Milling edge lit signs

This post is about the creation of one of those cool looking, completely transparent, glowing signs you can see in bars, restaurants and other shops.

acrylic_base_for_edge_lit_signs__45374-1377584970

These objects leverage light diffraction to create that nice glowing effect and I wanted to create one for a colleague of mine who was getting into a new job position.

I had a piece of 5mm acrylic lying around and my Bantam Tools Desktop PCB Milling Machine (Really? You couldn’t find a shorter name?) sitting on my desk was begging for something to do, so…

A plan got quickly depicted by my mind: engrave the acrylic, create a PCB hosting a few LEDs to slip under the sign and mill some pine wood to create the base, a USB cable and there it is!

The first step was to create the design that gets engraved on the transparent plastic, so I got into Inkscape and let the ideas flow: this is what I ended up with.

bitmap

Marcello, here abbreviated to “Marcè”, is my colleague’s name and, if you wonder what DSI stands for, is the acronym of my department: Direzione Sistemi Informativi

Time for milling the acrylic, with the mindset this would be the most challenging task and I had to leverage what I’ve learned in my past experiences, so I turned on my wonderful OtheRPM addon and started the Bantam Tools software.

SVG is the default output format in Inkscape and is also a supported input format for the software provided by Bantam, so I only had to fiddle a bit positioning and scaling the image so to fit on my acrylic scrap and set the engraving depth at 0.2mm as I had no idea if depth had any effect on the final result.

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Once ready for breaking a few bits I used double sided tape to lock the acrylic sheet in place leaving the protective film on the bottom side but removing the film from the top side, where engraving is going to occur. I then readied the endmills:

  • a 1.0mm, single flute, flat carbide endmill is used for coarse engraving at 8000 RPM at 1100mm/min surface speed and 200mm/min plunge speed, pass depth of 0.2mm
  • fine details have been engraved using a 0.2mm 30°, v-shape engraving carbide endmill at 8000 RPM at 2000mm/min surface speed and 500mm/min plunge speed, pass depth of 0.2mm
  • finally, a 1.5mm, single flute, flat carbide endmill is used for cutting out the outline at 9000 RPM at 600mm/min surface speed and 100mm/min plunge speed, pass depth of 0.6mm

Cooling is of the highest importance to prevent the plastic melt and stick to your bits, so I kept the BitFan installed on each and every tool change: trust me, it makes the difference.

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This one contained a typo that forced me to redo it… (facepalm)

Before proceeding further I did a quick test with one LED and a battery, just to check I didn’t mess up everything.

edge-lit-test

Glow intensity seems directly related to the number and power of the LEDs, but not related to the engraving depth…

It was time for the PCB, quickly designed in CircuitMaker with the simplest possible approach: just stick the RGB LEDs in parallel and use the current limiting resistors to determine the output colour. In my case I was aiming at a deep blue glow, so one resistor per LED should have done the job, plus I wanted to test my soldering skills and aimed at using those tiny 0603 resistors (? Yes, they are crazy tiny!) I ordered a few months earlier.

I went a for double-sided design to avoid any complication in routing the board, so about an hour later I had those lying on my desktop: they are 11mm wide and 120mm long, 4 LEDs each. Power is going to be provided by a USB cable stripped out from a dead mouse (no, not the animal).

Since I got the tinning solution around, I couldn’t pass on a quick tinning process, just for the fun.

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What was left at this stage was just some soldering (those tiny bastards!)

When I went into designing the base I had to adjust my initial thought of just carving out a single piece of wood: the LEDs alone, not counting the PCB additional gap, are wider than the thickness of the acrylic, so I had to opt for a 5 pieces assembly, designed in Fusion360.

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Laying the parts to fit within my work area wasn’t too difficult, but creating the correct toolpaths while trying to keep the work time within acceptable boundaries has been a different level of challenge. Nonetheless, I came out with a plan of 2 hours of milling which allowed me to produce a decent amount a wood dust:

  • A 7.62mm × 3mm, 2 flutes flat HSS endmill was used for the rough clearing
  • Most of the work has been done with a 3.175mm × 8mm, 2 flutes, flat carbide endmill
  • The smooth rounding leveraged a 3.175mm, 2 flutes ball carbide endmill
  • The side finishing and final cutout have been done with a tall 2.0mm × 22mm, 2 flutes flat carbide endmill, leaving 5mm × 2mm tabs for breaking the pieces apart.
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Plenty of operations and a good amount of work time on display

My Othermill Pro, ops, Bantam Tools Desktop PCB Milling Machine, had to carve deeply into the stock of wood, which I had double taped onto the bed, to bite away all the excess while my family happily enjoyed the over 2 hours of high pitch noise…

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What followed was a little sanding, just to remove the milling leftovers, a good clean with a wet cloth and then some vinyl glue, clamps and a little (12 hours at least) time.

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The thing looked so cool in the office I got some requests…

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