Work holders – Holder inserted within work aperture
Reexamination Certificate
1999-09-27
2001-09-11
Watson, Robert C. (Department: 3723)
Work holders
Holder inserted within work aperture
C269S100000, C269S303000, C269S305000, C269S315000, C269S900000
Reexamination Certificate
active
06286823
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to the clamping of workpieces to a surface and, more specifically, to a system for indexing and clamping a workpiece to a base plate that leaves the opposite surface free for machining or the like.
When raw material or preforms are to be machined it is necessary that the workpiece be tightly clamped to a base plate or an intermediate sub-plate to hold the workpiece stable at a precise position while resisting the forces on the workpiece produced by the machining operation. Typically, such machining includes drilling, boring, honing, grinding and milling.
In numerical controlled machining, a machine tool, such as a milling head, is programmed to follow a very precise path. The workpiece must be located extremely precisely relative to certain machine datum points and/or datum planes from which the machining path is related or measured.
A number of different indexing and clamping systems have been developed to permit a workpiece to be positioned at a specific location on the support plate. Generally, the clamps have fingers and straps or other means that extend over the side of the workpiece opposite the supporting surface. While generally effective, these upper surface clamps must be carefully positioned to avoid interference with the movement of the machine tool, such as a milling head, across the workpiece.
Insufficient clamping-pressure, or the use of too few clamps, may allow the workpiece to shift during machining, resulting in wasted, out-of-tolerance products. Further, errors in clamp placement may allow the machine tool cutter to run against the tool, resulting in damaged clamps and serious damage to the machining tool and/or to the machine itself and could be a safety hazard to nearby personnel.
A wide, generally flat workpiece that is clamped to a support plate around the edges may lift slightly at the center due to machining forces. Thus, the central dimensions of the product may be thinner and out-of-tolerance.
It is absolutely essential, especially with numerical controlled machining, that the workpiece be positioned precisely relative to datum points or planes on the support plate. Failure to establish and maintain the precise workpiece position will result in an improperly machined and rejected parts.
A variety of components have been used to accurately locate workpieces. Among these are index pins mounted on or in the support plate and extending into precisely located holes in the workpiece. Round pins inserted into round workpiece holes may have problems such as lack of the necessary concentricity, a hole that has an axis that is slightly off the pin axis (angularly or otherwise), pins that tend to interfere or be loose in productions holes with variable diameters, etc. These problems, and others, largely keep full round index pins from having the necessary overall accuracy and flexibility.
Thus, there is a continuing need for improved indexing and clamping systems for workpieces to be machined, systems that have improved reliability, accuracy and versatility and do not extend over or obstruct the workpiece surface being machined, generally the surface opposite the surface in contact with a support or base plate and hold the workpiece firmly in place during machining.
SUMMARY OF THE INVENTION
The above-mentioned problems, and others, are overcome in accordance with this invention by an improved workpiece clamping and locating system for use in machining, in particular numerical control machining. The clamping system basically includes at least one index pin for locating a workpiece on a base plate, and edge clamping arrangement and a central clamping pin. The indexing and clamping pin may cooperate with other clamping devices that clamp workpiece edges to the base plate, etc. Typically an edge clamping portion may basically include a plurality of clamps, each having a simple clamp arm pivotally mounted on a support block that can be fastened to a base plate and preferably is longitudinally movable or adjustable on top of the base plate surface toward and away from a side of a workpiece supported on the base plate. Each clamp arm has a cam surface adjacent to the axis of rotation with the cam surface configured to move towards and away from an adjacent workpiece side as the arm is pivoted. The clamp arm includes means for pivoting the arm about the axis, preferably an extended handle for manual pivoting or a powered gear segment engageable with a powered drive for exact automatic movement toward and against the workpiece.
In the invention as claimed in this application, a workpiece is precisely positioned on a base plate by one or more indexing and clamping pins fastened to the base plate. Each pin has a head at the pin distal end extending above the base plate and an extension at the pin proximal end extending into a corresponding hole in the base plate. Each pin preferably has a generally spherical head with at least two radially spaced flats on the head, the flats lying generally perpendicular to the base plate when the index pin is installed on the base plate. The workpiece is positioned by lowering holes in the workpiece over correspondingly sized pin heads. The pins preferably have 2-4 spaced flats. In an optimum embodiment, two pairs of contiguous flats are provided, with the pairs on opposite sides of the head, so that the head has a generally diamond shaped plan view. While the spherical head is optimum for ease of insertion and accurate indexing, if desired the head could be cylindrical.
The index and clamping pin has an axial hole therethrough, a more narrow threaded hole in the extension and a wider threaded hole in the head. At least two, preferably four, equally spaced axial slots extend from the distal end of the head through the head to approximately the base of the head. Similarly, two or preferably four, equally spaced axial slots extend from the proximal end of the extension to almost the base of the head, with out intersecting the head slots.
The threaded holes in the head and extension receive setscrews, sized to expand the diameters of the head and extension as detailed below to bring the head and extension into forceful contact with the workpiece and base plate, respectively.
In a preferred embodiment, the head (where cylindrical) and/or extension have a plurality of protuberances extending outwardly toward the workpiece and base plate to press against and indent the hole walls and further secure the index/clamping pin in place against any forces tending to lift the workpiece above the base plate. The plural protuberances may have any suitable shape. For best results, the protuberances are shaped as normal bolt threads. The tips may spiral as in a bolt thread or may be circular, Optimally, the thread angle will be approximately 90° on the thread side toward the pin distal end and an a lesser angle toward the pin proximal end for best engagement with hole walls.
As an aid in remotely clamping a workpiece, an eccentric central index clamping pin may be provided if desired.
Any suitable base plate may be used with the clamping system of this invention.
Depending on the size of the workpiece, at least two index/clamping pins are used. If desired in particular circumstances the other clamping means as disclosed in this application may be used with those index/clamping pins.
REFERENCES:
patent: 1685899 (1928-10-01), Andrew
patent: 4387845 (1983-06-01), Mefferd
patent: 4500079 (1985-02-01), Morghen
patent: 5090608 (1992-02-01), Jones
patent: 5186441 (1993-02-01), Berti et al.
patent: 5527023 (1996-06-01), Starr
Harms Donn K.
Watson Robert C.
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