How do I make an edge-lit acrylic sign: which side to engrave, mirroring, and how deep?

How it works

Light from an LED strip travels inside the clear sheet and only escapes where the surface is roughened. The engraving goes on the back face, so the viewer looks through a smooth front. That means the artwork must be mirrored before toolpathing.

Steps

1. Material. Use cast acrylic. Extruded acrylic turns gummy when routed.
2. Mirror. Select everything, open the Mirror form and flip horizontally (shortcut H). Forgetting this is the classic mistake, so check that the text reads backwards on screen.
3. Engraving toolpath. A V-Carve toolpath with Flat Depth checked keeps the cut shallow and even. One published CNC build engraved only 0.2 mm deep in 5 mm sheet and found that brightness depended on the LEDs, not on engraving depth. Deeper is not better; consistent is.
4. Bits. A 60 degree engraving bit is a common recommendation. The same build used a 30 degree engraving tip for fine lines and a 1 mm single flute flat cutter for filled areas.
5. Cut out. A 1/4 inch O-flute, at least 18,000 RPM, a feed somewhere in the 75 to 300 inches per minute range depending on bit size, pass depth about half the bit diameter, and a ramp entry.
6. Protective film. Peel the film from the face being engraved and leave it on the other face until assembly.
7. Base. Measure the sheet thickness and the width and height of your LED strip, then pocket a slot in the base to suit, and prove the fit on scrap.

Pro tips

• The sheet must be flat to the spoilboard. A 0.2 mm engraving shows every bit of bow. Surface the spoilboard and hold the sheet down across its area.
• Keep chips clearing with air. Melted acrylic welds to the bit quickly.
• Clean with a soft cloth only: no paper towels, no ammonia or alcohol cleaners.
• See the entry on cutting acrylic without melting for bit and feed details.