Method study: recording techniques and charts

Why it matters

You cannot improve a job you have not first described exactly. Method study charts turn a messy shop-floor activity into a clean, standard picture of who does what, where and for how long, so wasted travel, waiting and handling become obvious and the proposed method can be compared with the present one on paper before anything is moved.

Key ideas

Method study is the systematic recording and critical examination of existing and proposed ways of doing work, to develop easier and more effective methods and reduce cost. It is the half of work study that reduces work content; work measurement (time study, work sampling, PMTS) then sets the time for the improved method.

Procedure (SREDIM). Select the job (high cost, bottleneck, high scrap, many complaints, safety risk); Record the present method with charts and diagrams; Examine it critically; Develop the improved method; Install it (train, trial, adjust); Maintain it (follow-up so workers do not drift back). Some texts add Define (write the new standard practice) before Install.

Critical examination: the questioning technique. Each recorded activity is challenged with primary questions (Purpose: what is done and why? Place: where and why there? Sequence: when and why then? Person: who and why that person? Means: how and why that way?) and then secondary questions (what else could be done, where else, when else, who else, how else, and what should be done). The aim is to Eliminate, Combine, Rearrange or Simplify (ECRS) each step. Only operations add value; transports, inspections, delays and storages are candidates for elimination.

ASME process-chart symbols

• Operation (large circle): changes the object physically or chemically, or adds information or plans.
• Inspection (square): checks quality or quantity; does not change the object.
• Transport (arrow): moves the object, worker or equipment from one place to another (not part of an operation).
• Temporary delay (D shape): waiting that is not authorised by an order, e.g. a part waiting for the next machine.
• Permanent storage (inverted triangle): kept under authorised control; removal needs a requisition.
• Combined activity: a circle inside a square, when operation and inspection are done together.

Process charts (sequence only)

• Outline (operation) process chart: only operations and inspections, plus points where materials or parts enter. Gives a bird's-eye view of the whole product.
• Flow process chart: all five symbols for one subject, with distance and time columns. Three types: man (worker) type, material type, equipment type. Its summary table (count, distance, time) is used to compare present and proposed methods.
• Two-handed (left-hand/right-hand) process chart: the activities of a worker's two hands side by side, using operation, transport, hold and delay; used for bench and assembly work.

Time-scale charts

• Multiple activity chart (man–machine chart, gang chart): activities of several workers and/or machines on a common time scale, showing idle time and how many machines one operator can tend.
• SIMO (simultaneous motion cycle) chart: hand and body motions recorded in therbligs against a time scale read from film or video (micromotion study). For short, highly repetitive cycles.

Diagrams (movement on the layout)

• Flow diagram: the flow process chart drawn on a scale plan of the area; shows back-tracking and congestion.
• String diagram: a scale plan with pins at work stations; a thread traces the worker's or material's path, and its length gives distance travelled. Best for irregular movement among many points.
• Travel chart (from–to chart): a matrix of trips or loads between stations; used to arrange stations to cut travel.
• Cyclegraph and chronocyclegraph: light-trace photographs of hand paths; the chronocyclegraph uses an interrupted light so speed and direction can be read.

Formulas

Distance saved (%) = (D_present − D_proposed) / D_present × 100

• D = total transport distance from the flow-process-chart summary (m).

Ideal number of machines per operator n′ = (a + t) / (a + b)

• a = concurrent activity time when operator and machine are both engaged (loading, unloading) (min); t = independent machine running time (min); b = independent operator time per machine (walking, inspecting, packing) (min). Assumes identical machines and a fixed cycle.

Cycle time T_c = a + t if n ≤ n′ (operator idle); T_c = n(a + b) if n > n′ (machines idle)

Operator utilisation = n(a + b) / T_c; Machine utilisation = (a + t) / T_c

Worked examples

Example 1 (standard). A material-type flow process chart of the present method shows transports of 12, 20, 8, 15 and 30 m and delays of 10, 5 and 15 min. The proposed method keeps transports of 12, 8 and 10 m and one delay of 5 min. Find the % reduction in distance and in delay time.

1. Present distance: 12 + 20 + 8 + 15 + 30 = 85 m; proposed 12 + 8 + 10 = 30 m.
2. Distance saved = (85 − 30) / 85 × 100 = 64.7 %.
3. Present delay 10 + 5 + 15 = 30 min; proposed 5 min.
4. Delay saved = (30 − 5) / 30 × 100 = 83.3 %.

Example 2 (GATE level). Identical semi-automatic machines: load and unload a = 2 min (operator and machine both busy), automatic machining t = 8 min, operator walk and inspect b = 0.5 min per machine. Find the ideal number of machines per operator, and the cycle time, utilisations and output per hour with 3, 4 and 5 machines.

1. n′ = (a + t)/(a + b) = (2 + 8)/(2 + 0.5) = 10/2.5 = 4 machines.
2. n = 3 (n < n′): T_c = a + t = 10 min; operator utilisation = 3 × 2.5/10 = 75 %; machine utilisation 100 %; output = 3 × 60/10 = 18 parts/h.
3. n = 4: T_c = 10 min; both 100 %; output = 4 × 60/10 = 24 parts/h.
4. n = 5 (n > n′): T_c = 5 × 2.5 = 12.5 min; operator 100 %; machine utilisation = 10/12.5 = 80 %; output = 5 × 60/12.5 = 24 parts/h.

A fifth machine adds no output, only idle machine time. Whether 3 or 4 machines is cheaper depends on the cost of operator time vs machine time per part.

Common mistakes

• Mixing up delay (unplanned waiting, D symbol) and storage (authorised, triangle).
• Calling a movement that is part of an operation (e.g. moving the tool to the job) a transport.
• Using a flow diagram where movement is irregular among many points; a string diagram suits that better.
• Treating the SIMO chart and the two-handed chart as the same: the SIMO chart uses therbligs on a time scale from film; the two-handed chart uses four process symbols with no time scale.
• In man–machine problems, forgetting that the machine is also busy during loading and unloading.

For GATE PI

Expect one-mark identification questions (which chart or symbol suits a situation, what a symbol means, the steps of method study) and short numericals on man–machine charts: number of machines per operator, cycle time, utilisation and cost per piece. Practise drawing a quick flow-process summary table and the n′ = (a + t)/(a + b) logic for both n < n′ and n > n′.

Quick check

1. Which symbol denotes a part waiting at a machine for its turn?
2. Which chart shows only operations and inspections for the whole product?
3. Which diagram best records a storekeeper's irregular trips among 15 racks?
4. With a = 1 min, t = 5 min and b = 1 min, how many machines can one operator ideally tend?

Answers: 1. Temporary delay (D); 2. Outline (operation) process chart; 3. String diagram; 4. n′ = 6/2 = 3 machines.