CHAPTER 18: SCHEDULING
Effective Production
Control
Plan--->Develop
production plans and schedules
Control-->Monitor
Control-->Revise
as necessary to achieve conformance to plans and stay "on" schedule.
*
An effective information system is essential to success. (Computerized Information systems can be very
helpful)
Scheduling--developing
and assigning specific dates for the start and/or completion of the necessary
work tasks.
Controlling production--collecting and analyzing the data necessary to evaluate the
progress on jobs relative to plans, and initiating corrective measures as
necessary.
Expediting--is
the application of personal attention to improve progress on specific
jobs. (A corrective measure)
Routing--specifying
the flow through work stations necessary to produce a product.
Job shop
variable
Batch Processing
semi-variable
Repetitive
fixed
Objectives of work center Scheduling:
1. Meet due dates
2. Minimize lead times
3. Minimize setup time and cost
4. Minimize work-in-process inventory
5. Maximize machine and labor utilization
Scheduling
is a 2 stage process
1.
Loading
2.
Dispatching
Loading--assigns
work to a work center but doesn't establish a sequence of jobs through that
workcenter.
We
use load reports to determine what capacity is available for use.
Dispatching--sequencing
of the work through a work station.
(n--no. of jobs/1--no. machines)
Which job first, second,...
*
Use some decision rule to decide which of the n jobs at a machine goes first.
Select
1 of n jobs
then: select 1 of n-1 jobs
then: select 1 of n-2 jobs
Dispatching Rules:
1.
Johnsons Rule (2 or 3 work centers)
2.
FIFO
* 3. SPT Shortest processing
time (
4.
EDD Earliest Due Date
5.
LAS Least Average Slack
Johnsons Rule
1.
List processing time for each job.
2.
Select job with shortest time at either W.C.
3. If shortest time is in 1st W.C.
schedule ASAP
If shortest time is in 2nd W.C.
schedule ALAP
If tie:
1st station ASAP
2nd station ALAP
If same job
schedule arbitrarily
4.
Remove job(s) from list and continue.
Job
Shop Scheduling is quite complex.
Often
an order is received for a product we have never built. In such cases we must first design the
product, develop specifications, B.O.M., estimates of labor costs, material
costs, lead times, etc. to come up with a bid.
THESE ACTIVITIES MUST BE SCHEDULED ALSO.
Therefore there is a L.T. associated with work that must be done before
production begins and there are costs associated with those activities.
Proposal from Customer
Design
and Specifications
Estimating
Procedure
Bid
Production
Delivery
*
The success of a company depends on how well they can do not one but all
of these things.
A
company may use:
a. BACKWARD scheduling--start with due date and back up as you schedule using lead times
b. FORWARD scheduling
Production Control in Batch Processing.
MRP
System is very helpful in this environment because it is very helpful for
planning and control. It doesn't just
schedule material orders and receipts==>it can schedule production
activities. Accurate info. is
critical--must be timely also.
With
scheduling done the focus shifts to Production Activity Control 2 Major components
1. Input/Output Control=> Balancing workloads and capacity at work
centers to prevent starvation & Queues (W.I.P.)
2. Priority Control=> attempts to make sure
that jobs worked on are the ones that
are most needed.
OBJ.
1. Keep lead times predictable
2. Utilize capacity efficiently
3. Keep Queues to a minimum
Expediting
(de-expediting): necessary because in a
job shop things don't always go as planned. (most of the time in the majority
of job shops.)
There
would not be many problems if orders of materials were received on time, the
various work stations worked at a predictable rate all the time, nothing got
scrapped, people were predictable, and capacity was adequate. This is generally not the case and queues
build up at some work stations and other work stations get starved. Constant monitoring is necessary and problems
must be uncovered and solutions developed => Shop Floor Control
We
attempt to assure that priorities are met and it is a pretty much continuous
job of expediting (making changes to hurry things along) and de-expediting
(halting or slowing the progress of a job--maybe because a delay has caused
part of assembly needed to not be available when needed.)
Priority
systems such as the critical ratio.
CR
= Due date - Today's date
divided by
Days required to complete job
<
1 late => maybe it gets 1st priority at W.S.
The
nature of production control and scheduling is somewhat different for different
types of manufacturing.
Job
Shop---Batch Processing---Repetitive
JOB SHOP
*
most complex
*
deals with everything from design to delivery
*
major issues: bidding, scheduling, lead
time, queues, bottlenecks
*
keeping track of where everything is and at what stage of completion is a major
challenge
*
Q control is useful here-- computer based tool to maximize capacity use and
minimize queues.
O.P.T.
can be useful.
BATCH PROCESSING
*
Queues are a major issue--means that priority control is very important. Bottlenecks also.
*
We would like to keep queues as small as possible so that lead time is short
and W.I.P. is low.
*
MRP System can be helpful in this situation as can J.I.T. and O.P.T.
REPETITIVE
*
Least complex
*
The efficient design of products and processes is critical.
*
In system design engineers and management make decisions that influence system
efficiency (Line Balancing). Balancing
the flow is a major issue--the pace of the system influences the level of output.
*
Output level is adjusted by cutting back on, or decreasing run time.
* MRP, JIT, OPT