Page 48 - MetalForming-Jan-2018-issue

The Science of Forming
Table 2—Severity Evaluation Criteria
Effect
Rank
Customer Effect
Manufacturing/Assembly Effect
Hazardous- without warning
Very high severity ranking when a potential failure mode affects safe vehicle operation and/or involves noncompliance with government regulation without warning
May endanger operator (machine or assembly) without warning
10
Hazardous- with warning
Very high severity ranking when a potential failure mode affects safe vehicle operation and/or involves noncompliance with government regulation with warning
May endanger operator (machine or assembly) with warning
9
Very High
Vehicle/item inoperable, with loss of primary function
100 percent of product may have to be scrapped, or vehicle/item repaired in repair department with a repair time greater than 1 hr.
8
High
Vehicle/item operable, but at reduced level of performance; customer dissatisfied
Product may have to be sorted and a portion (less than 100 percent) scrapped, or vehicle/item repaired in repair department with a repair time between 0.5 and 1 hr.
7
Moderate
Vehicle/item operable, but comfort/ convenience item(s) inoperable; customer experiences discomfort
A portion (less than 100 percent) of product may have to scrapped with no sorting, or vehicle/item repaired in repair department with a repair time of less than 0.5 hr.
6
Low
Vehicle/item operable, but comfort/ convenience item(s) operable at reduced level of performance; customer experiences some dissatisfaction
100 percent of product may have to be reworked, or vehicle/item repaired offline but does not go to repair department
5
Very Low
Fit-and-finish/Squeak-and-rattle item does not conform. Defect noticed by more than 75 percent of customers
Product may have to be sorted, with no scrap, and a portion (less than 100 percent) reworked
4
Minor
Fit-and-finish/Squeak-and-rattle item does not conform. Defect noticed by 50 percent of customers
A portion (less than 100 percent) of product may have to be reworked, with no scrap, online and out-of- station
3
Very Minor
Fit-and-finish/Squeak-and-rattle item does not conform; defect noticed by discriminating (less than 25 percent) customers
A portion (less than 100 percent) of product may have to be reworked, with no scrap, online and in-station
2
None
No effect
Slight inconvenience to operation or operator, or no effect
1
Potential Causes or Mechanisms for Each Failure Mode
Each failure mode can have several causes. For example, using the wrong sheetmetal grade can have many caus- es. These include:
• Service center shipped the wrong product
• Purchasing ordered the wrong material grade
• Receiving placed the wrong label on the coil
• Operators pulled the wrong coil from inventory
• The forecast was incorrect, so the correct material was not available when production needed to make parts.
Occurrence—The Frequency of Each Failure Mode
The assembled team should assign a ranking related to the likelihood that each of the potential causes will occur. A design or control change of potential causes represents the only way to change an occurrence rating. Table 3 lists possible failure rates and their rat- ings. As with severity ratings, the actual thresholds should be customized for your specific needs and concerns.
Design Controls for Detection and Prevention
Next, we list the controls in place to detect and/or prevent each of the failure modes. These can include placing a notch into the blank shape to ensure proper orientation; laser etching a bar- code linked to tensile properties on the outer lap of every coil rather than relying on the oil-drenched paper certs taped to the outer lap; or scheduling a weekly call with a customer to gain insight on the latest usage forecast.
Detection Assesses Effectiveness of Design Controls
If it occurs, are we likely to catch it before damage is done? Detection assesses the effectiveness of each design control. For example, having two operators check the certified metal properties helps to confirm that the correct grade will be processed, but
Table 3—Possible-Occurrence Evaluation Criteria
Rank
Failure Rates
Probability of Failure
10
> 100 per thousand pieces
Very High: Failure almost inevitable
9
50 per thousand pieces
8
20 per thousand pieces
High: Repeated failures
7
10 per thousand pieces
6
5 per thousand pieces
Moderate: Occasional failures
5
2 per thousand pieces
4
1 per thousand pieces
3
0.5 per thousand pieces
Low: Relatively few failures
2
0.1 per thousand pieces
1
< 0.01 per thousand pieces
Remote: Failure is unlikely
46 MetalForming/January 2018
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