Sub-Assembly Stock

What Is Sub-Assembly Stock?

Between raw components and finished product sits a category that is complete at its own level while still being an input to something else. A motor built and tested, a wiring harness assembled, or a pump ready for installation are each finished articles waiting to become part of a larger one, and they are held deliberately rather than caught mid-process.

That completeness is what separates them from work in progress and it simplifies both counting and valuation considerably. A sub-assembly can be counted as whole units, because it either exists or it does not, and it can be valued at a determinable cost, because every operation contributing to it has been completed and absorbed. Work in progress requires a stage of completion to be assessed and a proportion of cost attributed to it, and both halves are open to argument. Where a business holds a large sub-assembly balance, the questions that arise are about obsolescence and ageing rather than about how far through the process anything is.

Which Sectors Use Sub-Assembly Stock and Why

The category appears wherever a product is built in stages and intermediate units are held between them.

• Automotive component manufacturing, where modules are built ahead of final assembly and buffered.
• Engineering and machine tools, holding completed gearboxes, spindles and hydraulic units awaiting a machine build.
• Electronics manufacturing, where populated boards are finished, tested and stocked before going into an enclosure.
• White goods assembly, buffering motors, compressors and control units against line demand.
• Capital equipment manufacture, where long build cycles make intermediate stocking necessary rather than optional.
• It has no application in process industries producing a continuous output, nor in trading and distribution, where nothing is built and every item arrives complete.

How Sub-Assembly Stock Works in Practice

1. Components are drawn from stores against a works order and built into an intermediate unit through a defined sequence of operations.

1. The unit is tested or inspected at completion and booked into stock under its own part number, because it is finished at its own level even though it is destined to become part of something larger.

1. Cost is absorbed and settled at that point. Material, labour and overhead for every contributing operation are complete, so a determinable cost attaches to the unit rather than an estimated stage of completion.

1. It is held until a higher assembly requires it, at which point it is issued exactly like a purchased component.

1. The holding is analysed by age, because units made for a line that has since been dropped have nowhere left to go. Those are identified and written down, which is the main valuation question this category raises.

Sub-Assembly Stock: A Worked Example

A Coimbatore machine builder holds stock at four levels on the same shop floor.

The two middle rows are the ones that get lost. A gearbox that is built, tested and waiting is neither raw material nor a finished machine, and if the record recognises only those two states its Rs 62.56 lakh either disappears or is counted at raw material value. Sub-assemblies also consume labour and overhead that must be in the valuation, so they cannot be valued from the purchase ledger. At a count they are physically obvious and easy to classify wrongly, since a tested gearbox on a rack looks much like one still being worked on.

Common Mistakes With Sub-Assembly Stock

Its completeness is an advantage that is frequently thrown away.

• Reporting it inside work in progress, which drags a countable population into a category requiring stage assessment and makes both harder to verify.
• Valuing it with an estimated completion percentage, when every contributing operation is finished and a determinable cost exists.
• Overlooking ageing, since assemblies built for a product that has been discontinued will never be consumed and are carried at full cost.
• Counting the assemblies without checking they are complete, so units missing a component are recorded as finished.
• Carrying heavy balances to buffer the line and never revisiting them, which commits working capital by default rather than by decision.
• Building assemblies to a forecast rather than to firm demand, which is how the obsolete portion of the balance is created.
• Storing them without protection, so units complete on paper need rework before they can be used.

Related Stock Audit Terms

Material out at a processor is the same population in somebody else's hands, so job work stock is the entry to read next. Every other term in this cluster is listed in the A-Z of stock audit terms, grouped by the part of the work it belongs to.