Finally, the way in which the tasks are apportioned should be re-examined, and the following questions answered:
- Are the tasks divided up in the best way?
- Is the changeover being performed by the right number of people?
Implementing the improvements suggested by this line of questioning can reduce changeover time by 30% to 50%. Although separating external and internal tasks is a fairly elementary approach, it is surprisingly effective when combined with establishing clear procedures for individual tasks. If these two actions are done properly, the variability in changeover times can be considerably reduced.
The aims of improving changeovers in this way are to:
- Eliminate problems occurring after startup.
- Standardize changeover procedures so that anyone can complete them within a
- Identify any mechanical problems with jigs, tools and methods.
Like other improvements, changeover reduction must be implemented step by step. Making improvements at random, without following the procedure explained above, will not be very effective.
(2) Convert internal tasks to external ones
Examining the tasks involved in a changeover and working out how to convert internal ones (ones performed with the equipment shut down) into external ones (ones performed while it is running) is a powerful method of reducing the changeover time.
For example, a jig that is usually changed, assembled, and adjusted while the machine is stopped can be pre-assembled while the machine is still operating, or parts that are usually adjusted on the machine can be adjusted off-line using standard gauges. Methods such as the following can be used to convert internal tasks to external ones:
1 Pre-assemble. Rather than installing several parts one by one while the machine is stopped, assemble them beforehand and replace the entire assembly.
2 Use standard, quick-fitting jigs. Compare the shapes of the jigs used for different products and see whether any of them (or any part of them) can be standardised for use across the product range, and whether quick-release mechanisms can be incorporated that enable them to be replaced with a single action.
3 Eliminate adjustments. Wherever possible, make adjustment an external task.
4 Use intermediary jigs. Whenever a cutting tool is changed, it must be centered.
However, the need for centering can be eliminated by setting the tool in a standard, high-precision intermediary jig which is installed on the tool post instead of mounting the tool in the toolpost directly.
(3) Shorten internal tasks
Internal tasks themselves can often be shortened by using standard, quick-fitting jigs and better assembly and fastening methods, and by eliminating adjustments.
1 Improve clamping mechanisms. Reduce the number of bolts, for example, or replace screw-type clamping mechanisms with hydraulic ones.
2 Work in parallel. Two people working simultaneously can sometimes complete a changeover more efficiently than a single person performing one step at a time. Although timing and coordination are crucial to the success of this approach, changeover times can often be halved with the same number of labour-hours, even in the most difficult cases.
3 Use the most appropriate number of workers, and allocate the tasks in the best way. Complex changeovers must sometimes be performed by dozens of people. In such cases, considering the following points can reduce setup and adjustment time drastically:
(a) What is the optimal number of workers for each task?
(b) How should the tasks be shared?
(c) What is the critical path for the changeover? What are the constraints preventing it from being shortened? (use of crane? labour resources?)
(d) What is the best way of using the labour available?
Table 4.15 shows some examples of strategies for improving setup.
(4) Eliminate adjustments
Many adjustments can be converted into ‘condition-setting’; that is, setting the required conditions in one go, without the need for trial and error. First, all the current adjustments should be examined to determine whether they are avoidable or not by looking at what their purpose is, what gives rise to the need for them, what exactly they consist of, and how effective they are. The items that should be taken into consideration when doing this are explained below.
Adjustments are usually performed for one or more of the following purposes:
- Positioning: adjusting height, or position on the X or Y axes (for example, setting a press die on a bolster and adjusting its height).
- Centering: centering cutting tools, workpieces, etc.
- Dimensioning: adjusting the depth of cut of a machine tool to achieve the specified dimensions, for example.
- Timing: adjusting the timing of various equipment mechanisms.
- Balancing: adjusting lateral pressure, balancing with set screws or springs, etc.
Adjustments are needed in the following circumstances:
(a) Accumulated errors
This is the most common reason why adjustments are needed, when all the small individual errors in the equipment or the jigs and tools add up to produce a large error necessitating adjustment. Sometimes the error is just in the machine itself, but more usually it is a combination of the cumulative errors in the machine and the associated jigs and tools.
(b) Lack of standardisation
Adjustment is required when there is insufficient standardisation- for example, when reference surfaces are not set, dimensions are unquantified, the amount to be removed in one pass of a grindstone is not specified, etc.
(c) Insufficient rigidity.
Even if everything is satisfactory when the machine is stationary, errors may be produced if the equipment flexes during operation because it is not stiff enough.
(d) Mechanical deficiency.
Some mechanisms are designed to be adjusted by a human operator and would have to be redesigned to eliminate it.
As stated earlier, right-first-time changeover produces good product from the start, without the need for trial processing. This is radically different from the usual type of changeover, where the parts are changed, some product is produced and checked, adjustments are made based on the results of the checks, more product is produced and checked, further adjustments are made, and the process is repeated, usually at least two or three times, until the product satisfies the specifications. In right-first-time changeover, the parts are changed and all the settings are mechanically locked into position, so that no adjustments are required (see Figure 4.30).
The effectiveness of adjustment tasks should be analysed in order to determine whether they are really necessary or not (see Table 4.16 and Figure 4.31). The procedure described below should be followed in doing this: