Lean manufacturing and value stream mapping

Why it matters

In a typical batch plant a part spends weeks in the factory but receives only minutes of real work; the rest is waiting, moving and sitting in stock. Lean manufacturing attacks that gap, and value stream mapping (VSM) is the one-page picture that shows where the time and inventory are. Lean thinking is now standard in Indian automotive, electronics and process industries, and interviewers expect graduates to talk about it with numbers.

Key ideas

Lean is a way of designing and running operations so that every step adds value the customer will pay for, with the least waste of time, material, effort and capital. It grew out of the Toyota Production System; JIT and jidoka are its two pillars.

Five lean principles (Womack and Jones).

1. Value – define it from the customer's point of view.
2. Value stream – identify all the steps (value-adding and not) that bring a product from raw material to customer.
3. Flow – make the remaining steps flow without interruptions, batches and queues.
4. Pull – produce only when the customer (or the next process) pulls.
5. Perfection – improve continuously (kaizen); there is always more waste to remove.

Activities.

• Value-adding (VA): changes the form, fit or function of the product, the customer would pay for it, and it is done right first time (machining, assembly).
• Necessary but non-value-adding (NNVA): needed under present conditions (inspection required by law, set-up), to be reduced.
• Non-value-adding (NVA): pure waste, to be eliminated.

Seven wastes (muda) – "TIMWOOD": Transportation, Inventory, Motion, Waiting, Over-production, Over-processing, Defects. A frequently added eighth waste is unused employee skills. Over-production is considered the worst because it creates most of the others. Lean also targets mura (unevenness) and muri (overburden).

Takt time is the pace of customer demand: available working time divided by demand. Each process's cycle time should be at or below takt. If a cycle time exceeds takt, that process cannot keep up without overtime or more capacity.

Value stream mapping (VSM) – drawn with pencil and paper while walking the actual flow (door to door).

1. Select a product family (products sharing similar processes).
2. Draw the current-state map: customer and supplier, process boxes with data boxes (cycle time C/T, changeover C/O, uptime, operators, shifts, batch size), inventory triangles with quantities, material flow (push arrows, supermarkets, FIFO lanes), information flow (orders, schedules, forecasts) and the timeline.
3. Compute the timeline: lower steps = VA (processing) times; upper steps = waiting time from inventory, converted to days of demand (inventory ÷ daily demand, from Little's law).
4. Draw the future-state map: produce to takt, continuous flow where possible, supermarkets and kanban pull where flow is not possible, schedule at one pacemaker process, level the mix and volume, reduce changeovers.
5. Make an implementation plan with kaizen bursts.

Little's law. For any stable system: WIP = throughput × lead time. In VSM it turns piles of inventory into days of lead time.

Process cycle efficiency (PCE) = VA time ÷ total lead time. Typical batch plants are well below 1 %; a lean target is much higher.

Common lean tools. 5S (sort, set in order, shine, standardise, sustain), standard work, visual management, SMED, poka-yoke (mistake-proofing), TPM, cellular layout, kanban, heijunka, kaizen events, andon.

Formulas

Takt time = available working time per period / customer demand per period

• In s/unit or min/unit. Available time excludes planned breaks.

Minimum number of operators (or stations) = total work content / takt time

• Round up. Work content in the same time unit as takt.

WIP = throughput × lead time (Little's law) Lead time (days) of an inventory pile = units in inventory / daily demand

• WIP in units, throughput in units/day, lead time in days.

Process cycle efficiency PCE = value-added time / total production lead time

• Both in the same time unit; dimensionless or %.

Worked examples

Example 1 (standard) – takt time and staffing. A cell works 2 shifts of 8 h, each with 30 min of planned breaks, and must deliver 450 units per day. Total manual work content is 9.5 min per unit.

1. Available time = 2 × (480 − 30) = 900 min/day.
2. Takt time = 900/450 = 2.0 min per unit.
3. Minimum operators = 9.5/2.0 = 4.75 → 5 operators, each loaded to about 1.9 min of the 2.0-min takt.

Example 2 (GATE level) – reading a current-state VSM. A welded frame passes through cutting (C/T 40 s), welding (60 s), painting (45 s) and assembly (50 s). Inventories counted on the floor: raw material 2000 units, after cutting 800, after welding 1200, after painting 600, finished goods 1600. Customer demand 400 units/day; one shift of 7.5 h available time per day.

1. Takt time = 7.5 × 3600/400 = 27,000/400 = 67.5 s. All cycle times are below takt, so capacity is not the problem.
2. Lead time of each pile = inventory/daily demand: 5.0, 2.0, 3.0, 1.5 and 4.0 days.
3. Production lead time = 6200/400 = 15.5 days.
4. Value-added time = 40 + 60 + 45 + 50 = 195 s.
5. PCE = 195/(15.5 × 27,000) = 195/4,18,500 = 0.047 % (working-time basis).
6. Future state: with flow between welding and painting and supermarkets elsewhere, total inventory is cut to 1500 units. New lead time = 1500/400 = 3.75 days and PCE rises to 195/(3.75 × 27,000) = 0.19 %, four times better, with no change to any machine.

Common mistakes

• Using total time (including breaks) or calendar time when computing takt time.
• Confusing takt time (demand pace) with cycle time (process pace) and with lead time (time through the whole stream).
• Adding inventory quantities to processing times without converting them to days of demand.
• Treating inspection, transport and set-up as value-adding.
• Drawing VSM for the whole plant instead of one product family.
• Seeing lean as head-count cutting; it removes waste, and freed people are redeployed to improvement.

For GATE PI

Expect conceptual questions on the seven wastes, lean principles, VSM symbols and lean tools (5S, SMED, poka-yoke, TPM), and short numericals on takt time, minimum number of workstations, Little's law and lead time from a VSM. Practise converting inventories into days of lead time and keeping all times in the same unit.

Quick check

1. Available time 420 min/day, demand 840 units/day. What is takt time?
2. WIP 300 units, throughput 60 units/day. What is the lead time?
3. Which waste is considered the root of most others?
4. VA time 12 min, lead time 4 days of 8 h. What is PCE?

Answers: 1. 0.5 min (30 s) per unit. 2. 5 days. 3. Over-production. 4. 12/(4 × 480) = 0.625 %.