Parts Waiting Between Operations: WIP Cash in an Indian CNC Machine Shop

In a typical auto-component machine shop, a part might spend a few minutes on each machine and several days waiting in bins between turning, milling, heat treatment, grinding and inspection. That waiting is cash: material plus every operation done so far, tied up in parts you cannot invoice yet. If you measure the queues one at a time and value each at its cumulative cost, a shop shipping 2,000 shafts a day can easily find ₹40 lakh or more sitting on the floor and at the heat treater. Halving the waiting frees about half of it.

By the Mama Editorial Team. Mama scopes camera and floor-visibility projects for Indian factories. Last verified on 13 October 2026, against Little's original 1961 paper, CGST Act s.143, CBIC Circular 88/07/2019-GST on job work, and the RBI's October 2026 policy resolution. Every rupee figure in the worked example is an assumption for you to replace with your own.

Key takeaways

• Most of a machined part's lead time is waiting, not cutting. Time the queues before you buy another machine or squeeze cycle times.
• Value WIP at cumulative cost, queue by queue. A shaft waiting for grinding has absorbed more cost than one waiting for milling, so the later queues hurt more per piece.
• Little's Law turns days into rupees. Average WIP = daily throughput × average days in the system. Cut the days and the WIP falls in proportion, at the same output.
• Outsourced heat treatment is usually the biggest queue, and it is also on a GST job-work clock.
• Long queues also widen the defect window. A drift found at final inspection puts every part made since it started under suspicion.
• Cameras show where bins and trolleys pile up and for how long. Part numbers, quantities, quality status and anything at a job worker's premises come from your routing cards or ERP.

Why a machine shop holds more WIP than it thinks

Most CNC owners know their spindle hours. Far fewer know how long a part takes from first lathe to dispatch box.

Take a routing that is common for auto-component shafts, flanges and hubs. The part is turned on a CNC lathe, milled or drilled on a VMC, sent out for induction hardening or case carburising, ground, inspected and packed. Between operations, the part sits in a bin on a trolley:

• After turning, it waits until the VMC finishes its current lot and the setter changes over.
• After milling, it waits until there is a full truckload or a full furnace load to send to the heat treater.
• At the heat treater, it waits in the job worker's own queue, the furnace cycle and the return trip.
• After heat treatment, it waits for the grinder, which is often the shop's constraint.
• Before final inspection, it waits for the inspector or a CMM slot (first-piece approval after setup is a separate wait).

Each wait is "only a day". Add them up and, in the example below, a part needing about an hour of work spends nine days in the system.

Little's Law: the arithmetic that links days to rupees

In 1961 John Little proved a simple relationship for any stable system with things flowing through it: the average number of items in the system equals the arrival rate multiplied by the average time each item spends inside (Little, Operations Research, 1961). On its 50th anniversary he revisited the result with an emphasis on practical applications, operations management among them (Little, Operations Research, 2011).

On a shop floor it reads:

Average WIP (pieces) = throughput (pieces per day) × lead time (days)

If you ship 2,000 pieces a day and a piece takes 9 days from first operation to dispatch, about 18,000 pieces are somewhere in the system at any moment. Cut the 9 days to 4.5 and, at the same 2,000 a day, you hold about 9,000. No extra sales, half the cash back.

Two cautions: the law works on averages over a stable period, so measure over weeks, not one bad day. And it tells you how much is in the system, not where. For that, count queue by queue.

Worked ₹ example: a turned, milled and hardened shaft

Every figure below is an assumption for illustration. Replace them with your own costing and your own counts. The shop ships 2,000 shafts a day. "Cumulative cost" means the raw forging plus the machine-hour cost, tooling and outside processing absorbed up to that point. (Our machine hour rate guide explains how to build that number.)

The heat treater holds the biggest block: a third of all WIP, outside your gate, valued at ₹245 because hardening is billed when done.

Later queues cost more per piece: ₹285 waiting for inspection against ₹215 waiting for milling. Fix the queue with the most rupees, not the most pieces.

The interest is the small part. Since October 2019, new floating-rate bank loans to micro and small enterprises have had to be linked to an external benchmark such as the RBI repo rate (RBI circular, 4 Sept 2019). The repo rate went up to 5.50% on 7 October 2026 (RBI MPC resolution, Oct 2026). Assume your cash-credit line costs about 10% a year all-in. Then freeing ₹22 lakh by halving lead time saves about ₹2.2 lakh a year in interest. The bigger gains are the freed ₹22 lakh itself, a shorter delivery promise, less floor space and less handling damage.

The second cost: a long queue widens the suspect window

WIP is also time during which a quality problem goes unnoticed.

Suppose a tool insert on the CNC lathe wears and a diameter drifts toward its limit. If final inspection is the first point where that diameter is measured properly, and the part takes about eight and a half days to get there, then that much turning output passes through the system before anyone notices. At 2,000 a day, that can be up to 17,000 parts, spread across the VMC queue, a furnace load at the job worker and the grinding queue. Every one must be traced, segregated and re-checked.

A shorter queue does not stop the drift; it shrinks the suspect batch. We work through the full cost of one rejected part in what a 3% rejection rate really costs a CNC shop.

Heat treatment: the queue with a tax clock

Most mid-size Indian machine shops send hardening, carburising or nitriding to a specialist heat treater nearby, and parts go out on a job-work challan. Two things make this queue different.

It batches by nature. A furnace runs a load, and a truck runs a trip. So parts are held back until there is "enough" to send, and they come back in big lumps. The VMC queue grows while the shop waits for a full load, and the grinder swings from starved to flooded. Much of this WIP is a scheduling choice. Smaller, more frequent dispatches cost more freight per piece but take days out; run the numbers with your own rates.

It sits on a statutory clock. Under section 143 of the CGST Act, inputs sent to a job worker must be brought back, or supplied onward from the job worker's premises, within one year (three years for capital goods). Otherwise, the original dispatch is deemed to be a supply by the principal on the day the goods went out, with interest payable for the intervening period (CBIC Circular 88/07/2019-GST). For job work, "input" includes intermediate goods arising from processing, so your half-machined shafts count (CGST Act s.143, Explanation). A healthy flow never gets near a year, but lots that come back short or are left as "rework, will send later" age quietly. Have your accountant reconcile challans against returns.

How to measure your queues this month

You need a fixed walk and a notebook, not software.

1. Mark the queue zones. Paint or tape a rectangle for each waiting point: after each machine group, the heat-treat dispatch bay, the returns bay and the inspection inbox. If WIP has no fixed home, you cannot count it.
2. Count twice a day at fixed times. Note bins or trolleys per zone and, where packing is standard, the pieces per bin.
3. Tag the oldest bin. Write the date and time on a card when a bin enters a zone. The oldest card in each zone tells you more than the average.
4. Value it. Multiply each zone's pieces by its cumulative cost, as in the table above.
5. Check with Little's Law. Total pieces divided by daily dispatch gives your lead time in days. If that number is far from what the planner believes, the floor count is usually closer to reality.

After four weeks you will know which queue is structural (a genuine grinder bottleneck) and which is habit ("heat treatment goes only on Fridays"). If the structural one is a machine, read before you buy another CNC first.

What cameras show, and what needs ERP data

Honest limits. A camera sees bins, not parts. Stacked crates hide what is behind them. Mixed bins of two part numbers look identical, and a half-full bin looks full from above. A camera cannot tell why a queue exists, see your job worker's floor or read a challan. Whether six hours of dwell is a problem is your production manager's call. And because every camera that covers a queue zone also sees the people working near it, display the worker CCTV notice. Our DPDP worker notice template covers that.

Where Mama fits

Mama reads the cameras a shop already has, or a few added ones over marked queue zones. Each morning it sends the owner a short, plain-language note on where work pooled yesterday: which zone held trolleys longest, which machine sat starved or blocked and for how many hours, and whether the heat-treat dispatch bay is filling up. It reports machines, zones and flow. It aggregates and does not name or rank people. The rupee value and part-level detail still come from your own costing and ERP. If you want to know which of your queue zones existing cameras already cover: Leave an email or phone in the form below, and Mama will get back to you with a camera plan.

FAQ

What is a reasonable WIP level for a CNC auto-component shop? There is no universal number; it depends on routing, outsourcing and batch sizes. A more useful test is the ratio of total lead time to total machining time. If a part needs one hour of work and spends nine days in the system, nearly all of that time is waiting, and you can reduce it without new machines.

Should WIP be valued at raw material cost or at full cost? For a cash view, value each queue at cumulative cost: material plus the machine-hour cost, tooling and outside processing absorbed up to that point. That is the money already spent. Your books may value it differently; use the cumulative view for floor decisions.

Does reducing WIP risk starving the bottleneck machine? It can, if you cut blindly. Keep a small, deliberate buffer in front of the constraint, often the grinder, and cut the waiting elsewhere.

Can cameras count the actual number of parts in WIP? Not reliably. They can count bins and trolleys in marked zones and time how long they stay. Piece counts need standard pack quantities or your routing cards and ERP.

Does WIP at the heat treater count as our inventory? Yes. Parts sent on a job-work challan remain the principal's goods, and under CGST section 143 they must come back, or be supplied onward, within one year for inputs. Track them as your inventory at an outside location.