Takt time
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Takt time can be defined as the maximum time allowed to produce a product in order to meet demand. It is derived from the German word taktzeit which translates to clock cycle. There is a logic therefore to setting the pace of production flow to this takt time. Product flow is expected to fall within a pace that is less than or equal to the takt time. In a lean manufacturing environment, the pace time is set equal to the takt time.
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[edit] Definition
Takt Time is defined as:
Where:
- Ta = Net Available Time to Work eg. [minutes of work / day]
- Td = Total demand (Customer Demand) eg. [units produced / day]
- T = TAKT Time eg. [minutes of work / unit produced]
Net available time is the amount of time available for work to be done. This excludes break times and any expected stoppage time (for example scheduled maintenance, Team Briefings etc).
As an example, if you have a total of 8 hours in a shift (gross time) less 30 minutes lunch, 30 minutes for breaks (2 x 15 mins), 10 minutes for a Team Brief and 10 minutes for basic Operator Maintenance checks, then;
Net Available Time to Work = (8 hours x 60 minutes) - 30 - 30 - 10 - 10 = 400 minutes.
If Customer Demand was, say, 400 units a day and you were running one shift, then your line would be required to spend a maximum of one minute to make a part in order to be able to keep up with Customer Demand.
In reality, people can never maintain 100% efficiency and there may also be stoppages for other reasons, so allowances will need to be made for these instances and thus you will set up your line to run at a proportionally faster rate to account for this.
[edit] Implementation
Takt time has direct implications concerning the allowable time for completing individual steps in a production process. This is the case for both steps that modify (form, assemble, finish…) the product and also the steps that observe and control (test, measure, adjust…) the process. Similarly steps which require a part or assembly of the product to have been put into an accurately fixtured position must be completed in less than the total takt time so that time is allowed for loading and unloading or positioning the part in addition to the time for actually performing the production step. The quicker that a measurement or test step can be completed, the less constraint is placed upon product motion between steps. For example, a measurement process that captures the entire information about a part at once will permit shorter total takt time and a higher pace of production flow. Elimination of the need to measure reduces this step best (See SMED).
An implication of using takt time can be that work packages get reorganised. If worker one performs actions A1 through A5 and worker two performs actions A6 through A8 then a reduction in takt time may mean that there are now three work packages required to fit the new shorter/faster pace. They might be package 1 (A1 to A4), package 2 (A5 to A6) and package 3 (A7 to A8). So now we will have three people working to do the work that used to be achieved by two. This subdivision of workpackages rather than parallel working on unchanged packages of actions is a new idea to many. This way of working requires:
- a very flexible workforce, that is willing to accept changes in their routines and workplace
- requires a multi-skilled workforce, since now people may be asked to 'pick-up' actions currently performed by others
- flexible workcells, since what is being done by two people today may need to accommodate three people tomorrow
- increases hand-offs, so these must have no significant overhead
- keeps the workflow simple and easy to manage, so whether the process will deliver is clear to all
- has been observed to speed up individual steps in production, because the new context of each action encourages innovation.
It will be obvious that this kind of capacity replanning is not something that will be desirable every week. It is therefore important that the varying part of Takt time, the customer demand, should have been leveled before this kind of work replanning is undertaken. That leveling is looked at elsewhere and that therefore this style of capacity modification should be undertaken to meet long term customer demand changes and not weekly forecasts.
