seamless join of plates at 135-degree angle?

I want to build an octagonal box, whose sides would be cut from something like laser-cut 1/8-inch lexan sheet. But I'd really like the
corners of the box to be as smooth and seamless as possible, and I'm a bit stumped as to how to accomplish that. Any suggestions?
I'm pretty sure the laser cutting machine can't miter the edges.
I've considered building the corners by gluing the side pieces (on the inside) to angle brackets, then covering the seam with some sort of tape. But that sounds like it will be hard to make it look good. I'd also be willing to consider something like a notched plastic dowel at each corner, where the sides fit snugly into the notch. But I haven't been able to find such a notched dowel, and can't think of a way to reliably make one.
Any other ideas? Any brainstorming will be appreciated!
Thanks, - Joe
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Joe Strout wrote:

There are 5-axis laser cutters that can cut material on an angle. You probably don't want to pay what they'd ask for, though.
If you don't mind rubber, there are gap-filling gaskets used for making displays that hide/protect edges of cut plastic (which can be quite sharp if they're done burnished down). Where to find this stuff is the question. Try the local plastics distributor and ask about their molded profiles. If my calculations are correct, you'll want a 45-degree profile: 45*860.
If you have access to a wood planer or radial arm saw or a good table saw you could cut your own profiles. Tilt the blade to 45 degrees and cut out strips from a block of nice wood or plastic.
-- Gordon
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Hum, not sure what you are talking about, but 45 deg doesn't sound right. Are you talking about making a piece that would fit in the corner to fill the gap between two sheets cut with normal perpendicular edges?
The inside angle for the corner of an octagon is 135 deg. The bevel for each piece is 22.5 degrees (90-(135/2)) off of perpendicular which is what you would set on something like a table saw or miter saw for cutting the bevel. Or if you were buying a router bit for beveling the edges you would buy a 22.5 deg chamfer bit.
If you wanted to cut a piece to fill the gap, the filler would have a 22.5 deg bevel on each side - or a total angle of 45 deg. Ugh, I guess you were right! :)

Well, setting it to 22.5 is probably what you want to do so you end with with 45 total after cutting both sides. Otherwise, you would end up with 90 deg total which would make a square box and not an octagon.

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Curt Welch wrote:

The profile needs to be a "V" where the angle of the V is 45 degrees. It stands to reason each stem of the V is at 22.5 degrees.
That's good in theory, but I'm imagining Joe is using not-too-thick pieces of polycarbonate (i.e. Lexan), and cutting strips to 22.5 degrees and joining them is a tough proposition. If the material is as much 1/4" thick we're talking really small profiles here, and staggeringly small if it's 1/8" thick. It's easier (IMO) to cut a single length at 45 degree angles, and touch up to get it smooth at the corners. One can start with a coarse sandpaper, and then switch to a high-grit wet/dry used wet to polish. A dressing rouge is preferred for the final burnish. However I will say if Joe's tools are precision, and he's skilled at it, he could do either two pieces at 22.5 degrees, or a single piece with A V at 22.5 each side.
But again, if the edges don't have to "meld" into one another, a rubber or other premade corner profile is sooooo much easier.
The 135 degrees is, of course, the inside angle, and the inside will already be flush, so this is not the angle to be concerned about.
-- Gordon
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(Answering my post because Joe's original is already marked as read and I'm too lasy to refetch it...)
Joe, It occurred to me you're in San Diego, right? If so, make a trip to San Diego Plastics, and talk to Karen or Daniel. Ask if they can cut your polycarb for you at the angles you need. They probably do this sort of thing all the time when constructing aquariums for folks. Do expect a cutting charge, but they are quite reasonable. They have a table saw that I believe can be adjusted to make oblique crosscuts. They also have an assortment of polycarb on hand, though it's not all Lexan brand. If they're not busy (unlikely) they might be able to cut your pieces while you wait.
SD Plastics is in National City, off the Mile of Cars offramp. Tell 'em I sent you.
-- Gordon
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I used to be -- lived there for almost ten years, but now I've settled down in Colorado.
Anyway, I'm hoping that this will evolve into a kit that may be sold in quantity someday, so I'm trying to avoid involving any custom manual labor (except for kit assembly), which is sure to drive the cost up. That's why I'm trying to stick to simple laser-cut parts.
Best, - Joe
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Having said that, he could also make the edge cut slightly more than 22.5 degrees per piece so that the outer edges mate cleanly and not worry about the inner edges, right ? (assuming it doesn't affect the method of joining them together ...) I've used this method on wood moulding around my house.
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Thanks to all for your thoughts (including those I haven't replied to directly). I haven't managed to find a premade corner profile which is suitable, but I think I can make my own by cutting them out of (say) half-inch lexan or acrylic, stacking them up, and gluing them together. These will look something like little ovals with eighth-inch indentations on two "sides" for the wall plates to fit snugly into. It's OK for my purposes for these corner pieces to extend into the inside of the box a bit; I can even use them as load-bearing columns, so I don't have to worry about the sides supporting any weight. The outside profile should then be smooth and round. Looks great, in my mind at least -- I'll let you all know how it turns out!
Best, - Joe
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Joe Strout wrote:

You're working way too hard here, trying to make something of multiple pieces of 1/8" polycarbonate and joining them. For that thickness, just bend the stuff. Make one long strip for all eight faces. Clamp the strip in a bending jig or, better, a sheet metal brake, where you need a bend. Hit the bend line with hot air from a heat gun, using the nozzle attachment that produces a narrow stream of hot air. Bend.
Practice on some scrap to get the time and temperature right. It's not hard.
                John Nagle
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I'm not sure what you are thinking here. I'm not talking about gluing strips together.
You simply set a saw blade to 22.5 and cut a sheet down the middle. Then you take the sheet, flip it around, and make a second cut at 22.5 so that you trim off a strip from the edge you just cut. The cross section of that strip is now a triangle with one 45 deg angle and two 67.5 deg angles.
That strip can them be used to join (aka fill) a corner of the octagon made out of sheets with normal 90 deg edges.
I don't see any way to make that strip with a saw set to make 45 deg bevel cuts.
But if you can make those cuts, I don't understand why anyone would bother with trying to create a filler strip like this. You just use the same saw to create beveled edges on the side pieces so they fit together correctly.
You use of the word "profile" means nothing to me in this context so I'm still not sure we are even talking about the same thing.

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I've not worked with Lexan (what is it? a plastic right?), but can't you just use some wood working tools to bevel it? Such as sand it, or cut it with a miter saw or table saw or jig saw or band saw or bevel it with a router?
Years ago in shop class I know you can use most wood tools for working with Plexiglas (take care not to melt it), and if you needed a clear finish you just had to sand and polish it with the correct compounds.
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    --Screw the laser; use a tablesaw and clean the cuts up by sliding them over a piece of, maybe, 240 grit sandpaper that's flat on the table and you'll get great results..
--
"Steamboat Ed" Haas : Proud to be the
Hacking the Trailing Edge! : family crackpot!
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Just calculate the joining angles, slice some Lexan on your table saw a bit oversized, then angle your router table plus fence and slowly run them through to get a smooth and perfect angle so you can glue them together after polishing the edges with 600 grit sandpaper/water.
Wayne

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