n lightweight sandwich mirror technology, quilting or print-through represents a permanent surface deformation (up-swelling or dimple) on the unsupported area of the thin face sheet attached to a cellular or foam core structure. It occurs during the machining or polishing of the face sheet when the force or pressure between the tool and face sheet is varying spatially due to the presence of the cell wall in the foam core sandwich mirror.The region with the wall support poses more resistance to the tool than the center of a pore, where the bending resistance of the face sheet is less.

Metal foams made by the melt route naturally have an interface region between the porous and nonporous structures. We investigate here the possible use of monolithic foam structures with such interfaces wherein the porous structure provides needed light-weighting while the nonporous region provides a functional surface such as the one needed for optics. Aluminum A357 alloy foams were made by the melt route.Then the cylindrical foam rods of diameter 19.5 mm were cut from bulk foam cylinder by wire electrical discharge machining (EDM). The internal pore structure of foam specimens was analyzed using micro X-ray computed tomography.

Face turning process was conducted at a cutting speed of 360 m/min, feed of 0.005 mm/rev, and depth of cut of 0.1 mm in the nonporous region very near the interface to the porous areas. After each cutting pass, the machined surface was characterized using 3D optical profilometer to assess the presence of quilting effects. Studies show that pore as near as 400 μm to the machined surface can cause significant quilting. This study could pave the way for the use of such structures for lightweight telescopic mirrors.