Data processing: structural design – modeling – simulation – and em – Structural design
Reexamination Certificate
1999-08-03
2003-01-14
Frejd, Russell (Department: 2123)
Data processing: structural design, modeling, simulation, and em
Structural design
C703S007000, C700S182000, C700S193000
Reexamination Certificate
active
06507806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the art of fabricating objects using machine tools, and more specifically to a Computer Aided Design (CAD) system for unambiguously constructing Feature Control Frames (FCF) and to Computer Aided Tolerance Analysis (CATA), Computer Aided Manufacturing (CAM) and Computer Aided Inspection (CAI) systems for automatically constructing Datum Reference Frames (DRF) for machine parts.
2. Description of the Related Art
A machine part or object made of metal or other material is conventionally formed or machined using a motorized tool such as a press or a milling machine by immobilizing the part in a holding fixture, and engaging appropriate surfaces of the part with forming or cutting tools to move or remove material and thereby form the part into the required shape.
The precision with which a part must be manufactured can be extremely high, with tolerances often expressed in microinches. With such precision comes the need to accurately determine the location of the cutting tool relative the other features on the part. For example, if a hole must be drilled at a certain distance from an edge of a part, means must be provided to establish a frame of reference in which to measure this distance and accurately position the cutting tool.
This need is fulfilled by a Datum Reference Frame (DRF), which is a Cartesian coordinate system relative to which the locations and attitudes of machine part features are defined. Whereas one or more DRFs may be defined in each part, a DRF is not a physical entity, but rather an imaginary construct to which physical features on a part are geometrically related.
A system of standards has been established for dimensioning and tolerancing using DRFS. These standards are presented in a publication entitled “Dimensioning and Tolerancing”, ASME Y14.5M-1994, American Society of Mechanical Engineers 1995. Further standards are set forth in a publication entitled “Mathematical Definition of Dimensioning and Tolerancing Principles”, ASME Y14.5.1M-1994, American Society of Mechanical Engineers 1995. These publications are incorporated herein by reference in their entirety.
A DRF is defined by a small number, typically three, of specially selected features on a part called Datum Features (DF), which, if engaged by a holding fixture, render the part immobilized. The immobilizing components of a holding fixture or a functional gage can be seen as the inverses of the datum features, and are referred to as Datum Feature Simulators (DFS) The origin, axes, and planes of the DRF constructed with the help of said Datum Features are referred to as Datums.
For example, a planar surface of a part can be used as a primary datum feature to eliminate pitch, yaw, and one degree of translational freedom, with other datum features being used to eliminate the three remaining degrees of freedom which are roll and two additional degrees of translational freedom. The precise geometrical orientations and locations of the remaining features of the part are then controlled relative to the DRF so constructed.
Prior to machining or inspecting a part, a holding fixture or functional gage is produced. The part is clamped in said fixture such that its datum features mate with the datum feature simulators of the fixture, whereby the DRF of the part is brought into alignment with the DRF of the fixture and therefore with the coordinate system of the machine or measuring tool. This enables the features of the part to be reliably machined and inspected using the dimensions specified in the engineering blueprint or formal drawing.
The concept of a DRF can be better understood through the presentation of an illustrative example, which takes a simple part from the concept stage, through all the ensuing drawing, manufacturing and inspection stages.
At the outset, it is useful to make a simple perspective sketch of the part, which is shown in FIG.
1
and designated by the reference numeral
10
. Further illustrated are an origin O and the X, Y and Z axes of a DRF which will serve to control the location of the part's features.
If the part
10
were handled at this point, it would be discovered that it has six Degrees of Freedom (DOF); it can pitch, yaw and roll (three degrees of rotational freedom), and translate in the X, Y and Z directions (three degrees of translational freedom). Since the coordinate system is attached to the part, it goes wherever the part goes, making it clear that coordinate systems also have six degrees of freedom.
The six degrees of freedom are illustrated in FIG.
2
. In the particular case shown, pitch is about the Y axis, yaw about the X axis and roll about the Z axis. Translation is indicated by a coordinate system X′,Y′,Z′ which is translated from the coordinate system X,Y,Z by offsets &Dgr;X,&Dgr;Y,&Dgr;Z.
In order to be manufactured, the conceptual part must be defined in a formal drawing or Computer Aided Design (CAD) data base. In addition to creating its general outlines and dimensions, it is important to select certain features to determine the coordinate system responsible for locating and orienting the other features.
The most reliable of such “datum” features are probably (1) the bottom of the part
10
which is designated by the reference character A, which can eliminate pitch, yaw, and one degree of translational freedom in the Z direction as indicated above; (2) a long edge B of the part
10
which can eliminate roll and one more degree of translational freedom in Y; and (3) a short edge C of the part
10
which can eliminate the last degree of translational freedom in X.
These edges are selected to constitute datum features A, B and C respectively, and control the remaining features using ASME Y14.5M tolerancing tools. A formal drawing of the part
10
is shown in
FIG. 3
, in which Feature Control Frames (FCF) for position and surface profile incorporate all the information required to construct the intended coordinate systems.
As viewed in
FIG. 3
, the part
10
includes a hole
12
that is to be drilled 2.750 inches from the bottom edge B, and 3.000 inches from the left edge C. The formal drawing includes an FCF
14
for the hole
12
, which specifies that the hole
12
is to have a diameter of 1.000+0.020 inches, and that the center of the hole
12
must lie within a cylindrical tolerance zone of diameter 0.015 inches at Maximum Material Condition (MMC) having its axis at the basic location (3.000,2.750). The concept of MMC will be described in detail below.
The FCF
14
further includes datum feature references
14
a
, in this case to the planar surfaces A, B and C. This specifies that a DRF for the part
10
is to be constructed using the datum features A, B and C in the order or sequence listed in the FCF. The formal drawing, including the FCF
14
with datum feature references
14
a
, is prepared by the engineer who designs the part
10
, and must be adhered to exactly during all stages of the manufacture, inspection, etc. of the part
10
.
Assuming that the part
10
is to be manufactured from a rough forged billet, it is secured in a milling machine vise (not shown), and the top surface is cleaned up using a rotary cutting tool
16
as illustrated in FIG.
4
. This results in datum feature A, which coincides with the machine's X-Y base plane as soon as the Z axis of the machine's digital readout is reset to zero.
In the same set-up, the front surface is milled perpendicular to A and parallel to the machine's X axis. This results in datum feature B, which coincides with the machine's X-Z base plane once the Y axis of the machine's digital readout is reset to zero, after correcting for the tool radius. Repeating the process for the right hand surface, the datum feature C is produced.
FIG. 4
illustrates the results of the process as described thus far.
Finishing the part at this point would consist of milling the remaining two sides, boring the hole
12
, flipping the part
10
over, and cleaning up the
Arter & Hadden LLP
Frejd Russell
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