The manufactured print head had two kinds of thermal problems, despite a Finite Element Analysis conducted during the design process. Focus shifted with temperature. We cured that by using two different metals to hold the lenses in front of each laser and defining just where the assembly glue would grip them by adding grooves. Also, the laser spots moved around with temperature cycling. Not surprising, since there was nothing but friction to keep iron and aluminum components centered on each other! We solved this problem by putting facing concentric grooves on the meeting surfaces, then filling them with glue after fine adjustment. What we then had were slightly resilient cast-in splines that (by symmetry) kept the optics centered after that! Note: a finite element analysis is only as good as its boundary conditions, and at best is less valuable than mechanical symmetry in achieving a successful design. 019