There are other threads similar to this one, but they are over a year old and not exactly the same.
I am trying to understand the example of Manufacturing Lead Time Calc in the MRP Tech Ref Guide (p, 97-98). I thought I understood it conceptually, but the math in the example doesn’t match my understanding, so I am trying to figure out where my thinking has gone awry.
In the example (as I read it) from the top down:
Part A is composed of part AC + 2 days of manufacturing part qty time
Part AC is composed of part AB + 14 days of manufacturing part qty time
Part AB is composed of P1 (lead time = 14 days) + P2 (lead time = 21 days) + 7 days of manufacturing part qty time.
Going from the bottom up, I would expect the following
AB’s Cumulative lead time = 28 days.(longest purchase lead time + this level lead time of 7 days)
AC’s Cumulative lead time = 42 days (cumulative lead time of AB + this level lead time of 14 days)
A’s cumulative lead time = 44 days (cumulative lead time of AC + this level lead time of 2 days)
However those are not the calculations in the guide.
Part A
A is calculated as AC + 2 days; the number is different but the methodology is the same, so I understand that.
Part AB
For AB, even though the Cum Lead Time is 28 (which I understand) in the top section, the explanation of the calculation is: “The Cumulative Lead Time for part AB (28 days) and the This Level Lead Time (7 days) divides the total time it takes to receive purchase parts P1 and P2. P1 has a purchasing Lead Time of 14 days and P2 has a purchasing Lead Time of 21 days for a total of 35 days.”
Why are purchase Lead Times sequential instead of simultaneous?
What is meant by “divides” – in “This Level Lead time (7 days) divides the total time…”
Why is the total in the explanation 35 but the Cumulative Lead time 28?
Part AC
The explanation for AC’s Cumulative Lead time is: " The Cumulative Lead Time for Part AC (49 days) is the sum of Part AB Cumulative Lead Time (28 days) and This Level Lead Time (7 days) and part AC’s This Level Lead Time (14 days)."
Why are we adding Part AB’s This Level Lead Time (7 days) again? Isn’t it included in the 28 days of Cumulative Lead Time for Part AB?
Thanks for whatever light can be shed on this… I need to be able to explain to others how it works…
Example in test (trying to keep it as simple as possible)…
Part XY is made up of X (purchased, lead time = 60) and Y (purchased, lead time = 60)
Production per scheduling block is about 10 min – 0.167 hour/piece (only one operation)
Setup time = 0 (method tracker shows this)
Production time = 0.167 (method tracker shows this)
Queue time = 0 (for resource group)
Move time = 0 (for resource group)
Include subassemblies: checked (even though there aren’t any)
Include (checked in Site Maint)
Lead Time: values for purchased parts as described above
Transfer Lead Time = 0
Receive Time = 0 (in all parts and in Part Class)
Kit Time = 0 (Part Maint)
Not include (not checked in Site Maint)
Rough Cut Parameters
System calculations:
Cume Mfg Lead Time = 84 (where does this number come from???)
This Level Lead Time = 0
I would expect a Cume Mfg Lead Time of 60 – the longest lead time on a purchased part + the 0.167 of production time.
OK, on this one, I was told that Lead Time (for purchased parts) is calculated in calendar days but Manufacturing Lead time is calculated using the production calendar (5-day work weeks for us). So the system expands 60 days of purchase lead time into 60 work days, which results in 84 calendar days.
I haven’t done the math, but that’s the explanation I received.
I know that this is a dead post but I think that this is a very important point of discussion and that’s why I’ve decided to revive this post, in case any one who’s hunting on information for mfg lead time calculation stumbles upon this post.
We tried to understand how mfg lead time calculation works and we found out that mfg lead time calculation will not work if you set your calendar to a 5 day calendar based on the explanation above. This creates heaps of confusion. You must change the calendar to a 7 day calendar before running mfg lead time calculation.
But the problem is that you don’t want a 7 day calendar when MRP runs, otherwise the scheduling engine is going to schedule jobs for Saturday and Sunday too (if you only work Mon-Fri)
The workaround we have got right now is that we are running MRP first with a 5 day calendar and then running an automated SQL script to update to a 7 day calendar and then running mfg lead time calculation and switching back to a 5 day calendar. Not happy with the confusion and little explanation around how mfg lead time calculation works though.