5.3. Bimetallic effect of plating aluminum If a protective coating is being applied to an optical surface, then bimetallic bending effects may need to be considered. In the case when plating with a CTE different from aluminum is used, the aluminum base material can be warped or deformed over temperature excursions. To get a gauge of the extent of the bimetallic bending, consider the basic bimetallic bending relationship for radius of curvature of a bimetallic strip: where Using the above relationship, a Grade A QQ-N-290 nickel coating (0.0016 inches thick) on one side of a 1/8” thick piece of aluminum would result in a radius of curvature of 25meters given a 50°C temperature change. This bending could be eliminated by using symmetry, which involves applying an identical nickel coating to the other side of the aluminum. Unfortunately, in cases where symmetry between the two sides of a piece of aluminum does not exist, such as lightweighted aluminum mirrors that have pockets milled out, a plating can cause substantial adverse effects such as waffling of the front face. It can also be mitigated by using a plating with a CTE similar to the aluminum alloy, such as by alclading. 5.4. Specifying protective coating requirements Surface treatment of aluminum can result in dimensional changes in the part; sometimes increasing dimensions such as when a coating is applied and sometimes negative when a material conversion or etching/cleaning/abrasion is employed. The end user of the part is generally only concerned with the final dimensions, so it is common to include a note on mechanical drawings such as: ALL DIMENSIONS APPLY AFTER FINISH Machine shops, plating houses, and finishers generally will have more expertise in achieving final dimensions than the part designer, so leaving the adjustment of the final dimensions to the part vendors is preferred if they will agree to those terms.
5.3. Bimetallic effect of plating aluminum If a protective coating is being applied to an optical surface, then bimetallic bending effects may need to be considered. In the case when plating with a CTE different from aluminum is used, the aluminum base material can be warped or deformed over temperature excursions. To get a gauge of the extent of the bimetallic bending, consider the basic bimetallic bending relationship for radius of curvature of a bimetallic strip: where Using the above relationship, a Grade A QQ-N-290 nickel coating (0.0016 inches thick) on one side of a 1/8” thick piece of aluminum would result in a radius of curvature of 25meters given a 50°C temperature change. This bending could be eliminated by using symmetry, which involves applying an identical nickel coating to the other side of the aluminum. Unfortunately, in cases where symmetry between the two sides of a piece of aluminum does not exist, such as lightweighted aluminum mirrors that have pockets milled out, a plating can cause substantial adverse effects such as waffling of the front face. It can also be mitigated by using a plating with a CTE similar to the aluminum alloy, such as by alclading. 5.4. Specifying protective coating requirements Surface treatment of aluminum can result in dimensional changes in the part; sometimes increasing dimensions such as when a coating is applied and sometimes negative when a material conversion or etching/cleaning/abrasion is employed. The end user of the part is generally only concerned with the final dimensions, so it is common to include a note on mechanical drawings such as: ALL DIMENSIONS APPLY AFTER FINISH Machine shops, plating houses, and finishers generally will have more expertise in achieving final dimensions than the part designer, so leaving the adjustment of the final dimensions to the part vendors is preferred if they will agree to those terms.