Inspection of Machined Metal Surfaces
The fabrication of metal components generally requires some type of machining operations. The exposed surface after the machining operation can expose internal features that might be of concern for different reasons. Some of the features might be considered cosmetic flaws and other functional flaws. Regardless of the category that these flaws fall in to, they must be identified and removed from the product stream. The isolation and identification of the flaws requires a quality standard that is defined by a specification.
The generation of this specification usually becomes the responsibility of the Product Engineer. The Product Engineer creates a specification that will cover ever potential failure mode of the component to insure that it will not experience a failure during the life of the product or at least its warranty period. If the product has been in use long enough to experience failures the specification may be changed to insure that particular failure mode does not occur again. After several revisions to the specification it will cover the majority of the failure modes. Although the revised specification is complete in its definition of the characteristics of an acceptable product it is usually very difficult to inspect the product as defined using human inspection techniques. The majority of surface inspection processes are still performed using human inspection.
Human inspection is still the primary choice for a couple of reasons. 1) If the production quantity is low, then the automation cost may be seen as prohibitive management. 2) The scope of the human inspection can be verbally described in general terms and easily taught to the inspection team. 3) The human inspector is adaptive to changing conditions. Let’s examine each of these topics a little closer to determine under what circumstances they hold true.
The volume is a major concern in the determination of whether automation should be implemented in the production environment. It cannot be emphasized enough that Manual Inspection is hard work! Many reports have been written describing the efficiency of manual inspection as a function of the inspection requirements, time and task performance. Most reports on the subject indicate that Human Inspection is at best 70% efficient for a typical inspection period of one shift. Granted it is difficult to perform as well as the Human Inspector at the beginning of the inspection period, but the Human Inspector rapidly become less efficient as the inspection period progresses. When the inspection involves Critical or Safety related components this drop off in efficiency is not an acceptable practice.
Case in point, the FDA in the last couple of years has stressed the use of automated inspection equipment wherever possible. This is an industry that takes the safety of product very seriously.
Manual Inspection is less costly to implement than automated inspection equipment. This is especially true when there are no or little requirements for the testing of inspection team’s capability. Companies that do not have an annual or semi-annual testing program for the qualification of inspection team members do not experience the cost associated with documentation and retraining. However, this may prove costly if a faulty product makes it to the customer and is associated with an assembled product. The customer may request or require a complete sort of all products in a lot or those manufactured during a specific time frame.
It is important to have a good description of the inspection procedure and requirements, most often referred to as an Engineering Specification (ES). This document will instruct the inspection personnel as to what is and what is not allowed in the determination of product quality. The specification usually contains a description of what areas should be inspected and what constitutes a defective condition in each area. The author of the specification will evaluate all of the potential failure modes of a product and then include a special case to protect the product from escaping the inspection process without detection. Often there are different inspection criteria for the various regions of a component. This is where the responsible party of the ES must be careful. It is one thing to include a list of every special case but it is another to expect that members of the manual inspection team will be able to capture all instances. If the product is complicated and has multiple surfaces that must be inspected the time allocated for the inspection process will be a concern. You don’t want the inspection team to short the inspection process because of time constraints but this is a common occurrence when members of the human inspection team are distracted.
A large percentage of machined components are manufactured from aluminum alloys. Aluminum is a pretty reactive material and tends to form oxides on machined surfaces quickly. The appearance of the machined aluminum surface can change depending on a number factors in the environment including: moisture, impurities, cutting and cooling fluids, tool quality, cutting speeds and endless others. The Human Inspector has the ability to change their interruption of the inspection requirements as the conditions in the manufacturing environment change. This may be considered as a “Good” characteristic by some and as a “Bad” characteristic by others, depending on which side of the “Quality Fence” you are on. The Quality Engineer wants to accept product that meets or exceeds ever item in the Engineering Specification for shipment to the customer. The Manufacturing Engineer also wants quality product but must insure that the required number of the components are shipped to the customer at the specified time. Sometimes there may be a tendency to “cheat” by loosening the inspection requirements so that production requirements can be reached. This is easier to achieve without the documented inspection report associated with each individual component.