Optical CMM Gauge Capability Evaluation for Sub Millimeter Molded Features
Optical CMM capability on sub-millimeter molded features requires material-specific bias correction and telecentric backlighting to maintain GR&R under 20%.
Tolerance Band Consumption
Meaning
The proportion of a part’s total allowable design variation that is taken up by measurement uncertainty reduces the usable tolerance range for manufacturing. This tolerance band consumption occurs because measuring instruments are not perfectly accurate, meaning that a portion of the design limit must be reserved to account for potential measurement errors. This reduction in the usable tolerance makes it harder for the production line to produce acceptable parts.
Measurement Uncertainty
Instrument errors and environmental variations contribute to the total measurement uncertainty of the inspection system. When tolerance band consumption increases, the effective limits for accepting parts must be narrowed to avoid accepting out of tolerance components. This narrowing reduces the margin of error for the manufacturing team.
Production Yield
Narrowing the acceptance limits can lead to the false rejection of acceptable parts that fall close to the boundaries. This increase in false rejects lowers the production yield and increases the costs associated with scrap and rework. Choosing a more accurate measuring instrument can help reduce the tolerance band consumption and restore the usable tolerance.
Quality Policy
Establishing a clear policy on how to handle measurement uncertainty is essential to ensure consistent quality standards. Many manufacturing standards require a four to one ratio between the product tolerance and the measurement uncertainty to keep tolerance band consumption low. When this ratio cannot be met, the company must accept a higher risk of false rejections or invest in high precision inspection equipment.
This decision requires balancing the cost of more accurate instruments against the potential savings from higher production yields.