Dodecahedral, man!

[Will1987]

HI
I'm new to this forum so this could work as an intro as well as some specific stuff I want to get some opinions on. My name is Will MacBride, and I live in East Amherst, NY (near Buffalo, NY). Most of my experience is in rough carpentry, remodeling, framing, house repair and so on. I'm pretty new to fine woodworking, but of course familiar with it to some degree.
I don't know how many of you know what a pentagonal dodecahedron is, but its a twelve sided polyhedron and each side (each face) is a pentagon. If you want to see what one looks like, go here - its the second one in the list: http://mathworld.wolfram.com/Dodecahedron.html. also if you scroll down there are interactive graphics where you can spin it and things.


I think i've already figured this out but I figured it would be an interesting forum topic anyway and I just want to double check: I'm building a dodecahedron box and I of course needed to know what the angles were. here's the most interesting property - each side of the pentagon has an angle of 108 degrees or 72 degrees depending if you're describing it as obtuse or acute). 5 x 72 is 360. however, a cross section of the dodecedron shows that cut in a certain plane it will be ("slices" of it will be) a hexagon, and in another plane it will be a decagon (10 sided figure). Thats 6 x 60 = 360 degrees. Isn't it strange that the same figure can contain both pentagonal and hexagonal geometries working together perfectly? I thought it was cool, anyway.

So it turns out that the angle (and i found that it is, indeed, based on the hexagonal plane if I'm corect) BETWEEN faces is 120/60 degrees. I need to mitre the pieces of wood ill be using so that when put together they'll add up to 120 degrees. thus i need to mitre each side of the pentagonal piece of wood at 60 degrees. I put togetehr a cardboard model of the thing and though rough thats pretty much what the angles measured at.
Anyway I'm fairly certain I'm right about those mitres but if anyone has anything to say or just comment feel free.

Merry xmas!

Will:D

[mmwood_1]

If I'm not mistaken, the dodecahedron is what Buckminster Fuller based his dome constructions on, is it not? Anyway, I'm pretty rusty on my terminology, so your geometric musings lost me a bit. What I do know is that if you have to actually make 60 degree cuts, I wish you luck. Most saws will cut to 50 degrees at most. However, given that on a tablesaw you reference off of 90 degrees instead of 0, you may be cutting at 30 degrees, which is entirely feasible. Good luck on this one. Nothing like a good challenge.

[Don]

Hey, Will and welcome to the forum! You've presented a good challenge and puzzle. It would be a good competition between some of the forum members to see who could come up with the best "repeatable" solution.

As Mark eluded to, you can get lost in the geometric permutations, but this problem boils down to two pretty simple challenges. First, producing multiple copies of a perfect pentagon, and secondly, producing the required angle between the facets. On a table saw we're in luck because, as you stated, the required angle between the faces are 120 degrees. This is larger than and an easily attainable 90 degree angle which we can achieve by tilting the table saw blade to 45. So, if we decrease the angle to 30 degrees on the saw (we get the reverse angle then, 60 degrees) you can achieve the required 120.

So, I've got a method for doing this but I don't want to give it away too easily. I could have this done in an hour and even have the pieces splined together for easy assembly - something you won't realize is a problem until you try to put it together. Lets see what others come up with first, I tend to jump in too quick.

Maybe I'll jump out to the shop for a "just for fun" project.

Okay, Guys. Here's a challenge - lets see some pictures.

dawjr

[Terry Mac]

A do. . . . da. . . What? This is geeonamatry? :eek:

This sounds kinda interesting though; looking forward to see what you guys come up with.

*passing the buck because of a headache*

[Will1987]

Thanks guys for your responses!

Don and mmwood you're both right - you can get 60 degrees easily. the blade upright is considered 0 so you set it to 30 and its at 60 degrees to the table.

As for a repeatable perfect pentagon, I've got it fairly accurate just by meausuring carefully and using a protractor (designed for woodworking) to fine tune the angle. Then I basically end up with a pretty perfect template peice and trace that over and over.

There's a fancy way of getting a pentagon by doing various steps inside a circle, and I tried that but I found it ended up being time consuming and actually somewhat less accurate. you have to make more measurements and draw more lines so there's greater room for (albeit minute) error. But Don, if you have some better technique, out with it man. You won't ruin anything, I donno how many people are gonna actually take this "challenge" or whatever it is, but I sure could use the ideas.

I'd also definitely like to hear what you have to say regarding splining. I don't even have a biscuit cutter yet but I'm gonna get one, its the next tool on my list. I can anticipate the problem already - getting the slots on the two adjoining mitred sides to line up. they'll have to basically each be at a 90 degree angle to the mitred edge (as a line) itself (or actually, any reasonable angle as long as its the same on the other mitred edge), and equidistant from each corner.


The Dodecahedron was believed to have mystical significance and I'm starting to see why - it's somehow a very intriguing, almost beguiling form.

Thanks again and I hope to hear more from you all soon,

Will

[Don]

Okay, Will. I'll get out in the shop and make this happen. You've got me challenged. I've got a way of making the pentagon(s) without drawing one or making a template. I'll go see if it works. The spline issue is simplicity itself, you already have the tool for it. Back with pictures in a while.

dawjr

[Don]

Here you go guys. I've done this twice now and it takes about an hour. A good, "I'm having a beer," exercise. Watch those fingers!

First, I placed the mitre gauge in the vise upright and zeroed the digital angle finder.

http://www.internetcp.com/gallery/data/503/medium/IMG_0410.JPG

Then, placed it on the face of the mitre slide and set to 18 degrees, 360/5=72 90-72=18

http://www.internetcp.com/gallery/data/503/medium/IMG_0411.JPG

Then zeroed on the table surface,

http://www.internetcp.com/gallery/data/503/medium/IMG_0412.JPG

Then set bevel to 60 (actually used 59 degrees to encourage acute angle touch

http://www.internetcp.com/gallery/data/503/medium/IMG_0413.JPG

I had previously ripped some scrap to 6" wide for the project. I then cut a group of twelve, this is the first cut of the pentagon.

http://www.internetcp.com/gallery/data/503/medium/IMG_0414.JPG

Here is that stack of parts with the first cut showing.

http://www.internetcp.com/gallery/data/503/medium/IMG_0415.JPG

Rotate clockwise so the first cut is on the face of the mitre gauge. Angle stop block is set wide at this point.

http://www.internetcp.com/gallery/data/503/medium/IMG_0416.JPG

Here is that stack of parts with the second cut made.

http://www.internetcp.com/gallery/data/503/medium/IMG_0417.JPG

[Don]

Now set the stop block to the desired pentagon side size. Notice the stop block has a matching angle cut, this allows the cut pieces to "nestle" into the crook nicely. This is important, it gives you a visual and feel that the pieces are in place for each cut.

http://www.internetcp.com/gallery/data/503/medium/IMG_0418.JPG

Keep rotating the pieces.

http://www.internetcp.com/gallery/data/503/medium/IMG_0419.JPG

Ready to make the last cut on the pentagons.

http://www.internetcp.com/gallery/data/503/medium/IMG_0420.JPG

The stack of all the pentagons now cut. But, how do we put them together?

http://www.internetcp.com/gallery/data/503/medium/IMG_0421.JPG

Without changing the bevel, slide the rip fence over. Set up the fence and blade to make a saw kerf near the center of the edge.

http://www.internetcp.com/gallery/data/503/medium/IMG_0423.JPG

You will end up with a space for a spine between the parts, like so.

http://www.internetcp.com/gallery/data/503/medium/IMG_0424.JPG

Here's a test fit of spline material, make it a loose fit.

http://www.internetcp.com/gallery/data/503/medium/IMG_0426.JPG

Pile of splines.

http://www.internetcp.com/gallery/data/503/medium/IMG_0427.JPG

[Don]

Now the fun begins.

http://www.internetcp.com/gallery/data/503/medium/IMG_0428.JPG

http://www.internetcp.com/gallery/data/503/medium/IMG_0429.JPG

http://www.internetcp.com/gallery/data/503/medium/IMG_0430.JPG

http://www.internetcp.com/gallery/data/503/medium/IMG_0431.JPG

Here's the two I made, the top one is simply taped together, the bottom one splined (no glue at this point)

http://www.internetcp.com/gallery/data/503/medium/IMG_0434.JPG

Try it, you'll like it. Have fun.

All the best, Don

[Will1987]

Wow! what can I say. I never expected all that. This site does sure does have a buzz to it. Looks good. I almost feel a little steamrolled. I haven't even gotten around to building the thing myself. Glad you got a kick out of it though and thanks for taking the time. A lot of interesting info there. Now I'm gonna have to outdo you somehow.
Tell me, exactly how did you make those splines? (in a little more detail - I really am in a sense pretty new to this. In fact I've never splined anything together before.) Also do you think when you glued what you made there you'd use a strap tightener clamp?
I don't have an electronic mitre guage. Anyway I gotta look at it further i'll get back to you after I look over what you did a little more.

Will

[Will1987]

hang on I meant to say I don't have an electronic angle finder. By the way how much do those things cost? what do you mean exactly by a "mitre slide
"?

[Will1987]

I'm kinda understanding this in stages. I see what you're doing now - the electronic gauge uses gravity to find the angle, so you had to put it in an upright 90 degree position in a vice, right? then you came off that 18 degrees leaving the 72 degree angle.
I still dont quite get how that "angle stop" works. Is that just something I can build or do you actually usually buy an angle stop? I assume its more or less a 90 degree angle turned to fit whatever angle you're cutting - turned relative to the saw blade line.

[Don]

Hey, Will. Here are a couple more pictures.

Here is a picture of the splines being cut to length. They are simply accurate rips of pine from the table saw. Make them loose or you'll be sorry once the assembly starts.

http://www.internetcp.com/gallery/data/503/medium/IMG_0391.JPG

Here is a close up picture of the spline groove being cut in the pentagons. Don't change the bevel on the blade. Simply stand the pieces up, adjust the fence and blade height (not angle) to cut about an 1/8"+ deep.

http://www.internetcp.com/gallery/data/503/medium/IMG_0397.JPG

Here is a picture of both of them assembled. The birch ply one is not splined, simply glue on the edges and pulled together with blue tape. A bit of a pain in the behind to assemble. The OSB one is splined and goes together in about 2 minutes, glue, splines and all. The beauty part of the splines are that it allows for "adjustment" after it's all together. Bumping the sides lightly with a dead blow hammer is all it takes. As for clamping - as you can see, stretch wrap is your boy here. It allows you to see through to your joints and still allows for a little adjustment if needed. More than enough clamping pressure for the job. A strap or band clamp will be useless here.

http://www.internetcp.com/gallery/data/503/medium/IMG_0398.JPG

Here is the link to the angle finder, they are $40. http://www.woodcraft.com/family.aspx?familyid=5894 You are correct in the theory of how the mitre slide is set up. I'm not so sure that the gauge works on gravity or not. Some digital items like this work on the optical gyro theory. You are actually zeroing the device in space, it reads the movements thereafter similar to inertial navigation on an aircraft.

The mitre slide is this, I'm sure one came with your saw, http://www.woodcraft.com/family.aspx?familyid=3702 this one is simply a little fancier and allows for adjusting out the play you may have between your mitre slide and the table saw slot. Not absolutely necessary. If your slide had minimal play you will be okay, the more play you have the more the errors will "compound" as each facet is cut. If you have play you can minimize your error by always putting clockwise or counterclockwise tension (pick one and stay with it) on the slide at each cut.

When I say "stop" it is nothing fancy, if you look at this picture closely you will see the angle cut block that I'm using to "butt" the cut pieces to as they are rotated.

http://www.internetcp.com/gallery/data/503/medium/IMG_0420.JPG

Don't be steamrolled by the project. Some of us guys will make this stuff look easy, only because we've worked those tough towns before. After you make the first one you'll smack yourself on the forehead. The bigger challenge is going to be coming up with a practical use for these things. They are kinda neat to hold in your hand and marvel at. Birdhouses, planters, piggy banks, conversation pieces?

All the best, Don

[Will1987]

yeah this stuff is a different kind of challenge. I'm used to working on roofs with 50 or 60 degree angles, rather than cutting them, hahaha. But I see how you made that stop now. Its not at 90 degrees to the 2x4 there, its at the 72 degrees, or that is to say 108. Then its set at the distance to the blade that you want the side to be. That's a time saver probably but the question is, is it a wood saver? I'm trying to minimize wastage as much as I can.
how did you make those round holes at the top there. in fact I'm gonna post something else because I could use some tips on inside cutting.

thanks again,
Will :D

[Will1987]

Hey Don I was just wondering, this relates back to the dodecahedron, when you were using that "angle stop block" what exactly did you do, I can't quite see from the pictures. is that just a block to make a 90 degree angle clamped onto the 2x4 thats screwed onto the mitre guage? or is there something more to it?

Also I was wondering about the splining - how loose do you go on the splines? Seems to me you'd want something relatively tight or else whats the point?

Will

[Don]

Hey, Will. You are seeing two pieces of 2x4 in this picture. One is simply screwed to the mitre guage and used as a fence. The other piece is cut at the proper angle and clamped in place. Where you clamp it determines the size of the polygons.

http://www.internetcp.com/gallery/data/503/medium/IMG_0420.JPG

The splines are very loose or you'll be sorry when trying to assemble it. I would say that a spline and 3-4 pieces of paper should be a snug fit. You will be able to "fine tune" your edges as you wrap/clamp. Remember, the glue will swell a spline - this is where most get in trouble and they don't realize it until final assembly, then it's a PITA.

After you read this, lets's merge it with the original thread, okay?

Have fun, dawjr

[Will1987]

Hey Don
The thing is it looks like you have the mitre guage also at an angle. Or maybe its just that way in the pic because you were showing it to the camera. but it seems to me you would either do it by putting the stop block onto the mitre guage at a right angle, then turning the mitre guage to the 78 degrees, or you could cut the stop block at the 78 degrees and have the mitre guage be at a right angle. Am I correct in assuming you just have both set at angles in that picture just because its a picture - not the way you really would have it?


:confused:
Will

[Don]

Hey, Will. Keep at it. It's going to come to you in a rush, like the kiss at the end of a wet fist. Once you get it, it'll seem very simple.

This is a better picture. Both the mitre gauger and stop block are at an angle, this is your 72 degree angle needed to make the polygon. Of course, the mitre gauger is set at 18 degrees (90-72=18) and the stop block is cut to that same angle. This allows the cut pieces to "nestle" into the crook of the stop block and fence for more accurate sizing. After all, you are rotating the same piece to cut the angles, any error in setup or method will compound after three more cuts.

Don't get hung up on the angle of the stop block, it could be square and work just fine. After you cut a bunch yourself though, you will see the beauty in it.

Let's keep after this until you get one made, keep asking questions.

http://www.internetcp.com/gallery/data/503/medium/IMG_0417.JPG

dawjr

[Will1987]

Hey Don,
Ha I finally got the idea. of course both angles have to be the 18 degrees because its gotta both be flat against the miter gauge which will be turned at the angle to the BLADE as well as sit against the stop, which will be at an angle to the MITER GAUGE, or that is, the 2x4 that's up against it. [slaps forehead with a muted, skull reverberting whack!] ok now that I've got it it shouldn't be a problem, but I'll let you know when I do get it set up.

Thanks man :D
Will

[Don]

I knew you would get it - now let's see one.

dawjr

[Will1987]

I'm finally getting on the dodecahedron. Don, there's some things I'd kinda like to get some advice on. I guess your kinda like my consultant on the project. I learned quite a bit from that series of photos. I've done all that - the angle stop block worked fine, the pentagons look great, I even bought the digital angle finder and its a really big help.
The only problem is I'm making a much larger dodecahedron than you made - it's 13.5 inches on a side. That makes a dodecahedron thats got a 3 foot horizontal span from point to point and is somewhere around 31 or 32 inches high. I may even go bigger on the next one. The problem I have now is that, having cut those saw kerfs and made loose fitting splines just like you recommended, they are in no way strong enough to hold the panels in place. I made them out of spf 2x4. in fact I don't really understand how something loose fitting and less than an 8th inch thick even held the smaller ones you made in place. But certainly its not gonna work here, just way too much weight.
So I ended up buying some bendable L brackets and screwing them in on the inside. that holds it fine but I don't want brackets all over the inside of the thing.
So: 1. If I take them off do you think glue and those weak splines will hold it together?
2. Is there some other way of approaching this you'd recommend?

Another thing I wanted to ask you -

What I'm working on now is just a practice run - with OSB plywood just like you used. But when I do the real one out of hardwood, The pieces have to be around 2 foot square for me to be able to cut the pentagon panels out of it. So I was wondering would I glue 2 12 inch pieces together and THEN cut the pentagons out of it? or do it afterwards? Or is there something else you'd recommend? i.e. is it even POSSIBLE to find hardwood in 24 inch widths?

Thanks a lot
Will

[Don]

Whoa, there, big guy! I made Christmas tree ornaments, you're making weather balloons. Let's look at the issues one at a time.

First, you wrote that your pentagons looked great. The true test is to take any two and place them face to face. Are they an exact match, especially as you rotate to a new facet? I'd be surprised if the answer was yes. How much are they out? Deviations here will be multiplied as you try to assemble. I would say that my little jig technique would only be accurate enough for about a six inch side pentagon. If I was going to make larger ones than that, I'd use a different technique. The new method would involve making a perfect pentagon a couple of inches larger than the biggest size you would ever make. This one has to be perfect. You attach this "template" to the face of your rough sized material and now make five passes on the rip fence. The blade is at the same angle as before but only raise enough to cut through your material and not the template. Do you see what I'm describing? If not let me know and I'll make a sketch.

Second, the splines are not to hold it together, they are there for alignment purposes first and construction second. If you don't think you need it for alignment, try putting one together without them, you'll see what I'm writing about. But . . . as you go larger, so will the splines. For the size you are trying to do, they need to be about 1/8" x 3/4" and now you can get away with them being a bit more snug - only because you can now use a rubber hammer to persuade the polygons. This is really going to be a learning experience on your part. Your machines, talents and the sizes you're after will dictate something that's ultimately comfortable with you.

The brackets inside is a recipe for a world of hurt - I wouldn't go there.

For assembling, even this size. it would be glue, splines, and some 3M 2" wide painter's tape. This stuff will stretch, about 3 or 4, 12" strips per edge, pulled tight, glue and spline - you'll be amazed. But, you have to put the whole thing together at once, or damn near. You're not going to be able to assemble sections then hope they go together. It's going to be messy, so control your glue amounts and get your mind right, then dive in. Assemble it dry with the splines for a test run, always.

You won't find hardwood in 24" widths that will be usable for something like this. I'd shy away from using hardwood, plywood is the medium to use for this. If made out of hardwood you may simply find that the movement of the wood across the grain breaks the joints and panels. It would be an interesting experiment, but probably expensive too.

What is the ultimate use? This will tell you what they need to be made out of. How about MDF and then veneered? This would give you a world of possibilities to explore with grain and inlays, etc. MDF would be the cat's meow of making these. At any rate, you would assemble your panels and then cut to size, you don't want to make things too hard.

Let's keep at it.

All the best, Don

[Will1987]

Hey Don
Damn it's good to hear from you. I've been out there on this cold day with the propane heater going and its hassle after hassle. I was just on the point of going out and buying some aluminum flashing which I figured I'd cut into strips and screw along the joints just to hold the damn thing together. I figured I could take them off when the glue was dry and then put felt over the interior (which I'd planned to do anyway) which would over the screw holes. Then I thought I'd check and see if you'd replied.
But you're right, those pentagons are not all exactly the same. It's just a close enough sorta thing but that's not gonna cut it for the real project itself.
I think I see what you're talking about with the template thats larger than the pentagon. Cool idea, hadn't thought of that. That's elegant in its simplicity. but first off how would I attach the template to the board I'm gonna cut? I fail to see how I could glue, screw, or clamp it. maybe again I could screw it down to the side thats gonna be the inside and then cover it with felt.
The use of this is going to be some kind of kids toy chest or clothing box. something where you open the lid and throw stuff in that doesn't need to be all stacked up right. So yeah it should be fairly strong. I like the MDF idea. Kinda heavy but it would work nicely, you're right.
Thing is, I still don't see how I'm gonna even "put it all together at once" splines, glue or no. I even tried duck tape with the OSB practice one and it wouldn't hold at all. Do you mean gluing and splining it and basically "clamping" it with the tape?
I think I might just go and get some aluminum and do it that way with this practice dodecahedron. I just wanna see how it looks roughly together and tight. I figured I could cover the inside with felt even if the aluminum was still on. Maybe I'm thinking more like a rough carpenter here, heheh.
anyway I just don't really see how to put it together all at once like you're saying without being an octopus. MDF will be even heavier. There's just no way to hold it in position at that 60 degree angle without some kind of assistance. If you tried it you'd see.
Thanks again. I Look forward to your response to this.

Will

[Don]

Hey, Will. On the new template method, a couple of finish nails through the template and into your material will hold things together for the sizing. Be very careful with this method, make sure you "rough size" you material so that you have clearance under at the fence. You don't want the scrap shooting out at you, you won't be able to control it.

As far as putting this together, I don't understand why it would be so tough. The splines do all of the work. As soon as you get three parts together, it's all downhill from there. study my progress pics and see what we're missing. The pieces are holding themselves together, no glue or tape. Tape on the outside is the way to go, I've built a lot of stuff with only tape clamps, the surface has to be clean, free from dust. If you are in the cold, that might make things a bit tougher. There are other ways to assemble something like this, but it involves gluing on temporary angled glue blocks for hand screw clamps. Keep at this, Will. I don't like the aluminum deal, that's going to go sour on you the further you go, and you will not be able to pull the joints tight.

Don't make me come in there.

Can you post some pics of where you are now?

dawjr



http://www.internetcp.com/gallery/data/503/medium/IMG_0428.JPG

http://www.internetcp.com/gallery/data/503/medium/IMG_0429.JPG

http://www.internetcp.com/gallery/data/503/medium/IMG_0430.JPG

[FrankAtl]

You guys have way too much time on your hands!;)

[Don]

Too right!

[Terry Mac]

That there is one strange soccer ball. How does it roll? :p


Okay, all kidding aside, that is a very interesting project. Very well done. Must admit I had absolutely no clue what was going on in this thread until seeing it built.

Nice job.

[Will1987]

Hey there dude (i.e. Don),
For the size I'm talking about for the panels the thing will not hold together with splines. I tried it. They weren't 3/4 inch wide, more like 3/8ths, so maybe I'll give that a shot, but there's too much weight. I don't see how going another 3 16ths on either side into the panels is going to change that. It simply falls over irrespective of the delicate spf in there, as it obviously would. Maybe I took your idea of a loose fit a bit too seriously, but its not even that loose. When I push it into the kerf between 2 panels it encounters a little resistance. The only thing I can think of is to make the splines out of some thin (slightly more then 1/8th) maple plywood I have but then I'd have to make the saw kerf larger by cutting into it twice - a hassle. And even then I donno if it would hold.
This is an item I want to be fairly sturdy for ordinary use, so I'm not so sure the aluminum strips is such a bad idea. It's not supposed to be a museum piece but something you could stash things in, move from room to room, roll over and so on. I would cover the aluminum strips with the felt that will cover the interior, so you wouldn't see them.
Yeah I'll try to snap some pics to give you a sense of what it looks like. How do you add a pic to this forum board? Oh I see, insert image there. Ok stay tuned. Or maybe by the time you read this there will be some pics following.
;)
Will

[Will1987]

Ok I went out and snapped a few shots. Here it is sitting on my workbench: http://viewmorepics.myspace.com/index.cfm?fuseaction=user.editAlbumPhoto&albumID=1713412&imageID=22930902&MyToken=e488688a-2a7b-447d-b527-75acf5c42f5d

[Will1987]

damn it doesn't seem to how up in this window. I uploaded it to myspace but it seems to just take you to myspace and tell you you have to log in. How do you get individual pics to show up here in this forum window?

Will

[Don]

Here is a link to picture posting, http://www.woodworkingbuzz.com/forums/showthread.php?t=42&highlight=picture

Hey, Will - I'll make another stab at trying to explain. For the size that you wanting, you are correct, the splines are NOT going to hold two parts together, regardless of the size of the spine. But - that is not the intent here. The splines are there for speed of alignment which puts them in SHEAR, we're not concerned about the tensile strength. If you were going to put this thing completely together at once, there would be 30 joints to contend with, an almost impossible task without some alignment method.

Now, as in this picture - http://www.internetcp.com/gallery/data/503/medium/IMG_0428.JPG

Once, three parts are "together" the splines will indeed help hold the assembly together since there are always two adjacent edges in play. Are you assembling it this way?
This picture is exactly half of the assembly,

http://www.internetcp.com/gallery/data/503/medium/IMG_0429.JPG

If I was making this thing big enough to stand in, you would only need to "hold" together five perimeter joints. This could be done with duct tape and loose splines and the thing would sit on the floor like an open flower with no problem. Now keep in mind, the tape is on the outside of the joint, tape on the inside will be useless. I'm probably overstating the obvious here, don't take offense - just want to make sure we're on the same page and that I'm helping you get where you want to be.


dawjr

[Will1987]

Hey Don, thanks for stickin' with it. I'm not as dumb as I hope I don't seem. Thing is I tried holding it up with duck tape and it was difficult. The splines were probably not wide or even thick enough. By now the problem is I've already cut the kerfs and even recut some of them. The pentagons are not as accurate as they could be also, so I'm basically just using this as a goof around study in the form itself. I'll just put it together with whatever and see how it looks.
But I see where you're coming from - if I do it accurately and make the splines and the kerfs the right depth and dimensions it should work. A 3/4 wide spline dimension seems to cut too deep into the edges though. It weakens the edges of the upper side. 5/8ths seems to be about the furthest you wanna go.
I was wondering do you think there's anything really wrong with putting those aluminum strips on? I'm basically planning on using the splines, glue AND the aluminum. Seems to me that should make it really sturdy, rollable, break - proof, which is what I want.
Ok lemme try this picture thing. I got them up on photobucket.

http://i273.photobucket.com/albums/jj240/Dadarkilo/sittingonworkbench.jpg


Gad Zooks I've created a monster!

[Don]

Monster it is. I looked at all of your posted pics - looks like you are on the right track, it's just a matter now of refining the techniques that will allow you to build them as large as you would like. The OSB in that size will create challenges all by itself, namely dealing with out of plane panels, this means controlling the material right at the point of cutting to make sure you get the size you want and the kerfs placed correctly. This means using fingerboard and hold downs where needed.

You have all of the right theories now, you'll get better with each iteration. Yes, a 3/4 spline will be large on this thickness material, kerfs need to move to the acute side of the angle to allow larger splines in thinner material. If duct tape will not hold, you may want to make some sacrificial blocks with the correct outside angle. These little butterflies can be stapled in place during assembly and taken off after the glue dries. Done correctly, a spline and glue even at this size will be extremely strong given the supporting nature of the adjacent sides. Use the aluminum if you wish, it's whatever works for you at this point.

I'm posting your link to your album in case others want see - http://s273.photobucket.com/albums/jj240/Dadarkilo/

Well done, go forth and multiply

[Will1987]

Acute side meaning shorter side or longer side? I imagine shorter side. The more you move up toward the longer side (ie. where the span from side of the pentagon to another side would be longer rather than shorter) the more you risk chopping off the edge.
What exactly do you mean by hold downs and finger boards?
Yeah I was thinking of creating 60 degree angle blocks that could be clamped like that. I more thought I'd clamp them to the workbench or screw them down then have the dodecahedron just sitting in the middle of them. Then clamp those. Well I guess I'd clamp them to the dechedr first then clamp them down. but if they were attached TO the pieces that would work to I guess but it would leave staple marks. I guess those could be sanded out easily enough.
By the way I'm thinking of painting it. what kind of paint would you recommend for the mdf that I'll soon use to build the real one? Just primer and then interior latex? Some kind of acrylic? And by the way what's the deal with actual oil paint, is it ever really used on pieces like this? I want something with very glossy and solid sheen, where you wouldn't see the grain underneath much at all.
thanks again for all your help. You're a regular mentor.
:D

Will

[Don]

Yes, the acute side is the shorter side - for the reasons you stated.

You may get more responses to your finishing questions in that forum. Look around in there, there is a lot of information already on the forum about different finishes.

Good luck, look forward to seeing a finished project from you.

dawjr