Angle calculations

[Argee]

The current project is for a bird feeding do-dah with three bird tables, each of which is to have a pyramid-type roof. Beginning to wish I'd said "No" to this one now, as the calcualtions are doing my head in!

If the roof structure had been square-based it would have been so much less of a problem, but it isn't square and that's that. So far, the short sides are 350mm and I've cut two equilateral triangles for those. The long sides are 410mm and no matter how I try to calculate the size of the long-side triangles to complete the pyramid, it always comes out wrong.

There are, of course, a number of angles working together here, but I can't seem to visualise it in 3D from a 2D sketch. When the two short-side triangles are set apart by the long side base dimension, the peak is 227mm from the deck, if that's any help.

If I could calculate the base to apex (height) of the long side triangles, or even the base angles required, all would be well, but it's hair-tearing-out time right now! Can anyone shed any light - I don't have a 3D CAD programme (even if I did, I can't afford the time to learn to use one). TIA

Ray.

 

[cambournepete]

If the ends are equilateral triangles then all three sides are 350 mm and so the other sides of the long-side triangles are 350mm (and root 350*350-205*205 base to apex).

I must be missing something though 'cos this seems obvious when I sketch it...

 

[Sgian Dubh]

What it sounds like you need to know Argee is how to calculate the dihedral angles of the meeting planes to enable you to work out what the compound cut settings should be.

If you'll tell me how many sides are in your polyhedra-- three I think you said, and what angle the sloping sides describe to their base line I'll be able to calculate the necessary information for you. Slainte.

 

[Argee]

There it is, thanks Pete - that's what I was missing - I was trying to calculate angles, when all I needed to do was to swing two 350mm arcs from the 410mm base line - job done! It's been a long day - at least I knew when to pack it in.

Thanks for the offer Sgian Dubh, but it looks like I'm back on track. They're four-sided, BTW.

Ray.

 

[Chris Knight]

Ray, Is this the sort of thing?

You will still need mitre angles for cutting?

 

[jasonB]

If you are also aiming for a specific pitch just cutting equilateral triangles will not give a specific pitch, with what you have done the end pitch will be 46.76 deg and the side pitch 51.91 deg. You will have to have a ridge if all angles are to be the same.

Jason

 

[tim]

with what you have done the end pitch will be 46.76 deg and the side pitch 51.91 deg

Which should be a doddle to set the saw to :lol: :shock: :shock:

Cheers

Tim

 

[mailee]

I don't know if this is of any help but when I am faced with a problem like this I turn to the corn flake pack. I fabricate the said item out of thin card with sellotape and then dismantle it and measure the pieces, true Blue Peter fashion. I don't have an old pair of Val's nickers or sticky back plastic but make do. :wink:

 

[Argee]

That's exactly it, Chris and where I was getting confused/frustrated/$*%" off, was that I got hung up on the base to apex height, whereas all I needed to do, as Pete reminded me, was to make the long side sides the same length as the short side length!

I wasn't aiming for a specific pitch, Jason, just trying to replicate a "back of *** packet" sketch from the client. However, I had to start somewhere, so equilaterals on the short ends seemed to fit the bill. Regarding the mitering, I'll tickle it with a block plane until it fits. This is the sheathing, BTW - needs shingling afterwards - oh, deep joy!

I've done card mock-ups before, mailee (well, sounds like "mock-ups"), but on this occasion I was using some leftover ply. There's none left over now!

New day tomorrow, I'll get it right at last - thanks everyone.

Ray.

 

[Travis Byrne]

Hello Argee

Thank you for not calling on me. My head hurt just reading the question.
I could pool all the heads of my buddies and myself and it wouln't make a puddle.
Travis

 

[Sgian Dubh]

Regarding the mitering, I'll tickle it with a block plane until it fits.

Argee, it's a fact that those polyhedra can be tricky, so it's good that you've managed to work something out, and as you say there'll be shingles or whatever on the top they will hide any gaps.

Still, there are formulae for eliminating the guesswork entirely and the, er, 'tickling' bit for high quality show work, ha, ha. Slainte.

 

[Argee]

Update! I've now finished this project and thought I'd be brave enough to show you the result. Firstly, the back-of-a-*** packet sketch from the client:

Finally, the finished thing - equilateral triangular base with 1220mm sides. I've had to prop it level, because the drive slopes.

The inverted flowerpots are an attempt at baffling the squirrels, but they've also given rise to me calling this thing "Saturn 3."

Ray.

 

[DaveL]

Ray,

Very nice.

I am not sure the flower pots are big enough to stop the squirrels, but its a good try. 8)

Just needs a down lighter in each of the roof spaces and you have a cool looking night club, not sure that the penguins will be able to reach the tables to serve. 8-[ :wink:

 

[Waka]

ray

Thats posh for the birds, like the layout.

 

[Jaco]

Nice JOB!!!!

Think i will give that one a miss ......

 

[Chris Knight]

Well done Ray - The flowerpots are a good idea too. I hit on the same notion to baffle the squirrels but used an old washing up bowl and it doesn't look half as a nice as yours!