLSB, of a limit switch lLS, and a limit switch 2LS, to provide an improved surface on the sides of the workpiece W during the grinding operation.

The abrasive discs 14L and 14R continue advancing toward each other while the work carrier 24 continues to reciprocate until the facecut infeed limit switches 3LS and 4LS are closed, and facecut infeed is terminated.

The closing of the limit switches 3LS and 4LS complete a circuit through the contact 4TR1 to energize a sparkout motor 4TR. Continued energization of the sparkout motor 4TR ultimately closes a timed contact 4TR4, which energizes an -air injectortimer relay 6TR, closing a -time delaycontact 6TR1 which energizes an -air injectorrelay 10CR. A contact 10CRl is closed which energizes a solenoid SOL F, and the valve 38 (FIG. 2) is shifted to the right.

An un-metered volume of air is now directed from the line 36, through the valve 38, and through the lines 39 and 46 into the hollow spindle 13R to supplement the continuous metered air flow already therein. The volume of air through the valve 38 is directed into the coolant with a sufficient force and suddenness to eliminate the tendency for any momentary vacuum to be formed between the abrasive discs 14L and 14R as they are being separated rapidly. Therefore, the abrasive discs 14L and 14R can be retracted moreassuredly in unison when workpiece size is reached at the end of the sparkout" operation.

The timed contacts 4TR1, 4TR2 and 4TR3 are simultaneously opened when the timed contact 4TR4 is closed when the sparkout clutch 4TR times out, as shown in FIG. 4. The opening of the contact 4TR1 deenergizes the sparkout motor 4TR.The opening of the contact 4TR2 deenergizes the relay 7CR, and the contact 7CR1 opens to deenergize the solenoid SOL D, which reverses the hydraulic pressure status in the facecut feed mechanisms 10L andlOR. The opening of the contact 4TR3 deenergizes the relay SCR. The contact SCRl opens to deenergize the sparkout clutch 4TR and the relay 6CR which permits the sparkout clutch 4TR to reset. The contact 6CR1 opens to deenergize the solenoid SOL C to efi'ect retraction of the work carrier 24 and the workpiece W from between the abrasive discs 14L and 14R, which are being retracted.

The abrasive discs 14L and 14R are retracted by the feed mechanisms 10L and 10R at a rapid rate. The

work carrier 24 and the workpiece W are then retracted. The solenoid SOL F is deenergized when the timer relay contact 6TR1 opens, after a time delay following retraction of the abrasive discs 14L and 14R which are retracted in unison. The valve 38 is then returned to its original position by spring pressure and the un-metered air supply is blocked until the above opera tion is repeated for the next workpiece W.

It is to be understood that the air of supply line 37 could be connected directly to one of the abrasive discs 14L or 14R rather than into one of the coolant lines. Also, it is not essential that air be supplied during the entire grinding operation. It is important, however, that air be supplied at a point in time which is early enough to prevent grinding to an undersize.

It is also to be understood that only a preferred embodiment of the invention has been specifically illustrated and described, and variations may be made thereto without departing from the invention, as defined in the appended claims. I claim:

1. A double disc grinding machine for abrading the parallel sides of a workpiece mounted on a workholder comprising a pair of abrasive discs each mounted on a rotatable spindle,

means for mounting said abrasive discs in coaxial spaced relation,

means for rotating said discs,

means for infeeding said discs to effect stock removal,

a source of liquid coolant,

means for directing the liquid coolant to at least one of the faces of said abrasive discs for cooling the workpiece during stock removal,

a source of pressurized air, and

means for selectively introducing a controlled amount of air from said air source into the liquid coolant within said directing means during a stock removal to substantially maintain the equality of the hydrodynamic forces acting on opposing sides of the wo rkholder.

2. A double disc grinding machine according to claim 1, further comprising means for preventing the passage of liquid coolant from said delivering means to said air source.

3. A double disc grinding machine according to claim 1, further comprising means for selectively introducing a volume of air into said directing means with sufficient force and suddenness at the conclusion of stock removal and prior to abrasive disc retraction to eliminate any vacuum existent between the abrasive discs.

Claims (3)

1. A double disc grinding machine for abrading the parallel sides of a workpiece mounted on a workholder comprising a pair of abrasive discs each mounted on a rotatable spindle, means for mounting said abrasive discs in coaxial spaced relation, means for rotating said discs, means for infeeding said discs to effect stock removal, a source of liquid coolant, means for directing the liquid coolant to at least one of the faces of said abrasive discs for cooling the workpiece during stock removal, a source of pressurized air, and means for selectively introducing a controlled amount of air from said air source into the liquid coolant within said directing means during a stock removal to substantially maintain the equality of the hydrodynamic forces acting on opposing sides of the workholder.

2. A double disc grinding machine according to claim 1, further comprising means for preventing the passage of liquid coolant from said delivering means to said air source.

3. A double disc grinding machine according to claim 1, further comprising means for selectively introducing a volume of air into said directing means with sufficient force and suddenness at the conclusion of stock removal and prior to abrasive disc retraction to eliminate any vacuum existent between the abrasive discs.

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