What are standard work and line balancing?
Standard work is the best way we know today to do a job, written down, followed by every shift, and improved over time. Line balancing is sharing the work between operators so each has about the same amount, close to the customer's pace, and nobody waits or rushes.
Walk through a plant and you will see this. The day shift makes 30 pieces and the night shift makes 22 on the same machine. A new operator learns by watching for three months. Three operators stand at three machines, mostly watching them cut.
None of this is the fault of the people. The best method lives in the head of one or two experienced operators, and the layout ties each person to one machine.
What are the three parts of standard work?
All three are needed.
- Takt time. The customer's pace: how often one finished piece is needed.
- Work sequence. The steps in the order the operator does them, every cycle, with safety and quality points.
- Standard work-in-process. The smallest number of parts that must be in the cell for the operator to work smoothly.
If takt is missing, people work at machine speed and overproduce. If the sequence is missing, each person invents his own. If the standard stock is missing, parts pile up between machines and hide problems.
An SOP alone is not standard work. An SOP says what is done and by whom. Standard work adds the pace, the timed sequence and the stock limit for one operator.
How do you calculate takt time?
Takt time = available time ÷ customer demand
Example: a shift has 450 minutes after lunch and tea. The customer takes 25 pieces a day. Takt is 450 ÷ 25 = 18 minutes. One finished piece is needed every 18 minutes.
Do not subtract breakdowns or setups from the available time. Those are our problems, not the customer's. If you subtract them, you hide them.
If demand rises to 45 pieces a day, takt becomes 450 ÷ 45 = 10 minutes. Check takt every month, or when demand moves by about 10%.
What is the difference between cycle time and operator time?
Cycle time is how often one piece comes out of a machine. Say a CNC machine takes 8 minutes per piece. But the operator is needed for only about 1.5 minutes of that, to unload, load and start. The machine cuts on its own for the other 6.5 minutes.
In most shops the operator stands and watches those 6.5 minutes. Once you separate manual time from machine time, he can use the machine time to run another machine. That is how one person can run a small cell.
How do you time a job properly?
- Tell the operator you are studying the method, not him.
- Break the job into small elements that you can see start and stop.
- Time ten cycles. Write each element.
- Take the lowest time that repeats, not the fastest single cycle and not the average.
- Note odd events, such as a chip jam or a search for a gauge. These are problems to fix, not part of the standard.
A phone video of ten cycles, timed calmly with the operator afterwards, is often easier than a stopwatch at his shoulder.
How many operators do you need?
Operators needed = total operator work per piece ÷ takt time
Count only the operator's own time: manual work and walking. Machine cutting time does not count. Always round up, then improve.
Balance % = total operator work ÷ (number of operators × takt) × 100
Example (figures for illustration): three machines, one operator each, and each operator has about 3 minutes of work per piece. Total work is about 9 minutes, takt is 18, and three operators are used. Balance is 9 ÷ (3 × 18) = about 17%. Each person is busy about 3 minutes in every 18.
Put the three machines side by side in a small cell and let one operator run all three. His work is about 10 minutes with the walking added. One operator, balance about 55%. The other two move to new work.
What if demand grows?
Suppose takt falls to 10 minutes and the cell work is 10.5 minutes. One operator cannot keep up. There are two choices.
- Add a second operator. It works, but both are half idle.
- Take the waste out first. Use a go/no-go gauge instead of a micrometer. Fix a deburring tool at the machine. If the work falls to 9.2 minutes, one operator is enough, at a balance of 92%.
Remove waste first, then add people. Adding people first builds the waste into the new standard. Also keep a little margin. A balance of 100% leaves no room for the small problems every shift brings.
Balance the work against the customer's pace, never by asking people to hurry.
How do you teach and keep the standard?
A standard that nobody was taught is only paper. Teach in four steps: prepare the worker, show one step at a time, let him try while you watch, and follow up. Write a job breakdown with three columns: important step, key point and reason.
Then keep it alive:
- Post the sheets at the cell, signed by the supervisor and the operator.
- Watch one cycle a shift. Ask "what stopped us?", never "who stopped us?"
- Revise after every improvement and remove the old sheets.
- Train at least three people on each standard. One trained person is a risk. Three is a system.
How to start in your plant
- Write today's takt on the board.
- Pick the bottleneck operation.
- Video ten cycles and time them with the operator.
- Separate manual, machine and walking time.
- Calculate the operators needed and the balance %.
- Remove one waste, write the new sequence and train all shifts.
Key points
- Standard work has three parts: takt, sequence and standard stock.
- Takt is the customer's pace: available time ÷ demand.
- Operators needed = operator work ÷ takt. Count only manual time and walking.
- Take out waste before adding people, and never use balancing to cost a job.
- A standard works only when it is taught, checked daily and revised after each improvement.
