Value Stream Mapping Q&A

By Ron Pereira Published on October 20th, 2008

Sunil, a reader of LSS Academy, recently asked me several value stream mapping questions via email.

I have been to your site and was quite impressed by your e-book on Lean Manufacturing. I need your guidance related to VSM as per below. I will be grateful for your help.

  • Q1: How do we calculate the waiting/processing time for the inventory which is build between two workstations? Does it depend on the cycle time of the process before it is build or does it depend on the customer demand per day or exit rate per day or takt rate?

Generally speaking, the standard way to calculate the days of supply for a certain amount of inventory is by dividing the number of pieces on hand by the average daily demand for the product.

So, for example, if a given product had a daily demand of 100 units and we counted 50 pieces of inventory between process A and process B, we would have 0.5 days of supply between those two process steps.

  • Q2: If a particular workstation has 90 minutes of tool change time which happens once in a shift, how do we account for this in the process lead time?

This is an interesting question. If we consider tool change time as non value added we could simply include it on the VSM timeline as non value added time similar to the way we note days of supply.

Another technique, and something Jon Miller and I recently discussed, could be to include this changeover time in our takt time calculation. In other words, we would deduct this change over time from the net available time which would then adjust the overall takt time accordingly.

With this said, one thing is for sure, no matter how we “account” for this changeover time we’ll definitely want to attack this 90 minutes of non value adding changeover time with a strong dose of SMED.

  • Q3: If a particular workstation has 200 minutes of tool change time which happens once in a 7 days, how do we account for this in the process lead time.

Again, as stated above you could add this to the VSM timeline although it’s trickier since you don’t deal with it everyday. You could “level load” the time over the week or have different lead times depending on the day.

One of the things I’m interested in is whether or not this long changeover only happens once a week since the organization likes to set things up and run long batches since the changeover takes so long.

If this is the case you’ll want to attack this changeover time in such a way that it no longer takes so long. And when this happens you may be able to level out your production using the concept of heijunka.

I am definitely speculating a bit and would need to know more details in order to really help.

In the end, dealing with things like calculating production lead-time and days of supply is not as important as dealing with things like long changeovers, poor scheduling, and waste in general.

What do you think?

Would you offer Sunil any other advice?  If so, please share it in the comments section below.

  1. Rick Foreman

    October 21, 2008 - 8:32 am

    I agree with your last comment on the most meaningful action is not the “how to” of measuring but actually going to the gemba and attacking the waste. I’ve seen way to many companies get more caught up on the rolling out lean improvements with a emphasis on the tools and graphs without really just engaging the work force to identify the opportunities and just make the small improvements daily. Can the set up be reduced? Yes. Can the inventory between processes be reduced or improved? Most likely, yes. I’m not saying throw everything to the wind because you need to know where you are before you decide where you are going. The best thing is to get going with the team.

    • lean?

      August 31, 2013 - 4:54 pm

      Charts, graphs, no action? Stop talking about my place of work.

  2. gaurav

    August 31, 2013 - 12:10 am

    How can I apply takt time concept in call center industry.
    Example: I have 480 minutes in a day and the team takes 26000 calls per day.
    480 excludes breaktime, lunch (it is just the talk time)
    when 480/26000 = 0.018 minutes, which is not the case.
    Please help me understand this.

    • Ron Pereira

      September 2, 2013 - 8:15 am

      Hi Gaurav, I will answer your question… but to help me do a better job with my example can you please tell me how many operators work in your call center? In other words, how many folks do you have working the telephones?

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