There are essentially four types of standard base assembly machines: Dial indexing machines
34 Assembly Process: Finishing, Packaging, and Automation
Floating work platform machines Continuous motion machines
To meet specific assembly automation needs, we add custom tooling. For maxi- mum cost effectiveness, we can choose from a stock of standard operating stations such as those for feeding, orienting, inspecting, and acceptance or rejection testing. Standard base machines and operating stations are outgrowths of the industry’s experience in designing, building, and using machines for high-volume production programs.
Dial Indexing Machines
The dial indexing automated assembly machines incorporate a mechanical drive that rotates a circular dial table, or base plate, and indexes with a positive cam action. A cir- cular, nonrotating table simultaneously raises and lowers a reciprocating upper tooling plate, usually mounted in the center of the larger rotating base plate. Assembly nests are installed around the outer edge of the dial table. Parts feeding, assembly, and inspection stations are installed around or above the assembly nests or on the upper tooling plate.
These machines offer the following advantages:
Greater machine accessibility and minimum floor space. The basic circular layout of dial-type machines is inherently more compact. High machine accessibility increases operator efficiency and simplifies maintenance. Greater adaptability to a variety of operations. The dial types of automated
assembly machines, containing central indexing mechanisms and recip- rocating tooling plates, offer simplified rotary and up-and-down tooling motions for high adaptability to many automated assembly operations. Figure 2.1 shows an elementary rotary indexing machine. The dial type described above has the added feature of a reciprocating tool table mounted above the indexing table.
In-Line Machine
In-line automated assembly machines feature a rectangular chassis housing an indexing mechanism driving an endless transfer chain. Nests that hold and transport the product during the various assembly operations are fastened to the transfer mechanism. The parts feeders, workstations, and inspection stations are then arranged along the work flow. Parts are fed into the assembly nests as required, and work and inspection opera- tions are performed in sequence along the length of the machine until the product is completed.
These machines have the following advantages: Unlimited number of workstations.
Efficient operator loading. The rectangular configurations permit machines to be placed side by side with an aisle in between. The operator can efficiently monitor all stations from the central aisle.
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Work can be performed from two or three directions simultaneously. Figure 2.2 shows an in-line indexing machine.
Floating Work Platform Machines
In floating work platform machines, parts flow into a manifold where they are located, assembled, and inspected. This system uses divergent flow channels for tandem or par- allel operations to achieve line balancing and consists of two major elements—a parts transporting element and a modular assembly element.
The parts transporting element moves the floating work platforms sequentially to the various modular assembly elements. Each modular assembly element consists of an independently powered unit containing one or more workstations. Use of a simple transporting band permits flexibility within the system. Modular assembly elements can be placed in remote areas such as cubicles, barricaded hazard rooms, holding or curing rooms, or storage banks and can be returned to the main system by the parts transportation element for further processing.
FIGURE 2.1 Rotary indexing machine. (From
Boothroyd, G., Assembly Automation and Product Design, Marcel Dekker, New York, 1992. With permission.)
Parts feeder Stationary workhead Work carriers Indexing table
36 Assembly Process: Finishing, Packaging, and Automation
Continuous band motion also permits the routing of parts onto a constant-motion machine for nonstop performance, as well as for routing to in-line or dial-type machines.
The quality of each assembly may be verified by inspection probes placed in tandem following the work performance at each of the workstations. Stations may be used for assembly function testing.
This type of machine has the following advantages:
Banks of parts may be accumulated between the workstations to cope with short station stoppages.
Work can be removed from the system, performed at a hand station, and returned to the machine.
Continuous-Motion Machines
Continuous-motion automated assembly machines provide for nonstop performance of operations. Such systems may be capable of up to 1200 assembly operations per minute. Parts are swept from a conveyor belt, oriented, and fed into the machine. Following assembly, inspection, and function testing, the assemblies are oriented and returned to the conveyor belt.
This type of machine has the advantage of higher production rates that can be achieved with other types of bases. The free-transfer machine shown in Figure 2.3 shows
FIGURE 2.2 In-line indexing machine. (From Boothroyd,
G., Assembly Automation and Product Design, Marcel Dekker, New York, 1992. With permission.)
Parts feeder Stationary workhead Completed assembly Work carriers indexed
Assembly Automation 37
a buffer position between the two workheads. The buffer parts could be shuttled off-line to another work position, and then back to the main feed line, in either system.
In general, the higher the production rate, the lower the per-unit cost of a product. Assembly automation systems are designed to fit production rates to specific needs. Good machine design considers more than the production rate; it also considers the overall production capability, including such factors as minimum maintenance, system efficiency over years of continuous operation, minimum training required for operators, and production of a consistently high-quality product. Flexibility is important in every assembly automation system. Modularized workstations, idle sta- tions, and standardized motions make systems adaptable to product changes with minimum downtime.
2.4.2 Robots
A robot can be broadly defined as a machine that copies the function of a human being in one respect or another. Industrial robots are generally equipment with a single arm, and they are used to perform assembly-line operations and other repetitive tasks such as feeding parts into another machine.
FIGURE 2.3 In-line free-transfer machine. (From Boothroyd, G., Assembly
Automation and Product Design, Marcel Dekker, New York, 1992. With permission.)
Partly completed assembly transferring to next station
Buffer stock Work carrier
Stationary workheads Parts feeders
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The assembly machines we described in the previous paragraphs are designed to handle large numbers of standard workpieces. These devices could be regarded as the forerunners of the modern industrial robot, although unlike most robots, they are controlled by the machine to which they are attached and may not be readily used to perform another function. The cost of reconfiguring the grippers or end effectors plus the reprogramming cost may make the robot less flexible in assignment than is sometimes imagined.
The real differences may be in the eye of the beholder—or in the mind of the reporter or author. The term robot comes from the Czech work robota, meaning work, and was first used in a play called “Rossum’s Universal Robots” written in 1920 by the Czech author Karel Capec. I personally find that the term robot to be rather imprecise and one that should normally be replaced by a more descriptive term. Perhaps the best definition is by example. A robot consists of three basic assemblies:
1. Motion system 2. Controller system 3. Heads and work tools