Line-side inventory is calculated, not counted

The BOM says each unit uses 4 bolts. Once the line confirms a unit, the system deducts 4.

On the floor, one bolt got dropped and one got swapped. The system doesn’t know.

This is the most common way parts are deducted in line-side logistics. It saves a lot of work, and it also builds in a gap: line-side inventory is calculated, not counted. This post covers where that gap is and how to clear it.

1. Posting parts to production takes two steps

From the warehouse to being installed on the product, the books go through two steps:

• Step 1, delivery to the line: warehouse stock goes down, line-side stock goes up. The line-side location is a storage location in the system that records “how many parts are left at the line”.
• Step 2, production confirmation: the line tells the system “these units are done”. When the system receives the confirmation, it deducts the parts from the line-side location automatically. This is called backflush: the parts are used first, and the books are posted backwards once the work is done.

How much gets deducted is simple arithmetic. Say a production order is for 10 units, and the BOM says each unit uses 4 bolts:

• In the morning the line confirms 6 finished units, and the system deducts 6 × 4 = 24 bolts from the line-side location
• The 16 bolts for the 4 units not yet confirmed are not deducted; they stay on the line-side books
• In the afternoon the last 4 units are confirmed, the system deducts another 16, and only then are all 40 bolts on this order deducted

The system only knows two numbers: how many units were confirmed, and how many parts per unit the BOM says. Whether those 6 units actually used 24 bolts or 26, it doesn’t care.

Why do it this way? The line runs at a fast takt; nobody can scan a barcode every time a part is installed. Backflush lets workers just install, and the system deducts by standard.

That is also the cost: the books deduct by standard, the floor consumes by reality. Parts that were dropped, damaged, swapped out, or changed by an add or removal are not in the standard.

So whether you can trust the line-side books comes down to two things: whether step 1, the receipt, is accurate, and whether step 2 uses the right BOM.

2. Four errors, all growing from the same gap

① Negative stock. The parts are physically at the line, but step 1 never finished and they were never moved into the line-side location. When the confirmation deducts, the line-side location goes negative. On an automated line, this step happens when the AGV delivers the parts to the line and the system automatically moves them from WMS stock into the line-side location. If the AGV task didn’t complete properly, or someone carried the parts up by hand, the step never happened and the books fall behind.

② Backflush fails, and the order can’t close. Backflush is mostly done in the MES, the system that runs production on the line. Some systems don’t allow negative stock: if the deduction can’t go through, a failure record is logged and left hanging. As long as there is an open failure, that production order can’t be closed and its cost can’t be settled. By financial month-end close, these orders are either forced through by hand or pushed to next month.

③ Too much or too little deducted. After the order was released, parts were added or removed, but backflush still used the old BOM. Removed parts get deducted anyway, and added parts don’t. The adds and removals from my previous post land here if only the physical parts were changed and the BOM wasn’t. When the BOM changes, delivery and deduction have to change together; change only one side and it goes wrong.

④ Books above reality. Scrap, losses, and old parts sent back to the warehouse were never recorded in the system. The physical parts are gone, but the books still carry them in the line-side location.

None of these four is about someone being lazy. They all grow from the gap between “standard” and “actual”. If the gap isn’t cleared, it keeps growing, and at the month-end count it all blows up at once.

3. How to clear it: let the system run one round, then hand over to people

We did it in two rounds:

Round 1, automatic. Parts that were already delivered to the line but not yet received into the line-side location get an automatic goods receipt; then failed backflush records are re-posted. Whatever the system can clear, it clears first.

Round 2, by hand. What the system can’t clear goes to the line feeders, who handle it one record at a time.

Which round each of the four errors belongs to:

• ① Negative stock: mostly round 1, filled in by the automatic receipt; if that doesn’t fix it, a line feeder checks whether the parts were actually delivered
• ② Backflush fails: round 1 re-posts it; if it still fails, it’s usually because ① or ③ hasn’t been fixed
• ③ Too much or too little: by hand only, adjusting the deduction to the order’s latest BOM
• ④ Books above reality: by hand only, recording the scrap and losses after the fact and returning old parts to the warehouse

The order can’t be reversed. If people start checking first, the line feeders get buried in records the system could have cleared on its own, and the exceptions that actually need judgment never get their turn. Let the system do everything it can first; only what’s left goes to people.

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How often do you clear your line-side inventory: every day, every week, or does it pile up until month end?