I admit there are still lessons to be learned around these two widely discussed, better said, debated parameters.
Most important is to not discuss too much until you know exactly how they work. This blog post wants to contribute to precise understanding.
To see the bigger context of these parameters, I recommend the article by Conrad Volkmann on Technet
In the following story, I am MRP and I am doing my thing, using these two parameters.
Negative days: If I am the MRP logic I would talk like this: Aha, I see a new demand that I have not covered yet. Let me check my “dynamic negative days” first. It is not checked. So I have to look at negative days for this item and start counting from the demand date forward and scan the horizon for any existing order I could use before I decide to create a new planned order. In order words, how many days am I allowed to wait, or, how many days am I allowed to have a projected negative inventory? Let me see. Aha, negative days = 0. I am not allowed to use ANY existing orders. Okay, I will create a planned order now for the same date as the demand. Negative days = 365. Okay, let me check a year into the future from the demand date if any existing supply order can be used for this demand, if I find one, I will create an action message to “advance” and “increase” if the quantity of that existing order is not sufficient for my new demand.
Now let’s see how I would reason when my “dynamic negative days” parameter is checked. I need to look at the situation differently. What is the lead time of this item? Let us say it is 20 calendar days. I will add these 20 days to TODAY and see if there is any existing Purchase order that I could use. Otherwise I will behave the same way as above.
Positive days: If I am the MRP logic I would talk like this: “I see a new demand that I have not covered yet. Should I create a planned order with a date equal to that demand date? I already looked into the future as far as the negative days allowed me and I saw nothing. Now I look over my shoulder, the time period BEFORE this new demand, is there maybe an existing supply to cover my new demand? This includes On-hand inventory that I have today. Am I allowed to use that existing supply? How close, in calendar days, do I have to be to a projected positive inventory in order to be allowed to use it?
Oh, positive days = 0. Well I guess not, I will create a new planned order then. I hope the user knows what he is doing. There is a pile of available on-hand, but.. I can’t use it. Aha, for the next item the positive days = 365. Okay, excellent, I see there is a pile of available on-hand inventory, I will create a pegging relation with the on-hand and be done. Or if there is no on-hand but a Purchase order is supposed to arrive a week before my new demand that is also perfect, I will create an action message to postpone and increase if needed.
Advice
- If you don’t have experience with Dynamics AX Master Planning, the best is to put both parameters equal to 365 days. Negative and Positive days both 365 days will make sure the end balance of the Net Requirement screen will be zero or equal to safety stock. This makes the MRP results much easier to understand.
A more logical setting is “dynamic negative days”, although we have to add that one would only see a big difference if planned orders are firmed far out of lead time, which is certainly not a best practice.
Only then would “dynamic negative days” protect you from in-appropriate and unusable action messages for orders far into the future.
2. There is no such thing as “dynamic positive days” that would protect us from using existing supply and/or on-hand for a demand that is far enough in the future to leave time for placing a new order. With positive days = 365 days, this is entirely possible. But in real life we typically do not see demand for make-to-stock items very far into the future (beyond lead time). You are not likely to get any inappropriate action messages if you leave the positive days to 365. But the parameter makes sense if we look at the example below that will lead us to the next topic, the rather unpopular "explosion" reservation.
EXAMPLE: we have a Make-to-Stock item with on-hand inventory. A customer places an order for shipment 6 months from now. The lead time of my make-to-stock item is only 3 weeks. If we use automatic reservation, a reservation will be made. This could be undesirable. This reservation goes to the detriment of more urgent sales orders that follow with dates much closer to today. Positive days could work as a “reservation horizon”. This is why one of the reservation parameters is called “explosion”.
The “explosion” reservation
This reservation is using the positive days value to decide whether to make a reservation on existing inventory or create a new planned order ( or use an existing order with an earlier receipt date)
This is exactly the same functionality as we see in the MRP-run.
NOTE: the “explosion” reservation competes with CTP as both run MRP for the sales order line.
Using both would not make sense. I see almost no company using this “explosion” reservation and I am not sure why that is. More news around this parameter in one of the following blogs.
