US3820439A

US3820439A - Tracer control circuit for machine tools

Info

Publication number: US3820439A
Application number: US00261591A
Authority: US
Inventors: S Cudnohufsky
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-06-12
Filing date: 1972-06-12
Publication date: 1974-06-28
Anticipated expiration: 1991-06-28
Also published as: JPS4950582A; DE2329420B2; DE2329420A1; GB1396061A; CA966758A; DE2329420C3

Abstract

A tracer control circuit for machine tools of the type having two perpendicularly related hydraulic cylinders for controlling longitudinal and transverse feed of a cutting tool. The circuit includes a tracer valve which is variably restricted in accordance with movement of a stylus following a template. The tracer valve is connected to a feed line from a high pressure pump which also extends to one of the cylinders. A feed line from a low pressure pump extends to the other cylinder. The drain lines from both cylinders extend to a stability valve, the outlet of which is connected to sump through a flow control valve for determining the maximum feed rate of both cylinders. The stability valve is designed to respond solely to the volume of flow permitted by the flow control valve to create the necessary pressure drop between the two drain lines to cause the stylus to accurately follow the contour of the template without oscillation.

Description

United States Patent |19| Cudnohufsky 1 June 28, 1974 TRACER CONTROL CIRCUIT FOR MACHINE TOOLS [76] Inventor: Sylvester Cudnohufsky, 1290 S.

Lake Angelus Dr., Pontiac, Mich. 48055 22 Filed: June 12, 1972 21 App]. No.1 261,591

Primary ExaminerEdgar W. Geoghegan Assistant Examiner-A. M. Zupcic Attorney, Agent, or FirmBames, Kisselle, Raisch &

Choate [5 7] ABSTRACT A tracer control circuit for machine tools of the type having two perpendicularly related hydraulic cylinders for controlling longitudinalv and transverse feed of a cutting tool. The circuit includes a tracer valve which is variably restricted in accordance with movement of a stylus following a template. The tracer valve is connected to a feed line from a high pressure pump which also extends to one of the cylinders. A feed line from a low pressure pump extends to the other cylinder.

[56] References cued The drain lines from both cylinders extend to a stabil- UNITED STATES PATENTS ity valve, the outlet of which is connected to sump 2,940,263 6/ I960 Cudnohufsky 91/37 through a flow control valve for determining the maxi- 2,960,l09 l l/l960 Wilson l38/46 mum feed rate of both cylinde s The tability valve is 3,093,159 6/1963 Royle 138/46 designed to respond solely to the volume of flow per 3,104,591 7/1963 Cudnohufsky mined by the flow control Valve to create the neces g g g fk' sary pressure drop between the two drain lines to u no u S y cause the stylus to accurately follow the contour of the template without oscillation. 17 Claims, 9 Drawing Figires 66 w az 60 /o r r i- 7 p 6 fix J .L Hm 9/3 I2 I; 58 9a /06 4 M 31 M 96 e m /:?4 j 94 3 5- we L EE' 'J E TXIIE hi5)? -72 11. 08 7 P /ZZ e2 ggrg uz //2 /l0 76 J PATENTEDJUHZB mm SHEET 1 [IF 7 PAn-imt'nm mw 3820 439 SHEET 3 [IF 7 TRACER CONTROL CIRCUIT FOR MACHINE TOOLS This invention relates to improvements in tracer control circuits for governing the automatic operation of pattern controlled machine tools.

More specifically, the present invention has to do with the general type of tracer control circuits disclosed in my prior U.S. Pat. Nos. 2,940,263, dated June 14, 1960 and No. 3,104,591, dated Sept. 24, 1963.

Many tracer control circuits of the prior art rely upon the use of one or more valves responsive primarily to pressure for controlling the feed rate of longitudinal and transverse feed cylinders. Normally the reliance primarily on the use of pressure responsive control valves for controlling the relative rates of movement of two angularly related feed cylinders is undesirable for several reasons. For example, if the viscosity of the oil in the system changes the flow rate of the cylinders will change unless adjustments are made to compensate for the change in oil viscosity. Thus, when operation of a tracer controlled machine is initiated, the machine must be operated in idling condition for a predetermined time period to permit the oil to reach the temperature and viscosity for which the controls are set. If in the course of operation a heat exchanger should fail to function properly and, thus, result in a change in the oil viscosity, a feed rate varying from the desired predetermined rate will result.

In a tracer control circuit where the feed rates are determined by valves which are primarily responsive to pressure, numerous valves and other components must be employed in the circuit to achieve proper operation and the adjustment of these valves is very critical in order to obtain a smooth (as distinguished from a rippled) finish on the part being machined during particular modes of operation such as when machining a radius or a tapered or inclined surface that involves the simultaneous feeding of both cylinders.

The primary object of thepresent invention is to simplify substantially the design of tracer control circuits and reduce the number and types of valves required by employing a valve connected to the feed cylinders, the valve being responsive primarily to the rate of flow rather than the pressure of the oil in the circuit to control the rate of feed of the two cylinders.

A further object of the invention resides in the provision of a tracer control circuit utilizing a minimum of hydraulic control components and eliminating the need for critical adjustments of valves, etc. in order to obtain a fine finish on the part being machined during the various modes of operation.

The present invention generally contemplates a tracer control circuit employing a high pressure pump and a low pressure pump. The high pressure pump is connected to a tracer valve which is variably restricted by the action of a stylus engaging a contoured template. The pressure created by the restriction in the tracer valve is exerted against one of the cylinders while the other cylinder is connected to the low pressure pump. The drain lines from the two cylinders are connected to a stability valve which is designed to respond primarily to the rate of flow from the two cylinders and direct the oil from the two drain lines to sump through a pressure compensated flow control valve.

Other objects and features of the present invention will become apparent from the following description and drawings, in which:

FIG. 1 is a diagrammatic view of a tracer control circuit embodying the present invention, the various components being shown in the positions assumed when the circuit is energized and running idly;

FIG. 2 is a view similar to FIG. 1 showing the control circuit conditioned to cause the tracer stylus to approach the template;

FIG. 3 is a view similar to FIG. 1 showing the control circuit conditioned for initiating the tracing cycle and with the tracer stylus engaging an axially or longitudinally extending edge portion of the template;

FIG. 4 is a view similar to FIG. 3 showing the control circuit operating with the tracer stylus retracting along a radial or transverse shoulder on the template;

FIG. 5 is a view similar to FIG. 3 showing the control circuit operating with the tracer stylus retracting along a surface on the template which is inclined in both a radial and an axial direction;

FIG. 6 is a view showing the tracer control circuit conditioned for retracting the stylus away from the template back to a starting position;

FIG. 7 is a sectional view of the stability valve employed in the tracer control circuit of the present invention, the valve spool being shown in the position it assumes while the tracer control circuit is operating under the conditions illustrated in FIGS. 1 and 2;

FIG. 8 is a view similar to FIG. 7 and showing the valve spool in the position it assumes at a predetermined flow rate when the circuit is operating under the conditions illustrated in FIGS. 3, 4 and 5;

FIG. 9 is a view similar to FIG. 8 and showing the position of the spool valve at a flow rate higher than that illustrated in FIG. 8.

In the drawings the tracer valve is generally designated l0 and has a finger or stylus l2 pivotally mounted thereon for tracing the contour of a pattern or template 14, which contour is desired to be machined on a workpiece. Tracer valve 10 is of the type disclosed in my prior US. Pat. No. 2,940,263 and need not be described in detail. For the purpose of this description it will suffice to state that valve 10 has a restrictable orifice 13 therein and the degree of restriction of orifice 13 depends upon the pivotal movement of the stylus 12. In the embodiment illustrated the hydraulic pressure within valve 10 tends to pivot stylus 12 in a clockwise direction to reduce the restriction at orifice l3. Pivotal movement of stylus 12 in a counterclockwise direction increases the restriction in valve 10.

To illustrate the operation of the improved control circuit of this invention it may be assumed that the tracer is used on a lathe. On a machine tool such as a lathe the template 14 would be preferably mounted in a fixed position on the bed of the lathe and tracer valve 10 would be preferably mounted on the cross slide of the lathe which is generally designated 16. A cylinder 18 mounted on the carriage 20 of the lathe has a piston 22 connected by a rod 24 to cross slide 16. A cylinder 26 mounted on the bed of the lathe has a piston 28 connected to carriage 20 by a rod 30. Actuation of piston 28 causes cross slide '16 and stylus 12 to be fed in a longitudinal or axial direction while actuation of piston 22 in cylinder 18 causes movement of cross slide 16 and stylus 12 in a transverse or radial direction normal to the longitudinal feed direction. The cutting tool is diagrammatically shown at 32 mounted on cross slide 16.

The source of pressure hydraulic fluid for operating the two cylinders comprises two

pumps

34 and 36. Pump 34 may be referred to as the low pressure pump capable of developing a relatively low substantially constant pressure. Pump 34, for example, is capable of delivering or 6 gallons of oil per minute at a pressure of about 400 pounds per square inch. Pump 36 is referred to as the high pressure pump capable of instantly developing a relatively high pressure. For example,

pump 36 is capable of delivering 5 or 6 gallons of oil per minute at a pressure of about 1000 pounds per square inch. Both pumps are driven by an electric motor 38.

In the circuit illustrated the means for controlling the direction of flow of hydraulic fluid through the circuit comprises three solenoid controlled, pilot operated

valves

40, 42 and 44. The pressure port 46 of valve is connected to the outlet of low pressure pump 34 by a hydraulic line 48. The tank port 50 of valve 40 is connected to sump as at 52. As illustrated, valve 40 may be a four-way, three-position valve. Port 54 of valve 40 is connected by a feed line 56 to a pressure port 58 of 25 valve 44. Port 60 of valve 40 is connected by a drain line 62 with the exhaust port 64 of tracer valve 10. The inlet port of tracer 10 (designated 66) is connected to high pressure pump 36 by a feed line 68. A branch line 70 from feed line 68 connects with a line 72 which extends between the tank port 74 of valve 42 and drain line 62. A one-way check valve 76 is located in line 72 between its connection with branch line 70 and drain line 62. Port 80 of valve 42 connects with the rod end of cylinder 18 by a line 81 and port 82 of valve 42 connects with the head end of cylinder 18 by a line 83.

Port 84 of valve 42 is connected by a line 86 to port 88 of a valve 90 which is pilot operated in response to a particular position of the spool of valve 40. The broken line 92 illustrates the connection between

valves

40 and 90 for pilot operation of the latter. The opposite port 94 of valve 90 is connected by a line 96 with a tank port 98 of valve 44. Port 100 of valve 44 is connected to the head end of cylinder 26 by a line 102 and port 104 of valve 44 is connected to the rod end of cylinder 26 by a line 106. Valve 90 includes a passageway'108 which bypasses the spool of the valve and incorporates a one-way check which permits free flow in a direction from port 94 to port 88, but which blocks the flow in the opposite direction.

The circuit of the present invention also includes a stability valve 112 which is shown diagrammatically in FIGS. 1 through 6 and which is shown in section in FIGS. 7 through 9. Valve 112 includes a body 114 formed with a cylindrical bore 116 in which there is arranged for axially sliding movement a spool 118. Spool 118 is biased axially toward one end of bore 116 by a ,relativelylight compression spring 119 so that in the absence of flow through valve 112 spool 118 assumes the position at one end of bore 116 shown in FIG. 7 regardless of the pressures obtaining at the various ports of the valve. Body 114 of valve 112 has three ports therein communicating with bore 116. The first port 120 is connected by a line 122 with line 86 between port 84 and valve 42 and port 88 on valve 90. The second port 124 on valve 112 is connected by a line 126 with line 96 between port 94 of valve 90 and port 98 of valve 44. The third port 128 is connected to a drain line 130 which extends to the inlets of a plurality of pressure compensated, flow control valves. In the illustrated embodiment of the control circuit two such pressure compensated, flow

contorl valves

132 and 134 are shown. As a practical matter a series of perhaps three or four such flow control valves would be utilized in the circuit, but for the purposes of illustration only two such valves are shown. Valves 132,134 may be of the type illustrated in my prior US. Pat. No. 3,524,386. For the purpose of this description, however. it will suffice to state that each of these valves are adjustable to control the quantity of flow therethrough and are controlled by solenoid valves 136,138, respectively, so as to either permit flow to occur through either of these valves or to block the flow through either of these valves. In the condition of the circuit shown in FIGS. 1 and 2 both of the valves are blocked to prevent flow therethrough by reason of pressure directed from a pressure source through the respective valves. Valves 132,134 are set to control the maximum rate of flow from port 128 of valve 112. For example, valve 132 may be adjusted to produce a maximum flow of oil of one gallon per minute and valve 134 may be adjusted to produce a flow of two gallons per minute. In the description that follows the operation will assume a desired flow of 1 gallon per minute. Thus, flow control valve 134 will aways be blocked by the pressure from the source 140 through the solenoid valve 138 and the flow through valve 132 will either be blocked or permitted, depending upon the actuation of solenoid valve 136. In FIGS. 1, 2 and 6 solenoid valve 136 is shown blocking valve 132 and in FIGS. 3, 4 and 5 solenoid valve 136 is shown in a condition permitting flow through valve 132 to sump 52.

Since

pumps

34 and 36 are capable of delivering a higher rate of flow than permitted by

valves

132, 134, a relief valve 142 is connected with feed line 48 from low pressure pump 34 and a relief valve 144 is connected with feed line 68 from high pressure pump 36. In the event that the pressure in these two feed lines exceeds the settings of the respective relief valves 142,144 the excess oil will be directed to sump.

Referring now specifically to FIGS. 7 and 8, port 124 of stability valve 112 is located at the end of bore 116 against which spool 118 is normally biased by spring 119. A riser 146 at the end of spool 118 prevents the spool from blocking port 124. The spool is provided at one side thereof with an axially extending slot 148 engaged by a pin 150 on the body of the spool to maintain the spool in a position fixed against rotation as it slides axially in bore 116. In the embodiment illustrated

ports

120 and 128 are located on diametrically opposite sides of the body. A groove 152 in spool 118 registers with port 128 at all times and extends axially from one end of the spool to the other so that the opposite end portions of bore 116 are in direct communication at all times. Thus, neglecting the slight bias of light spring 119, the pressures obtaining at opposite ends of the spool in bore 116 are substantially the same. Groove 152 may have one portion 154 of uniform cross section and a second portion 156 adjacent the end of bore in which port 124 is located of progressively increasing cross section. I

In the embodiment illustrated groove 152 is assumed to be of generally rectangular shape in cross section and the slope of the inclined inner side 158 is such that spool 118 is shifted axially away from the end of the bore containing port 124 in response to an increase in the rate of flow through port 128 as determined by the setting of pressure compensated, flow control valves 132,134. At any given flow rate through port 128 the position of spool 118 within bore 116 is solely dependent on the viscosity of the oil and completely independent of the pressures obtaining at

ports

120 and 124 by reason of the hereinafter mentioned tapered passageway 162. Thus, if flow control valve 132 is operative to permit a flow of one gallon per minute through port 128, spool 118 will assume the axially shifted position shown in FIG. 8 regardless of the pressure at

ports

120 and 124. On the other hand, if flow control valve 132 is rendered inoperative and flow control valve 134 is rendered operative so that a flow of two gallons per minute is permitted at port 128, spool 118 will shift to the position illustrated in FIG. 9 regardless of the pressure conditions obtaining at

ports

120 and 124. Likewise, with a constant flow rate as determined by

valves

132 or 134 spool 118 will shift axially in response to changes in oil viscosity.

Diametrically opposite groove 152, spool 118 is provided with a fiat surface 160 inclined to the axis of the spool and cooperating with bore 116 to define a tapered passageway 162 communicating with port 120. Surface 160 has an axial extent such that even when spool 118 is located at the end of bore 116 in the position shown in FIG. 7 the passageway 162 is connected to port 120 by a small orifice 164 which in size may be equivalent to a 0.030 inch diameter orifice. Thus, the pressure in bore 116 is at all times reflected by the pressure at port 120 since bore 116 is at all times in communication with port 120 through orifice 164. However, in response to a flow of hydraulic fluid at port 128 spool 118 is shifted axially as described above and as shown in FIG. 8, for example, so that orifice 164 becomes of increasingly larger size as the spool is shifted further against the bias of spring 119. Passageway 162 is detennined in size and shape so that at a given pressure at port 120 and a given pressure at port 124 the pressure drop at orifice 164 will be substantially constant regardless of the rate of flow through port 128. For example, flat surface 160 is inclined to the axis of the spool such that if the pressure at port 120 is 700 pounds per square inch and the pressure at port 124 is 400 pounds per square inch, orifice 164 is sized to produce a pressure drop between

ports

120 and 124 of 300 pounds per square inch regardless of whether the spool is in the position shown in FIG. 8 or in the position shown in FIG. 9. This feature of stability valve 112 results in a uniform smooth operation of the tracer irrespective of the feed rates of the two

cylinders

18 and 26.

For the purposes of simplification, the oil flow lines of the control circuit are shown as single solid lines. However, the oil lines which are operative during any particular portion of the cycle are illustrated in the drawings in heavy lines while the oil lines which may be considered as inoperative or idle during any particular portion of the cycle are shown in light lines.

The operation of the tracer as controlled by the circuit illustrated in FIGS. 1 through 6 will now be described. Referring first to FIG. 1, the circuit is there illustrated in the energized idling condition. The circuit is set in this condition by closing a main switch (not illustrated) which energizes electric motor 38 that drives pumps 34,36. In this condition valve 40 is spring centered so that the oil from the low pressure pump is caused to flow directly to sump through pressure relief valve 142. Oil from high pressure pump 36 flows through feed line 68 to the inlet port 66 of tracer valve 10. Since orifice 13 is wide open at this time and since the flow through flow control valve 132 is blocked, all of the oil delivered by pump 36 is directed to the tracer valve and back to sump through drain line 62 and through

ports

60 and 50 of the center section of valve 40. In the idling

condition valves

42 and 44 are deenergized so that no oil is permitted to flow to either cylinder 18 or cylinder 26.

When it is desired to have the stylus approach the template an approach switch (not illustrated) is depressed. This in turn energizes the proper solenoid to shift the spool of valve 40 to the right and shift the spool of valve 42 to the left, the positions indicated in FIG. 2. The solenoid of valve 136 remains de-energized so that no flow is permitted through flow control valve 132. With the valves in these positions low pressure pump 34 delivers oil at low pressure through feed lines 56 to the interconnected ports 58,59 of valve 44, across check valve 110 and to the head end of cylinder 18 through

oil lines

86,83. The rod end of cylinder 18 discharges through lines 81 and, since this oil is prevented from flowing directly to drain line 62 by reason of check valve 76, the oil from the rod end of cylinder 18 flows through line and merges with the oil being delivered through feed line 68 from pump 36 to the inlet of tracer valve 10. Since the stylus 12 has not yet contacted the template the orifice in tracer' 10 is wide open and accommodates substantially the full flow of oil through drain line 62 and back to sump 52 through

ports

60,50 of valve 40. Under the idle conditions of FIG. 1 the restriction at orifice 13 in tracer valve 10 is very little so that the pressure in feed line 68 may be a relatively low value, for example, pounds per square inch. However, in the approach condition illustrated in FIG. 2, since a greater quantity of oil is flowing to the inlet of tracer valve 66 the pressure in the feed lines and in cylinder 18 would be at a slightly higher value, for example, 200 pounds per square inch. For the purpose of this description we shall neglect the load imposed on the cylinders by carriage 20 and cross slide 16 and we will also neglect the difference in the effective areas of the opposite sides of

pistons

28 and 22. In any event, the pressure at the head end of cylinder 18 is slightly higher than the pressure at the rod end of cylinder 18 and the cross slide 16 is advanced so that stylus l2 approaches template 14 and cutting tool 32 approaches the work (not shown).

Eventually stylus 12 engages the template and is pivoted thereby to immediately increase the restriction at orifice 13in tracer valve 10. By reason of this increased restriction the pressure in feed line 68 immediately rises to a substantially higher value (for example, 400 pounds per square inch) and a pressure switch (not shown) in line 68 energizes valve 44 to shift, the spool thereof to the right as shown in FIG. 3. Thus, feed line 56 is connected to the rod end of cylinder 26 and drain line 102 from the head end of cylinder 26 is connected with line 96 through passageways in the spool of valve 44.

Assuming for the moment that valve 136 remains in the position shown in FIGS. 1 and 2 so that flow control valve 132 remains blocked, then the head end of cylinder 26 is connected directly to the head end of cylinder 18 when the stylus engages the template. By reason of the variable restriction of the orifice in tracer valve produced by pivotal movement of stylus 12, the pressures on the opposite sides of piston 22 in cylinder 18 immediately substantially equalize so that the stylus remains stationary on the template and the pressure on the opposite sides of piston 28 in cylinder 26 are substantially equalized so that longitudinal feed is not initiated.

When stylus 12 engages the template valve 136 can be actuated either manually or automatically to allow flow through flow control valve 132. As soon as flow control valve 132 is opened oil from the head end of cylinder 26 is permitted to flow to port 124 of stability valve 112. However, since the pressure at port 124 and the pressure on the opposite sides of piston 22 and cylinder 18 remain at approximately the same value (for example, 400 pounds per square inch as determined by the setting of relief valve 142), there is no movement of the cross slide, but the differential pressure across piston 28 of cylinder 26 causes carriage to move to the left, thus producing a flow at port 124 of stability valve 112. Spool 118 thus shifts from the position shown in FIG. 7 to the position shown in FIG. 8 in response to the flow of oil through passageway 152. Spool 118 shifts upwardly so that the flow through port 128 is sufficient to supply the 1 gallon per minute flow to which flow control valve 132 is adjusted. Orifice 164 between passageway 162 and port 120 is slightly open and the pressure at these ports is substantially the same. Thus, for all intents and purposes,

ports

120 and 124 are externally connected, but, since orifice 164 is a very small opening (the degree of taper 160 being shown highly exaggerated in FIGS. 7 9), it prevents oscillation of stylus 12 and all the oil discharging from port 128 enters the stability valve at port 124. The pressure at port 124 balances the pressure on the downstream side of orifice 164 to prevent flow at port 120. Thus, the stylus follows the longitudinally extending surface 166 of template 14 very closely without oscillating and, thereby, the cutting tool 32 produces a fine finish on the workpiece. In view of the fact that the position to which spool 118 of valve 112' is shifted axially varies with the 'flow rate through passageway 152, the feed rate of carriage 20 to the right is independent of the viscosity or temperature of the oil.

Eventually the stylus 12 engages the right angle shoulder 168 and immediately produces a substantially greater degree of restriction at orifice 13 in tracer valve 10. The back pressure in feed line 68 immediately increases to a substantially high value (for example, 700 pounds per square inch) which is directed through valve 42 to the rod end of cylinder 18..Again neglecting the load on the carriage and cross slide and the difference in the areas of opposite sides of the pistons, piston 22 in cylinder 18 immediately retracts discharging oil from the head end of cylinder 18 through drain 83 at a relatively high pressure (close to 700 pounds per square inch). This oil flow at high pressure from the head end of cylinder 18 is directed to port 120 of stability valve 112. As mentioned previously, the tapered face 160 on spool 118 in stability valve 112 is designed so that the size of orifice 164 under this condition will produce a pressure drop of about 300 pounds per square inch (based upon the pressure example given above.) Thus the pressure of 400 pounds per square inch obtaining at port 124 is balanced by the 400 pound pressureon the downstream side of orifice 164 so that all the oil discharged through port 128 is now flowing into the stability valve through port and, while the pressure at port 124 remains the same, there is actually no flow of oil through this port. Thus the crossslide retracts smoothly in a straight line as the stylus follows the transverse surface 168 of the template.

While passageway 162 is illustrated as being defined by a flat tapered surface on the periphery of spool 118, it will be appreciated that it could also be formed as a tapered groove in the periphery of the spool. The size of the orifice in passageway 162 at port 120 varies in accordance with the axial position of the spool. The required size of the orifice for various positions of the spool can be determined mathematically or by the cut and try methods. It is important, however, that when the tracer stylus is tracing a straight shoulder (such as shown at 168) that the pressure drop across the orifice is such that all of the oil entering port 120 of the stability valve is discharged at port 128. If the pressure drop across orifice 164 is not sufficiently high there would be a tendency to discharge oil through port 124 and this in turn would tend to cause the stylus to move slightly to the right away from shoulder 168 of the template, thus, decreasing the restriction at the tracer valve and producing a new balanced condition. This, however, will produce a slight oscillation of the stylus and correspondingly produce a slight ripple in the finish produced on the machined part. The size of orifice 164 under operating conditions is based upon the operating pressure selected for the two cylinders. With the values given above (namely, above 400 pounds per square inch at the carriage cylinder and 700 pounds per square inch at the cross slide cylinder), when the stylus finger is following a straight shoulder, the orifice should be designed to produce a pressure drop of about 300 pounds per square inch so that there will be absolutely no flow of oil at port 124.

Referring now to FIG. 5, there is illustrated a condition wherein the template is provided with surface inclined at about an angle of 45 to the longitudinal axis. When tracing a surface of this configuration it will be appreciated that both the carriage cylinder and the cross slide cylinder must be actuated. As the stylus encounters this inclined surface, which is designated 170 in FIG. 5, the restriction in the tracer valve is less than the restriction produced when the stylus finger is tracing the square shoulder 168 shown in FIG. 4. Since the restriction is less, the back pressure in feed line 162 will drop to a valve less than 700 pounds per square inch (for example, to about 550 pounds per square inch). This pressure will appear at port 120. However, under this condition the pressure at port 124 from the carriage cylinder as developed by pump 34 will remain at about 400 pounds per square inch. Passageway 162 is designed such that with a pressure of 400 pounds per square inch at port 124 and a pressure of about 550 pounds per square inch at port 120, one-half of the oil discharged through port 128 is entering the stability valve through port 124 and the other half is entering through port 120. Thus, assuming that both cylinders are of the same size, the longitudinal feed rate of carriage 20 will be exactly the same as the transverse feed rate of cross slide 16. This is true because under these conditions (a lower pressure at port 120) the orifice 7 9 would result from blocking the flow at flow control valve 132 and permitting the flow through flow control valve 134. Regardless of the position of spool 118, passageway 162 is designed to produce the same pressure drop for the same pressure differential between

ports

120 and 124. Thus the feed rate of the two cylinders can be varied at will without any adjustment in any of the hydraulic components in order to avoid instability and oscillation of the tracer stylus. Accordingly, the feed rate can be changed during a machining operation, depending upon the diameter of the part being machined, so as to obtain approximately the same surface speed per minute. This enables the machining of a fine finish along portions of different diameters so as to eliminate the need for grinding after the part is turned on a lathe.

In practice the solenoids of

valves

136, 138 and of additional such valves, if employed, can be actuated by limit switches which are tripped as the carriage reaches certain predetermined positions where the diameter of the part may change.

When the machining operation is completed a retract button or switch is actuated either manually or automatically to shift the spool of valve 40 to the position shown in FIG. 6. When this occurs, pilot pressure in pilot line 92 shifts the spool of valve 90 so as to directly connect the head end of cross slide cylinder 18 with the head end of carriage cylinder 26. At the same time valve 136 is actuated to block the flow through flow control valve 132. Under this condition oil from both

pumps

34,36 is directed to the rod end of cross slide cylinder 18 so as to retract the stylus. At the same time all of the oil discharged from the head end of cross slide cylinder 18 is caused to flow directly to the head end of carriage cylinder 26. The oil in the rod end of carriage cylinder 26 is discharged directly to sump through valve 40 without restriction. Thus the cross slide and the carriage retract at substantially the same rate which in effect produces retraction of the cross slide at about a 45 angle to the longitudinal as indicated by the arrow 172 in FIG. 6. It will be appreciated that, since the oil supplied by both pumps is directed through both cylinders without restriction, the retraction stroke is relatively rapid as compared to the feed stroke. It will also be appreciated that suitable switches can be located so as to be actuated when either the carriage or cross slide reaches its fully retracted position. The location and utilization of such switches are obvious and need not be described. It will sufi'iceto say that, if the cross slide engages i'ts limit switch first, then valve 42 will be actuated to shift the spool to the position shown in FIG. 1 so that the oil from both pumps will thereafter be directed exclusively to the carriage cylinder. On the other hand, if the carriage engages its limit switch first, then valve 44 will be actuated to shift its spool to the position shown in FIG. 1 so that all of the oil from both pumps will be directed to the cross slide cylinder.

Thus it will be seen that the control circuit disclosed herein incorporates a minimum number of valves and the utilization of the stability valve 112 not only eliminates oscillation of the stylus, but also eliminates the need for accurate readjustment of numerous valves in response to a change in the viscosity and/or temperature of the oil flowing through the circuit.

I claim:

1. In a tracer control circuit for governing the operation of a pair of feed cylinders arranged to feed a cutting tool in angularly related directions, the combination comprising two sources of hydraulic pressure, one being capable of delivering a substantially higher pres sure than the other, a tracer valve, a feed line connecting the tracer valve with the source of higher pressure, a pattern controlled actuating means to control the tracer valve so as to variably restrict the flow therethrough in accordance with the contour of the pattern being traced and thereby vary the pressure in said feed line, means connecting the feed line with one end of said cylinders, a feed line extending from said source of lower pressure to one end of the other cylinder, the other ends of the two cylinders each having a drain line connection thereto, a stability valve comprising a body having a bore therein and a spool shiftable axially in said bore, said body havin three ports therein communicating with said bore, the first port being located axially between the opposite ends of said bore and connected to one of said drain lines, the second port being connected to the other drain line and communicating directly with one end of said bore, the third port being located axially between the ends of said bore and being connected to sump through a flow control valve means, said third port being arranged such that all of the fluid entering said bore through the first and second ports is discharged through said third port, said spool having a first passageway therein establishing communication between said third port and said one end of said bore, means establishing communication between the opposite ends of said bore so as to substantially equalize the pressure therein, said spool being shiftable axially in said bore to a predetermined position therein in response to the volume of flow through said passageway as permitted by said flow control means, said spool having a second passageway therein establishing communication between said first port and said second port, said second passageway forming an orifice of predetermined size between said first and second ports in response to location of said spool in said predetermined axial position to produce a pressure drop between said first and second ports in response to a differential pressure between said first and second ports resulting from a differential pressure between said two drain lines, the pressures obtaining in said two drain lines being in opposed communicating relation through the orifice in said last-mentioned passageway.

2. A tracer control circuit as called for in claim 1 wherein said flow control valve means are designed to two drain lines and thereby vary the feed rate of the two cylinders.

4. A tracer control circuit as called for in claim 2 wherein the first passageway is designed to shift the spool axially in response to an increase or decrease in the flow rate at said flow control valve means to increase and decrease, respectively, the size of the orifice in said second passageway.

5. A tracer control circuit as called for in claim 4 wherein the second passageway is designed to vary the size of the orifice therein to produce substantially the same pressure drop across the orifice irrespective of the flow rate at said flow control valve means when the pressures in the two drain lines are at correspondingly predetermined values.

6. A tracer control circuit as called for in claim 2 including relatively light resilient means biasing said spool toward said one end of said bore.

7. A tracer control circuit as called for in claim 6 wherein said first passageway extends axially between opposite ends of the spool so that, neglecting the effect of said light resilient biasing means, the pressures obtaining at the opposite ends of the bore are substantially the same when the flow rate at said third port is at a constant value.

8. A tracer control circuit as called for in claim 6 wherein said first passageway has a portion of tapered decreasing cross section in a direction axially away from said one end of said bore and said third port is located so as to register with said tapering portion of said passageway when a predetermined minimum flow is established through said stability valve.

9. A tracer control circuit as called for in claim 8 wherein said second passageway has a portion of tapered decreasing cross section in a direction axially away from said one end of said bore, said first port registering axially with said tapered portion of said second passageway to form said orifice which increases in size as the spool moves axially away from said one end of said bore in opposition to said biasing means in response to the rate of flow in the first passageway.

10. A tracer control circuit as called for in claim 9 wherein said second passageway comprises a flat surface on the periphery of the spool, said surface being inclined to the axis of the spool.

11. A tracer control circuit as called for in claim 1 wherein said first port is connected to the drain line extending from said cylinder connected to the high pressure feed line and the second port is connected to the drain line extending from the cylinder connected to the low pressure feed line.

12. A tracer control circuit as called for in claim 1 including means for blocking the flow through said flow control valve means to sump and valve means for connecting said drain lines directly to sump to produce a feed rate in either or both of said cylinders greater than the feed rate obtainable when the flow from the drain line is directed through said stability valve and said flow control valve means.

13. A tracer control circuit as called for in claim 1 wherein said two drain lines are also connected together through a one-way check valve which permits flow therethrough when the pressures in the drain line extending from the cylinder connected with the high pressure feed line is less than the pressure in the feed line connected to the low pressure cylinder.

14. In a tracer control circuit for governing the operating of a pair of hydraulic feed cylinders arranged to feed a cutting tool in angularly related directions, said circuit being of the type having two sources of hydraulic pressure, one being capable of delivering a substantially higher pressure than the other, a tracer valve, a feed line connecting the tracer valve with the source of higher pressure, a pattern controlled actuating means to control the tracer so as to variably restrict the flow therethrough and thereby vary the pressure in said feed line, means connecting the feed line extending from the source of lower pressure to one end of the other cylinder, the other of said two cylinders having a drain line extending therefrom, that improvement which comprises a stability valve having a body provided with a bore and an axially shiftable spool in said bore, said body having three ports therein communicating with said bore, a first port adapted to be connected to the drain line extending from the high pressure cylinder, a second port adapted to be connected with the drain line extending from the low pressure cylinder and a third port connected to sump through a flow control valve means for controlling the volume of flow from said drain lines through said valve and to sump, said spool having a first passageway therein extending from one end thereof and establishing communication between said third port and one end of said bore, said spool having a second passageway therein establishing communication between said first and second ports, said first and third ports being located axially between the end of said bore and said second port being located adjacent said one end of the bore such that it is always in communication with said first and third ports through said two passageways, said first passageway being adapted to shift said spool axially in a direction away from said one end of said bore to a predetermined position in response to a predetermined rate of flow from said one end of said bore to said third port, said second passageway communicating with said first port to define an orifice therein which varies in size with the extent to which said spool is shifted axially away from said one end of said bore.

15. That improvement called for in claim 14 wherein said second passageway is adapted to produce substantially the same pressure drop across said orifice in response to the same pressure differential between said first and second ports irrespective of the axial position of said spool.

16. A control circuitas called for in claim 14 wherein each of said passageways has a portion of tapered cross section with a decreasing area in a direction axially away from said one end of said bore.

17. A control circuit as called for in claim 14 including lightly resilient means biasing said spool toward said one end of said bore.

:UNlTE'D STATES PATENT OEFICE CERTIFICATE OF CORRECTION Patent No, D t d June 28, Inventor) CUDNOHUFSKY, Sylvester R.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 10, Line ZO After "of" insert -'one of Column 12, Line 16 After "feed line" insert with one end of one of said cylinders, a feed line Signed and loalod this 29th day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents

Claims

1. In a tracer control circuit for governing the operation of a pair of feed cylinders arranged to feed a cutting tool in angularly related directions, the combination comprising two sources of hydraulic pressure, one being capable of delivering a substantially higher pressure than the other, a tracer valve, a feed line connecting the tracer valve with the source of higher pressure, a pattern controlled actuating means to control the tracer valve so as to variably restrict the flow therethrough in accordance with the contour of the pattern being traced and thereby vary the pressure in said feed line, means connecting the feed line with one end of said cylinders, a feed line extending from said source of lower pressure to one end of the other cylinder, the other ends of the two cylinders each having a drain line connection thereto, a stability valve comprising a body having a bore therein and a spool shiftable axially in said bore, said body havin three ports therein communicating with said bore, the first port being located axially between the opposite ends of said bore and connected to one of said drain lines, the second port being connected to the other drain line and communicating directly with one end of said bore, the third port being located axially between the ends of said bore and being connected to sump through a flow control valve means, said third port being arranged such that all of the fluid entering said bore through the first and second ports is discharged through said third port, said spool having a first passageway therein establishing communication between said third port and said one end of said bore, means establishing communication between the opposite ends of said bore so as to substantially equalize the pressure therein, said spool being shiftable axially in said bore to a predetermined position therein in response to the volume of flow through said passageway as permItted by said flow control means, said spool having a second passageway therein establishing communication between said first port and said second port, said second passageway forming an orifice of predetermined size between said first and second ports in response to location of said spool in said predetermined axial position to produce a pressure drop between said first and second ports in response to a differential pressure between said first and second ports resulting from a differential pressure between said two drain lines, the pressures obtaining in said two drain lines being in opposed communicating relation through the orifice in said lastmentioned passageway.

2. A tracer control circuit as called for in claim 1 wherein said flow control valve means are designed to accommodate variable predetermined flow rates to increase or decrease the rate of flow through said two drain lines and thereby vary the feed rate of said two cylinders.

3. A tracer control circuit as called for in claim 1 wherein said flow control valve means comprises a plurality of flow control valves of different flow rates and means for rendering each of said flow control valves selectively operable to vary the rate of flow through said two drain lines and thereby vary the feed rate of the two cylinders.

16. A control circuit as called for in claim 14 wherein each of said passageways has a portion of tapered cross section with a decreasing area in a direction axially away from said one end of said bore.