Process Planning in Manufacturing Systems
Manufacturing systems students usually complete lab or shop projects for class requirements. The projects often involve significant amounts of manufacturing processes, and require the students to work very hard, sometimes extra hours at school or home to meet deadlines. When they do work at home, they often use their parents’ tools, and purchase materials with their money. During the period of, and after the, construction of the project, their parents’ yards and garages might be disorganized and some tools may be missing, damaged or misplaced.
Although these technically-minded students often score highly on their projects, several lessons could be learned from the way they often undertake them:
1. They have no idea how many resources (time, tools and materials) would be required to build the project until they got into building it.
2. Their tasks are, for the most part, technically unorganized (out of sequence) and unplanned.
3. They cannot estimate the dollar worth of their labor in the whole project.
4. They hardly think that housekeeping is very important to their safety.
5. They seldom think that manufacturers can’t make any profits operating like this.
Every manufactured product has some planning associated with it. In manufacturing systems, this is technically referred to as process planning. One of the first tasks of the manufacturing personnel when they receive new drawings is to perform the process plan. This task, when completed, generally directs both the organization of needed resources and the actual production of the product.
In many school shops and laboratories however, the notion is that manufacturing begins when a designer produces and releases full-proof working drawings to the manufacturing personnel. Erroneously, most manufacturing students envision manufacturing as something a company just jumps into without adequate planning and preparation. This paper outlines the basics of process planning and provides examples for school shop and laboratory instructors who may need to teach their students the basic principles of process planning.
The practice of process planning in manufacturing provides precise and clear sequential directions about how the product is to be routed and fabricated in a manufacturing facility. In advanced manufacturing, this will influence how the facility will be designed and laid out in preparation for the new product. For school shops and laboratories, this mostly helps to guide students from one process to the next logical one, thus simplifying their manufacturing project activities. This explanation will begin with the technical drawings.
CAD or Manual Drawings: The first step in preparing a process plan is to secure a good drawing or drawings of the project. Because the drawings represent the initial ideas and plans for the product, “The design of production processes starts with the product designer” (Wright, 1990, p. 412). In many schools today, computer-aided drafting (CAD) is taught and can be used to create the student’s ideas into quality, dimensioned drawings with achievable specifications. Either manual or CAD drawings are suitable as long as students know how to read and interpret them. The dimensioned drawings should contain important information including the following: complete and clear graphics, material types, part name, drawing number, owner name, date, units, appropriate set of views showing all required dimensions, tolerances with reasonable values for each dimension, clear titles and labels, and should be easy to read.
Study and Separate the Drawings into Parts: The drawings should be carefully studied the way one reads a manual, to understand all the details contained in them. It is important to separate the drawings into their parts at this juncture. Each part should have a clear label so that it can be identified. After separating the parts, the reader should try to answer such questions as: “How should each of these parts be processed?”; “What types of tools and machines will be needed to process each piece?”; “How many units of each part should be fabricated?”, and “How long should it take to process each piece?” These questions can be addressed by studying the wooden stool shown in Figure 1. The stool has one seat, four legs and four supports, resulting in a total of nine parts. These parts will be used for illustrations in the following sections.
In a typical manufacturing setting, each of the different parts, i.e. the seat, legs and supports will have their separate drawings, dimensions and notes. Each drawing will bear the features associated with it. For example, the legs will have the holes features at the correct spots. Moreover, the drawings for the legs and supports will include a notation that four legs and four supports are required respectively. For the raw material used for illustrations in the following sections, it is assumed that a 1” thick X 12” diameter seat will be fabricated out of a 1.25” X 13” X 13” piece of lumber, the 2” diameter by 24” long legs out of a 2” diameter X 100” long dowel, and the 1” diameter by 12” long supports out of a 1” diameter X 50” long dowel.
Figure 1: Wooden
Figure 1: Wooden Stool
Identify, List, and Sequence Required Operations for Each Part: The identified tasks or processes required to fabricate each part should be listed below it. This step requires a careful study of each part and determining the various manufacturing processes needed to fabricate it into the shape shown in the drawing. Successful completion of this step often requires a good knowledge of manufacturing processes and shop processing equipment, but students who are not familiar with shop processes and machines can consult their instructors at this stage.
The listed tasks are then sequenced so that they follow the order in which they will be performed. The sequencing is important because the listing simply listed the identified processes required to fabricate each part, but did not arrange them in the order or sequence in which they will take place during the fabrication process. Again, successful completion of this step will require a good knowledge of manufacturing and shop processing equipment, but students who are not familiar with shop processes and machines can consult their instructors at this stage. Table 1 illustrates what this may look like for the wooden stool.
Seat Legs Supports
(4 Required) (4 Required)
1. Plane stock to thickness 1. Mark lengths of legs 1. Mark lengths of support
2. Layout seat circumference 2. Cut out legs 2. Cut out supports
3. Saw rough circumference 3. Form the tapers 3. Sand smooth
4. Smoothen seat edges 4. Drill two holes 4. Stain
5. Round seat edge 5. Sand smooth 5. Apply finish
6. Sand smooth 6. Stain 6. Dry
7. Stain 7. Apply finish 7. Store
8. Apply finish 8. Dry
9. Dry 9. Store
It is important to number the sequenced processes at this stage. In Table 1, the numbers (also called task numbers) indicate the sequence in which the processes will take place. For example, the circumference of the seat must be sawed, and then the edges rounded before sanding takes place.
Sometimes a part can have a flexible sequence of operations. For example, operations 2 (cut stock to length) and 4 (drill two holes) could be reversed for the legs in Table 1. When such a situation arises, the process planner should employ the sequence that will yield greater benefit to the person, company or customer.
Assign Time Data, Equipment, and Tooling to the Sequenced Processes: To complete the planning, it is necessary that the machines and tooling needed to process each part, as well as the time it takes to complete each process are assigned. The time unit should be in minutes and all machines and tools must be clearly identified. This is illustrated with the stool’s seat in Table 2. Completed process plans for the legs and supports are not included here but can be ideal classroom or lab exercises for students.
Determining how much time a process takes to complete (also called standard time) is beyond the scope of this discussion. Therefore, the assigned time for each process should be an educated estimate of how much time that process should take to complete. This method is ideal for classroom purposes. But in industrial application, published standard time data, time study results and experts’ opinions are used. The estimated time should include times taken to retrieve tools, set up equipment and perform other related but not specified tasks. The total time is also calculated for each part.
Task Time Machine Tooling
1. Plane stock to thickness 5 Planer Goggles
2. Layout seat circumference 7 NA Ruler/dividers
3. Saw rough circumference 12 Band saw Eye goggles
4. Smoothen seat edges 20 Wood lathe Skew
5. Round seat edge 10 Wood lathe Skew
6. Sand smooth 5 Wood lathe Sand paper
7. Stain 5 NA Brush
8. Finish 5 NA Spray can
9. Dry 15 Blower NA
10. Store 60
Total time 144 minutes
The discussions so far have been on basic process planning. As has been observed, as new information emerges, it can be added to the plan. For a detailed process planning, however, the process chart is very useful for documenting all the
A Process Chart
PRODUCT NAME: Seat
Planing the stock
Storage of part
PREPARED BY: John Doe
APPROVED BY: James Doe
important details needed to accomplish safe and efficient fabrication of the seat discussed above. According to Meyers and Stephens (2005), “The process chart is used for just one part, recording everything that happens to that part from the time it arrives in the plant until it joins the other parts” p. 146. A process chart usually has standard operation symbols and other important elements in it for use by the process planner.
Figure 2 shows what a more comprehensive and detailed process plan for the seat would look like when performed with a process chart. As can be seen in the chart, operations such as basic inspection and setup of needed machines are sequentially included to systematically show when each process takes place. The process chart also identifies beginning and ending operations, the key personnel involved in fabricating the part, the date the chart was completed, and total number of each type of process used.
In Figure 3, an operations (or assembly) chart shows how the stool’s various parts are assembled. This chart helps manufacturing personnel such as assemblers to visualize the proper sequence of operations in assembling a product. In advanced manufacturing which is beyond the scope of this paper, the chart will include all the processing time elements for each step, and the total number of minutes required to assemble the entire product.
Figure 3: Operations (Assembly) Chart
Plane stock to thickness Mark lengths of legs Mark lengths of support
Layout seat circumference Cut stock to length Cut stock to length
Saw rough circumference Form the tapers Sand smooth
Smoothen seat edges Drill two holes Stain
Round seat edge Sand smooth Dry
Sand smooth Stain
stool Pack stool
Join supports to legs Install
Join supports to legs
Process selection is influenced by several factors, including required quantity, materials that the parts are made of, surface finish requirements, as well as the specified tolerances. Selecting a process without considering the influence of these factors on it could adversely affect the cost, quality, and ease of manufacturing the parts. These factors and their influences are listed and explained in Table 3.
Factor Potential Influences
Quantity Large lot sizes justify expensive tooling/process
Materials Some materials require a different process. For example, aluminum can be cast, wood cannot
Surface finish requirement Different processes produce different finishes. A process with a better finish can help in avoiding unnecessary secondary processing
Specified tolerances Tight tolerances require expensive tooling
Some Uses of the Process Chart
It can be seen from the foregoing discussions that the process chart has some useful purposes in the manufacturing industry. One of the primary uses is for documenting and filing the procedures for processing new and existing products. Most companies have in their possessions similar documents filed for documentation and for training new employees who may not be familiar with the procedures for fabricating the product. It is also a good source of reference material for companies to use during programs.
The time element of the process chart is an invaluable piece of information that is often used in product cost estimation. When an order is received by a job shop, a cost estimation is usually generated so that the customer will know how much it will cost the company to fabricate the parts contained in the order. The wooden stool’s seat, for example, would cost about $45.5 in labor cost to fabricate if it took 186 minutes to fabricate and the operator charged $15 per hour to do the job. The material and overhead costs are also added to this estimate to generate the final cost to the customer.
The process chart also helps in the preparation of route sheets. Route sheets are instruction papers that are attached to parts lots as they make their way around a plant during processing. They contain instructions, specifying which workstation to route the parts to and which comes next, until the parts are completed. The process chart makes the task of preparing the route sheet a simple one in that the route sheet is a simplified version of the process chart. The route sheet does not contain all the other details like processing time and set up of machines.
One of the most important uses of the process chart is for improving manufacturing processes. The chart helps to reveal unnecessary and time-wasting processes, which can be eliminated, combined or modified. In today’s highly competitive manufacturing industry, engineers use such charts to help them get a better picture of how their existing manufacturing systems look like compared to their target. In advanced manufacturing, this type of analysis helps in developing ideal facility layout and placement of machines for efficient operation. Often, this also helps in the preparation of tooling and purchase of ideal equipment needed for production.
Implications for Manufacturing Programs
It has been noted that process planning involves determining the most appropriate manufacturing processes and the order in which they should be performed to produce a given part or product specified by design engineering. There are apparent advantages in the foregoing discussions. What is left at this juncture is how to take them to the students who are engaged in lab and shop processing activities in schools.
One apparent advantage is the safety aspect for students, since they will be directed to work in a more systematic, organized and meaningful manner during and after classes. Simplicity will be incorporated in their manufacturing tasks leading to increased productivity and more likeness for manufacturing programs. There will be more structure and orderliness in the labs and school shops as systematic instructions become the order of the day. Significant learning will also result.
Manufacturing programs are
some of the most fulfilling human activities in the
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Upper Saddle: Prentice Hall.
Rehg, James A. & Kraebber, Henry W. (2005). Computer-Integrated Manufacturing.
(3rd Ed.) Prentice-Hall:
Wright, R. T. (1990). Processes of Manufacturing.