High volume production systems

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Chapter 2: high volume production systems, class presentation of Computer Integrated manufacturing for 6th sem BE degree, VTU.

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High volume production systems

  1. 1. 4/2/2014 1Hareesha N G, Dept of Aero Engg, DSCE, Blore
  2. 2. Syllabus 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 2
  3. 3. Automated Production Lines  Automated production lines are used for high production of parts that require multiple processing operations.  Each processing operation is performed at a workstation, and the stations are physically integrated by means of a mechanized work transport system to form an automated production line.  Machining (milling, drilling, and similar rotating cutter operations) is a common process performed on these production lines, in which case the term transfer line or transfer machine is used.  Other applications of automated production lines include robotic spot welding in automobile final assembly plants, sheet metal press working, and electroplating of metals.  Automated production lines require a significant capital investment. They are examples of fixed automation, and it is generally difficult to alter the sequence and content of the processing operations once the line is built.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 3
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  7. 7. • High production of parts requiring multiple processing operations • Fixed automation • Applications: – Machining transfer lines – Robotic spot welding lines – Sheet metal stamping – Electroplating of metals – Electronics assembly Features and Applications of Automated transfer lines 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 7
  8. 8. Where to Use Automated Production Lines? • High product demand – Requires large production quantities • Stable product design – Difficult to change the sequence and content of processing operations once the line is built • Long product life – At least several years • Multiple operations required on product – The different operations are assigned to different workstations in the line 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 8
  9. 9. Benefits of Automated Production Lines • Low amount of direct labor • Low product cost -because cost of fixed equipment is spread over many units. • High production rates. • Manufacturing lead time(the time between beginning of production and completion of a finished unit) and work-in-process are minimized. • Factory floor space is minimized. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 9
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  16. 16. Fundamentals of Automated Production Line  An automated production line consists of multiple workstations that are linked together by a work handling system that transfers parts from one station to the next, as depicted in Figure . 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 16
  17. 17. Fundamentals of Automated Production Line A raw workpart enters one end of the line, and the processing steps are performed sequentially as the part progresses forward.  The line may include inspection stations to perform intermediate quality checks.  Manual stations may also be located along the line to perform certain operations that are difficult or uneconomical to automate. Each station performs a different operation, so that the sum total of all the operations is required to complete one unit of work. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 17
  18. 18. Fundamentals of Automated Production Line Multiple parts are processed simultaneously on the line, one part at each workstation.  In the simplest form of production line, the number of parts on the line at any moment is equal to the number of workstations, as indicated in the figure. In more complicated lines, provision is made for temporary parts storage between stations, in which case there is on average more than one part per station. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 18
  19. 19. System Configurations Depending upon the workflow, the automated transfer lines are classified as below. 1) In-line (straight line) arrangement of workstations 2) Segmented in-line – two or more straight line segments, usually perpendicular to each other 3) Rotary indexing machine (e.g., dial indexing machine) 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 19
  20. 20. In-line (straight line) arrangement of workstations This configuration is common for machining big work pieces, such as automotive engine blocks, engine heads and transmission cases. Because these parts require a large number of operations, a production line with many stations is needed. The in-line configuration can accommodate a large number of stations.  In-line systems can also be designed with integrated storage buffers along the flow path. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 20
  21. 21. Segmented In-Line Configurations L-shaped layout U-shaped layout Rectangular configuration 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 21
  22. 22. Segmented in-line arrangement of workstations The segmented in-line configuration consists of two or more straight-line transfer sections, where the segments are usually perpendicular to each other. There are a number of reasons for designing a production line in these configurations rather than in a pure straight line, including: 1) Available floor space may limit the length of the line 2) It allows reorientation of the work piece to present different surfaces for machining 3) The rectangular layout provides for return of work holding fixtures to the front of the line for reuse. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 22
  23. 23. Two Machining Transfer Lines Figure: Line drawing of two machining transfer lines: At bottom right, the first is a 12- station segmented in-line configuration that uses pallet fixtures to locate the work parts. The return loop brings the pallets back to the front of the line. The second transfer line (upper left) is a seven-station in-line configuration. The manual station between the lines is used to reorient the parts. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 23
  24. 24. Rotary configuration The work parts are attached to fixtures around the periphery of a circular worktable, and the table is indexed (rotated in fixed angular amounts) to present the parts to workstations for processing. A typical arrangement is illustrated in Figure . The worktable is often referred to as a dial, and the equipment is called a dial indexing machine, or simply, indexing machine. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 24
  25. 25. Rotary configuration Although the rotary configuration does not seem to belong to the class of production systems called "lines," their operation is nevertheless very similar. Compared with the in-line and segmented in-line configurations, rotary indexing systems are commonly limited to smaller work parts and fewer workstations This configuration cannot accommodate buffer storage capacity.  The rotary system usually involves a less expensive piece of equipment and typically requires less floor space.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 25
  26. 26. Rotary Indexing Machine 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 26
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  29. 29. Work Transport Systems There are two basic ways to accomplish the movement of work units along a manual assembly line: (1) manually or (2) by a mechanized system. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 29
  30. 30. Manual Methods of Work Transport • In manual work transport, the units of product are passed from station-to-station by hand. • Two problems result from this mode of operation are starving and blocking. • Starving is the situation in which the assembly operator has completed the assigned task on the current work unit, but the next unit has not yet arrived at the station. The worker is thus starved for work. • When a station is blocked, it means that, operator has completed the assigned task on the current work unit but cannot pass the unit to the downstream station because that worker is not yet ready to receive it. The operator is therefore blocked from working.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 30
  31. 31. • To mitigate the effects of these problems, storage buffers are sometimes used between stations. • The work units made at each station are collected in batches and then moved to the next station. In other cases, work units are moved individually along a flat table or unpowered conveyor. When the task is finished at each station, the worker simply pushes the unit toward the downstream station. • Space is often allowed for one or more work units in front of each workstation. Hence, starving and blocking are minimized. • It can result in significant work-in-process • Workers are un-paced in lines that rely on manual transport methods, and production rates tend to be lower.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 31
  32. 32. Mechanized Work Transport Three major categories of work transport systems in production lines are: (a) continuous transport, (b) synchronous transport, and (c) asynchronous transport. These are illustrated schematically in Figure. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 32
  33. 33. continuous transport, synchronous transport, asynchronous transport 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 33
  34. 34. continuous transport system • A continuous transport system uses a continuously moving conveyor that operates at constant velocity, as in Figure (a). This method is common on manual assembly lines. • The conveyor usually runs the entire length of the line. However, if the line is very long, such as the case of an automobile final assembly plant, it is divided into segments with a separate conveyor for each segment. • Examples of this kind are overhead trolley conveyor, Belt conveyor, Roller conveyor, Drag chain conveyor.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 34
  35. 35. • Continuous transport can be implemented in two ways: (1) Work units are fixed to the conveyor, and (2) work units are removable from the conveyor. • In the first case, the product is large and heavy (e.g., automobile, washing machine) and cannot be removed from the conveyor. The worker must therefore walk along with the product at the speed of the conveyor to accomplish the assigned task. • In the case where work units are small and lightweight, they can be removed from the conveyor for the physical convenience of the operator at each station. • Another convenience for the worker is that the assigned task at the station does not need to be completed within a fixed cycle time.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 35
  36. 36. Overhead Trolley Conveyor • A trolley is a wheeled carriage running on an overhead track from which loads can be suspended • Trolleys are connected and moved by a chain or cable that forms a complete loop • Often used to move parts and assemblies between major production areas 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 36
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  40. 40. Belt Conveyor • Continuous loop with forward path to move loads • Belt is made of reinforced elastomer • Support slider or rollers used to support forward loop • Two common forms: – Flat belt (shown) – V-shaped for bulk materials (Support frame not shown) 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 40
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  46. 46. Roller Conveyor • Pathway consists of a series of rollers that are perpendicular to direction of travel • Loads must possess a flat bottom to span several rollers • Powered rollers rotate to drive the loads forward • Un-powered roller conveyors also available4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 46
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  52. 52. Skate-Wheel Conveyor • Similar in operation to roller conveyor but use skate wheels instead of rollers • Lighter weight and unpowered • Sometimes built as portable units that can be used for loading and unloading truck trailers in shipping and receiving4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 52
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  56. 56. synchronous transport systems • In synchronous transport systems, all work units are moved simultaneously between stations with a quick, discontinuous motion, and then positioned at their respective stations. Depicted in Figure (b), this type of system is also known as intermittent transport, which describes the motion experienced by the work units. • Synchronous transport is not common for manual lines, due to the requirement that the task must be completed within a certain time limit. This can result in incomplete units and excessive stress on the assembly workers. • Despite its disadvantages for manual assembly lines, synchronous transport is often ideal for automated production lines. • Examples of this kind are Walking beam transport equipment and Rotary indexing mechanisms. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 56
  57. 57. asynchronous transport system • In an asynchronous transport system, a work unit leaves a given station when the assigned task has been completed and the worker releases the unit. • Work units move independently rather than synchronously as in Figure (c). • Examples of this kind are Power-and-free overhead conveyor, Cart-on-track conveyor, Powered roller conveyors, automated guided vehicle system, Monorail systems, and Chain- driven carousel systems.4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 57
  58. 58. Workpart Transfer Mechanisms • Linear transfer systems: – Continuous motion – not common for automated systems – Synchronous motion – intermittent motion, all parts move simultaneously – Asynchronous motion – intermittent motion, parts move independently • Rotary indexing mechanisms: – Geneva mechanism – Others 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 58
  59. 59. Belt-Driven Linear Transfer System Side view of chain or steel belt-driven conveyor (over and under type) for linear transfer using work carriers 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 59
  60. 60. • Figure illustrates the possible application of a chain or belt driven conveyor to provide continuous or intermittent movement of parts between stations. • Either a chain or flexible steel belt is used to transport parts using work carriers attached to the conveyor. • The chain is driven by pulleys in either an "over- and-under" configuration, in which the pulleys turn about a horizontal axis, or an "around-the corner“ configuration, in which the pulleys rotate about a vertical axis. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 60
  61. 61. Walking Beam Transfer System 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 61
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  63. 63. • Many transfer lines utilize various walking beam transfer systems, in which the parts are synchronously lifted up from their respective stations by a transfer beam and moved one position ahead to the next station. The transfer beam then lowers the parts into nests that position them for processing at their stations. The beam then retracts to make ready for the next transfer cycle. The action sequence is depicted in Figure. (1) work parts at station positions on fixed station beam (2) transfer beam is raised to lift work-parts from nests (3) Elevated transfer beam moves parts to next station positions. (4) Transfer beam lowers to drop work parts into nests at new station positions. Transfer beam then retracts to original position shown in (1). 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 63
  64. 64. Geneva Mechanism with Six Slots 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 64
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  67. 67. • The Geneva mechanism uses a continuously rotating driver to index the table through a partial rotation, as illustrated in Figure. • If the driven member has six slots for a six-station dial indexing table, each turn of the driver results in 1/6 rotation of the worktable, or 60o. • The driver only causes motion of the table through a portion of its own rotation. For a six-slotted Geneva, 120° of driver rotation is used to index the table. The remaining 240° of driver rotation is dwell time for the table, during which the processing operation must be completed on the work unit. In general, Where θ= angle of rotation of worktable during indexing (degrees of rotation), and ns = number of slots in the Geneva. • The angle of driver rotation during indexing = 2θ , and the angle of driver rotation during which the work table experiences dwell time is (360-2θ). • Geneva mechanisms usually have four, five, six, or eight slots, which establishes the maximum number of workstation positions that can be placed around the periphery of the table. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 67
  68. 68. Given the rotational speed of the driver, we can determine total cycle time as: Where Tc = cycle time (min), and N = rotational speed of driver (rev/min). Of the total cycle time, the dwell time, or available operation time per cycle, is given by: Where Ts = available service or processing time or dwell time (min), and the other terms are defined above. Similarly, the indexing time is given by: Where Tr - indexing time (min). 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 68
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  71. 71. Cam Mechanism to Drive Dial Indexing Table •Various forms of cam drive mechanisms, are used to provide an accurate and reliable method of indexing a rotary dial table. •Although a relatively expensive drive mechanism, its advantage is that the cam can be designed to provide a variety of velocity and dwell characteristics. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 71
  72. 72. Ratchet and pawl mechanism • A ratchet is a device that allows linear or rotary motion in only one direction, while preventing motion in the opposite direction. • Ratchets are used in many other mechanisms, including clocks, jacks, and hoists. • Ratchets consist of a gearwheel (marked with a "b" in the diagram to the left) or linear rack with teeth, and a pivoting spring loaded finger called a pawl (marked with an "a" in that same diagram) that engages the teeth. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 72
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  74. 74. • Either the teeth, or the pawl, are slanted at an angle, so that when the teeth are moving in one direction, the pawl slides up and over each tooth in turn, with the spring forcing it back with a 'click' into the depression before the next tooth. • When the teeth are moving in the other direction, the angle of the pawl causes it to catch against a tooth and stop further motion in that direction. • Because the ratchet's teeth can only stop 'backward' motion at discrete points, a ratchet does allow a limited amount of 'backward' motion, or backlash, to a maximum of the spacing between its teeth. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 74
  75. 75. Rack and pinion mechanism • A rack and pinion is a pair of gears which convert rotational motion into linear motion. • The circular pinion engages teeth on a flat bar - the rack. Rotational motion applied to the pinion will cause the rack to move to the side, up to the limit of its travel. • The rack and pinion arrangement is commonly found in the steering mechanism of cars or other wheeled, steered vehicles. • This arrangement provides a lesser mechanical advantage than other mechanisms such as recirculating ball. 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 75
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  79. 79. Storage Buffers in Production Lines A location in the sequence of workstations where parts can be collected and temporarily stored before proceeding to subsequent downstream stations • Reasons for using storage buffers: – To reduce effect of station breakdowns – To provide a bank of parts to supply the line – To provide a place to put the output of the line – To allow curing time or other required delay – To smooth cycle time variations – To store parts between stages with different production rates 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 79
  80. 80. Storage Buffer Storage buffer between two stages of a production line 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 80
  81. 81. Storage Buffer )( 1 k )( 2 k 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 81
  82. 82. Control Functions in an Automated Production Line • Sequence control – To coordinate the sequence of actions of the transfer system and workstations • Safety monitoring – To avoid hazardous operation for workers and equipment • Quality control – To detect and possibly reject defective work units produced on the line 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 82
  83. 83. Applications of Automated Production Lines • Transfer lines for machining – Synchronous or asynchronous workpart transport – Transport with or without pallet fixtures, depending on part geometry – Various monitoring and control features available • Rotary transfer machines for machining – Variations include center column machine and trunnion machine 4/2/2014 Hareesha N G, Dept of Aero Engg, DSCE, Blore 83

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