Let’s be honest – the term “rework” is most commonly associated with delays in production. Technically speaking, rework means correcting failed, defective, non-conforming items. It is essential to reduce manufacturing and production rework and repairs in order to save money and achieve higher quality across the board. However, this requires detailed planning and strategies that must be implemented at specific stages in the manufacturing process in order to ensure quality is upheld from start to finish.
It is a process that can be carried out either during or after the inspection phase, and there are usually one of several reasons for this.
- Not up-to-date instructions – operators may be using manuals that aren’t entirely up to date with any modifications made to the production process.
- Human error – a high level of production or temporary increases in production capacity can often lead to errors creeping into the production line. This can be avoided by automating certain stages of the workflow or employing more operators to deal with the uptick in work, however temporary it may be.
- Machine malfunction – as well-equipped as your maintenance team may be, machines are likely to produce errors from time to time, leading to delays in production and a potential need for rework. It is worth implementing or increasing the frequency of preventive machine maintenance in an attempt to reduce this.
- Using the wrong tool or material – if errors are made at the material selection stage, this can lead to issues further down the line.
Although the potential causes of reworks are understandable and simply part of the manufacturing process to a certain extent, they nonetheless have a negative impact on OEE, production costs, and overall higher quality products.
While rework might be necessary on rare occasions, it is advisable to minimize it. Sometimes as little as two reworks on a product may lead to it becoming scrap. Therefore, it should become a company-wide priority to avoid reworks in the first place. It is important that they are well managed using a clearly defined process so as to keep disruption and overall impact to an absolute minimum.
Defects and reworks definition and documentation
The first step in the process is to prepare the defects master data, starting with basic information including defect name, defect code, and defect category. Once this stage is complete, we should then move on to provide more detailed information of what has malfunctioned, including photo evidence and digestible descriptions that can be easily understood by all workflow parties concerned.
After defects documentation preparation, defects should be assigned to items, items group, or operation type. The final step of documentation definition is to prepare the workflow.
This essentially means that after detecting a specific defect on a particular operation, there should be clear instructions written to guide operators with regard to changes in the workflow:
- Scrap – there are cases when products will need to be scrapped altogether, as the defects detected may have no solution or are too tricky or costly to remedy for the client in question,
- Rework – if the product in question is deemed appropriate for rework, then the operation item/batch should be clearly indicated and sent to the appropriate operation or machine.
Of course, to speed up the rework and detection process, rework labels should be printed automatically, and the same defects should be grouped into one bigger production order and resolved together. This will boost efficiency and prevent any further preventable hold-ups in the production process.
On the other hand, the production of excess amounts of scrap seriously impacts the OEE of any business. Therefore, it is important to keep scrap to a minimum in order to ensure an optimal score for your manufacturing line. That said, scrap in some quantity is inevitable, and planning for this can help to keep the production system operating as smoothly as possible.
Reworks management – support during the defects detection
The operator should be supported during the defect detection process. This support can be provided via the following means:
- displaying reminders for defects detection after some events (production launching, breakdowns, failure, etc.), time (15 minutes, 1 hour, etc.), or produced pieces (100/1000 produced goods, etc.),
- displaying instructions with documents, movies, pictures, etc., to make the process as simple as possible for all those involved,
- automatic defects reporting if it can be read from the PLC and manual if not (with optional location pointing on item’s drawing/picture),
- after a defect occurs, help in making a decision regarding rework or scrap,
- managing the rework flow – what production processes should be applied if the defect is reported on the particular operation. The user should decide to what operation such a defective item should return to,
- plan in the RTS or APS blocks for reworks.
In order to keep costs down, it is also worth not only backing up any paper files digitally but replacing them with digital form altogether. Using files in a digital format also makes it much easier to track revisions, ensuring your employees are referencing the most up-to-date, accurate information at all times. Investing in project management programs can also reduce the risk of ordering incorrect components or building to an old revision level.
Reworks analysis influencing OEE
Since reworks influence OEE and increase production costs, you should prepare a reduction plan. There are several Pareto Charts that can help to understand and analyse root causes of the reworks, such as sharing of reworks for the whole facility or department. The sharing of reworks for a particular production line, machine, or item would be another cause. Moreover, OEE waterfall could be used, resulting in showing the contribution of reworks in OEE drops.
The two most common ways to include rework in OEE are:
- To treat the time needed for rework as unscheduled time. Then rework has no impact on OEE, but will reduce TEEP. This option seems most reasonable in cases where reworks are done in batches after production. OEE would be reduced by decreasing the quality factor when the part would be first in the production process.
- To treat rework as a new raw material that returns to the operation specified in the technology. This approach impacts the OEE. It leads to all the coefficients being calculated a second time. This option seems to be the most reasonable in the case when repairs are made during the current production order. In this case, you should keep a closer view of the reworked parts to avoid a situation of repeatedly repairing items. You may consider scrapping items after 2-3 reworks.
What are the benefits of managing reworks?
The proper management of reworks enables you to better monitor progress, size, and time spent on reworked production in order to better optimize processes and improve productivity. In addition, establishing a standardized rework flow can help to minimize impact on the company’s OEE and ensure that future incidents are dealt with effectively and efficiency.
By taking a proactive rather than a reactive position and pre-empting any potential quality control issues, it can be helped to keep things running as smoothly as possible with minimal delays.