Plastic article or earthenware shaping or treating: apparatus – Distinct means to feed – support or manipulate preform stock... – Female mold type means
Reexamination Certificate
1999-08-31
2004-02-10
Hong, John C. (Department: 3726)
Plastic article or earthenware shaping or treating: apparatus
Distinct means to feed, support or manipulate preform stock...
Female mold type means
C029S527600, C029S855000
Reexamination Certificate
active
06688871
ABSTRACT:
This invention was made with government support under Grant Number DMI-9702913 awarded by the National Science Foundation. The government has certain rights in the invention.
FIELD OF THE INVENTION
This invention relates to fixturing systems for workpieces which are to be machined, and more particularly to a method and apparatus for encapsulating a workpiece to be machined to form an encapsulation block of standardized size and shape so as to facilitate universal automated fixturing for workpieces of varying sizes and shapes.
BACKGROUND OF THE INVENTION
Machined parts can have complex shapes and can include thin and fragile areas. Some of the parts are also quite small. All of these factors complicate machining of the parts, and in particular the fixturing of the parts to facilitate such machining. Proper fixturing of a part for machining generally involves a number of factors including:
(a) Immobilization of the workpiece to allow aggressive machining of the part without spurious movement thereof. Without proper immobilization, machining must be done gently in order to avoid spurious movement of the part during machining, which movement can result in the destruction of the part. This significantly increases the time required to complete a machining operation.
(b) Unobstructed machining paths. Obstructions in the machining path significantly complicate the generation of tool paths and other computer aided manufacturing (CAM) data for the manufacturing process, thereby increasing both the time and expense for manufacturing the part.
(c) Locate the workpiece with absolute accuracy, particularly where parts are being manufactured with very fine tolerances. Even a small fixturing error in locating the part for a machining operation can result in parts being out of tolerance, and therefore worthless.
(d) Support the workpiece entirely. This is a particular problem where a significant portion of the original workpiece is removed during the machining operation, leaving fragile points on the part which must be thoroughly supported to avoid cracking, bending or breaking during subsequent machining operations. Fixturing to fully support a partially machined part so as to prevent damage thereto is frequently a difficult and expensive procedure. At times, it is not possible to properly fixture such parts and it is therefore necessary to manufacture them in two or more pieces which must subsequently be secured together.
Achieving the above objectives is further complicated by the fact that many, if not most, parts need to be machined on all six surfaces of for example an originally rectangular workpiece, or on at least five such surfaces. Since fixturing to achieve the above objectives generally requires gripping and holding the workpiece on several surfaces, typically four or five surfaces, it is normally necessary to refixture a part several times during the machining thereof. Therefore, multiple fixtures are generally required for the machining of a single part, as many as six different fixtures being required in the extreme case.
However, the design of custom fixtures and the fabrication of such fixtures for a particular part is an expensive procedure. While this procedure can be justified where the cost is spread out over the manufacture of many thousands of parts, the cost of developing and fabricating fixtures can be prohibitive for custom parts having a small run and is a particular problem where parts are being prototyped or are being made for use in a prototype product.
A need therefore exists for a universal fixturing system which eliminates the need for designing and fabricating custom fixtures for each part, and in particular designing multiple custom fixtures for each part to accommodate machining on various sides thereof, while still meeting all of the fixturing requirements indicated above. A proposal for such a universal fixturing system is provided in “A Methodology for Integrated CAD and CAM in Milling,” a Ph.D. thesis of S.E. Sarma, University of California, Berkeley, 1995 and in “Reference Free Part Encapsulation: A New Universal Fixturing Concept,” S. Sarma and P. Wright, Journal of Manufacturing Systems, Vol. 16/No. 1, 1997 (“the Sarma Papers”), which discuss the concept of encapsulating a workpiece to be machined in a material having a lower melting temperature than that of the material for the workpiece, the encapsulated block containing the encapsulated workpiece having a known, standardized size and shape which fits in a standardized fixture. Further, the concept involves re-encapsulating any portions of the encapsulated block, including the workpiece embedded therein, which are machined away during the machining of a given side or sides. This re-encapsulation accomplishes two functions. First, it assures that the block being fixtured is always of standardized size and shape, which can be mounted in a standardized fixture and can be precisely located therein. Second, it assures that all portions of the workpiece, regardless of how heavily machined and how fragile, are fully supported within the encapsulation material so that machining may be performed aggressively without risking cracking or breaking of the part, and without requiring complicated specialized fixturing.
However, while the concepts presented in the Sarma Papers represent an interesting approach to the universal fixturing problem, there are many problems involved in implementing such an encapsulation and re-encapsulation process, and the Sarma Papers do not describe a practical system for performing the encapsulation process which overcomes these various problems. Thus, while the encapsulation technique discussed in the Sarma Papers represents a promising approach to universal fixturing of parts or other objects to be machined, a need exists for a practical implementation of this approach.
SUMMARY OF THE INVENTION
In accordance with the above, this invention provides a mold for use in encapsulating a workpiece of a material having a density D
1
in an encapsulant having a density D
2
to form an encapsulated block. The mold includes a structure defining four sidewalls of an encapsulation cavity in the mold, a top plate and a bottom plate, which plates mate with the structure on opposite sides thereof to fully define and seal the cavity. One of the plates is a gate plate having a plurality of openings formed therethrough and spaced over the area of the gate plate adjacent the cavity, with encapsulant being applied to the cavity through the gate plate openings. The openings may be substantially evenly spaced over the area of the gate plate; and are, in any event, sufficient in number and are positioned so that at least one opening overlies the smallest feature to be machined in the workpiece. The diameter at least at the cavity side of each of the gate plate openings is preferably no more than approximately 0.03 inches. Each of the openings preferably has a selected small diameter extending for a short distance from the cavity side of the opening and then flares to increasing diameters for its remaining length. The gate plate is preferably formed of a material having good high temperature characteristics but poor thermal conductivity, such material for example being a ceramic.
For one embodiment of the invention where D
2
is greater than D
1
, the mold is oriented with the top and bottom plates being substantially horizontal, and an orienting structure is formed on the cavity side of the top plate, the workpiece floating up into orienting contact with such orienting structure when encapsulant is applied to the cavity. The orienting structure may for example be a depression formed in the top plate, which depression preferably has chaffered walls. The workpiece is preferably oriented in the cavity so that the thickness of encapsulant on the workpiece is substantially uniform. For preferred embodiments, the top plate is the gate plate. A recess having rounded corners may be formed in the bottom plate.
The cavity preferably has a substantially rectangular shape, with sidewalls that are substantially per
Lee Elmer C.
Sarma Sanja E.
Hong John C.
Massachusetts Institute of Technology
Wolf Greenfield & Sacks P.C.
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