Visual inspection is a process that is highly dependent on humans and is prone to human error and inspector variation. To prevent them, it is important to clearly define the line between good and defective products. Here, we will introduce inspection methods, inspection standards, and how to select an inspection method that suits your work and environment.
About inspection standards
As with all inspections, it is important to clarify the inspection standards in advance. In particular, visual inspections such as visual inspections are difficult to quantify like dimensional inspections, and judgment criteria tend to be ambiguous, so caution is required. Also, if there are multiple inspectors, it is important to accurately communicate to all inspectors.
Therefore, instead of using letters and photographs like specifications and inspection standards, “limit samples” that make it possible to visually understand the pass / fail judgment criteria, “dot gauges” that clarify the pass / fail judgment of scratches and foreign substances, etc. Use the tools of.
In sensory tests such as visual inspection, there are no clear criteria such as numerical values, and it is difficult to understand the pass / fail judgment criteria, so we will prepare a limit sample for comparison with manufactured parts and products. In addition, free samples should be properly managed and updated regularly.
Among the limit samples, the one that shows the conditions of the rejected product is called “defective sample“. Visual inspection makes it easy to catch large changes, but it has the problem that small changes are hard to notice. Therefore, let’s “visualize” defects that are easily overlooked in advance with a “defect sample”.
Standard swatches show quality standards. The standard sample only conveys the standard quality of parts and products, and we do not know the limit of quality, “how much should be a good product”. The tools to answer that question are “limit sample” and “bad sample”.
The dot gauge is a reference gauge that displays the shapes of foreign substances and scratches in various areas. Since it is printed on a transparent sheet, you can check it directly on the work when you are uncertain about the judgment. It is a tool that is easy to use in the field because it can directly compare foreign substances and scratches, and it is less likely to cause individual differences compared to measurement with calipers.
Inspection method according to work/environment
In visual inspection, it is necessary to consider the target work and environment and select the optimum method from the viewpoint of quality improvement, cost reduction, and production efficiency improvement. Here are some points to check when conducting a visual inspection.
100% inspection and sampling inspection
The method of inspecting all manufactured parts or products is called “100% inspection”, and the method of extracting and inspecting parts or products by the method specified from the inspection lot is called “sampling inspection”. 100% inspection inspects literally all the workpieces produced, so the quality of the parts and products is completely guaranteed.
However, if 100% inspection is performed on inexpensive and large numbers of parts such as screws and bolts, the time and cost are often not worth the time, and the increased inspection amount increases the possibility of defective outflow due to human error. ..
On the other hand, sampling inspection is an inspection method in which an arbitrary sample is sampled from an inspection lot and all lots are passed if the inspection criteria are met. Since we inspect only the sample, we cannot guarantee the quality of all parts and products. It is a major premise that the product quality is already stable, the rate of defective products is low, and it is checked that the product is non-defective.
In-line inspection and offline inspection
In-line inspection is a method of inspecting on the production line. It is efficient because the inspection can be performed without stopping the production line, but if the visual inspection takes a long time, the speed of the line should be slowed down. In recent years, there have been an increasing number of cases where sensors are mounted on the line and automated in order to increase productivity.
Off-line inspection is a method of unloading the workpiece from the production line and inspecting it. Since the inspection process is independent of the flow of the production line, we can handle many confirmation items, but it takes time, effort, and labor to inspect all mass-produced products.
Visual inspection and automated inspection
There are two types of visual inspection methods: visual inspection by the human eye and automated inspection using sensors and image processing systems. For example, there is a limit to visual inspection of 100% in-line. Therefore, when 100% inspection is performed on mass-produced products, the number of cases where automated inspection is adopted is increasing.
Inspection method selection points
The above is just an example, but it is important to select the inspection method in consideration of the unit price and quantity of parts / products, required quality, delivery date, etc. For example, visual or microscopic 100% inspection of cheap, mass-produced products such as bolts and nuts is inefficient and increases production costs. From the next page, we will explain in detail the method of visual inspection.