Process Sequencing and Economic
Processing Considerations
Dr. Pulak M. Pandey
http://paniit.iitd.ac.in/~pmpandey
Traditional Process Sequences
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Years of process planning teaches engineer that
certain processing sequences are almost standard
For example, the tolerance on a given hole might
require a certain amount of reaming. Before
reaming can begin , the hole must be drilled. So in
this case processing pattern of drill –ream is
established.
Virtually all production operations tend to fall into
such standards, or traditional processing sequence.
This means that most known production processes
are often preceded by other preparatory processes.
Advantages of Process Sequencing
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By arranging the processes necessary in
the production of any given part, the
process planner can achieve one or more
of the following economic benefits:
1. Proper use or maximizing of all dimensional
tolerances given on the product drawing
2. Process elimination
3. Utilization of in process operations, which, in
effect results in additional processing without
increase in the part’s direct labor content
Process Elimination
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Earlier companies have developed process plans in hurry
for quick entry into the market. This practice was based
on the fact to get into market first and then keep on
improving the processing.
This plan of doing it later is very costly in terms of
capital equipment, durable tooling, direct labor, and
administrative overheads.
In the today’s scenario of competitive manufacturing
environment, a modern process planner must analyze,
from the outset, all required processes for possible
combination or eliminations.
Example: Die cast part
Sample routing
Revised routing
Die cast part: In the original routing roll
forming taps are used for taping. It need
additional operation of chip blowing. Once
roll forming taps are replaced by cut taps
operation 20 gets modified. It results into
increased production rate and hence
improved productivity.
One minor change in process sheet result
into increased quality and lowered cost.
Example: sample part
The second routing eliminates the de-burring
operation and its cost. Surface B was milled
after the hole was drilled So the burr created
through drill breakthrough was cut off by
milling operation 30.
In this example, inexpensive changes
eliminates processing labor and its related
equipment costs. This kind of thinking cannot
be a afterthought; it must be a conscious and
deliberate effort that takes place as the original
process plan is developed.
In-Process Operations
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It is characterized by one or more of the
following attributes:
– It happens automatically
– It takes place on the route between other definite
operation
– It involves little or no direct labor
– It happens secondarily as the result of another
required operations
In-process operations
This figure shows how a series of in-process operations can be
placed between two direct labor operations. In-process operations
such as these tend to lower production costs while increasing
quality. The process planner should take advantage of this concept.
Profitability of each operation
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What machine is required?
Will the machine be capable?
Is there capability in existing machine?
Can the cost of new machine is justified?
How should the machine be tooled?
What kind of hourly production can be expected
from one machine?
How does the plant’s labor-and-burden rate affect
other required decisions?
Machine Selection
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Since there are a variety of complicating variables, selecting a
machine capable of accommodating the individual processing
needs of each operation can be a difficult task.
Process planner should choose the best machine tool being one
of the capital investment. The best choice will be contingent
upon other economic factors such as the following:
1. Tooling cost
2. Set-up cost
3. Frequency of change over
4. Versatility requirement
5. Production rate
6. Basic machine cost
7. Ease of automation
8. Direct labor cost
Break-Even Charts
Pay back comparison
More elaborate methods of financial analysis do exist and
sometimes used. However, a majority of manufacturing
companies have selected the payback comparison method for
use in justifying capital purchase. A two year pay-back period
seems to be a common benchmark used in judging investment
proposals.
Example
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A machine tool needs to be purchased.
The cost of machine is $100,000.
At 100% efficiency, machine produces 20 parts per
hour.
A plant-wide efficiency of 75% can be expressed.
The plant runs 3 shifts per day, yielding 21.5 productive
hours of work each day.
The plant runs 250 days per year
The sales dept. will like to charge 30 cents per part as
profit.
A long term contract will be awarded.
The company like a 2 year payback on all capital
investments.
Existing Machinery
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If existing machinery in the process planner’s plat
can be used, great financial benefits will be
realized due to the following factors:
¾The equipment was previously justified and
paid for with prior of existing business.
¾The equipment’s true capability and potential
for productivity is known.
¾The equipment is in-house; therefore, there are
no waiting or delivery problems.
¾People in the plant are already familiar with the
equipment. The need for training is thereby
minimized and potential for immediate
production is enhanced.
Machine Capability
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A process engineer must assure about the capability of
machine tool to repeat and hold the necessary tolerances
before releasing the payment of new machinery or before
selecting an existing machine in the plant.
These checks are carried out by using statistics and control
charts. These items allow the engineer to draw statistically
sound conclusions from small amount of data.
A process capability study is used to to measure those
variations in the process that result from common or normal
causes. This definition means that a broken or dull tool
would not be considered a common cause. In effect, the
process-capability study is only accurate when everything is
set up and running correctly.
Average and Range Chart
If all subgroup
plot points falls
randomly within
the control limits
on both the
average and
range chart,
without having a
pattern, then
process is said to
be under a state
of statistical
control.
Capability Vs. Tolerance
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Once the process is established as under the state of
statistical control, engineer should check the capability
of process to hold the tolerances.
The capability of the process is defined as six standard
deviations (6S) of the subgroup variation.
Process capability percentage is the 100 times ratio of
6S to tolerance range.
A process capability of 75% or more is generally
accepted. It means that 99.73% of the time, all parts
run will fall within a range only three quarters the size
of the part print tolerance.
Some companies want to work at lower process
capability percentage so that less inspection and
machine adjustment will be required.
Three sigma limits
Other economic factors
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Tooling costs
– Estimating design time and cost
– Estimating build time and cost
– Estimating material cost
– The volume effect and cost
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Production Rates/costs
– Direct labor content and overheads