If the problem is due to human error, how should we analyze and rectify it? Examples include appearance defects, incorrect labeling, quantity shortages, missed processing steps, or incorrect material issuance.
01. Appearance Defects
Generally speaking, appearance defects require 100% visual inspection, which can easily lead to missed defects. However, we can still find the causes and develop improvement measures from the following aspects:
■ Are the appearance inspection standards specific, clear, and agreed upon with the customer?
Some factories simply state "no burrs, no scratches" in their appearance standards. However, in actual operation, operators and production managers may believe that "no scratches are impossible," thus applying a vague standard based on their own understanding.
The proper approach is to classify the product by function, determining acceptable and unacceptable standards for each possible defect. These standards should then be clearly described using both text and images. The standards should be clear, easy to understand, unambiguous, and realistically feasible. Avoid vague statements like "See limit sample for appearance standards."
If limited samples are used, there must be written descriptions. These must clearly state which aspect and which defect is being addressed, whether it's a defective or acceptable sample, and the criteria for determining defect and acceptance.
■ Have the appearance standards been clearly and explicitly communicated to employees?
Avoid vague statements like "training is inadequate" and simply training an employee on this one defect, as similar issues may recur at different workstations.
If the issue is related to employee training, the root cause must be identified and rectified from the perspectives of training methods, processes, post-training assessment methods, peer review and mentorship for new employees during their initial onboarding. This will prevent similar incidents from recurring.
For example, training on appearance defects should include explaining appearance standards or work instructions to new employees, as well as providing ample physical samples of defects for practice in defect identification. Additionally, products with appearance defects can be artificially introduced into normal production to test employees' ability to effectively identify defects.
When employees first start working, they should consult with a master craftsman or quality control personnel to assess parts near standard limits. A system for key position management and on-the-job training should be established to prevent new employees from being hastily put on the job after only temporary training.
■ Is the job work plan reasonable?
For example, is the lighting intensity of the light source at workstations requiring visual inspection appropriate? Is the height of the visual inspection workbench suitable for workpiece turning and observation? Has the order of employees' visual inspection observation been planned and standardized?
In other words, when inspecting the appearance of parts, which surfaces and points should employees look at? Is the workpiece turning order and eye movement path fixed?
Can visual inspection and production operations be completed within the defined cycle time?
■ Are quality warning cards posted for frequently occurring visual defects to remind employees to pay special attention?
■ Most importantly, are the causes investigated at the source to reduce and eliminate visual defects?
For example, regarding the issue of bumps and dents, will the product transport rollers on the assembly line bump into parts?
Will there be bumps and dents between products?
Are the logistics and turnover tooling reasonable? Is there any stacking of workpieces?
Are there risks of bumping or knocking during employee handling and workpiece placement?
02. Incorrect Labeling
Incorrect labeling is a serious problem in assembly plants, potentially leading to line stoppages. Improvements can be made from the following perspectives:
■ On-site printing: Print as needed, prohibit pre-printing labels.
Generally, strict OEMs require suppliers to install label printers at the packaging station on the production line. If the factory has multiple production lines, each line should have its own independent label printer at the downstream packaging station. Avoid printing a large number of labels in the office.
Some factories handle various products, possibly from different batches, and prefer to print a large number of labels in the office, then bring them to the production line and sift them out during packaging. Alternatively, they package the boxes first, stack them together, and then label them all at once. Some factories even leave the packaged boxes for several days before labeling them before shipment.
These incorrect practices are bound to cause problems.
■ If the customer doesn't explicitly require a separate printer, and the factory can't yet produce a dedicated label printer, changeover error-proofing can be used to ensure label accuracy.
Simply put, when multiple products are produced on the same line, before switching to process a different product model, all raw materials, finished products, semi-finished products, tooling, cutting tools, auxiliary tools, various work instructions, record sheets, packaging materials, including the programs and process parameters used in the equipment, and of course, the labels, are switched to match the model of the product to be produced.
Anything that doesn't match is removed from the production line and from the operator's reach, or sealed away. This makes misuse impossible; this is called changeover error-proofing.
During changeover, operators use the "Changeover Error-Proofing Checklist" to check and record that all hardware and software changes have been implemented, and team leaders conduct mutual checks.
03. Quantity Shortage Issues
Increasing mutual checks among personnel can reduce efficiency. Therefore, besides using counters, improvements can be considered from the following perspectives:
■ For regular products, they should be arranged neatly.
A fixed number of rows, columns, and layers in a packaging box or material bin transforms quantity inspection into a visual check of the arrangement, making quantity confirmation very intuitive.
■ If there are requirements for protection against bumps and crushes, consider custom-made blister boxes or material bins with dividers to arrange products, one for one place. If a space isn't filled, or a part is missing, it indicates insufficient quantity.
■ Weighing method.
However, when there are many parts, and the weight of each part fluctuates within a certain range, it may lead to inaccurate quantity determination by weighing.
In this case, consider setting up several smaller packages within the larger package. The number of smaller packages should be set so that the sigma of the total weight fluctuation range is less than one-sixth of the weight of a single part.
04. Missing Processing Steps
Besides setting up continuous production lines and implementing process error prevention, improvements can be considered in the following aspects:
■ Increase self-inspection and mutual inspection.
Before releasing each process, the operator should self-inspect whether the processing of that process is complete, and release it or place it back into the material rack or material bin after inspection. Before processing, the next process should inspect whether the processing or assembly work of the previous process is complete.
■ Fixed-position management and storage.
For single-machine operations, parts to be processed and parts already processed must be placed separately and in a fixed position at each workstation. Generally, parts to be processed are on the left and parts already processed are on the right.
Mixed storage is strictly prohibited. Parts to be processed and parts already processed must not be placed on the same rack or box. In the work-in-process turnover area, work-in-process in full boxes should also be placed in separate areas according to different process stages, and clearly labeled.
■ Hang process tracking cards.
For single-machine production lines, tracking cards should be hung on racks or carts, indicating the process stage.
05. Wrong Material Issuance
The most common error is that one or two different models of products are mixed into a large package of mostly correct products. This is generally caused by the following reasons; correcting these aspects will largely eliminate the problem of mixed materials.
■ The most fundamental reason for this problem is the lack of proper changeover error prevention.
There are instances where different models of finished products and work-in-process exist simultaneously at production workstations and along production lines.
■ Similarly, different models of products may coexist in packaging and inspection areas, scattered and mixed together.
Therefore, fixed-position management and 5S are still fundamental requirements, not just simple cleanliness and tidiness. Besides the above issues, another common reason cited by employees is that "test parts were mixed in with qualified parts and submitted."
We often ask, how did you determine that the part was a test part?
Does the part have a traceability number?
Do the inspection records for the test part correspond to the traceability number?
Even if a test part was indeed mixed in with qualified products, the factory needs to analyze and improve its management methods in several areas, including the fixed placement and labeling of non-conforming products, the fixed placement and labeling of test parts, pre-start inspection and confirmation, and first-piece confirmation, rather than simply attributing it to operational errors.
