i should have mentioned that that is 6061 aluminum alloy. also, when you chuck up the piece of aluminum in your lathe, if the chucking force deforms the work piece much you may find that what was just the right diameter and size in the chuck may not be after you remove it from the chuck. ie, et may be roundish with 3/4 lobes. therefore, when you make the cut which will house the lenses, be sure that the cell is nearly unstressed. note that if you cannot measure accurately, a snug fit will do ie, not tight, not loose, enough to have a strip of xerox paper in the cell at one point of the edge. also, be careful that if the elements bind in the cell, dont panic. in this case, a plastic dishpan with water warm/hot to the touch will allow you to gently ease the lens loose without damage.