Visual Inspection of Tires - Evaluation and Improvement
Mohammad Zendehrooh[1], Ahmad Ghasemshirazi
Kerman Tire & Rubber Company (BAREZ TIRES), Jupar road, Kerman, IRAN
, +98(341) 2222259 Ext 2243,+98(341) 2223853 (fax)
ABSTRACT
Visual inspection is a key quality process in many manufacturing companies, so the effectiveness and efficiency assessment of this process is important to guide process improvement.
In this study we used an attribute measurement system analysis method (gauge R&R cross tab method) to evaluate this process in the tire industry.
The kappa parameter, which indicates the extent of interpreter agreement, was calculated based on the observed results. The calculated Effectiveness of the measurement system, False Alarm Rate, and Miss Rate were used as criteria for assessing the visual inspection process.
Based on initial results of this study, the several improvements were undertaken. These included retraining of inspectors, changing the inspection path, using visual aids (appearance masters or distinct image), and optimizing luminous intensity of evaluation areas.
These changes resulted in a decrease of overall MR compared with the initial situation. In addition, the general agreement of inspectors was increased.
Key words: visual inspection of tire, measurement system analysis, visual inspection performance
Introduction
Visual inspection is a key quality process in many manufacturing companies. In tire manufacturing this process has more importance by reason of many safety features that must detect during visual inspection.
Develop a reliable visual inspection of tires constitutes a quality challenge in consequence of the large amount of characteristic to be considered in planning judgments. The effectiveness and efficiency assessment of the visual inspection process is important to guide process improvement.
Visual inspection is an attribute measurement system the measurement value has only two possible results.
There is a quantifiable risk where using any measurement system in deciding. Since the largest risk is at the categories boundaries (area II). See figure 1
Figure 1: Three areas that tires selected from it
Then whether the part is in area II the decision is depend on the skill level of inspectors. So to considering different situation the selected tires covered all areas. Approximately on third of tires was conforming, on third nonconforming and one third was marginal (marginal conforming and marginal nonconforming.)
This means the selected tires has obtained throughout the process range.
The calculated %Effectiveness of the measurement system, %False Alarm Rate (FAR), and %Miss Rate (MR) were used as criteria for assessing the visual inspection.
Where:
Effectiveness % = / Number of correct decisions / * 100Total opportunities for a decision
False Alarm Rate % = / Number of false alarm (that is rejecting good tire) / *100
Total opportunities for false alarm
Miss Rate % = / Number of miss (that is accepting rejected tire) / *100
Total opportunities for miss
Calculation and discussion
Assessing initial situation
Base on attribute measurement system analysis method (gauge R&R cross tab method) for evaluation of visual inspection efficiency for each type of tire defect the 30 different tires was selected (from three areas that introduce before) for evaluating % FAR, and % MR.
Five inspectors/ appraisers are used with each inspector making 5 decisions on the each tire.
The results have been shown in the table 1.
Table 1: the MSA results for visual inspection of tire before improvement
*) the tire inspectors rely on his/ her touch for detecting this defect
Interpretation of first Initial evaluation of results
According to the fist results the average %MR value is 8.21 that mean the inspectors had some bad judgment about the quality of tires.
Also base on average %FAR value (7.58), we expected for finding some good tires in rejected tires that judged by inspectors.
Also In initial evaluation the results shows the average % MR=8.93 and average % FAR = 7.95 for detecting some tire defect such as air trapped tires that inspectors rely on their touching sense. (See all data in table 1)
Finding out best condition
The main road map for improvement to create best facility for good discriminating among tires are: retraining of inspectors, changing the inspection path, using visual aids (appearance masters or distinct image), and optimizing luminous intensity of evaluation areas, defect catalog, and revising priority-ranking procedure.
Priority-ranking procedure
In the first step, we try to ensure priority-ranking procedure that used inspector used for tire ranking by clarification.
Using visual aids
Clear defining of each defect and define all guidelines for good judgments. For instance, a one-page of defect catalog and relative guideline has been shown on figure 2
Figure 2: a sample of tire defect catalogue and relative guideline.
Define the control path
For the deferent type of tires the inspectors used individual and accustomed control path. (Figures 1)
In some applied control path, the miss rate will be increased because of some ergonomic subject.
For instance, in visual inspection of heavy tires (agriculture, truck and bus tires) the inspection was occurring during whirling of the tire on the shop floor and often the inspectors recline for detecting defect closer.
In the light tire such as PCR tires also some wrong habit such as spinning tire by one hand. Inspector weary is a main cause for improper judgment.
For instance; improved control path for two types of tires and the changes have been shown in figures 3 -5
a) Initial control path:
b) Improved control path:
Figure 3: explore optimum control path for inspection of different typre of tires. (Small tires)
a) Initial control path:
b) Improved control path:
Figure 4.explore optimum control path for inspection of different typre of tires. (big tires)
Figure 5: a sample of official work instruction for clarifying optimum control path
Optimizing luminous intensity
Luminous intensity in defect place on a tire was measured by HAGNER EC1. The direction of luminous intensity measurement device was on the inspector's eyes.
The results have been shown on a table .2.
Table 2: optimum Luminous intensity for visual inspection of tires
Tire group / Tread area / Internal area(behind the tread) / Out side area
(sidewall) / Internal area
(behind the sidewall)
Truck & Bus / 2800-2900 / 500-600 / 600-700 / 360
Light truck / 2000 / 1300-1400 / 2700 / 700-800
Agriculture / 1630 / 800 / 600 / 300
Passenger / 3100 / 700 / 2560 / 300
Over inspection tire / 3800 / 240 / 3000 / 250
Touching improvement!
For improving touching inspection! We considered the problem of the defect detection of air trapped by a touching inspection of the touched tire surface. The procedure for qualification of an inspector was revised. And the make-up classes and strengthening practice for inspector was arranged by skilled inspectors.
In particular we propose a special test to evaluating natural ability of inspectors to recognizing this defect.
Also by a simple test, The 20 different defective tires were used to evaluating the natural ability of inspectors to recognizing the defective spot. And the pass of this exam was introduced as one of qualification criteria for visual inspection job.
Second Assessing of visual inspection
The process of visual inspection of tire was evaluated by the same method. And the results have been shown in the table no. 3.
Table 3: the MSA results for visual inspection of tire after improvement
*) the tire inspectors rely on his/ her touch for detecting this defect
The comparison the results have been shown on table no.4. the establish improving actions resulted in a decrease of nearly 30% in the overall MR and a 40% improvement in MR for detecting potentially dangerous failures, such as internal air-trapped tires (that has been introduced as the root cause of some car accidents), compared with the initial situation. In addition, the general agreement of inspectors was increased.
Table 4: Improvement in visual inspection of tire
Total defect / Defect that detect by touching%MR / %FAR / %Effectiveness / %MR / %FAR / %Effectiveness
Initial evaluation / 8.21 / 7.58 / 80.97 / 8.93 / 7.95 / 79.53
Evaluation after improvement / 5.70 / 4.81 / 87.58 / 5.35 / 4.82 / 86.96
% Improvement / 30.57 / 36.54 / 8.16 / 40.09 / 39.37 / 9.34
The MSA results prove that the defect detection achieves more classification capability and better defect detection performances.
Conclusion
a) Attribute measurement system analysis method (gauge R&R cross tab method) is a good tool for evaluating the visual inspection process in the tire industry.
b) False Alarm Rate (FAR) and Miss Rate (MR) is a good criterion for evaluation visual inspection performance.
c) Using visual aids helps for general agreement.
d) Optimized the tire inspection path will improve the performance of tire visual inspection.
e) The luminous intensity in defect place on tire to reach the better tire visual inspection performance figured out and optimized.
f) A devised simple test for comparison of appraisers' touch ability help to choose inspectors and improving touching inspection performance for detecting air tapped defective tires.
g) False Alarm Rate (FAR) and Miss Rate (MR) analysis shows marked improvement touching inspection process performance with qualified inspectors according to the new test.
References:
1. Measurement system analysis, reference manual, third edition, published by AIAG, USA
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[1] Presenting author