What a Material Shortage at the Line Really Costs
Trace the causes of line stoppages and material shortages show up nearly as often as equipment failures. The hardest kind to catch is the shortage where the part is in the warehouse but not at the line. Book inventory looks healthy, so neither purchasing nor the warehouse sees anything wrong.
The cost becomes clear once you run the numbers. A line with a 60-second takt time that waits 10 minutes for parts loses 10 units of output. Even without a full stop, 20 operators each spending 15 minutes per shift looking for material adds up to 5 hours per shift, or 220 hours a month on two shifts and 22 working days.
Stacking extra material beside the line to avoid shortages creates different problems. Work space shrinks and walking distances grow, first-in-first-out breaks down so old lots get mixed in, and look-alike parts sitting side by side raise the risk of misassembly.
Choosing a Supply Method: What to Send and How
No single method fits every part. Most plants combine three.
Three factors decide which method fits.
Keep in mind that kitting does not eliminate picking labor. It moves that labor from the line to the warehouse.
Designing the Milk Run
A milk run is a delivery loop that follows a fixed route on a fixed interval. The point is that it leaves on schedule, not on request, and that is what makes line-side inventory calculable.
The interval starts from the loop time. Say the route is 400 m at 1 m/s, which takes about 7 minutes. Add 12 stops at 1.5 minutes each for 18 minutes, plus 5 minutes of loading, and one loop takes 30 minutes. A 30-minute interval leaves no slack at all, so set it at 40 minutes for a 75% load factor.
Check carrying capacity against consumption. If the whole line uses 48 containers an hour, a 40-minute interval needs 32 containers per trip. Three carts holding 12 containers each carry 36, for a fill rate of about 89%. Size line-side stock to two intervals of consumption (80 minutes) so the line survives one missed loop.
When to move from a human-driven tugger to AMRs depends more on standardization than on technology. It is worth evaluating when the route and stop positions have been stable for months, containers and carts follow one standard, and the loop ties up at least one dedicated person per shift. If routes change often and containers vary, standardize first.
E-Kanban and MES Production Reporting
The number of kanbans comes from a simple formula: hourly consumption × replenishment lead time × (1 + safety factor) ÷ quantity per container. At 120 pieces an hour, a 1.5-hour lead time, a 20% safety factor, and 50 pieces per container, the result is 4.32, so you run 5 cards.
The strength of e-kanban is its link to MES production reporting. When output is reported, line-side stock is deducted according to the BOM, and a replenishment request is created automatically once stock falls below the reorder point. If defects and scrap are not reported, though, the error accumulates, so many plants add a physical trigger.
The Boundary with the Warehouse System
The MES knows what is needed and when. The WMS knows where to pick it and how to ship it. The interface between them should carry, for each replenishment request, the part, quantity, delivery location, required time, and a request ID. The request ID prevents duplicate issues on retransmission, and the WMS has to report back issue and arrival status so the MES does not raise the same request again.
You also have to decide how line-side stock appears on the books. Expensing everything at the moment of warehouse issue is simple, but line-side stock then disappears from the ledger. Registering the line side as a location works better for tracing shortages: the move from warehouse to line is an inventory transfer, and actual consumption is deducted when production is reported. This approach needs regular counts of the line location to correct drift.
How POLYGLOTSOFT Helps
Line-side supply cannot be completed from the MES side or the WMS side alone. POLYGLOTSOFT builds both MES and WMS, so we design the whole flow together, from classifying parts by supply method and sizing milk-run intervals to e-kanban triggers and the interface between the two systems. If time spent waiting for parts is becoming visible on your line, contact POLYGLOTSOFT. We will start by assessing your current supply flow with you.