Description:
BACKGROUND OF THE INVENTION
The present invention relates to a process for machining castings and minimizing the warping attendant to machining portions thereof having relatively slender or shallow cross sections. It has particular application to the machining of propeller castings. In such castings, warpage of the blade portions adjacent to and outboard of the roots may cause severe deflection of the tips from their intended plane of rotation.
A principal cause of such warpage during machining is the uncontrolled release of some of the stresses locked in on cooling of the casting. Typically, when cast metals cool, they shrink. However their outer surfaces solidify sooner than the portions inwardly thereof, and cool to ambient temperature more quickly. As the surfaces solidify, they first assume a relatively unstressed length commensurate with the volume of the still molten metal at the interior. As the interior continues to cool, it solidifies; then, as its temperature lowers to ambient temperature, the interior tends to shrink further. Such further shrinkage of the solidified interior is resisted by the previously cooled surfaces, resulting in compression stresses in the surfaces and tensile stresses in the interior. If the casting is shallow its shape when cooled is in part dependent upon the balance of such compression and tensile stresses.
In such a situation, machining the surface removes some of the material in compression, disturbing this balance of stresses. Considering a shallow section, if this balance is disturbed more on one side than on the other, the casting will bend or warp.
One aspect of this problem is reviewed in my prior U.S. Pat. No. 3,295,190 dated Jan. 3, 1967, entitled "Method of Machining Metal Castings for Screw Propellers and the like." That patent sets forth a method of machining propeller blades which is suitable if machining pressures are light or if the outer portions of the propeller blades are of sufficient thickness to require no support during machining. In contrast, the present invention provides for rigidly restrained, undeflecting support of shallow casting portions during machining, and minimizes their warpage when released from such restraint.
SUMMARY OF THE INVENTION
Summarizing generally and without limiting the scope of invention, the present process is best applied to metal castings of relatively shallow cross section, that is, those having portions defined principally by two opposed surfaces spaced from each other more closely than their width. Typical are the blades of a marine propeller. Utilizing the present process, the casting is mounted and aligned and restrained in position for machining one of such opposed surfaces, for example, the convex blade surfaces of a propeller casting. While maintaining such restraint, such an opposed surface is machined to contour, and is then peened, as by shot peening. The peening increases the bending resistance of the metal at the machined surface, to compensate in part for the bending resistance of the metal removed by machining. The casting is then released from restraint, and mounted, aligned and restrained in position for machining its other surface or set of surfaces; in the example mentioned, the opposite blade surfaces. After such machining, such new machined surfaces are similarly peened; and the casting is then released from restraint. The peening of the opposed surfaces, while restrained from warping, affords them added resistance to bending out of the alignment in which the casting has been restrained. This added bending resistance lessens the warpage which would otherwise occur on release from restraint.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of a marine propeller casting mounted aligned in position for machining the concave surfaces of its blade.
FIG. 2 is a plan view thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Typical of the castings for which the present process has been devised is the casting generally designated 10. As cast, it includes a hub portion 12, which in the machining process is first drilled to provide an axial bore 13 which establishes a plane of rotation. To mount the casting 10 for machining, it is also necessary to provide the hub 12 with end faces 14, machined precisely perpendicular to the bore 13.
Projecting from the hub 12 are three blade portions 15. Each blade has two sets of surfaces, a concave surface 16 shown uppermost in the drawings and a convex surface 17 opposite thereto. Along the outer edges of each of the blade portions 15, positioned approximately on a radial line from the hub axis extending through the blade root juncture 18, is an integral restraint pad portion 20, thicker than the blade portion 15 and having fillets 21 merging with its upper and lower surfaces 16, 17.
A somewhat simplified mounting fixture is shown schematically in FIGS. 1 and 2. On a heavy base plate 25 is a cylindrical hub mounting platform 26. The base plate 25 and hub mounting platform 26 have a central bore 27 and the base plate 25 has a counterbore 28, holding a large central mounting bolt 29 whose upper end 30 is threaded to receive a mounting nut 32 and washer 33, by which the hub 12 of the propeller casting 10 is mounted.
Outboard along the base plate 25, spaced at equal angular and radial distances from the bore 27, are rigid blade restraint posts 35, positioned to restrain and support the machining pads 20 of the blade portions 15. In the simple embodiment illustrated, each has a central vertical tapped bore 36 to receive a restraint bolt 37. Bored spacers 38 may be provided on each of the posts 35, of such thickness as may be required to provide the desired planar alignment of the restraint pads 20.
In preparing a casting preliminary to machining its blade portions 12 to contour, after boring the hub 12 and providing its upper and lower faces 14, each restraint pad portion 20 is machined to provide upper pad surfaces 22 in planar alignment with each other, and lower surfaces 23 similarly aligned. Restraint bores 24 are drilled through each of the pads 20 at precisely equal radial distances from the bore 13 and at equal angular distances from each other.
The casting 10 is then mounted by its hub portion 12 as shown in the drawings, in position for machining one of its two sets of blade surfaces, for example, its concave surfaces 16. The blade portions 15 are secured in desired alignment by inserting the bolts 37 through the restraint bores 24 and bolting securely to the posts 35, using spacers 38 to hold to desired planar alignment. The upper outer surfaces of the hub and the upwardly presented set of blade surfaces 16 are then machined to final contour either entirely or at least at and outboard of the root junctures 18.
As stated earlier of this specification, such machining removes the surface metal which, by virtue of differential rates of cooling and other factors, is likely to be stressed in compression; and the result of the removal is to disturb the balance of stresses, as might typically tend to cause the blade portions 15 to warp upwards. The bolts 37 hold the restraint pad portions 20 so as to counteract this tendency to bend upward. The most critical region for such upward bending of each blade is that commencing at the root juncture 18 and extending outboard roughly halfway to the outer blade edges, for example, to the dashed line b. If warping curvature in this region can be minimized, such curvature nearer the tip portions will cause no serious deflections.
Therefore, after such machining I shot peen the surfaces so machined. Conventional shot peening apparatus, which propels shot against the machined surface portion, serves to harden the surface. With the blade portion held restrained from warping such hardening increases the bending resistance of the peened surface portion, to lessen its tendency to warp upward and thus to compensate in part for the effects of machining.
The restraint bolts 37 are then released, and the large nut 32 removed, freeing the partially machined casting to warp as much as it will. The casting 10 turned over and again bolted in place on the hub mounting platform 26. Spacers 38 of approximate thickness are utilized to fix planar alignment of the pads 20, and the restraint bolts 37 are again tightened, so that the unwarped desired alignment for machining is restored. The second set of blade surfaces 17 is them machined, at least those portions at and outboard of the blade root junctures 18 approximately to the line b, and such surface portions are peened to afford them additional bending resistance. Finally the casting is released from restraint. Any warpage which previously appeared will be found to be substantially offset by such second machining and peening under restraint. The pads 20 and their fillets 21 are cut off, and the edges marking the intersections of the blade surfaces 16, 17 are finished, as well as those other portions of the casting which may require finishing.
The blade casting 10 is typical of metal castings at least part of which include cross sections defined by outer surfaces comprised principally of two opposed surfaces spaced more closely than their width. By machining the surface of such a casting portion while restraining it from warping out of alignment, and peening the machined portion without releasing the restraint, there results a substantial degree of compensation for the unbalance in stresses which accompanies the removal of metal during machining of the surfaces.
Utilizations other than on marine propellers will be apparent to those familiar with the problems of machining metal castings.