US3460239A - Method and apparatus for locating a casting preparatory to machining thereof - Google Patents

Description

Aug. 12, 1969 M. G. NIX 3,460,239

A METHOD AND APPARATUS FOR. LOCATING A CASTING PREPARATORY T0 MACHINING THEREOF Filed July 19, 1965 5 Sheets-Sheet 1 M. G. NIX 3,460,239 METHOD AND APPARATUS FOR LOCATING A CASTING Aug. 12, 1 969 PHEPARATORY TO MACHINING THEREOF 5 Sheets-Sheet 2 Filed July 19, 1965 Invenlar M 6. /V/'x Bil AL/ I v or 'neyg Aug. 12, 1969 M. G. NIX

METHOD AND APPARATUS FOR LOCATING PREP'ARATQRY TO MACHINING THE Flled July 19 1965 Fig.4

lnven lor I 7. 6. Nix

ilorneys Filed July 19. 1965 Aug. 12, 1969 M G NIX 3,460,239

mmnuu AND APPARATUS i0R LOCATING A CASTING PREPARATORY TO MACHINING THEREOF 5 Sheets-Sheet 4.

H97 Wm )VI') Attorneys Aug. 12, 1969 M G. NIX

METHOD AND APPARATUS FOR LOCATING A CASTING PREPARATOHY T0 MACHINING THEREOF Filed July 19, 1965 5 Sheets-Sheet 5 lnve nlor lV/X

A tlomey:

United States Patent US. Cl. 29-559 18 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of accurately locating a hollow piston casting preparatory to machining the exterior surface thereof, which method comprises locating the casting by means of radial locating supports which engage the interior of the casting at a plurality of circumferentially spaced, radially fixed, points divided into two groups in two axially spaced annular zones of the interior of the casting to provide transverse centralising location, and centralising the casting by urging the points of each group axially with respect to the casting and to the other group, to cause the points to come into wedging engagement with the parts of the interior of the casting in said spaced zones.

The present invention relates to a method of locating hollow or tubular castings, more particularly the castings of pistons for internal combustion engines, compressors or the like, to enable a machining operation to be carried out thereon, and also to a fixture for locating or holding such castings during these operations. The invention also relates to castings adapted for carrying out the method.

The object of the invention is to provide a method of and means for accurately fixturing or locating a hollow or tubular casting from its cast internal shape or surface to enable machining operations concentric with the internal shape to be carried out on its external surfaces, and which makes allowances for variations in castings produced by different dies or cores, or due to wear of the die cores which takes place during the continuous casting.

From one aspect, the invention consists in the method of locating a piston casting for machining purposes, comprising locating the casting by means of radial locating supports which engage the internal surface of the casting at a plurality of points, divided into two groups, circumferentially spaced and radially fixed in two axialy spaced annular zones of the interior of the side wall of the casting to provide transverse centralising location, and centralising the casting by urging the two groups of radial locating supports axially with respect to the casting and to each other to cause said points to come into wedging engagement with parts of the interior of the casting in said spaced zones.

In carrying out the method, the locating and wedging action may be conveniently obtained by shaping the interior of the casting in said zones with a tapering or frusto-conical shape, diverging towards the open end of the casting through which the supports are introduced, against which the ends of the radial locating supports of the two groups, each group being separately movable in a guided longitudinal path relative to the longitudinal support, are urged longitudinaly by resilient or other appropriate means. The taper angle of said zones may approximate the usual casting draft angle, and is conveniently about 5 with reference to the longitudinal ice axis of the piston casting. In addition to tapering the zones, or as an alternative, the ends of the radial locating supports may be tapered.

The axial movement of the radial locating supports compensates for variations in the internal dimensions of different castings, due to their having been made in different dies or to wear of die cores.

The invention also consists in a fixture for locating a casting for machining, comprising a longitudinally extending member over which the casting is adapted to be positioned, and two radial support members each having a plurality of circumferentially spaced, radially fixed, casting-engaging points constituting the radial location supports, at least one of said radial support members being movable along the longitudinally extending member towards and away from the other radial support member, and resilient means urging said two radial support members apart.

The invention also consists in a casting, more particularly a piston casting, provided with two internal axially spaced circumferentially extending generally frustoconical surface zones diverging towards the open end of the piston skirt, one zone being disposed between the underside of the piston crown and the gudgeon pin bosses, and the other zone being disposed adjacent the open end of the piston skirt.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic longitudinal section of a piston casting according to the invention, and showing the seven points by which it is located, according to this invention, during a machining or like operation;

FIGURE 2 is a side view of a fixture according to one embodiment of this invention for locating a piston casting at seven points, a piston casting being indicated in section in its position of location on the fixture;

FIGURE 3 is a partial axial section of the fixture shown in FIGURE 2;

FIGURE 4 is a partial longitudinal section of a fixture according to a second embodiment of the invention;

FIGURE 5 is a longitudinal section of an embodiment of a piston according to the invention;

FIGURE 6 is a view looking in the direction of the arrow A in FIGURE 5, and showing the positions of the transverse locating points;

FIGURE 7 is a fragmentary longitudinal section of the skirt of a modified form of piston;

FIGURE 8 is a longitudinal section of a piston casting showing diagrammatically a modified form of longitudinal location support; and

FIGURE 9 is a diagrammatic plan view indicating the positions of the core pieces and longitudinal location supports of the casting shown in FIGURE 8.

The method of the location, according to one embodiment of the invention, consists in locating the axial position of a piston casting 'by a support or supports engaging with the underside of the piston crown and centralising the piston radially by two groups each of three supports disposed angularly around the piston axis and engaging the internal surface of the piston side wall in each of two transverse planes, one plane being disposed adjacent the bottom of the piston skirt and the other plane being disposed between the underside of the crown and the piston pin bosses. The three support points, which are preferably approximately apart in each plane, are preferably orientated in one plane relative to the other plane. The parts of the internal surface which they contact are constituted by two frusto-conical zones, centred about the longitudinal axis of the piston, which diverge towards the open end of the piston skirt With a taper, conveniently of about with respect to the longitudinal axis of the piston.

This is illustrated in FIGURE 1, which diagrammatically shows a piston casting for example an aluminium alloy die-cast piston casting, having a crown 1, a skirt 2, and formed internally, according to the invention, with a pair of axially-spaced circumferentially extending frusto-conical internal bands or zones 3 and 4 which diverge towards the open end of the skirt 2. The angle of taper may be the same for both zones 3 and 4, and may for example be of the order of 5 with respect to the longitudinal axis of the casting.

One of the location support points, represented by the arrow 5, is disposed on the longitudinal axis of the piston casting against the underside of the piston crown 1, and serves longitudinally to locate the casting. Three of the radial support points, represented by the arrows 6, are disposed against the upper zone 3, angularly spaced about the longitudinal axis in a common transverse plane. The remaining three radial support points, represented by the arrows 7, are disposed against the lower zone 4 in a common transverse plane adjacent the bottom of the piston skirt. The support points in each plane are shown as being mutually angularly spaced by approximately 120, and the three points in one plane are shown as being displaced angularly with respect to the points in the other plane by approximately 180. Such angular relationships are, however, not always convenient in practice.

One embodiment of the fixture or device according to this invention for supporting and locating the casting is shown in FIGURES 2 and 3. This fixture comprises a longitudinal location support member in the form of a stepped pillar or post 8, rigidly mounted on, or integral with, a base 9, and formed with a rounded upper end 8a which is adapted to engage the undeside of the piston crown 1 and constitutes the support point 5 in FIGURE 1.

Slidably mounted on the post 8 are two radial location support members, namely upper and lower members and 11 respectively, each comprising a body part 10:: and 11a and three radially extending arms 10b and 11b respectively, the radially outer ends of which are adapted to engage the conical zones 3 and 4 (FIGURE 3) respectively, and constitute, respectively, the radial support points 6 and 7 shown in FIGURE 1. The arms 10b and 11b, although they may be integral with the body parts 10a and 11a, are in this embodiment, secured thereto by counter sunk screws 20. The arms 11b and one of the arms 10b are radially disposed and are individually secured to their respective body part. However, the two remaining arms 1% comprise opposite ends of the cross piece of a T- shaped element 21 secured by its vertical limb to the member 10.

The upper member 10 is prevented from sliding off the upper end of the post 8, when the piston casting is removed, by means of a circlip 26 located in an annular groove in the upper end of the post 8, and engageable with the bottom of a transverse slot or recess 10c in the upper end of the member.

The member 10 is provided with a recess 10c Within which is located the upper end of a helical compression spring 12 surrounding the post 8. The lower end of the spring 12 bears on a shoulder 8b of the post 8, and normally resiliently urges the upper member 10 towards the upper end 8a of the post. The member 11 and a shoulder 8d on the base 9 are provided with pairs of aligned recesses 11g and 8e respectively, and a helical compression spring 13 is trapped in each pair of recesses. The springs 13 normally urge the lower member 11 in the same axial direction as the member 10.

The members 10 and 11 are so arranged that they are axially but not rotationally movable relative to each other and the post 8. To this end the member 10 is of rectangular cross-section and is keyed to the lower member 11, by its lower end 10d, which is lecated in a transverse slot 11d in the upper end of the lower member 11. The interengagement of the end 10d and slot 11d thereby prevent rotation between the members 10, 11 whilst accom modating axial movement therebetween. To prevent relative rotation, but to allow axial movement, between the post 8 and lower member 11, the lower member 11 is provided with a pair of downwardly extending arms 11e having flat parallel and vertical inwardly facing surfaces 11] which engage correspondingly disposed flat side surfaces 8c of a rectangular shoulder 8d at the lower end of the post 8.

The outer rectangular surface of the body part 10a of the upper member 10 is so dimensioned that it provides a clearance, but a reasonably close fit, between the inner faces of the piston pin bosses 16 of the piston casting. Thus the casting is prevented from rotating during machining relative to the members 10 and 11, and therefore relative to the fixture.

As previously mentioned, the groups of three arms 10b and 11b of the fixtures previously described are ideally angularly spaced apart by in each transverse plane, and the arms 10]: are ideally angularly displaced from the arms 1111 by In practice, however, and as shown in FIGURES 2 and 3, the piston design itself determines the positions of the arms in each plane and their relationship to the arms in the other plane, and this relationship will vary from piston design to piston design. In particular the positions of the arms is dependent upon the size and position of the piston pin bosses 16 and any internal projections or recesses within the piston body or skirt.

The fixture is used as follows:

The fixture is either rigidly secured by its base 9 to a machine table if the fixture is to be maintained stationary during the machining operation, or is fixed to the spindle nose of the machine if the fixture is to be rotated during machining. The piston casting is placed over the device and pressed towards the base 9 (downwardly in FIG- URE 2) whilst orientating the casting so that the arms 1% do not foul the piston pin bosses 16. This pressure brings the radially outer ends of the arms 10b into engagement with the conical zone 3 beneath the piston crown centralising the piston casting about its core adjacent the orown. The casting is pressed further towards the base 9, causing the member 10 to slide along the post 8 towards the member 11, against the action of the spring 12, until the radially outer ends of the arms 11b of the member 11 engage with the conical zone 4- adjacent the open end of the piston skirt 2. The piston casting is now accurately located centrally about its longitudinally axi at both locating planes, the fixture allowing for permitted variations in internal diameters of different castings, due to ear of the casting die core within casting tolerances, by the axial movement of the members 10 and 11 in relation to one another along the piston axis. The casting is then pressed further towards the base 9, simultaneously axially displacing both members 10 and 11 against the action of their respective springs 12 and 13, until the outermost end 8a of the post abuts against the underside of the piston crown, thereby longitudinally locating the casting with respect to the fixture.

The casting is held in this position, against the actions of the springs 12 and 13, by means of a clamping bar or member 17 which is urged against the external face of the piston crown 1 to clamp the latter against the post 8. This bar 17 may be separate from and operated independently of the fixture, for example it may form part of a machine on which the fixture is mounted. In this case the bar 17 may be located on an axis coincident with the axis of the post 8 or piston casting, or the axis of the machine spindle.

During machining, the casting rotates slightly due to forces imposed by the cutting tool until the flat external surfaces of the body part 10a engage the inner faces of the piston pin bosses 16 as shown, and thereby prevent further rotation of the casting relative to the fixture during the machining operation, and provide for a driving torque therebetween.

The machining operation is then carried out. If the fixture is used in conjunction with a tool holder carrying two tools at 180 apart or three tools at 120 apart, or more than three tools equally radially spaced, the cutting action further presses or steadies the piston symmetrically on the locating points.

Due to the provision of the two axially guided members and 11, there are no jaws or pins which are expanded to grip and distort the casting. The casting is held in place on the fixture by the clamping bar 17 which acts through the outside of the piston crown to clamp the piston crown to the post 8, permitting the piston casting to take up its natural unrestricted position on the two sets of three arm 10b and 11b.

The fixture itself, since the members 10 and 11 and therefore the two sets of three radial supports are movable along the piston axis, not only accommodates variations in diameter within the casting tolerances on the two locating zones 3 and 4, but also, by reason of the three point support, always makes contact with the three points in each transverse plane (in the manner by which a three legged stool sits evenly on a rough floor) and averages out out-of-roundness within the casting itself, and the diameters of the piston turned at the same time will be concentric with the two locating zone diameters on any one casting. Thus by the introduction of these two locating zones with the seventh support point underneath the piston crown to determine the position of the inside of the crown in order to face the crown to the correct thickness, any piston design can be consistently both concentrically and longitudinally located. The fixture will also accommodate a certain amount of reduction in size on the core of the die due to wear on the core during use in the casting operation. In allowing for this wear, the casting will always locate on the three points in each of the two planes, but at different axial positions in each zone. For example a variation of .010" on the internal radius of piston castings can be accommodated by an axial movement of the members 10 and 11 within a range of 0.111" if the zones 3 and 4 have a taper of 5. Such a variation can easily be accommodated if the zones 3, 4 have an axial length of bout 0.25". The final positioning of the inside of the crown at the seventh support point will determine in every case the thickness of the crown during the machining operation, since this point is a fixed dimension from the datum of the lathe or machine on which the crown is being turned, and is used for positioning the facing tool.

FIGURE 4 illustrates a modified fixture which may be used to locate a piston casting from which the frustoconical zones 3 and 4 have been omitted. In the embodiment, these zones are assimilated by modifying the ends of the arms 10!] and 11b so that the said ends are inclined at an angle, approximating to 5'. These inclined ends cooperate with suitable diameters, shoulders or the like on the internal surface of the piston.

The fiixture shown in FIGURE 4 also differs in that the arms 10b and 11b are integral with the body parts 10a and 11a. Moreover, the two members 10 and 11 are located against rotation relative to the post 8, independently of one another, by keys 81 secured to the post 8, and slidable in axial slots 1011 and 11/1 in the members 10 and 11. In addition, the springs 12 and 13 shown in FIGURES 2 and 3 are replaced by equivalent resilient means in the form of O-rings 12a, 13a made from a resilient material, for example natural or synthetic rubber, and enclosed in metal housing 12b and 13b.

The fixture shown in FIGURE 4 is operated in the same manner as described with reference to FIGURES 2 and 3.

One practical form of piston design having an internal shape for location by a fixture as shown in FIGURES 2 and 3 is illustrated in FIGURES 6 and 7. The radially outer ends of the arms 10b and 1111 are shown diagrammatically in FIGURE 7 by the arrow heads. The angular relationship of the arms 10b is such that the piston casting may he slid axially onto the device until the arms 10b engage the zone 3 (see FIGURES 2 and 6) without fouling the piston pin bosses 16.

As shown the angle between the two arms 1% on the same side of the piston axis is of the order of In practice, however, the device has been found to work very satisfactorily when these two arms are only spaced apart by 40, and the other remaining arm is spaced by an angle of 160 from the other two arms. Since the arms 11b do not have to pass any internal projections on the casting, by virtue of the fact that the zone 4 is disposed at the open end of the piston skirt 2, these arms may be usually disposed at approximately with respect to each other.

In FIGURE 6, the outer end 8:: of the post is shown diagrammatically as an arrow head and is located on a rib 18 cast integrally with the piston casting, which serves to reinforce the piston crown 1.

FIGURE 8 shows a construction in which the lower zone 4 is spaced from the lower end of the piston skirt to accommodate cut-away portions 19 which provide crankshaft clearance.

The piston die core which determines the internal dimensions of the piston casting, generally consists of four sections surrounding a rectangular wedge-shaped centrepiece, the function of the latter being to hold the four sections in their correct position ready for casting. During manufacture of the core, these four sections are mounted on the centre-piece which is centred at both ends, and those surfaces shown as diameters in the piston casting are turned in a lathe. Thus all of the outer diameters of the core (inner diameters of the piston casting) are concentric with one another. During this turning operation on the core, the two frusto-conical zones for producing the zones 3 and 4 on the piston casting are also turned. There is thus obtained an accurate location which can only change within the normal casting tolerances or due to core wear.

In the modification shown in FIGURES 9 and 10, the two sets of three radial supports which locate against the zones 3 and 4 remain unchanged, and have been omitted from these figures. However, the longitudinal support corresponding to the arrow 5 in FIGURE 1 has been divided into a pair of supports represented diagrammatically at 5a, each of which is arranged to engage the underside of the piston crown. These two supports are arranged to float or rock about a fulcrum 22 in order to permit variations within the permitted casting tolerances of the underside of the piston crown.

As previously mentioned, the piston core comprises four pieces positioned by a rectangular wedge-shaped centre piece. As shown in FIGURE 10, two of the four pieces, namely boss cores 23 carrying piston pin boss cores 23a, together constitute approximately 320 of the piston core, and the portion of the piston casting defined by these boss cores 23 constitutes 85% of the total piston casting weight. These boss cores 23 are separated by side cores 24 which constitute the remaining 40 of the piston core. In turn, the two boss cores 23 and the two side cores 24 are positioned by the wedge-shaped centre piece 25, which is also shown in FIGURE 9.

The effect of contacting the piston crown by the supports 5a at positions where the internal surface of the crown is formed by the boss cores 23 as shown, further decreases the possibility of casting weight variations compared with the single axially aligned support correspond ing to the arrow 5 in FIGURE 1 which engages the portion of the internal surface of the crown formed by the centre piece 25, since any wear taking place on the jointing faces of the five core pieces will in this embodiment, only permit the centre piece 25 to move axially towards the piston crown within its permissible tolerance (0.005" for example), thus increasing the weight of the piston casting only by the volume of metal represented by the cross-sectional area of the crown-forming surface of the centre piece 25 multiplied by the amount of axial movement. With one support in line with the centre piece 25 only, wear on the core jointing faces has the effect of allowing the boss cores 23 to move axially in the reverse direction, rapidly increasing the weight of the casting by an amount proportional to the cross-sectional area of the crown-forming surface of the two boss cores 23 multiplied by the amount of movement.

It will be understood that various modifications may be made without departing from the scope of the present invention. For example the angle of the frusto-conical zones, although illustrated as being of the order of may be more or less than 5, and the angle of one zone may dififer from that of the other zone. Each zone may be circumferentially continuous, or may comprise partannular segments separated or interrupted, for example by the internal ribs or other parts of the piston body. Further, such segments need not all be on the same radius provided that the arms of a group are of such lengths that the casting will be centralised on the fixture.

Although, ideally, two groups of three radial location points are employed, the number of points may be reduced or increased. For example, where radial location is required within a portion of the casting which is not circular, four arms or points may be employed.

Moreover, one or each member 10 and 11 may be in the form of a disc or cone (or segments thereof) which engage the internal surface of the casting continuously therearound, or along axis. In particular, the lower member 11 may be replaced by a frusto-conical member, which engages the open end of the piston skirt.

Axial or longitudinal location may be effected by means other than the end 8a of the post 8. For example, longitudinal location may be effected externally of the casting, using the open end of the skirt or the upper surface of the crown, or by providing a limit stop for axial movement of one or other of the radial location members.

In some cases, for example when the casting comprising a tube instead of a piston, longitudinal location may be omitted.

I claim:

1. A method of locating a tubular casting for machining purposes, comprising locating the casting by means of radial locating supports which engage the interior of the casting at a plurality of circumferentially spaced, radially fixed, points divided into two groups in two axially spaced annular zones of the interior of the casting to provide transverse centralising location, and centralising the casting by urging the points of each group axially with respect to the casting, and to the other group, to cause the points to come ino wedging engagement with the parts of the interior of the casting in said spaced zones.

2. A method as claimed in claim 1, which includes centralising the casting by means of two relatively axially movable radial locating supports, each of which engages the casting at three points.

3. A method as claimed in claim 2, wherein the radial locating supports engage tapered parts of the interior of the casting.

4. A method as claimed in claim 3, which includes locating the casting longitudinally by urging the casting into engagement with a longitudinal locating support.

5. A fixture for locating a tubular casting for machining, comprising a longitudinally extending member over which the casting is adapted to be positioned, and two radial support members each having a plurality of circumferentially spaced, radially inextendible, casting-engaging points, at least one of said radial support members being movable longitudinally of the longitudinally extending member towards and away from the other radial support member, and resilient means for normally urging said two radial support members apart.

6. A fixture as claimed in claim 5, wherein each radial support member is provided with three radial arms, the radially outer end of each of which constitutes a castingengaging point.

7. A fixture as claimed in claim 5, wherein the radial support members are adapted to be longitudinally movable into wedging engagement with tapered or frustoconical internal zones of the casting.

8. A fixture as claimed in claim 6, wherein the two radial support members are separately movable longitudinally on said longitudinally extending member, and are urged towards the free end thereof by resilient means.

9. A fixture as claimed in claim 8, wherein means are provided for preventing rotation of the radial support members relative to the longitudinally extending member.

10. A fixture as claimed in claim 9, wherein at least one of the radial support members is adapted to engage the casting, when located on the fixture, so as to prevent relative rotation between said member and casting.

11. A fixture as claimed in claim 5, wherein means are provided for longitudinally locating the casting on the fixture.

12. A fixture as claimed in claim 11, wherein said means comprises a longitudinal locating member constituted by the free end of the longitudinally extending member which is capable of engaging the casting internally.

13. The method of locating a piston casting for machining purposes which consists in locating the casting by locating supports which engage the casting at least at seven points on its internal surface, namely at least at one point on the underside of the piston crown to provide longitudinal location, and at six points divided into two groups, each of three, circumferentially spaced in two axially spaced annular zones of the interior of the side wall of the piston to provide transverse centralising location, holding the casting longitudinally against the longitudinal locating support at said one point, and centralising the casting by urging the radial locating supports at said six other points in wedging engagement with parts of the interior of the casting in said spaced zones.

14. A method as claimed in claim 13, including providing said zones with a tapering shape, diverging towards the open end of the skirt, and longitudinally and resilient- 1y urging the ends of the radial locating supports of the two groups against said zones, each group being separately movable in a guided longitudinal path relative to the longitudinal support.

15. A method as claimed in claim 14, wherein the taper angle of said zones approximates to 5 with reference to the longitudinal axis of the piston casting.

16. A fixture for locating a piston casting for machining, comprising a longitudinal location support member over which the piston casting is adapted to be positioned with the free end of said member constituting the longitudinal location support, and two radial support members each having three radial inextensible arms, of which the ends constitute the radial location supports, guide means along which both said radial support members can move towards and away from said longitudinal location support, and resilient means urging said two radial support members towards said longitudinal location support.

17. A fixture according to claim 16, wherein said resilient means comprise compression springs, disposed between the radial support members and the longitudinal location support.

18. A fixture according to claim 17, wherein said ends of the arms are inclined -with reference to the longitudinal axis of the piston casting.

(References on following p ge) References Cited UNITED STATES PATENTS Craver 269-52 X Anthony 279-2 X Schwarzmayr et a1. 269-47 X Anthony 279-2 X 10 3,272,041 9/1966 Anthony 279-2 X 3,304,078 2/1967 Hogo 269-52 PAUL M. COHEN, Primary Examiner U.S. C1. X.R.

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