Process Planning in Manufacturing Systems
Introduction
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.
Steps to 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
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.
Table 1
Sequenced List of Identified Processes Required to Fabricate a 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
10. 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.
Table 2
Process Plan for Seat
______________________________________________________________________
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 Process Chart
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
Figure 2:
A Process Chart
|
PRODUCT
NAME: Seat |
FLOW BEGINS Planing
the stock |
FLOW ENDS Storage
of part |
DATE |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
PREPARED
BY: John Doe |
APPROVED
BY: James Doe |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Total Time = 186 Minutes
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 advanced manufacturing, the process chart can be
automated, which helps to simplify the task of putting it together and updating
it when orders for parts with similar features are received in the future. In
such an environment all related information is on the computer and, the process information can be viewed on
computer terminals (Rehg &
Kraebber, 2005).
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
Seat Legs Supports
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
Stain Dry
Dry
Inspect
stool Pack stool
Join supports to legs Install
seat
Factors Influencing Process Selection
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.
Table 3
Factors Influencing Process Selection
______________________________________________________________________
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
References
Meyers, F. E. (2005). Manufacturing Facilities Design and Material Handling.
3rd. Ed.
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.
Willcox Company.