We’re building a TARDIS, and I need help figuring the compound miters for the little flat-topped pyramid that makes the roof.
The outside dimensions are 48 in. x 48 in. For estimation, I’ve been using 4/12 as the slope of the roof, which would be the slope of a true pyramid with a height of 8 in. and a base 48 in. x 48 in. I’ve sketched plans that call for the flat top of the roof to be 6 in. x 6 in., but truth is, I’ll make it whatever size it needs to be to accommodate the light fixture we put up there.
The only compound miters I need to cut are at the four sloped corners of the pyramid. I need to calculate both angles of the compound miter, the angle of the cut and the angle of the blade. The cuts will be done with a circular saw.
I found this site which gives me 45.07[sup]o[/sup] for the angle of the cut and 2.83 [sup]o[/sup] for the angle of the blade. I just wanted to get input from more experienced woodworkers to see if that sounds about right.
I’m not a Real Woodworker, but I don’t think that’s right. The page you linked to is kind of confusing, but the “slope of side to base” field I believe should actually be the angle of the side relative to the base. (The PDF table agrees, listing “side angle” as its second parameter instead of slope.) You used sides=4, slope=4; you actually want sides=4, slope=18 (really 18.43°=arctan(4/12)), for which the form gives a blade angle of 12.62° off vertical and miter angle of 46.44°.
I don’t know why the form only allows two digits for the angle, but if you calculate it using the exact value of arctan(4/12) you get a blade angle of 12.92° and miter angle of 46.51°. You can also get close to this by interpolating between 18° and 19°. But using the values for 18° just means the peak will be at 7.8" instead of 8", which you probably won’t notice when the Daleks are after you anyway.
Tack or lightly glue a block of wood under the raised edge of the board and then just make your cut at 45°. The rest of the angles will take care of themselves.
Thanks, guys. I had asked the mods to close this thread because the more I thought about it, the more I realized that this project is beyond the limits of the technology we will have available to us. Perhaps at some point in the future, we can upgrade, but for this year’s Gala, it will just have to be a flat-topped TARDIS.
Projammer, I suspect that your idea would work great if we had one of these, but alas, we only have one of these.
I should probably explain that I got your request to close the thread, but there are other interested people reading it and I’m curious what solutions will come out. No sense closing yours if someone else is going to open another.
My comment would be that the tolerances are irrelevant. Cut the angles too sharp, as Projammer said, so the outside (top) edges touch but the inside (bottom) edges don’t. Assemble the unit, fastening each of the four triangles along its bottom edge (on top of the outside walls of the tardis). Connect all four to your square top piece and it’ll be as solid as you’d like. Fill the gaps on the inside with putty, paint over it, and nobody will notice.
Actually, the technique I described would have created perfectly mated corners. Unfortunately, it won’t work for the OP and the tools he has available.
In his situation I’d do as you described. First mark the wood at 45°, then move the top mark out about 1" and mark again making the angle a bit more than 45°. Make the cut with the blade tilted somewhere between 20° and 30° so that when it’s assembled the outer edges meet while the inside is open. He can brace the pieces with some scraps and caulk any gaps big enough to notice.
As a pretty experienced woodworker, here’s what I would do: I’d guesstimate the angles and cut a few scrap pieces to refine the settings. In other words, I wouldn’t bother with the math at all. And generally, you’re not dealing with NASA-type tolerances; close enough is good enough.
exactly (pun intended). If it’s a 4 sided pyramid then get it close and let the glue deal with the tolerances. Heck, you could use gorilla glue with no cuts at all and let it foam up a bond that would support the weight of a car.
I’m confused here is this a prop? or an actual working thing? If it’s a prop I’d make it out of Styrofoam. If it’s a working thing and has to survive the elements, I’d wouldn’t miter the edges at all I’d leave the cracks and cap it with tin roofing material.
ummmm—I’m pretty sure that the miter angles are the least of your problems…
I’m more concerned about the fact that you had to read this whole thread before you realized that you DON’T actually have the technology to build a working time-travel machine camoflaged as a telephone booth.
geez …what’ wrong with you Brits, anyway?
Us Yanks have known for a long,long time that telephone booths aren’t for time travel…they’re for Superman to change clothes in
The carpentry is a snap. It is the convincing of stunning women to serve as your assistant that will be the hard part…well that and the whole time travel thing.
This for real. I’d go a step further and once the roof and walls are painted, I’d paint the capping to look like weathered copper, that would look amazing!
