MRP II is not a
very effective method of achieving control on the shop floor. The constraints
and limitations that cause this can broadly be divided into seven categories as
summarized in the table 1:
Limitation and
Constraint |
Functional
Requirement (FR) |
Design Parameter
(DP) |
Quality |
1.
Eliminate machine assignable causes 2.
Ensure operator human errors do not transfer to
defects 3.
Eliminate material assignable causes 4.
Improve capability of process |
1.
Selection / maintenance of equipment. 2.
Mistake proof operations (Poka-Yoke) 3.
Supplier quality program 4.
Design of experiments to check for poor capability |
Identifying and Resolving Problems |
1.
Identify disruptions where they occur. 2.
Identify disruptions when they occur. 3.
Identify nature of disruption 4.
Minimize delay in contacting correct support
resources. 5.
Supply descriptive information to support resources. 6.
Solve problems immediately |
1.
Simplified material flow paths. 2.
Increased operator’s sampling rate of equipment
status. 3.
Context sensitive feedback. 4.
Rapid Information transfer system 5.
System that conveys nature of problem. 6.
Standard method to identify and eliminate root cause. |
Predictable Output |
1.
Ensure availability of relevant production
information. 2.
Do not interrupt production for worker allowances. 3.
Ensure material availability. |
1.
Capable and reliable information system. 2.
Mutual Relief system with cross-trained workers. 3.
Standard material replenishment system. |
Delay Reduction |
1.
Provide knowledge of demand product mix (part types
and quantities) 2.
Produce in sufficiently small run sizes. 3.
Define takt time. 4.
Ensure that production rate is balanced with takt
time (rsmax=1/ttmin) 5.
Ensure that part arrival rate is balanced with
service rate (ra=rs) 6.
Reduce lot delay 7.
Reduce transportation delay |
1.
Information flow from downstream customer 2.
Design quick changeover for material handling and
equipment. 3.
Definition or grouping of customers to achieve takt
times within an ideal range. 4.
Subsystem enabled to meet the desired takt time
(design and operation) 5.
Arrival of parts at downstream operations according
to pitch. 6.
Reduction of transportation lot size
(single-piece-flow) 7.
Material flow oriented layout design. |
Direct Labor |
1.
Eliminate operator’s waiting on machines. 2.
Eliminate wasted motion of operators. |
1.
Human machine separation. 2.
Design of workstations / workloops to facilitate
operator tasks. |
Indirect Labor |
1.
Eliminate managerial tasks 2.
Eliminate information disruptions |
1.
Self directed work teams (horizontal organization) 2.
Seamless Information flow (visual factory) |
Facilities Cost |
Minimize facilities cost |
Reduction of consumed floor space |
The interaction
of these various issues causes the frequently observed problems of fixed lead
time and infinite capacity. From a systems perspective, MRP can be modeled as
an open loop system with a non functional feedback arm. Since information flows
downstream, in the same direction as the material flow, the actual reality on
the shop floor is different from that conveyed by the information system.
This discrepancy
gives rise to the large amounts of inventory often seen on the shop floor and
an inability to meet the customer demand.
A good shop
floor control system is one that has the goal of developing a dynamic and
flexible organization. Its design takes into account the structure of the
system and the individual work tasks on the shop floor. This introduces a
capacity for self design and lasting adaptability that enables the shop floor
control module to become an agile entity that can keep pace with ever changing
customer demands. MRP II fails in this respect because of its inability to work
well with randomness. It normally thrives well in systems characterized by
large backlogs, excellent forecasts, highly reliable vendors, large inventories
and short material lead times. Unfortunately, this is not the reality in
current production environments.
MRP II ‘s
problems cannot be solved by carrying out small improvements such as software
updates. Instead, it calls for radical steps that address the logic behind the
entire system. One solution is the use of hybrid systems that complement the
push characteristics of MRP with those of pull systems. Pull systems are
particularly effective as control modules in manufacturing systems. In this
arrangement, MRP can still be utilized as a planning tool to create demand for
the production system.
In the case
study provided, company X runs an MRP II system that has little control over
the shop floor. The system is characterized by large inventories, long delays
and an inability to keep up with customer demands using the inventories on the
shop floor. Company X nevertheless has an excellent infrastructure for
distributing its products and receiving orders. Several of its problems can be
solved by shifting its manufacturing system to ‘lean’. Using already existing
product families of centrifuges, company X can establish cells based on the
Tandem Push-Pull hybrid system or the Requirement Driven Kanban system
suggested by Karmakar.
The design of a
Kanban controlled cell at company X is a potential area for subsequent study.
This would involve the design of a cell using the existing machinery to
establish a cell cycle time that keeps pace with the customer demand. A second
area that needs further study is the value stream mapping for a single
centrifuge. This would involve tracking the process of putting together one
centrifuge from all its 200 different components (this study looked at only one
such component). These two areas would provide the foundation for converting
company X’s system to lean. Subsequent improvements would address issues such
as the size of inventory, reducing set up time for the different machines,
Coordinating production to enable throughput time for all the components to
equal the actual processing time.