Understanding beveled edges

Posted by Steve Rowe on

Beveled edgeA process that is often overlooked in precision glass fabrication that is not glamorous, but extremely crucial is the creation of a beveled edge on an optic. You may have heard the term chamfer, safety bevel, or break edge bevel. These are all just different ways of saying the same thing. Where two surfaces meet that edge is flattened to ease the transition of the two planes, generally at 45º.

For optics, a bevel provides a few benefits. The first of which is it prevents chipping of a sharp 90 degree edge, which leads to problems during the lapping/polishing process by inducing scratches. The second reason is safety, handling of machined glass for both fabricators and customers would certainly lead to cut fingertips without bevels.

A beveled edge can be added by hand or by machine. Adding a beveled edge by hand takes skill and it is usually for smaller optics where a simple easement prevents a sharp edge. For circular optics, an operator will place the optic in a finely textured concave bowl that rotates. The bowl shape allows the round optic to be ground down uniformly. 

Machined bevels are performed on a Sadai Automated Centering / Edging Machine. These machines optimize high volume manufacturing by using a “pick-and-place” capability. Pick-and-place generically is a computer-guided operation that allows for 100% repeatability on bevels, part to part, run to run. Esco uses several Sidai machines that are in constant motion for circular optics. In most cases, 45º is the standard, however custom bevels of any degree can be accommodated. For rectangular or square optics another specialized automated machine is used for straight and consistent edges.

Optical bevels, standard bevel, chamfer edge, safety bevel

Polished Edges

For certain applications, polished edges are beneficial versus standard ground edges. Polished edges create more durable optics because it reduces the number of microscopic surface deviations (sub-surface damage) on the edge of the material. This is especially important for high pressure or deep-space applications for two main reasons. Any imperfections in the structure of the glass are reduced as the edges are polished smooth thus minimizing the chances of a fracture or failure of the optic. Additionally, for an unpolished or ground edge, the microscopic pores of the glass can trap polishing compound or other debris. Any applied forces or changes in temperature or could release the contamination, causing failures or shorter lifetimes of the optics or overall system. For a polished edge, the optic will undergo a series of edge grinding at finer and finer grits until the final edge is completely smooth.

In conclusion, the choice of bevel shape, type, and tolerances will impact the pricing of the optic, but also overall performance in the actual application. Whether the optics are is used in standard commercial use, require custom machined bevels for high-pressure applications, or need polished bevels/edges for flight or semiconductor applications, Esco Optics can provide the correct edge treatment to maximize performance for your application.

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