[Ben Krasnow] is working on a project that uses an extremely expensive specialty mirror. He needed to cut curves into it, taking care not to chip or shatter the material. He’s found a reliable way of doing this with a CNC mill and is sharing his methods.
The material he’s working with is a cold mirror; it reflects visible light while allowing infrared light to pass through. He had to custom order it, breakage is not acceptable. [Ben] explains that the biggest risk when milling glass is the clamping method used. He built his own jig and uses shims, rather than clamps, to secure the material along the X and Y axes. It is held down on the Z axis using a bar of acrylic spanning from one side to the other with rubber feet on the bottom.
A diamond burr cutter does the work, spinning at 3000 RPM. [Ben] recommends moving the head at the slowest rate possible in order to give the cutter time to do its work. And of course the material needs to be kept cool by pumping cutting fluid across it. As he shows in the video after the break, what you get is a piece of glass that comes out with clean and smooth edges.
In case we’ve sparked your curiosity, this mirror will be used during an MRI scan. The patient looks at a monitor reflected in the mirror at a 45 degree angle. At the same time, an infrared camera records the patient from the other side of the mirror to monitor where they are looking.
