Planning manufacturing systems

 When planning manufacturing systems, the degree of automation that can economically be justified must be considered. Experience has shown that the most successful ones are those which are not fully automated. Part family concept  A part that has been designed for manufacturing usually has to be produced by several succeeding manufacturing operations. If there is a large spectrum of parts to be produced, it will be necessary for workpieces to share common processing equipment.  It is advantage to group parts together to families either according to their geometric similarities or to similar fabrication methods.  A change of parts would only require a new part program to generate a new contour. The parts form a design family, they are similar in design, and in this case can also be produced by a similar manufacturing process.

 The cubical parts which are not very similar any more; however, they also form a production family and can be made on the same multi-axis machining center.

 The dissimilar parts requiring at least one common process, which is to drill four holes. In this case, the other processes needed to shape the part would have to be done with different machine tools. These parts are typical for companies producing a wide spectrum of products.

 Two completely identical designed parts, one made from plastic and the other from steel. The manufacturing processes would be injection molding for the plastic reel and turning for the metal reel. In this case we have a common design family; however, the production processes are unrelated.  In group technology an attempt is made to find common manufacturing processes and to schedule parts through a manufacturing facility to maximize utilization of available machining resources and to emulate a flow line production process. A typical conventional layout of a manufacturing facility for similar rotational parts.

 The factory is divided into 10 different work centers. The raw material arriving in form of rods will be (1) stored in a warehouse, (2)retrieved and cut to slabs, (3) finishing machining operation, (4) milling, (5) sent to an external source for processing, (6) return to the factory a thread is cut, (6) a hole bored . From here (7) the part travels back to center (3) to be turned again, (8) deburring, (9) heat treating, (10) grinding, before leaving the plant is (11) inspected and (12) packed for shipping.

 If the parts were to be manufacturing according to group technology considerations, the plant would have to be realigned.

 The production process assumes a flow line operation with machine tools located in the flow line where they are needed. It can readily be seen that intraplant transportation is minimized. Setup operations and tool changes are also reduced. Classification Procedures  With group technology the workpieces and machining operations have to be classified. This implies that a suitable method of coding must be found which can easily be used for manual method or computer-aided classification procedures.  With the manual method the description of parts and processed are cataloged. When a workpiece is scheduled for production, a catalog search is made to find a suitable manufacturing process and sequence.  When the computer is used, the information about the part and the fabrication process is stored in a memory peripheral and the manufacturing data are retrieved automatically.  One of the main difficulties for coding is to decide which parameters are important for classification. No rigid rule can be given since the parameters may vary, depending on the part spectrum. A common practice is to separate rotational from non-rotational parts.  Classification by shape usually determines the manufacturing process, whereas the function is of interest to the designer. If a part that has similar functions is already in existence, the designer does not have to duplicate it.  The result of an industry survey where different parameters for typical workpieces were ranked in order of importance. The most important ones are candidates to be included in a classification system

 A similar study with sheet metal parts. The major shape, the material, and the material specification had the highest ranking.  For classification there are also many processing parameters that must be taken into consideration. This means that the types of processes within the plant and their limitations must be known.  Tool changes and setup times usually add considerably to processing cost. Similarly, the number of machining sequences should be kept to a minimum. The entire machining operation should be controllable by one operator; otherwise, difficulties may be encountered during rescheduling of production runs.  One of the most difficult tasks is to balance the load of available machine tools. In adverse conditions it may even happen that most operations are done on only a small number of machine tools and that the rest of the machines are underutilized. This may require an unduly large effort to balance machine tools.  There are many parts that cannot be handled by any known classification method. It is important to perform economic studies when group technology is introduced and to limit its application to parts that can be grouped together in a family reasonably well.

 In this example group technology can be applied to only about two-thirds of the parts at the lowest level. This figure shows the importance of investigating the entire product spectrum.

PRODUCTION LINES  Fundamentals of Production Lines  Manual Assembly Lines  Automated Production Lines Overview of Production Lines  Important class of manufacturing system when large quantities of identical or similar products are made  Total work is divided into small tasks, and workers or machines perform these tasks with great efficiency Examples:  Assembled products (e.g., automobiles and appliances)  Mass produced machined parts (e.g., engine blocks and transmission housings) Production Line A series of workstations arranged so that the product moves from one station to the next, and at each location a portion of the total work is performed

General configuration of a production line Production Line Operation  Production rate of the line is determined by its slowest station.  Transfer of product along the line usually by a mechanical transfer device or conveyor system.  Some manual lines simply pass the product by hand between stations.  Production lines are associated with mass production although some are used for batch production.

Product VariationsProduction lines can be designed to cope with variations in product models so long as differences between models are not significant (soft product variety)  Three types of line can be distinguished: 1. Single model line - produces only one model and there is no variation in the model 2. Batch model line –produces more than one model, but in batches 3. Mixed model line - multiple models are intermixed rather than produced in batches .Mixed Model LinesAdvantages over batch model line:  No downtime between models  Avoids high inventories of some models while other models are stocked out  Production rates and model mixes can be increased or decreased in response to changes in demand Problems with mixed model lines:  Assigning tasks to workstations so that all share an equal workload is more complex  Scheduling (determining the sequence of models) and logistics (getting the right parts to each workstation for the model currently at that station) are more difficult Methods of Work TransportManual methods - passing work units between stations by hand  Mechanized methods - powered mechanical systems to move work units along the line (e.g., lift-and-carry devices, powered conveyors)  Advantage: provides pacing of the line Mechanized Work Transport1. Continuous transfer systems - continuously moving conveyor that operates at a constant velocity (most common on manual assembly lines) 2. Synchronous transfer systems –work units are moved simultaneously between stations with a quick, discontinuous motion (common on automated transfer lines) 3. Asynchronous transfer systems - each work unit moves independently, rather than synchronously (permits tactical use of queues between stations) Two Basic Types of Production Lines1. Manual assembly lines –human workers at workstations performing assembly operations  Significant problem is line balancing 2. Automated production lines –workstations are automated  Significant problem is system reliability Manual Assembly Line

 A portion of a manual assembly line.  Each worker performs a task at his/her workstation.  A conveyor moves parts on work carriers from one station to the next. The Line Balancing ProblemProblem is to assign tasks to workers so that all workers have an equal amount of work  Total work to be accomplished on the line = the work content  The work content can be divided into minimum rational work elements, each element concerned with some small portion of the total work content  Minimum rational work elements must be grouped into logical tasks and assigned to workers at stationsWhy are Station Task Times Different?Different work elements require different times  When grouped into logical tasks and assigned to workers, the task times will not be equal simply due to the variable nature of element times  Restrictions on the order in which work elements can be performed, called precedence constraints  These and other limitations make it virtually impossible to achieve perfect balancing  Some workers will end up with more work, while other workers will have less Line Balance EfficiencyDefined as the work content time divided by the total available service time on the line Work content time Twc = sum of the times of all the work elements Total available service time on the line = w Ts Where w = number of workers on the line; and Ts = the longest service time on the line Thus, line balance efficiency Eb = Twc/wTsManning LevelNumber of workstations on a manual assembly line does not necessarily = number of workers  For a given station, manning level Mi = number of workers wi in the station  For the entire line, where M = average manning level for the assembly line; w = number of workers on the line; and n = number of stations Automated Production LineMultiple automated workstations connected by a parts transfer system whose actuation is coordinated with the stations’ operations  In the ideal, no human workers are on the line, except to perform auxiliary functions such as  Tool changing  Parts loading and unloading  Repair and maintenance

Operations Performed on Automated Production Lines  Operations performed by automated stations tend to be simpler than those performed by humans on manual lines  Tasks that are easy to automate consist of:  Single work elements  Quick actuating motions  Straight line feed motions as in machining

Types of Automated Lines Two basic categories: 1. Lines that perform processing operations  Example: transfer line 2. Lines that perform assembly operations  Example: automatic assembly machine

Layouts of Automated Production Lines  In-line layout as in a transfer line that performs machining operations  Circular around a dial-indexing machine  Carousel –common in automated assembly systems

A machining transfer line, an important type of automated production line

Configuration of a dial indexing machine Worktable rotates partially (indexes) at end of each cycle to position parts at the next station in the sequence Automated Assembly SystemsOne or more workstations that perform assembly operations, such as adding components and/or affixing them to the work unit Two basic categories: 1. Single station assembly cells - often organized around an industrial robot that performs a sequence of assembly steps 2. Multiple station assembly systems - used for mass production of small products such as ballpoint pens, cigarette lighters, flashlights, and similar items with relatively few components

Three common configurations of multiple station assembly systems: (a) in-line, (b) rotary, and (c) carousel