Robotic Welding Processes
1
Introduction
 Until the early 1950s, most operations in a typical manufacturing plant
were carried out on traditional machinery, such as milling machines, drill
presses, and various equipment for forming, shaping, and joining materials.
 Such equipment generally lacked flexibility, and it required considerable
skilled labor to produce parts with acceptable dimensional accuracy and
surface characteristics.
 Moreover, each time a different product was to be manufactured, the
machinery had to be retooled, fixtures had to be prepared or modified,
and the movement of materials among various machines had to be
rearranged.
 The development of new products and of parts with complex shapes
required numerous trial-and-error attempts by the operator to set the
proper processing parameters. Also, because of human involvement,
making parts that were exactly alike was often difficult, time consuming, and
costly.
2
 The important elements in improving productivity have been
mechanization, automation, and control of manufacturing equipment and
systems, as well as widespread adoption of communications and software.
 Mechanization controls a machine or process with the use of various
mechanical, hydraulic, pneumatic, or electrical devices; it reached its peak
by the 1940s.
 The next step in improving the efficiency of manufacturing operations was
automation, a word coined in the mid-1940s by the U.S. automobile
industry to indicate the automatic handling and processing of parts in
and among production machines.
 Efficiency became further increased by rapid advances in automation and
the development of several enabling technologies, largely through advances
in control systems, with the help of increasingly powerful computers and
software.
3
Figure1 Outline of topics described in Chapter 37.
4
Automation
Automation is an evolutionary rather than a revolutionary concept, and has been
implemented especially in the following basic areas of manufacturing activities:
• Production processes: Machining, forging, cold extrusion, casting, powder
metallurgy, and grinding operations.
• Material handling and movement: Materials and parts in various stages
of completion (called work in progress) are moved throughout a plant by
computer-controlled equipment, with little or no human guidance.
• Inspection: Parts are inspected automatically for dimensional accuracy, surface finish,
quality, and various specific characteristics during their production
(in-process inspection).
• Assembly: Individually manufactured parts and components are assembled
automatically into subassemblies and then into assemblies to complete a
product.
• Packaging: Products are packaged automatically for shipment.
5
History of Automation of Manufacturing
6
6-Axis KR030 KUKA Robot
Figure 2. (a) Schematic illustration of a 6-axis KR030 KUKA robot. The payload at
the wrist is 30 kg and repeatability is ±0.15mm (±0.006 in.). The robot has
mechanical brakes on all of its axes, which are coupled directly. (b) The work
envelope of the robot, as viewed from the side. Source: Courtesy of KUKA Robotics.
7
Devices Attached to End Effectors
Figure 3 Types of devices and tools attached to end
effectors to perform a variety of operations.
8
Types of Industrial Robots
Figure 4 Four types of industrial robots: (a) cartesian (rectilinear), (b) cylindrical, (c)
spherical (polar) and (d) articulated (revolute, jointed, or anthropomorphic)
9
Work Envelopes for Three Types of Robots
Figure 5 Work envelopes for three types of robots. The choice
depends on the particular application. (See also Fig, 37.17b).
10
Industrial Robot Applications
(a)
(b)
Figure 37.21 Examples of industrial robot applications. (a) Spot welding automobile
bodies with industrial robots. (b) Sealing joints of an automobile body with an industrial
robot. Source: Courtesy of Cincinnati Milacron, Inc.
11
Robot Gripper
Figure 37.24 A robot gripper with tactile
sensors. In spite of their capabilities,
tactile sensors are used less frequently
because of their high cost and their low
durability in industrial environments.
Source: Courtesy of Lord Corporation.
12
Spot Welding
•
•
•
A form of resistance welding, spot welding is one of the oldest welding processes
whereby two or more sheets of metal are welded together without the use of any filler
material.
The process involves applying pressure and heat to the weld area using shaped alloy
copper electrodes which convey an electrical current through the weld pieces. The
material melts, fusing the parts together at which point the current is turned off,
pressure from the electrodes is maintained and the molten “nugget” solidifies to form
the joint.
The welding heat is generated by the electric current, which is transferred to the
workpiece through copper alloy electrodes. Copper is used for the electrodes as it
has a high thermal conductivity and low electrical resistance compared to most other
metals, ensuring that the heat is generated preferentially in the work pieces rather
than the electrodes.
13
Principal Types of Resistance Welds
Electrodes
or Welding
Tips
Electrodes
or Welding
Wheels
Spot Weld
Electrodes
or Dies
Seam Weld
Projection
Welds
Projection Weld
14
[Reference: Resistance Welding Manual, RWMA, p.1-3]
Typical Equipment of Resistance Spot
Welding
(a)
(b)
[Reference: Welding Process Slides, The Welding Institute]
15
Advantages of Resistance Spot Welding
l
Adaptability for Automation in High-Rate Production of
Sheet Metal Assemblies
l
High Speed
l
Economical
l
Dimensional Accuracy
16
Limitations of Resistance Spot Welding
l
Difficulty for maintenance or repair
l
Generally, have higher cost than most arc welding equipment
l
Low tensile and fatigue strength
l
The full strength of the sheet cannot prevail across a spot
welded joint
l
Eccentric loading condition
17
Resistance Welding
• Resistance welding depends on three
factors:
– Time of current flow (T)
– Resistance of the conductor (R)
– Amperage (I)
– Electrode Force (N)
• Heat generation is expressed as
Q = I2R TK
18
Heat = I2 RTK
Where
I = Current (Amps)
R = Resistance (Ohms)
T = Time (Cycles 1/60
Second)
K = Heat Losses
Is a function of:
Transformer Tap Setting
Material Prop., & Pressure
Control Setting
Conduction, Convection,
Radiation
19
Factors Affecting Heat Generation (Q):
•
Welding pressure
– as welding pressure increases
both R and Q decrease.
•
Electrodes
– deformation of electrodes
increases contact area. As
contact area increases, both R
and Q decrease.
20
Resistance Varies with Pressure
Low Pressure
(a)
Medium Pressure
(b)
High Pressure
(c)
21
Surface Condition
Steel
Steel
(b) Rusted Conditions
Oils/Dirt
Oxide
Steel
Resistivity
(a) Pickled Conditions
Rusty
Polished
Pickled
Oxide
Oils/Dirt
Electrode Force
Steel
22
23
Where is Spot Welding Used?
Spot welding has applications in a number of industries, including automotive,
aerospace, rail, metal furniture, electronics, medical building and construction.
Given the ease with which spot welding can be automated when combined with
robots and manipulation systems, it is the most common joining process in high
volume manufacturing lines and has in particular been the main joining process in
the construction of steel cars for over 100 years.
24
Resistivity, mW-cm
Resistivity as a Function of
Temperature (self-stimulation)
130
120
110
100
90
80
70
60
50
40
30
20
10
HSLA
Low Carbon
100 200 300 400 500 600 700 800
Temperature, °C
[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]
25
Heat Dissipation
Water-Cooled Copper Alloy Electrode
Base Metal
Weld Nugget
Base Metal
26
Water-Cooled Copper Alloy Electrode
Initial Resistance Through Weldment
Top Electrode
Water
Distance
Weld
Nugget
Resistance
Bottom Electrode
27
Temperature Readings of A Spot Welding
Process
(Note: Temp at Electrode/Sheet Interface Higher than Bulk)
Workpiece
This illustration was taken
about 4/60th of a second
after the welding current
starts.
28
Nugget Solidification
29
WHY A ROBOT TO BE USED FOR WELDING?
1.
2.
3.
4.
Safety hazards
Undesirable job for workers (due to protective equipment that must be worn)
Heavy loads are sometimes involved
To achieve quality and product uniformity
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
30
BENEFITS OF ROBOTIC WELDING
•
•
Automatic line tracking is sometimes required, on assembly lines where welding is to
be done
Arc-on-time concept
• Manual welding (arc-on-time percentage is 2030%)
• Robotic welding (production level of 4 welders
•
“Weave Patterns” for weld paths
• Weave patterns ….. Zigzag path
• A robot can be programmed for weave patterns
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
31
MACHINE LOADING
•
Why robots to be used for machine loading?
•
Safety and relief
–
–
1.
2.
3.
4.
Safety and relief from handling heavy equipment
Risk of amputations while feeding punch press by hand
• To eliminate production slowdowns
• To achieve high operating speed
• Small clearances make manual feeding a tricky job
• Reduction of scrap is a side benefit
Single robot for single machine loading and unloading
Multiple robots for multiple machine loading
Sequential machine loading
Robots in forging and die-casting
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
32
MACHINE LOADING (Contd.)
• Positioning problem that
may occur (fig 10.4)
• A robot has to pick and
place on different
elevation levels where
robot is an axis-limit,
polar configuration robot
• Remedies
• Work envelop
• Suction cup pickup
devices
• Racks design
• End effecter with greater
flexibility
• Double handed grippers
• Figure 10.5
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
33
PROBLEM
(Robot Machine Loading System Productivity)
The robot machine loading / unloading system
diagrammed in figure has an eight seconds
cycle time and a daily two shift production level
of over 28,000 workpieces. Do the four milling
machines perform sequential operations upon
each workpieces in series, or do all four milling
machines perform the complete machining cycle
in parallel with each other – that is, is each part
processed completely by only one machine, not
all four?
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
34
PROBLEM
(Robot Machine Loading System Productivity)
Series operation:
Production rate / day
= 1part / 8sec X 60 sec/min X
60min / hr X 16hr/day
=7200 parts per day (two shifts)
Parallel Operation:
Production rate / day
= 1 part/8sec X 60 sec/min X 60
min/hr X 16hr/day X 4 machines
=28,800 parts per day (two shifts)
• => system runs on
parallel basis
• Discussion
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
35
SEQUENTIAL MACHINE
LOADING
• Single robot to load / unload
several machines (figures)
• One robot to load and unload
three machines with help of
two conveyors
• 60% increase in production
level is observed
• Double handed grippers are of
little value
– We intend to work on single
work piece sequentially
– When one job is complete on
one machine, the workpiece
would be unloaded and loaded
to the 2nd machine and so on
– Also because there is no room
for semi-finished parts in the
workcell
TEMPUS IV Project: 158644 – JPCR
Development of Regional Interdisciplinary Mechatronic Studies - DRIMS
ROBOTICS
36
What is a Dye Penetrant Testing?
•
Dye penetrant Testing or also known
as penetrant testing (PT), is a
commonly applied and low-cost
inspection method used to check
surface-breaking defects in all nonporous materials such as metals,
plastics, or ceramics. Penetrant can be
applied to all non-ferrous materials and
ferrous materials, although magneticparticle inspection (MPI) is often used
instead for ferrous components due to
its subsurface detection capability.
37
How Do you Perform Dye Penetrant Testing?
•
•
•
•
•
•
1. Pre-cleaning
2. Application of Penetrant
3. Excess Penetrant Removal
4. Application of Developer
5. Inspection
6. Post Cleaning
38