(The following is taken from Sheet Metal Made Lean and Clean: An Introduction to the Principles and Methods of Lean Production for Sheet Metal Shops.)
The second challenge in converting a shop to lean production is to make the work flow. There are a couple of questions to ask to determine if a "lean" work flow is appropriate for your shop. Are the things you are making going through the same sequence of operations time after time? If the way you are doing your work today is not repetitive, could it be made more repetitive by standardizing how the work is done?
If you can answer "yes" to these questions, implementing a lean work flow is worthwhile. A few shops, those specializing in fabricating unique architectural elements for example, may find that there is no common sequence of events that can serve as a standard (and) no flow chart that will consistently describe how products are created. If your answers to the questions above are "no," you may still want to consider using the "lean" work-flow approach to improve design or administrative processes that are repetitive.
To get started it is essential to understand your current situation; to get a good idea of how your shop operates today. You find out by developing a process-flow chart. The process-flow chart will help you assess and understand the value of things you are doing and the time involved to do them. It will help you determine a lot size to implement a smooth work flow.
Equal-size lotsHow do we make the work flow? What does a smooth work flow look like? They key to flow is to release small, equal-size lots of work to the shop floor and then to allow the lots to move through the shop to completion without being interrupted.
To illustrate, we will describe a fictional shop called Justin Thyme Sheet Metal. JTSM started to implement work flow by documenting and analyzing their production process. They observed activities in the shop carefully and drew a flow chart that reflected what they saw. The analysis of shop activities showed that when they made the most complex fitting they were likely to make, each operation took about 30 minutes to perform. They decided to try releasing work to the shop in 30-minute lots. Thirty minutes would ensure that a lot always contained at least one fitting, and most lots would contain more than one fitting.
At JTSM, the work starts at a computer-controlled plasma center. The computer is in an enclosed office that looks out over the shop. The cutting instructions are input by an experienced worker who is familiar with all shop operations. He is the one who divides the work into 30-minute lots. He and the shop foreman also determine the sequence for job releases throughout the day. The work flow looks like this:
Thirty minutes of work is released to the cutter. In 30 minutes, it is cut, labeled, put on a cart and moved to the brake. As the braking starts on the first lot, the cutter starts another 30-minute lot. After 30 minutes, the first lot is finished at the brake and moves to the Pittsburgh (machine). The second lot moves from the cutter to the brake and the cutter starts lot No. 3.
30-minute incrementsThirty minutes later, lot No. 1 moves to the TDF, lot No. 2 Pittsburgh, lot No. 3 to the brake and the cutter starts lot No. 4. Thirty minutes later, lot No. 1 moves to assembly, lots Nos. 2, 3 and 4 advance to their next operations and the cutter starts lot No. 5. Thirty minutes later, lot No. 1 is complete. The elapsed time is two hours and 30 minutes.
Now let's say that 10 minutes into the cutting of lot No. 5, a "hot" job arrives. It is equal to one 30-minute lot (to keep it simple). Nothing stops, but 20 minutes later when lot No. 5 is cut, the hot job becomes lot No. 6 and begins its journey through the process.
Two hours and 30 minutes later, it is done. From the time the hot job arrived until it is complete, the total elapsed time is two hours and 50 minutes. No work on the floor has been interrupted. No waiting or extra handling has occurred, and no significant scheduling or supervision (management cost) has been required.
The Justin Thyme Sheet Metal Co. has found that releasing small, equal-size lots rather than whole jobs at once provides the flexibility to handle hot jobs with minimal disruption to planned work. The hot job is completed in three hours or less. The impact of the hot job can be evaluated before it is accepted. The visibility and predictability of the system allows JSTM to give customers accurate information about job-completion times.
InexactObviously, calculating a 30-minute lot isn't an exact science. It is a skill that takes some time and practice to master. To help maintain the flow, the workers on the shop floor continuously adjust their activities to respond to the situation.
The continuous adjustment might be likened to players in the infield in baseball. Infielders constantly shift their positions and responsibilities depending on the circumstances of the game. Where they are is determined by runners on base, where a ball is hit and so on.
In the shop, it is easy to see the situation and respond to it as needed. The goal is to have all lots ready to move to the next operation at the same time. The workers make that happen by shifting positions to help each other and take up slack. It takes some practice and experience to perform well. Teamwork is everything, and a seasoned team plays much better than a rookie team.
Continuous adjustment also occurs over time through small improvements to the process and operations. If a particular operation is consistently constraining the work flow, special effort is made to find the cause and correct it.
(For information on ordering Sheet Metal Made Lean and Clean, write the Sheet Metal and Air Conditioning Contractors' National Association, 4201 Lafayette Center Drive, Chantilly, VA 20151, or call (703) 803-2980; fax (703) 803-3732; see www.smacna.org on the Internet.)