8. Planning: demand, the run, and firming
Everything before this chapter was bookkeeping with good habits: what you have, what you buy, how things are built, what is being made. This chapter is the reason the system is called MRP. It closes the loop from what we've promised to what we should start, and when.
The question
You have agreed to deliver two boats in December. A boat is a bill of materials several levels deep: hulls, a wing sail of sixteen printed sections, a bow node with a processor board and a printed housing, filament by the kilogram. Some of it is in stock, some is on order, some is half-built. What, exactly, is missing, and when does each thing have to start so that December is met?
Doing that by hand is where every small manufacturer loses their evenings. The planning run does it in a second, and does it again every night.
Demand
The run needs to know what is wanted. Record it on the Demand page: a part, a quantity, a due date, and whether it is an order (promised) or a forecast (expected). That is all. Two boats by 1 December is one row.
You do not record demand for the things inside the boat. That is dependent demand, and the run derives it from the bill of materials every time, so it is never stale.
Reorder points are demand too. A part with a minimum stock of five is a standing instruction to keep five, and the run honours it at every level. Use demand rows for what is dated and specific; use reorder points for what you always keep.
The run
Press Run planning, or wait for the nightly job. For every part, top of the tree first:
- Gross: what demand wants, plus what the parents already planned this run need.
- Net: gross, plus the reorder point, minus on hand, minus what purchase orders will bring (drafts included), minus what work orders will deliver (drafts included).
- Proposal: if net is positive, a make when the part has a routing, a buy otherwise. Makes round up to the routing's batch size, buys to the supplier's minimum order and pack size.
- Dates: needed-by is the earliest date wanting it; release-by is that less the lead time. A component is needed by the day its parent starts.
Then the proposal's quantity is pushed down through its bill of materials, and the next level is netted the same way. Phantoms, unstocked sub-assemblies, are looked through: their components are planned, they are not.
The result is a table: make ten wheels by 28 November; order ten bearings by 21 November; print sixteen wing sections, first one starting Tuesday. Each row says why, back to the demand that caused it.
Firming
The run never creates a real order. It proposes; you firm. Tick the rows you agree with, choose the location work orders should run from, and press Firm selected. Makes become draft work orders, one each. Buys become one draft purchase order per supplier. From there the ordinary pages take over: approve and send the purchase orders, release the work orders, receive and complete as usual.
Because drafts count as supply, the next run will not propose them again. Because firmed proposals are kept, you can always see what a work order was for.
Disagree with a row? The × cancels it until the next run. Change the demand, the stock or the BOM, and the next run reflects it.
What it will not do yet
There are no time buckets: two orders for the same part on different dates become one proposal needing the earlier date. There is no capacity check: the run assumes the printers and the people are available. And it does not plan across companies. All three are possible later without changing what you have entered.