Metal working – Method of mechanical manufacture – With testing or indicating
Reexamination Certificate
1999-11-15
2001-01-23
Hughes, S. Thomas (Department: 3726)
Metal working
Method of mechanical manufacture
With testing or indicating
C029S522100, C029S524100, C029S525060, C029S715000
Reexamination Certificate
active
06176000
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a low-voltage electromagnetic riveting apparatus and method, and more particularly to a method and apparatus for controlled and efficient low-voltage electromagnetic riveting.
2. Background Information
Riveting machines are well known and in wide use throughout the aerospace industry, as well as in other industries. Rivets provide the best known technique for fastening an aerodynamic skin to a frame to provide a strong, aerodynamically smooth surface. Rivets are also used in the interior structure of an aircraft, since they are the lightest and least expensive way of fastening structural components together.
One form of riveting uses a low voltage electromagnetic riveting (LVEMR) system
100
, as shown in FIG.
1
. The LVEMR system
100
provides a controlled amount of energy in a single pulse and is typically smaller and less cumbersome than a pneumatic or hydraulic system. Further, the LVEMR system has almost no mass so it only has nominal reactionary forces. The LVEMR system
100
shown in
FIG. 1
incorporates two electromagnetic actuators, a first actuator
101
and a second actuator
112
, which are positioned on opposite sides of first and second workpieces
114
and
115
, respectively. The first and second work pieces
114
and
115
are sandwiched together and a hole has been drilled through them to accommodate a rivet
93
. The first and second actuators
101
and
112
each include a body
116
in which is positioned a driver
118
and a coil
120
. A rivet die
92
is coupled to the driver
118
and is forced against the rivet
93
. Also, there may be a recoil mass
123
which is typically secured to a rear surface of the coil
120
. Extending from the recoil mass
123
is an air cylinder rod
124
, which extends out of the body
116
into a two-chamber air cylinder
126
. Associated pressure relief valves and other control elements are shown diagramatically as block
128
. The elements of block
128
are responsible for initially positioning the driver
118
and its rivet die
92
against a head of the rivet
93
.
Power is supplied to the system
100
by means of a power supply
130
. A DC output from the supply
130
is used to charge a bank of capacitors in circuit
132
to a selected voltage. The voltage selected is based on the force necessary to accomplish the desired riveting task. The circuit
132
includes an electronic switch positioned between the capacitors and the coil
120
.
A trigger signal from a firing circuit
134
activates the electronic switch, dumping the charge of the capacitor bank in circuit
132
into the coil
120
. A current pulse is induced into the coil
120
causing strong eddy currents in a copper plate
119
located at the base of the driver
118
. This creates a very strong magnetic field that provides a repulsive force relative to the coil
120
. The driver
118
is propelled forward with a large force causing the rivet die
92
to upset the head of the rivet
93
. A more detailed discussion of low voltage electromagnetic riveting can be found in U.S. Pat. No. 4,862,043, which is incorporated herein by reference.
Once the LVEMR system
100
has upset the rivet
93
, a fastened assembly
140
is created as shown in FIG.
1
B. The assembly
140
includes a deformed rivet
146
, having a head
142
and a tail
154
. The hole drilled into the first and second workpieces
114
and
115
includes a countersink
148
drilled into the second workpiece
115
to receive the head
142
of the deformed rivet
146
.
Unfortunately, the fastened assembly
140
, when produced by the LVEMR system
100
described above, has significant gaps
150
between the head
142
of the deformed rivet
146
and the countersink
148
. The gaps
150
are undesirable since they could lead to early corrosion of the deformed rivet
146
, causing it to weaken and prematurely fail. Accordingly, for the foregoing reasons, there is a need in the art for a controlled low-voltage electromagnetic riveting apparatus and process that mitigates the gaps
150
between the rivet head
142
and the countersink
148
.
SUMMARY OF THE INVENTION
In one aspect, the present invention is directed to a method for minimizing undesirable gaps in riveted assemblies including the steps of selecting a rivet having a head and a tail with identical forming characteristics, positioning the selected rivet in an assembly that is countersunk on one of two sides, and applying a force over time to tile head of the rivet and a force over time to the tail of the rivet that are equal and opposite, compensating for force-unbalancing characteristics of the countersink.
In another aspect, the present invention is directed to a method for mitigating gaps between a deformed head of a rivet and a countersink in an assembly that is coupled by a low-voltage electromagnetic riveter having a head side actuator and tail side actuator. The method includes the steps of selecting a rivet that uniformly deforms at a tail and at a head of the rivet, positioning the volume of the rivet within the assembly such that force applied over time to the head of the rivet by the head side actuator equals a force applied over time to the tail of the rivet by the tail-side actuator.
In yet another aspect, the present invention is directed to a method for mitigating gaps between a head of a rivet and a countersink within a first workpiece of two workpieces when the rivet is upset by a low voltage electromagnetic riveting process. The method includes the steps of extending a tail of the rivet out of a surface of a second workpiece of the two workpieces by a length from 0.9 to 1⅓ times a diameter of the rivet, extending the head of the rivet out of a base of the countersink by a length that is 5% to 10% less than the length the tail of the rivet was extended out of the second workpiece surface, and upsetting the tail of the rivet with a tail die having a shape substantially similar to a shape of the countersink within the first workpiece.
In still another aspect, the present invention is directed to a method for controlled low-voltage electromagnetic riveting of a primary workpiece including a countersink and at least a secondary workpiece with a rivet, having a head, a tail, and a diameter, using a head actuator having a head die to contract the head of the rivet and a tail actuator having a tail die to contact the tail of the rivet. The method includes the steps of selecting the rivet so the rivet is comprised of a homogenous alloy and the rivet has a uniform diameter, positioning the tail of the rivet so that it protrudes from an outside surface of the secondary workpiece by a length from 1 to 1.3 times the diameter of the rivet, positioning the head of the rivet so that it protrudes from the base of the countersink by a length that is 5 to 10 percent less than the length that the tail protrudes from the step of positioning the tail, upsetting the head of the rivet with the head die having a flat contact surface, and upsetting the tail of the rivet with the tail die, wherein the tail die has an upper diameter within 20% of the depth of the countersink, and wherein the tail die has an upper diameter within 10 degrees of the upper angle of the countersink.
In another aspect, the present invention is directed to a low-voltage electromagnetic riveter for controlling the force over time applied to a head and a tail of a rivet within an assembly having a workpiece that is countersunk to receive the head of the rivet. The riveter includes a head and a tail actuator that respectively apply a force over time to the head and the tail of the rivet. Each of the actuators includes a die which contacts the rivet, a coil which creates a repulsive force when electrical current is passed therethrough, a driver physically adjacent to the coil and movable along an axis of the rivet by the repulsive force created by the coil, and a load cell positioned between the driver and the die to measure the force over time applied to a designated end of the rivet
Kostenick Paul G.
Lulay Kenneth E.
Cozart Jermie E.
Hughes S. Thomas
Nelson Lawrence W.
The Boeing Company
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