Chapter 3 Assembly Systems. Screen Titles

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1 Chapter 3 Assembly Systems Screen Titles System Input / Output Methods of Assembly Manual Assembly Automated Assembly Flexible Assembly Choice of Assembly Assembly Economics Assembly Line Components Assembly Line Categories Indexing Line Factors Free Transfer Line Factors Continuous Line Factors Line configurations Rotary Indexing Table Free Transfer Carousel Continuous Moving Line System Matrix Review Exercise 1 Review Exercise 2 Off Line Exercise Chapter Summary Assembly Systems C.F. Zorowski 2004

2 Assembly Systems C.F. Zorowski 2004

3 1. Title Page Chapter 3 is an overview of generic assembly systems. It is important to have an appreciation of the different type of assembly systems in use to better apply the principles of design for assembly to be taken up in chapter 4. Topics covered in the chapter include assembly methods and their details, assembly economics, assembly line components, categories and configurations and schematic examples of three typical line layouts and operations. The development of a matrix of assembly methods versus line categories is provided for the reader to complete. 2. Page Index Listed below are all the pages in Chapter 3 by title. Each is hyperlinked to its specific page. It is recommended that the reader first proceed through all the pages sequentially. Any specific page can then be revisited by clicking on the title. Assembly Systems C.F. Zorowski 2004

4 3. System Input / Output Every assembly system irrespective of its layout or operation is subject to inputs and outputs. The inputs consist of the components that go into creating the product as well as the energy required to carry out the assembly process. The outputs are waste from the process and the product itself. The remainder of this chapter will deal with the details of the assembly system that is shown here as a generic process. 4. Methods of Assembly There are really only two basic methods of assembly. These are manual assembly and assembly by machines. Manual assembly is carried out by human operators and is normally done by hand. Assembly by machines is generally accepted as being made up of two separate categories: so called automated assembly and flexible assembly. Automated assembly is machine assembly by special purpose machines designed for a specific assembly function. It is often referred to as hard automation since it usually is a one of a kind devise performing one function. The second category of machine assembly is designed flexible automation or soft automation. These machine are programmable and can perform a variety of functions depending on their programming and capability. Assembly robots make up this category. Assembly Systems C.F. Zorowski 2004

5 5. Manual Assembly A number of the important characteristics of manual assembly are listed on this page. Tools used for manual assembly are usually simple and inexpensive. These might be hand tools like screwdrivers, wrenches, pliers or soldering irons as well as hand held power tools such as electric drills or power torquing devices. Downtime due to the existence of defective parts is almost negligible as the human operator serves to screen these out of the process. The cost per unit of product remains essentially constant with production rate. If more product is required more manual operators are hired. Their cost to produce a product effectively stays the same. The process is very flexible and adaptable due to the intelligence of the human operator that can be easily trained to handle assembly changes in the product. However, the speed and accuracy of the process is limited by the physical skills and ability of the operator as well as the number of coffee breaks they require in Monday mornings. 6. Automated Assembly Flexibility is severely limited by the very nature of the special design hardware that performs the assembly process. It may not permit any significant design change in the product. The cycle time and production rate of the process is also dictated by the design of the automated assembly device. However, high speed and accuracy can be achieved by appropriate design of the assembly machine itself. These special machines and the auxiliary devices that feed them with parts can be quite expensive to both design and build. Hence, there is a very high initial up front cost that results in the unit cost of assembly to decrease almost linearly with production rate. Finally, production on these special assembly machines can be seriously hampered by defective parts if no method is employed to screen bad parts out initially. Assembly Systems C.F. Zorowski 2004

6 7. Flexible Assembly Flexible assembly represents a compromise between the speed and accuracy of hard automation and the simplicity and adaptability of manual assembly. It possesses a greater degree of adaptability since it can be programmed to perform different assembly functions. This programmable capability permits multiple assembly functions to be performed at a given work station as compared to hard automation. Studies of the economics of robotic assembly indicate that its cost is between manual and hard automation on a unit cost basis. One of the reasons for this is that the initial design and development cost are spread out over the number of robotic machines built for general-purpose applications. Finally, robotic arms and work head can in general be modified to accommodate given needs with significantly less cost than hard automation. 8. Choice of Assembly Whether manual, automated or flexible assembly is chosen for putting together a specific product is dependent on a large number of variables. Four of the more important factors include the unit cost of the assembly process, the production rate required to meet market demands, the availability of appropriate skilled labor and the life of the product over which the assembly system can be amortized. This will be examined in more detail by comparing the economics of unit cost as a function of annual production rate for the three method of assembly described. Assembly Systems C.F. Zorowski 2004

7 9. Assembly Economics From the characteristics described for manual assembly recall that the unit cost is effectively constant with production rate since the rate is increased by hiring more operators but the unit cost of assembly stays the same. This is represented on the plot of unit cost versus annual production rate by the yellow horizontal line. In contrast the unit cost of hard automated assembly effectively decreases linearly with production rate and can be represented by the sloping green line. Where these two lines intersect represents the point where it is desirable to switch assembly methods to insure the lowest unit cost as a function of annual production rate. To the left of the intersection manual assembly should be used whereas to the right of the intersection hard automation provides a lower unit cost. Introducing robotic assembly is represented by the red curve. Its initial cost is less than hard automation but studies show that it tails off with production rate as illustrated. Considering all three methods there are now two intersection points where the method should be changed to provide the lowest unit cost. That is, for low annual production rates manual assembly is generally the best choice. As the production rate increases robotic assembly provides a lowest unit cost. For high annual production rates hard automation is the best selection. 10. Assembly Line Components Every assembly system requires three basic components to function properly. The first is a transfer devise or system that moves parts or assemblies from one workstation to the next. Different common types of transfers systems will be discussed later. Second, the assembly system requires parts feeders that deliver parts, components or sub assemblies to the workstation for the operator, robot or work head. These can vary from complex vibratory bowl feeders to simple boxes or magazines that hold the parts. Finally, the assembly system requires an assembly station at which either the manual, robotic or hard automation assembly operation is performed. Assembly Systems C.F. Zorowski 2004

8 11. Assembly Line Categories Generally one of three generic assembly line categories is employed in creating an assembly system. The first is called an indexing line. It is characterized by having all the products, in their various states of assembly, move at the same time to the work head at next fixed station in the line. That is, the product moves in a synchronized fashion through out all stations in the assembly system. It stops at each station while the assembly function is performed. The second type of line is called a free transfer line. It is characterized by having the product move on independent pallets or holders as needed by the work station. If a pallet leaves one work-station and the next work-station is still occupied the pallet is held in a buffer until the station becomes available. This system is obviously not synchronized. Work-stations accept the next job from the buffer in front of them when they become available to carry out their assembly function. The third type of generic assembly line is a continuous line. In this arrangement the product moves along continuously without stopping. This of course requires the manual operator or automated work head to move along with the product as it performs it assembly function. All three line categories have advantages and disadvantages that will be discussed next. 12. Indexing Line Factors In the discussions of line factors that follows on the next three pages positive factors are highlighted by a red check while negative factors will be marked with a red check that has been crossed out. Now an indexing line is generally the most compact of all three line types. It is particularly good for the automated assembly of small parts that can be mechanically fed to the work heads. It can also be designed to achieve high production rates with hard automation hardware. Some of its disadvantages include its rate of production being dictated by the slowest operation in the process since all movement is synchronized. It is also very susceptible to poor production or breakdowns due to defective parts if no screening devices are employed. It also requires station design and operation that locks all assembly operations together so they take place at the same time. Assembly Systems C.F. Zorowski 2004

9 13. Free Transfer Line Factors Free transfer lines can be used with all three methods of assembly. It is characterized by individual product movement on pallets or in some form of holder. This independent movement tends to average out variations in the line. The inclusion of buffers permits the individual stations to operate at different speeds independent of each other. This type of line is inherently safer for the operators and handles the effect of defective parts more easily since a single station can be shut down while the remainder of the line operates until all buffers are filled. The independence of the stations permits greater modularity in the automation and provides the opportunity for optional stations to be included. This allows different models of the same product to be made on the same line. The overall cost is generally higher than other lines because of the transfer hardware requirements, buffers and total line programming control needs. The production rates are also generally lower than an indexing line 14. Continuous Line Factors An assembly line that moves the product along continuously at a constant rate is both easier to design and fabricate. There is no stop and go movement of the product. This significantly simplifies its control. Both of these factors provide for lower initial cost compared to the other type of lines. The continuous line is better suited for larger products. The best example of this is the moving assembly line used to assemble automobiles. This was one of Henry Ford s biggest contribution to modern manufacturing. However, the pacing of the product is rigid as it moves along at the same rate as all other products on the line. It also requires the operator or assembly machine to move along with the product as the assembly function is performed. An important disadvantage of a continuous line is that any breakdown will shut down the entire line. Individual operator performance affects the overall production and quality of every product. An old story told in regard to this last item is don t buy a car built early Monday morning or late Friday afternoon. Assembly Systems C.F. Zorowski 2004

10 15. Line configurations There are three basic line configurations or geometry which are the most popular today. These are the rotary, carousel and in-line. The rotary is as the word implies. The product moves in a circular path through a number of fixed work stations normally coming on to and leaving the line at or near the same point or work station. The second type is called a carousel and is characterized by two parallel flow paths on which the product moves in opposite direction and is switched from one path to the other by crossover transfer devices. Again the product normally enters and leaves the line at or near the same point. This geometric arrangement provides the greatest flexible for the size and arrangement of the work stations. The inline configuration, which is effectively a straight path through the required workstations, usually takes up more space than the previous two. The product on an in-line system begins at one end and proceeds through each of the workstations until it is completed at the other end of the line. An automotive final assembly line is probably the best example of this geometric line configuration. 16. Rotary Indexing Table Schematic representations of three assembly systems displaying examples of different categories and configurations of lines are illustrated on the next three pages using common symbols for the work stations and the product. The first example is that of a rotary indexing table. The model system shown in plan view below consists of an indexing table, in blue cross hatch that rotates a quarter of a turn on each index movement. There are four fixed work stations, represented by the solid green circles equally space around the circumference of the table. The product is designated by small red circles. It goes through four stages of assembly as indicated by the numbers 1-4. Each time the table rotates the product proceeds on to the next station where the next assembly function is produced. All work stations are synchronized to operate at the same time when the product arrives. Following the assembly activity the product in indexed to the next required operation. Assembly Systems C.F. Zorowski 2004

11 17. Free Transfer Carousel The schematic model in plan view shown here is a free transfer carousel assembly system. Again the transfer line is in blue cross hatch, the fixed work stations are green circles and the smaller red circles represent the product. Added elements in this system are the buffers, represented by yellow rectangles, located between each work station. The movement of the product around this line is not synchronized. It is dictated by the availability of a workstation to accept it and the size of the buffer between stations. It is easily seen that control of this type of system is highly complex. In the example shown all buffers can only accept two products. Note that in the animation of the operation of the line some stations remain temporarily open and some buffers fill up as the product makes it way through the system. Unfortunately the animation gives the impression of synchronized flow of the product but this is only a graphical limitation of the model but not of a real system. 18. Continuous Moving Line This final schematic example is of a continuous moving line assembly system. The same symbols are used to depict the transfer line, the work stations and the product. This can be the simplest of all systems. Since the product moves continuously along the line it is recognized that the worker or assembly machine at a specific work station must move along with the product as the assembly function is performed. This is represented by the green dotted extensions to the work station symbols. Again the limitations of the graphical animation of the lines operation depicts a synchronized stop and go motion which is not true of a real system on which the product moves continuously. Assembly Systems C.F. Zorowski 2004

12 19. System Matrix To solidify what the reader has learned in this quick overview of assembly systems an exercise is now presented for completion by the viewer. The exercise is to establish a matrix of possible assembly systems matching assembly methods with system types. This will consist of a three by three matrix array as shown below. The reader is to decide which combinations of assembly method and system type can be put together by placing a yes or no in each square of the matrix as appropriate on the readers work sheet. After you have completed the entire matrix click on the button in any of the nine squares of the matrix to check your answer. When the button is clicked and held down either a yes or no will appear. When you release the button an audio explanation of that choice will be provided. You can click the buttons in any sequence you desire, however a button to move on to the last page of this chapter will only appear after the ninth button in the matrix is clicked. System Matrix Manual Indexing Manual assembly on an indexing line simply will not work well. It requires integrated sequencing and timing of the assembly function that would be difficult for a human operator to achieve and maintain with any real degree of consistency. 19. System Matrix (continued ) System Matrix Manual Free Transfer Manual assembly on a free transfer line works well since buffers can be included that allow the operators to work at their own speed and account for delays that may occur due to defective parts or other irregularities. System Matrix - Manual Continuous Manual assembly on a continuously moving assembly line is a very appropriate combination and is used particularly for large products like automobiles or motorized farm equipment. The operator simply moves with the product on the line as the assembly function is performed. System Matrix Automated Indexing This is an ideal combination particularly for small products or assemblies on a rotary indexing table with parts fed automatically to the automated assembly heads at each station. Such a system can achieve high production rates. System Matrix Automated Free Transfer This combination of assembly method and product transfer line can be used for a large range of product sizes. The line must be equipped with appropriate buffers between stations to balance the product flow and production rate. Assembly Systems C.F. Zorowski 2004

13 19. System Matrix (continued ) System Matrix Automated Continuous Automated work-stations on a continuous line is a combination not normally used. It would require a moving work-station that would have to be synchronized to move along with the product. This is not only physically difficult it represents an additional cost that is hard to justify. 19. System Matrix (continued ) System Matrix Flexible Continuous Although this combination of assembly device and transfer system can be made to work together it is not used very often. It requires the flexible robotic device to be programmed to synchronize the movement of its working head with the line in addition to the assembly function that it must perform. System Matrix Flexible Indexing A computer controlled flexible robotic work head can be easily combined with an indexing product transfer system. Since the work head is flexible it can also serve as either part of or the entire part feeding system in addition to being the assembly device. System Matrix Flexible Free Transfer This is the combination of assembly device and transfer system that is used most frequently with robotic work heads. Advantage can be taken of the programmable capabilities of both the robotic device and the free transfer system to achieve a high degree of flexibility permitting product variability on the same line. Assembly Systems C.F. Zorowski 2004

14 20. Review Exercise 1 This review exercise covers some of the important facts covered on assembly systems. It is completed like the review exercise in chapter 1. The reader types in an appropriate response in a blank when the audio finishes. Clicking the tab key will give an immediate feedback. If a negative feedback is provided enter another response. A double click on the tab key will move the cursor to another blank. If you need help with the answers click on the hot word in the question and the relevant page will pop up. When finished with all the blanks go on to the next page. 21. Review Exercise 2 In review exercise two you are to match the characteristics in the left column with one of the appropriate assembly methods on the right. Place the cursor over one of the items in the left column, hold down the left mouse and drag the pencil to the correct green dot on the right. Wait until the audio is completed before trying to do this. If the match is correct the red arrow will remain fixed to the green dot. If the match is incorrect the red arrow will disappear and you can try again. More than one arrow can be drawn to the same dot. When you have completed all the matches correctly go on to the next page by clicking on the page forward button. Assembly Systems C.F. Zorowski 2004

15 22. Off Line Exercise In this off line exercise you will apply what you have learned about assembly systems by proposing a system for a product you choose. Look around at some of the simpler everyday household or office products that you use. Select one that does not have too many individual parts and is relatively easy to put together or take apart. With what you have learned in this chapter and using engineering common sense propose an assembly system for putting this product together assuming some appropriate production rate. Be sure to take into account all the parts in your product and how they go together. In a brief report describe the system you propose and justify why you have selected it. Your system can be a combination of different kinds of systems if that is what your product requires. 23. Chapter Summary This chapter has provided the reader with an quick overview of generic assembly system and their characteristics. The subject is one that contains a great deal more detail and information than can be provided here. However, what the viewer should carry away from this chapter is an appreciation of the different methods of assembly, something about their economics, their required assembly components, assembly line categories and line configurations. With this information in hand we are ready to proceed to chapter 4 to take up the subject of the principles of design for assembly. Click on the menu button to return to the main menu and select chapter 4 to continue. Assembly Systems C.F. Zorowski 2004

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17 Assembly Systems C.F. Zorowski 2004