Do you have a preference for laser or CNC cut mirrors? Both processes are good processes and both processes have their advantages and disadvantages. You will be much happier if you select the right process for your application, and can avoid problems if you follow a few basic rules.

What is Laser Cutting?

Simply put, laser cutting involves accurately moving a laser beam with sufficient power and at the correct wavelength to melt its way through the mirror. So laser cutting is not really “cutting” so much as melting through a narrow region of the material. Because you melt through, when the melted material cools off, it re-solidifies into a nice clear glassy edge, almost as clear as if well-polished. Laser cutting is also a good means for engraving as one can adjust power (down) and speed (up) so as to only partially melt into the plastic, removing only a thin surface layer as desired for engraving.

What is CNC router cutting?

CNC is short for “computer numerically controlled”. So CNC cutting involves using a computer controlled cutting machine, usually a router, to mechanically cut through the acrylic. Very stiff and very sharp carbide router bits are used and produce a very smooth and clean cut edge.

What are the advantages and disadvantages of each?

Laser Cutting

Edge Quality – Clear polished look

Cutting Force – No mechanical force

Tolerance – Typically +/- 0.1 mm for small parts

Residual stress – Very high residual stress

CNC Router Cutting

Edge Quality – Smooth but with fine tool marks and not polished

Cutting Force – High mechanical force

Tolerance – Typically +/- 0.1mm (0.005 inch) for small parts, more for large parts

Residual stress – Little to no residual stress

A Few Basic Rules:

  1. Do not use laser cut parts on outdoor applications. The residual stress left behind at the melted and resolidified edge produces embrittlement. In the weather this can produce crazing (numerous small micro-cracks) and even large macro cracks that weaken the entire part.
  2. Do not try to CNC cut very small parts (under 25 mm or 1 inch for example) if you can avoid it. Fixturing and holding small parts can be very difficult.
  3. If you use sharp router bits of the correct type for cutting plastic mirrors, with a well-designed cutting fixture, you can achieve very clean and smooth edge cuts via CNC router cutting.
  4. Recognize that final part dimensional tolerance is not only affected by the precision of the machine, but also by the precision of the fixture, by residual stresses left in the part, by thermal expansion and contraction, and finally by moisture absorption and desorption.