Patent Application
20080276444
This disclosure generally relates to clamping and squeezing devices, and deals more particularly with a method and apparatus for performing fastener installation operations, especially the process of upsetting rivets.
Solid rivets are used extensively to fasten parts in a variety of applications. In the aircraft industry, rivets are commonly used to join structural members and to fasten exterior skins on airframes. In the case of relatively soft rivets, one end of the rivet may be upset using high impulse forces from a rivet gun. However, where the rivet is relatively hard, the upset process is performed by squeezing the rivet between an anvil and a forming die. One type of apparatus used to squeeze rivets includes a C-shape frame having a pair of opposing jaws defining a throat into which a workpiece may be placed. One of the jaws includes a tool in the form of an anvil that bears against one end of the rivet, sometimes referred to as the “factory head”. A linear actuator mounted on the other jaw includes a forming die that engages the opposite end of the rivet, sometimes referred to as the bucktail. With the factory head held against the anvil, the actuator pushes the forming die into the bucktail, deforming the end of the rivet to create a button, thereby upsetting the rivet in place.
The linear actuator described above may be hydraulic, pneumatic or electromagnetic, and usually includes an elongate cylinder or body that extends outwardly from the C-shape frame. In some applications, the geometry and placement of the actuator on the C-shape frame may result in physical interference between the actuator cylinder and the workpiece that prevents the jaws from being positioned around the rivets. For example, a workpiece to be riveted may be held and clamped in place on jigs and/or fixtures on an interior side of the workpiece where the bucktails of the rivets are located. These structural interferences and the size of the actuator may prevent the button forming jaw from being positioned in back of the rivet.
One solution to the problem discussed above involves moving the actuator to the opposite jaw so that the anvil is driven into the factory head of the rivet. This solution is not satisfactory in many applications where pushing on the manufactured head of the rivet may result in damage to the rivet and/or the parts being fastened.
Accordingly, there is a need for a method and apparatus for squeezing rivets that overcome the problems discussed above. Embodiments of the present disclosure are intended to provide a solution to these problems.
The disclosed embodiments provide a method and apparatus for squeezing rivets which place the actuator developing the squeeze force on the jaw of the C-shape frame that applies force to the factory head of the rivet. As a result, the opposite jaw holding the button forming die that upsets the rivet may pass freely in back of the workpiece where jigs and/or fixtures provide minimum clearance for entry of a jaw. The actuator holds the anvil on the factory head of the rivet while producing a force that displaces the C-shape frame relative to the anvil so as to cause the forming die on the other jaw to deform and upset the bucktail of the rivet. The translating movement of the C-shape frame during the button forming process is enabled by slidably mounting the C-shape frame on a support, such as a robotic arm. A counterbalance in the form of a spring or pneumatic cylinder, for example, biases the C-shape frame toward a standby position after the button is formed.
In accordance with one disclosed embodiment, a method of upsetting a rivet in a workpiece is provided, comprising the steps of: placing the workpiece between opposing jaws of a frame; moving an anvil from one of the jaws into engagement with a head on one end of the rivet; and, upsetting the other end of the rivet by linearly displacing the frame to force a die on the other jaw against the other end of the rivet. Engagement of the head by the anvil is maintained as the rivet: is being upset by the die. The frame is linearly displaced along an axis that is parallel to the longitudinal axis of the rivet.
In accordance with another disclosed embodiment, a method is provided of squeezing a fastener, comprising the steps of: positioning the fastener within the throat of a C-shape frame; linearly displacing an anvil from the frame into engagement with the first end of the fastener; and,
while the anvil engages the first end of the fastener, linearly displacing the frame to bring a tool on the frame into engagement with and apply pressure to a second end of the fastener. The frame and the anvil are displaced along axes that are parallel to the longitudinal axis of the fastener.
According to a further embodiment, an end effector for performing a rivet installation operation on a workpiece is provided, comprising: a support; a frame mounted on the support for movement along an axis between a standby position and a displaced position, the frame including a pair of jaws between which the workpiece may be positioned; an anvil mounted on one end of the jaws for linear displacement relative to the frame; a tool carried on the other end of the jaws for forming a button on a first end of the rivet; and, a drive for moving the anvil and the tool toward each other into respective engagement with the first and second ends of the rivet. The support may be a robotic arm, and the drive may include a linear actuator mounted on the one jaw. The anvil and the frame are each displaceable along a common axis parallel to the longitudinal axis of the rivet. The biasing device may be connected between the support and the frame, and may comprise a spring or a fluid cylinder such as a pneumatic cylinder. The end effector may further comprise a sensor for sensing the displacement position of the frame relative to the support. A controller may be provided for controlling the operation of the drive and the robotic arm.
In accordance with a further disclosed embodiment, apparatus is provided for squeezing rivets in an aircraft assembly. The apparatus comprises a computer controlled robot having a robotic arm; a frame having first and second jaws between which the aircraft assembly may be positioned; an anvil on the first jaw; an actuator for moving the anvil relative to the frame into engagement with a factory head on one end of the rivet; a tool on the second jaw for deforming the other end of the rivet; a slide mounting the frame on the robotic arm for linear displacement along the longitudinal axis of the rivet.
Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims.
Referring first to
As best seen in
A biasing device 32 has one end thereof connected to the robotic arm 26 by a bracket 33, and the other end thereof connected to the rear plate portion 22 by means of a clevis 35. The biasing device 32 comprises a pneumatic cylinder in the illustrated embodiment, however other forms of biasing means are contemplated, including electromagnetic, hydraulic or mechanical devices, such as a simple spring. The biasing device 32 provides a counterbalancing force that normally urges the end effector 20 to be displaced along axis 34a to a standby position when a rivet squeeze operation is not being performed.
A frame position sensing device 54 (
The C-shape frame 24 includes a pair of opposing jaws 24a, 24b defining an open throat 25 that may receive portions of a workpiece that are to be riveted. The C-shape frame 24 may be formed from any suitably rigid material such as high strength steel, aircraft grade aluminum, titanium or a composite material. The frame 24 may have configurations other than C-shape, providing the frame has a pair of opposing jaws 24a, 24b. The depth of the throat 25 should be sufficient to accommodate the workpieces to be riveted.
A linear actuator 38 is mounted on jaw 24a which may comprise a conventional, commercially available pneumatic, hydraulic or electromagnetic cylinder having a linearly displaceable output shaft 40. A tool which may be in the form of a flat anvil 42 is mounted on the end of the shaft 40, and is intended to engage the factory head of the rivet 50. In one embodiment, the shaft 40 and anvil 42 are linearly displaceable in the direction of the arrow 44 along an axis 34b. The other jaw 24b includes a button forming die tool 46 which is intended to engage and upset the bucktail end of the rivet 50. The exact configuration of the die tool 46 will depend upon the shape of the button that is to be formed.
Referring to
Referring now concurrently to all the figures, the first step in squeezing a rivet 50 using the end effector 20 is shown at step 56 in
Next, at step 58 (
After the anvil 42 has engaged the factory head of the rivet 50, continued extension of the actuator shaft 40 transmits a reactive force to the frame 24 as a result of the actuator 30 being mounted on the jaw 24a. As a result of this reactive force, the frame 24 begins translating along axis 34a, resulting in the displacement of jaw 24b and thus the die 46, toward the bottom end of the bucktail 50a, as shown at step 60 in
It should be noted here that steps 58 and 60 can be reversed, if desired. Thus, the robot 28 may move the C-shape frame 24 to bring the forming die 46 into close proximity or initial contact with the bucktail 50a. Then, the controller 52 may energize the actuator 38, resulting in the displacement of shaft 40 until the anvil 42 engages the factory head of the rivet 50, following which continued extension of shaft 40 results in a reactive force that is transmitted through the jaw 24b, causing the die 46 to deform the bucktail 50a.
As the actuator shaft 40 begins to retract as shown in step 64 and illustrated in
In the disclosed embodiment, the counterbalancing effect provided by the biasing device 32 should be sufficient in magnitude to overcome gravitational force when the axis 34a of movement is vertically oriented. Further, the counterbalancing force exerted by the biasing device 32 should be sufficient to maintain the frame 24 in its standby position while being moved and positioned to a rivet location by the robot 28. However, the force imposed on the frame 24 by the biasing device 32 should not be so great t h a t it adversely affects the rivet squeezing process. In other words, the frame 24 should effectively be “free-floating on the slide assembly so that a material lateral force is not imposed on the tools (anvil 42 and die 46) during the rivet squeeze process. The magnitude of the counterbalancing force exerted by the biasing device 32 may be adjusted by the controller 52, depending upon the attitude of the end effector 20, and/or can be eliminated or maintained during the rivet upset process, as may be required in a particular application.
In some applications, the biasing device 32 may not be required. For example, in an application where the frame 24 is maintained in an attitude such that the axis 34a is vertical, gravity will provide the force necessary to return the frame 24 to its standby position. In such an application, the force developed by the actuator 38 would have to be sufficient to effectively “lift” the frame 24 from its standby position and complete the squeeze process.
From the forgoing, it may be appreciated that the end effector 20 described above may provide successful rivet upsetting within close quarters as a result of several features. By placing the linear actuator 38 on the jaw 24b that faces the manufactured head of the rivet 50, interference with structures on the backside of the workpiece 48 may be avoided.
Further, by slidably mounting the frame 24 on the robotic arm 26 using a linear bearing slide, the C-shape frame 24 is allowed to translate linearly as the actuator arm 40 extends and retracts during the rivet upsetting cycle.
Finally, the use of a counterbalance provided by the biasing device 32 offsets the weight of the end effector 20 as the rivet 50 is being upset, resulting in a minimum amount of force being transmitted to the workpiece 48 and in any fixture/jigs that may be supporting it. The counterbalance force provided by the biasing device 32 also maintains the frame 24 in an extreme position of travel against a stop when in the standby position. This feature prevents the end effector 20 from sliding freely along axis 34 during changes in attitude of the end effector 20, when moving between rivet locations, and ensures that the die 46 is precisely aligned along the longitudinal axis of the rivet 50, and therefore is in a known location when being positioned on a rivet 50.
Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art. For example, although the disclosed embodiments have been described in connection with upsetting rivets, the embodiments may be employed to squeeze other parts, such as clamping workpiece parts.
