Making Practical Improvement in Visual Inspection EffectivenessTed J. Schorn - V.P. Enkei America
Visual Inspection is very inconsistent. While most people involved will feel that 100% visual inspection makes correct calls better than 90% of the time, studies show that in actuality it is closer to 60%. This leads to considerable expense, not only in the business risk of shipping flawed product, but a much greater expense is in unnecessary rework that is performed on product which is really acceptable, but was miss-called by visual inspection as requiring the re-work. We have shown that the effectiveness of Visual Inspection can be greatly improved by careful planning, choreographing the inspection procedure and establishing an orchestrated approach.
In tonight's presentation we will discuss several practical tips to accomplishing this:
- Inspection station Layout and Lighting
- Universal and Published Standards of Acceptance
- Establishing an inspection routine
- Training of Inspectors
- Effectiveness Measurement
I have published three Technical Papers by AFS (2006-2007) which deal with this issue and are available for subscription download:
- Economics of Inspection
- Factors that effect Visual Inspection
- Training for Visual Inspection
First let's talk about Layout and Lighting.
Studies have shown that diffuse fluorescent lighting and non-rolling inspection stations are preferred for quality visual inspections of wheels at Enkei. The height of the inspection table should be carefully considered for both visibility and minimum lifting or part handling which may fatigue the inspector. Tools should be suspended for easy access and minimum fatigue (and they cannot be misplaced that way). Wheels are delivered to the inspector with extra stock on face rims so the face is protected when transferring through the station (between heat-treat and painting). It is believed (not yet proven) that green light (mixed with white) helps the inspector identify texture. A study with Iowa University is currently underway to validate this. Keep all weight low (and lifting minimum) to reduce physical fatigue of the inspector.
For $40 you can download an IESNA Recommended Practice 7-01 as a pdf from the Web. This goes into a lot of detail on ideal lighting for specific tasks. Generally Visual Inspection requires at least 1000 LUX (100 ft-candles) of fluorescent lighting with deflectors or diffusers. It is important to remember that despite the specifications at installation, if tubes, diffusers, and deflectors are not kept clean, the actual light being delivered will not meet the requirement. Routine cleaning of all lighting fixtures must be practiced.
Next let's talk briefly about standards.
At Enkei we identified 63 specific things that can go wrong with our aluminum wheel product. These are organized by processes that cause them and are published for all workers in both English and Spanish.
The defects can be:
- related to Casting
- related to Spinning
- related to Machining or
- related to Painting
In addition the various conditions are considered to be either appearance dependent or cause dependent.
The "standard defect list" is numerically coded for:
-training
-reporting (allows analysis for every process)
-external communication (Educating Customers).
In addition to the "defect list" there are "disposition codes" which include "continue processing", "re-work" (at specific operations, and "other".
Defining Acceptability
Every customer wants to be unique. A major part of the job is to impress on customers that there is an Enkei standard and it is a high standard. Part of this is explaining what is OK and not OK and why. Size of specific types of defects or blemishes (Length, width, depth, and area) must be precisely defined. At Enkei a special plastic ruler template is issued to employees as a tool which can be used to accomplish reasonably precise measurement of any visual blemish. Employees must be appraised of the Schema associated with specific defects and why these are important to specific customers (ie. a particular rim area might be adjacent to the customers nameplate or logo making it a focal point). In other cases safety may be involved (such are items adjacent to the valve stem hole). The more the inspector understands, the better the inspection will be.
Training
Establish a precise inspection routine.
Identify critical surfaces.
Identify the proper part orientation for viewing each critical area.
String the inspection of the surfaces into an orchestrated sequence in an ergonomic manner. Generally an inspector must be given at least 1/5 second to dwell on each spot to be inspected.
Train the sequence of views and correct hand positions for each.
Optimize the eye-part distance and lighting of accurate inspection of each critical area.
Training should involve actually touching each of the critical areas in sequence. This help ensure the eye focuses on what is important, and the routine is learned.
At Enkei inspectors were originally inspecting wheels in 8-10 seconds and supposedly covering the 63 critical areas. Such inspection were creating massive amounts of re-work and cost, and still allowing unacceptable parts to be delivered to customers. By incorporating proper orchestrated techniques and training (and frequent re-training) the inspector now gets a 20 second cycle time to inspect a wheel but the cost savings of increased inspection accuracy (less re-work and customer dissatisfaction) has outweighed the added inspection cost many times over.
In addition to the orchestrated handling of parts during inspection, inspectors should be trained by becoming familiar with real examples of defects such as shrinkage, skin, mis-run, brushes, pin holes, cold shuts, drag, chatter, and various paint defect types. The more knowledgable the inspector is about the processes the better he do his job.
As part of the inspection station are touch screens when the inspector can with a few touches record the defect (one of 63), its location, and its disposition). No typing or writing is involved. It is similar to taking orders at a well run fast-food restaurant.
Inspector selection and training must include:
-Screening (visual, acuity, color, etc.)
-Pre-check
-Class room instruction (include touching critical areas)
-Floor time with trainer
-Post check (must make cycle time with accuracy)
-Re-calibration (regular check to make sure all inspector find perform equally and consistently at least monthly.)
This last item is very important because inspector can wear down, or have changes in health. They may need re-assignment to other duties if performance has changed. A good inspector doesn't necessarily remain good (just because he is regular and diligent). Actual metrics must be used to assess performance.
Measuring Effectiveness
Audit
Check Cycle Time
Check Rates of Fallout
Maintain Traceability of Returns - Consider counseling to help inspector improve.
Artificial Measurement (auto industry has AIAG ans MSA4 attribute study information).
Summary
-Test you assumption of inspection effectiveness
-Don't leave inspection methods up to chance
-Bad inspections are very costly (ie. effective [maybe slower] inspection is cost effective).
-apply quality control methods to inspections as well as production