Center to Center Tolerances
Center to center accuracy is determined photographically, rather than by the metal etching process.This tolerance is typically considered to be within +/- .0005 inches (.013 mm) over 6 inches (153 mm) for metal etching.
Photochemical Etching Dimensional Tolerances
It is not possible to establish hard and fast rules regarding photochemical etching tolerances since so many factors are involved. Some of the variables that affect accuracy are: type of metal; size of the sheet; size of the production run; and the number of critical dimensions per part. Typically, an etched dimensional tolerance of +/- 10% thinner to +/- 20% thicker of the metal is considered acceptable by the industry.
Parts that are designed with score lines for hand forming, such as RFI and EMI shields, are an exception to this guideline. The dimensional tolerance for hand-formed parts is approximately +/- .010 inch (.254 mm). Dimensional tolerances for parts that are formed with hard tooling after photochemical etching have to be determined on a part specific basis by the fabricator.
Finally, dimensional tolerances for dropout parts are different than for parts that remain on the sheet. Refer to the section under “Tabs” for more information on having parts with or without tabs.
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