DBR, A Step Ahead of JIT

Implementation of Just In Time (JIT) would require a change in the production process. Drum Buffer Rope scheduling (DBR), solves the problem…

For implementing JIT, the existing production processes of a company may have to be reworked. Since, the tool links production and procurement functions, such changes are imperative.

Changes to be made
The changes are in terms of :

• Enhancing quality of raw materials and the production process.
• Reduction in setup time to cut down carrying costs
• To reduce work in process inventory, ensure all workstations, in the process, have the same capacity and processing time (balanced work flow)
• To ensure smooth flow of a process, the layout should be changed to discourage movement of machinery
• To design performance measurement and rewards systems that discourage building of inventory

The Theory of Constraints (TOC)

According to TOC, every production process has a constraint that inhibits the system from performing at its optimal level. Hence, TOC focuses on identifying these constraints and managing them. The three main principles of TOC are:

• Increasing throughput: Throughput is “the rate at which an organisation generates money through sales”.
• Reducing inventory: Inventory is the amount of purchases made.
• Decrease the operational costs: Operation costs refer to all production costs, except for direct material, incurred for the throughput.

According to TOC, the primary objective of any company is to make money. Money is made through sales and not by piling inventory. Hence, inventory is a liability.

Drum-Buffer-Rope (DBR) scheduling

DBR is based on TOC. It realises the existence of constraints in production process. Also, it is difficult to maintain a balanced flow rate as in JIT. DBR scheduling, hence, is a mix of JIT and TOC.

• The drum: Drum in DBR is the constraint. According to DBR, non-constrained resources should help serving the constrained ones.

• The buffer: A time buffer is used for avoiding disruptions in the production process. These disruptions can be because of breakdowns, longer setup times, delay by suppliers and so on. Some companies also use shipping buffers to enhance on time deliveries.

• The rope: A schedule is drawn to release either materials or jobs into the system. The basic aim of the schedule or the rope is to ensure that all workstations are stretched to perform at the speed of the drum.

The process

The DBR process involves the following steps:

• Identify the constraint
• Determine the amount of buffer to ensure that drum is not idle on the basis of past experience.
• Lastly, the schedule is determined by working backwards from the due date of the job

DBR process

At Wellcare Hospital, patients in need of operations should undergo preoperative tests. To ensure that all tests are completed two days before the surgery, patients have to go to four different workstations. These are:

• Intake/ Registration: Here the patient fills the form and insurance details
• Blood test
• Chest X-ray
• Discharge: Here the patient is given a detailed description about his surgery

The hospital had only one X-ray room and hence it is the drum. Based on past experience, they determined the buffer to be three patients. The rope was based on the buffer. If there were more than three patients waiting for X-rays to be taken, no patient would be taken to the intake workstation till the situation eased. If the buffer were below three, then, new patients would be taken.

This system avoided queuing of patients before the X-ray room. The hospital, from its experience, had realised that patients kept waiting outside the X-ray room became irritable thereby disrupting the process. On the other hand, they were less disruptive at the intake workstation.

DBR success stories

Boeing, which adopted DBR for its printed board manufacturing operations, could reduce its lead time by 75% and throughput by 100%. Rockland Manufacturing was able to achieve an increase in throughput by reducing work in the process inventory. Also, it’s on time shipment rate increased by 100%. Bal Seal Engineering doubled its net profits after implementing DBR. This was achieved by reducing inventories by 50%, increasing output by 50% and reducing operating expenses.

DBR a better tool

There are two basic differences between DBR and JIT, according to P C Cook. First, to implement JIT it is essential to have a balanced workflow, which is not required in DBR. Second, JIT does not allow flexible batch amounts, as the capacities of the workstations are same. With DBR scheduling, flexibility is possible.

In simulation tests conducted by Cook, both JIT and DBR proved to be better than traditional tools. However, DBR outclassed JIT in a number of metrics. It requires less inventory, hence it leads to a reduction in manufacturing costs, enhances customer responsiveness and product quality. Also, Cook found that DBR helped in producing less number of defective products.

To conclude…
DBR scheduling results in flexible batch manufacturing, less investment in inventory and a reduction of work in process inventory. These factors make DBR superior to JIT.

Related reading :

1. “Using Drum-Buffer-Rope Scheduling Rather than Just In Time Production”: P Huff

2. ”Pass the Buck, Save a Few”: R T Christensen

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