Making things with a three-dimen-sional (3d) printer is quickly becoming a commonplace activity. In part this is because of the expanding range of "printable" materials, which now includes precious metals, biological tissue and even the ingredients for a pizza. But the machines are still not very good at producing smooth surfaces. This is because 3d printers deposit material layer by layer, which can leave ridges. Sometimes, as with a Margherita, rough surfaces do not matter. But many things need to be smooth to look good and work well. Machining or polishing 3d-printed items by hand costs time and money. Some companies have tried polishing 3d-printed metal and plastic parts in machines like those used for polishing pebbles, but it is a slow process. Now a way has been developed to print objects in a transparent material with a precise shape and a surface that is smooth enough to be used as a lens.
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