I'm not sure how Show4Pro did his acrylic groove, but I did my MB tray with a round sand drum with my Dremel on low speed going really slow because of the melt factor, then sanded to 600 wet and heat polished the edges ~ http://www.overclock.net/t/1441548/build-log-consummation-dual-htpc-water-cooled-audiophile-stereo-build/20#post_24175912
I apologize for the roughness of that job there, I was originally intending this to just be a test so the acrylic is really thin somewhat milky junk, but then I got a bit lazy and since the mb light tray quality isn't a major focus for my build, I'm not sure I want to remake it with good acrylic or not heh
That said, prior to the full on tray there, I'd done a bunch of smaller tests for making a nice solid light line. (For some reason I can't upload pictures, I'll try to come back and edit with my testing sample pics, they're not from my build here heh - update: looks like all I've got are light transmission testing pictures, no edge clean up pictures. >.< too many hard drives ago.)
I found the "router" v-groove bits just cracked the acrylic, even the scrap 1/4" stuff I had, the vbits are to sharp and to fast even on the lowest setting of my dremel. The drum sander works way better for not cracking, though it does take a long time because you have to go super slow and steady so the acrylic doesn't melt and make a smear, it also takes a lot of sanding to clean up and get the acrylic clear again. Sand from w/e grit drum to the 600 wet, then give it a heat polish. If you want to get real serious (which I had on a sample piece,) get a CPU lapping kit and finish off with Novus plastic scratch remover/polish.
I'd used a pen that happened to be the right size to sand the inside of the groove, just wrapped the sandpaper around it and rubbed while watching TV. For the heat polishing I used a butane torch on the tray in my post there, and it was way too hot so I've got a lot of bubbles and stuff (not that I took as much time with the drum sander as I should have either heh.) However, on one of my small test pieces I'd used a heat gun on it's low setting and been super slow and careful, it'd turned out way better so I'd recommend a heat gun by far over a "torch" or a lighter. I've also heard about the oven thing, and I'm sure that works alright for shaping acrylic, but you'll be risking your piece deforming since you're heating the whole thing (rather than just the edges you want to clear up.) Also do keep in mind that it will destroy your oven, the acrylic gases and puts a white film all over the inside, can also etch the glass and stuff, you can never get it off either. (I destroyed a decent oven shaping acrylic bracelets. Stunk like heck every time we turned it on, and we were seriously concerned that it was poisoning our food so we ended up buying a new oven >.<)
As a final thought - and if you want the light "dots" look this isn't for you - I found that scuffing up the outer edge and making that more of a rounded than a 45 angle helped solidify the light line. I did that by hand with a "jig" of sorts, they make sponge like sanding bars, if you maintain constant pressure it naturally rounds one side, then I finished up with a trick I actually learned making and polishing jewelry; emery board/manicure boards for fingernails - makes perfect sense if you think about it since most false nails are acrylic themselves. Anyway, the boards I use have 2 "grits" of fine "sandpaper" and 2 polishing sides, works great for sanding and polishing acrylics. I also found that covering the top and bottom with reflective surface helped a lot; I used adhesive backed brushed aluminum on mine, but this stuff (http://www.amazon.com/gp/product/B00LY81CAY?keywords=mirror%20vinyl&qid=1445096959&ref_=sr_1_1&sr=8-1
) would work better.
Also, I think you could sand the rounded groove into a v with some more work, just use the smallest drum then angle the two sides with a hand file. I've got a jeweler's diamond file set that works really well on the acrylic without cracking it and such. My jeweler's file kit even has a triangle file that could cut in a nice v-notch, but I think the problem with that's going to be the size; the files only like 2mm at widest but for 5050's you need 5mm at least. Could look around for a "fine" diamond infused file in a bigger size triangle though. One might also look into something like this - http://www.amazon.com/gp/product/B015PDQFJG?keywords=nail%20shaper%20and%20polisher&qid=1445096792&ref_=sr_1_2&sr=8-2
I've not tried it myself,(yet, I'm thinking about buying it just to test,) but as long as the things slow enough it won't melt the acrylic which is where the biggest problem I've found is.Edited by Mystriss - 10/17/15 at 9:28am