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.