end effector

Arc Welding. Arc welding is used to provide continuous welds rather than individual spot welds at specific contact points. The resulting arc-welded joint is substantially

stronger than in spot welding. Because the weld is continuous, it can be used in airtight

pressure vessels and other weldments in which strength and continuity are required. There

are various forms of arc welding, but they all follow the general description given here.

The working conditions for humans who perform arc welding are not good. The

welder must wear a face helmet for eye protection against the ultraviolet light emitted by

the arc welding process. The helmet window must be dark enough to mask the UV radiation. High electrical current is used in the welding process, and this creates a hazard for

the welder. Finally, there is the obvious danger from the high temperatures in the process,

high enough to melt the steel, aluminum, or other metal that is being welded. Significant

hand–eye coordination is required by human welders to make sure that the arc follows

the desired path with sufficient accuracy to make a good weld. This, together with the

conditions described above, results in worker fatigue. Consequently, the welder is only

accomplishing the welding process for perhaps 20–30% of the time. This arc-on time is

defined as the proportion of time during the shift when the welding arc is on and performing the process. To assist the welder, a second worker is usually present at the work site,

called a fitter, whose job is to set up the parts to be welded and to perform other similar

chores in support of the welder.

Because of these conditions in manual arc welding, automation is used where technically and economically feasible. For welding jobs involving long continuous joints that

are accomplished repetitively, mechanized welding machines have been designed to perform the process. These machines are used for long straight sections and regular round

parts, such as pressure vessels, tanks, and pipes.

Industrial robots can also be used to automate the arc welding process. The cell consists of the robot, the welding apparatus (power unit, controller, welding tool, and wire

feed mechanism), and a fixture that positions the components for the robot. The fixture

might be mechanized with one or two axes so that it can present different portions of the

work to the robot for welding (the term positioner is used for this type of fixture). For

greater productivity, two fixtures are often used so that a human helper or another robot

can unload the completed job and load the components for the next work cycle while the

welding robot is simultaneously welding the present job. Figure 8.12 illustrates this kind

of workplace arrangement.

The robot used in arc welding must be capable of continuous path control. Jointedarm robots consisting of six joints are frequently used. Some robot vendors provide manipulators that have hollow upper arms, so that the cables connected to the welding torch

can be contained in the arm for protection, rather than attached to the exterior. Also,

programming improvements for arc welding based on CAD/CAM have made it much

easier and faster to implement a robot welding cell. The weld path can be developed directly from the CAD model of the assembly [9].

Spray Coating. Spray coating directs a spray gun at the object to be coated. Fluid

(e.g., paint) flows through the nozzle of the spray gun to be dispersed and applied over

the surface of the object. Spray painting is the most common application in the category,

but spray coating refers to a broader range of applications that includes painting.

The work environment for humans who perform this process is filled with health

hazards. These hazards include harmful and noxious fumes in the air and noise from the

spray gun nozzle. To mitigate these hazards, robots are being used more and more for

spray coating tasks, particularly in high-production operations.

Robot applications include spray coating of automobile car bodies, appliances,

engines, and other parts; spray staining of wood products; and spraying of porcelain

coatings on bathroom fixtures. The robot must be capable of continuous path control to

accomplish the smooth motion sequences required in spray painting. The most convenient programming method is manual leadthrough (Section 8.5.1). Jointed-arm robots

seem to be the most common anatomy for this application. The robot must possess a

work volume sufficient to access all areas of the work part to be coated in the application.

The use of industrial robots for spray coating offers a number of benefits in addition to protecting workers from a hazardous environment. These other benefits include

greater uniformity in applying the coating than humans can accomplish, reduced waste of

paint, lower needs for ventilating the work area because humans are not present during

the process, and greater productivity.

Other Processing Applications. Spot welding, arc welding, and spray coating are

common processing applications of industrial robots. The list of industrial processes that

are being performed by robots is continually growing. Among these are the following:

• Drilling, routing, and other machining processes. These applications use a rotating

spindle as the end effector. The cutting tool is mounted in the spindle chuck. One of

the problems with this application is the high cutting forces encountered in machining.

The robot must be strong enough to withstand these cutting forces and maintain the

required accuracy of the cut.