Electric heating – Metal heating – Cutting or disintegrating
Reexamination Certificate
2002-01-30
2003-09-30
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
Cutting or disintegrating
C269S152000
Reexamination Certificate
active
06627833
ABSTRACT:
TECHNICAL FIELD
This invention relates to a fixture for securing a workpiece. Specifically, the present invention is a fixture for securing a workpiece within an electrical discharge machining (EDM) apparatus.
BACKGROUND OF THE INVENTION
EDM is a common method of machining features into workpieces. Common examples of workpieces that undergo EDM include gas turbine engine components such as blades and vanes.
FIG. 1
displays a schematic of a typical EDM apparatus
10
. The apparatus
10
includes an electrode
11
, a holder
13
for the electrode, and a fixture
15
for supporting a workpiece W. The electrode
11
, holder
13
and fixture
15
reside within a tank
17
filled with dielectric (e.g. deionized water).
An actuator
19
advances the electrode
11
towards the workpiece W to create a suitable gap. Once the electrode
11
and the workpiece W receive suitable electrical cabling (not shown), power is supplied to the cabling. The current through the cabling creates an electrical potential between the electrode
11
and the workpiece W. The electrical potential between the electrode
11
and the workpiece W creates a spark therebetween. The spark vaporizes a portion of the workpiece W.
By controlling movement of the electrode
11
relative to the workpiece W, the EDM apparatus can produce many desired shapes with significant accuracy. These shapes include notches, slots and holes. To ensure accurate placement of these shapes on the workpiece, the fixture
15
must positively position the workpiece W.
FIGS. 2 and 3
display parts of a conventional EDM apparatus
50
relevant to this discussion. A fixture
51
for the workpiece includes two discrete toggle clamp assemblies
53
,
55
used to retain the workpiece against a nest assembly
57
. The nest assembly
57
conforms to the shape of the workpiece and includes contact points
59
that engage the workpiece. To isolate the workpiece from the fixture
51
, the contact points
59
should be electrically non-conductive. Ceramic is a preferred material for the contact points
59
.
FIG. 2
shows, in phantom, the preferred workpiece—a turbine blade. Specifically,
FIG. 2
displays two cross-sections of the turbine blade. One cross-section is taken at a distal region of an airfoil section of the blade. The other cross-section is taken at a proximal region of the airfoil section of the blade. The nest assembly
57
has six contact points
59
, the typical number for orienting an object at a desired position.
Each of the toggle clamps
53
,
55
has a spindle arm
61
,
63
, lever arm
65
,
67
, link
69
,
71
, and mounting bracket
73
,
75
. Using pivot pins, the spindle arm
61
,
63
mounts to the bracket
73
,
75
and to the lever arm
65
,
67
. Likewise, the lever arm
65
,
67
mounts to the link
69
,
71
and to the spindle arm
61
,
63
. Finally, the link
69
,
71
mounts to the bracket
73
,
75
and to the lever arm
65
,
67
. In this arrangement, rotation of the lever arm
65
,
67
by an operator O away from the spindle arm
61
,
63
can actuate the spindle arm
61
,
63
toward the nest assembly
57
.
After placing the workpiece into the nest assembly
57
, the operator O actuates the toggle clamps
53
,
55
in series to secure the workpiece within the fixture
51
. Several drawbacks to this arrangement exist.
First, actuation of the toggle clamps
53
,
55
can pull the workpiece away from at least some of the contact points
59
in the nest assembly
57
. As seen in
FIG. 2
, the movement of the spindle arm
61
,
63
follows actuation path P. The actuation path P is directed away from some of the contact points
59
. When the spindle arm
61
,
63
engages the workpiece, the actuation path may cause a gap to form between one or more contact points
59
. Typically, gaps form at the contact points
59
located on the upper portion of the nest assembly
57
. The gaps occurs at these locations since the difference between actuation path P and the contact points
59
is most severe.
Second, sequential actuation of the toggle clamps
53
,
55
can pull the workpiece away from at least some of the contact points. Since the workpiece is elongated, actuating a toggle clamp at one end may lift the workpiece from the contact points
59
at the opposite end. Both of these drawbacks can reduce the accuracy of the EDM operation.
While ensuring accurate machining, the EDM apparatus should also have features ergonomically suitable to the operator O. In terms of ergonomics, several drawbacks to the EDM apparatus
50
exist.
First, a confined space exists between a rear of the fixture
51
and a wall
77
of a tank
79
containing the fixture
51
. The space is limited due to the size of the EDM tank
79
. The operator O must reach over the fixture
51
and into the confined space to reach the toggle clamps.
Second, the confined space limits the size of the toggle clamps
53
,
55
. A smaller clamp has a smaller lever arm. Thus, an operator must exert a larger force to the toggle clamp to achieve the same torque as a mechanism having a larger lever arm.
Third, the arrangement of the toggle clamps
53
,
55
requires the operator O to actuate the toggle clamps
53
,
55
along an engaging path E directed towards the rear of the EDM apparatus
50
. Residing within the tank
79
, the toggle clamps
53
,
55
may become wet. The operator's hand could lose grip on the toggle clamps
53
,
55
. Potentially, the operator's hand could then collide with the tank
79
.
Fourth, the toggle clamps
53
,
55
may pinch the operator O during release. The operator O disengages the toggle clamps
61
,
63
along a release path R directed towards the front of the EDM apparatus
50
. Since a tip of the spindle arm
61
,
63
is preferably made from rubber, the tip is under compression. As the operator releases the toggle clamps
53
,
55
, the actuation of the tip rotates the spindle towards the rear of the EDM apparatus
50
. In other words, the spindle arm
61
,
63
and the lever arm
65
,
67
approach each other during release of the toggle clamp
61
,
63
. Unfortunately, pinching may occur if the hand or finger of the operator O is positioned between the spindle arm
61
,
63
and the lever arm
65
,
67
during release.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide an improved EDM apparatus.
It is a further object of the present invention to provide an improved workpiece fixture.
It is a further object of the present invention to provide a fixture that positively locates the workpiece against each contact point.
It is a further object of the present invention to provide a fixture that simultaneously locates the entire workpiece against the contact points.
It is a further object of the present invention to provide a fixture having improved ergonomics.
It is a further object of the present invention to provide a fixture capable of actuation with only one hand.
It is a further object of the present invention to provide a fixture in which actuation of a clamping occurs in a direction oriented towards the front of the apparatus.
It is a further object of the present invention to provide a fixture in which the clamping assembly parts move in the same direction when engaging and releasing the workpiece.
It is a further object of the present invention to provide a fixture having a larger lever arm.
It is a further object of the present invention to provide a fixture than can retrofit into an existing EDM apparatus.
It is a further object of the present invention to provide a fixture useable on different EDM apparatuses.
These and other objects of the present invention are achieved in one aspect by a fixture for securing a workpiece. The fixture comprises a base; a plurality of locators on the base for accepting the workpiece; a plurality of arms, each secured to the base at a pivot point; and a lever for actuating the arms. The arms positively position the workpiece against the locators.
These and other objects of the present invention are achieved in another aspect by an electrical discharge ma
Varsell Richard W.
Voisine Donald Joseph
Evans Geoffrey S.
Hamilla Brian J.
United Technologies Corporation
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