‘Job shops’ have particular challenges in production planning and capacity utilisation.
No two jobs are exactly the same, so the sequencing of jobs to optimise factory utilisation takes on even more importance.
In the middle of an operational improvement project in a job shop environment with multiple possible routes for a given job, I found myself having a regular conversation with the factory management and machine operators about creating a sense of ‘flow’ through the factory, and how that related to machine cycle time, total process time, and Takt time.
There were multiple definitions of cycle, depending on who I was talking to, but none had any idea of what Takt time was.
Therefore I followed my own advice to ensure that as a first step, the language in any factory was absolutely common. This ensures that the meaning meant to be conveyed by specific words were exactly the way they were received.
Cycle time has many definitions depending on circumstances. In effect it is the time taken to do one cycle of a specified task. The cycle time of production of a Boeing 747 is many months, whereas the cycle time for production of a specific part may be hours. Both are valid definitions, so ensuring that the definitions being used in your context are exactly the same is essential.
In my job shop, there are many routes for a completed job via a range of different machinery, each with its own limitations.
A job may be done on a machine that produces 5/minute, a cycle time of 12 seconds, or a machine that does 60/minute, a cycle time of 1 second. Make sure you are talking of the same machine when discussing the routing.
The cycle time of a particular completed job may have a number of separate processes that require both a specific order and potentially variable routing, so knowing the cycle times of each machine option, and other limitations, such as manning, that may apply is essential.
Takt time. (derived from the German ‘Taktzeit’ referring to a musical beat measured by a metronome)
Takt time is similarly subject to differing interpretations, but less so than cycle time.
The classic definition of Takt time is:
Available time for production/Required units for production.
This is exactly right, but the confusion usually occurs in the definition of ‘available time’
Assuming a normal 8 hour day, we start with 8 hours X 60 minutes = 480 minutes.
However, there will also be standard times during the day when the machine is not available. For example, it may take 15 minutes to be set up in the morning, then there are 2 x 10 minute ‘smoko’ breaks during the day, a 30 minute lunch break, and a 40 minute wash up at the end of the day.
The available time then becomes: 480 – 15 – (2 x 10)-30-40 = 375 minutes/day.
This ‘down time’ might be managed by having split times for lunch and start-up/wash up, and would change the Takt time calculation, but essentially to be simple there are 375 minutes available in the day. This becomes the base for the Takt time calculations.
Let’s assume that the customer demand was for 50 units. You then have 375 minutes to produce 50 completed products, or an available time/unit of 7.5 minutes.
Let’s further assume that the product takes 8 minutes /unit to produce, you are therefore 30 seconds short per unit, or 25 minutes over the course of a normal day. This is typically made up with overtime used to produce the 50th unit, or producing only 49 units, which annoys the customer of the 50th who are short or late delivered.
The additional problem is the backlog of back orders builds up, resulting in customers cancelling, going elsewhere, or just losing confidence in you.
None are outcomes you would wish for.
In addition, there are always unexpected things that happen, a machine goes down, a well-meaning manager walks through talking to operators and slows down the speed, a productivity improvement meeting is called, and so on. All this impacts on the available time, but the customer demand does not vary from 50 units/day.
The operational improvement task, often referred to as ‘Lean Thinking’ is to apply continuous improvement to both the available time, and time required to produce the unit, so that it eventually matches or exceeds the Takt time available.
Most factories I have seen have no idea of takt time, and many see no need for it, but it has several benefits.
- It makes capacity calculations relatively easier, even through a complex set of sub processes. By calculating the capacity and cycle time of each process, and the alternative routes that may be available, the best fit to the demand is exposed.
- It enables the calculation of the best batch sizes to be done.
- It gives team members an idea of what ‘well done’ looks like for every bit of production, an enormously valuable outcome.
- It acts as an early warning system of an emerging problem.
Understanding the two times, Cycle and Takt enables a clear sense of priority that can be applied to planning the sequences of jobs, and an understanding the total and local (individual process) capacity utilisation necessary to be of maximum service to customers.
When you have that clarity, you can address the opportunities for improvement, leading to a greater sense of ‘flow’ through the factory.