A Normal, Variable and Extreme Variability Day’s Drive
The previous article (4 of 5) in this series talked about perfect execution. Our drive went 100% according to plan. Of course, we know that perfection is not the rule. For that reason, we have more to gain by considering how a normal, variable and extreme variability day’s drive might unfold.
A normal day’s drive might go something like this. As we drive, we find it necessary to use 1 minute of buffer in the first segment, 2 minutes in the second segment, 2 minutes in the third segment, and 1 minute in the fourth segment. In total, we have used 6 minutes of buffer. Since we have budgeted 17 minutes for our time buffer, we arrive at work 11 minutes early. We are on time, thereby avoiding penalty. Success!
A variable day’s drive might be defined by a late start. Recall that our plan is to leave at 7:00 and arrive at work by 8:00 a.m. So, if we depart home at 7:07 a.m., our time buffer has already lost 7 minutes. We are concerned about arriving late to work!
Suppose that we then get stuck behind a pokey driver, taking us 4 minutes longer than estimated to complete the first segment. We are more concerned about being late! The threat level has risen. Fortunately, we complete the second, third and fourth segments more or less within the estimate and arrive at work with 1 minute to spare. Whew - we got lucky!
An extreme variability day’s drive might include a snowstorm. We recognize that it’s snowing outside, so we decide to leave home 1 minute early; thus building our time buffer up to 18 minutes. At this point, we are unconcerned. The first segment takes 2 minutes longer than estimated, but since we’ve completed 23% of the total drive and have only used 6% of our buffer, we are less concerned than when we left home.
Unfortunately, the snowstorm worsens. The second segment of the drive takes 15 minutes longer than estimated. We have just 1 minute of buffer remaining but 42% of the drive to go. The threat level has risen precipitously. We are very worried about being late!
The third segment of the drive takes 6 minutes longer than estimated. We have used up all of our buffer and still have 14.0% of the drive to go. Even though it is not yet 8:00 a.m., we’re definitely going to be late!
The final segment of the drive takes 4 minutes longer than estimated. We arrive 8 minutes late. Although our employer is an understanding person, she’s not happy about our poor performance. We vow to use a bigger buffer in bad weather from now on!
What can manufacturers take away from this exercise? Here are a few things to consider.
- Variability is real - Manufacturers should recognize that perfect execution is rarely achieved in the real world. Variability must be managed by recognizing that it exists, and actively anticipate it, in a realistic way, in manufacturing scheduling systems.
- Penalties are to be avoided – Manufacturers who fail to deliver jobs on time will incur penalties. The penalty may or may not be encoded contractually with buyers, but it is certain that customers frustrated by missed due dates will seek out alternative suppliers.
- Time buffers are important - Time buffers that account for variability can help manufacturers avoid the penalties associated with missed due dates and keep their customers happy. But there needs to be a balance. Allocating too much buffer can cause even more problems than too little.
- Variability can take multiple forms - Manufacturers must consider many likely scenarios and align their scheduling practices accordingly. Time buffers are not one size fits all. They should be sized based on how likely variability is to effect certain product lines, part families, even part or job specific.
Has variability ever disrupted your daily drive? How about your shop - what kind of variability does your business most often experience? How do you calculate time buffers for driving and for business?
About Protected Flow Manufacturing™ (PFM)
Protected Flow Manufacturing™ (PFM) is a revolutionary new Manufacturing Execution and Planning System. Aimed specifically at the crucial area of “production execution”, Protected Flow Manufacturing™ continually directs production priorities to minimize wait time and maximize on-time delivery using data fed from a customer’s existing ERP system that customers can act on.
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