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Fireplace Chimney Cap

Posted: Fri Jan 12, 2018 12:01 pm
by mdwalker
Can anyone tell me how to figure the dimensions to cut this out? It will be a fireplace chimney cap. Looking at it from the side it will be 2" tall with a lip around the edge probably 1" to 2" lip. I can add the lip with no problem just having difficulty figuring the triangle pieces. Looking at it from the top it will be 33" square and would like to cut it all as one piece and bend and weld. Thanks for any help.

Re: Need help with formula to figure dimensions

Posted: Fri Jan 12, 2018 1:50 pm
by djreiswig
A squared plus B squared = C squared will give you the hypotenuse of the triangle in your left picture. That would be the length of the angled sides on your right picture. Then just draw it out. As far as cutting it out as one piece, I don't think that will work because your center point will not be connected once you cut all of your angles. You could do 4 identical pieces and then weld them together after you bend the edges down.
I find it easier to just start drawing things like this in CAD easier than trying to calculate the angles.

Re: Need help with formula to figure dimensions

Posted: Fri Jan 12, 2018 2:01 pm
by djreiswig
This should do it for you.
chimney cap.dxf

Re: Need help with formula to figure dimensions

Posted: Fri Jan 12, 2018 2:11 pm
by rdj357
DJ, by my math your cap would be 23.42" tall!!

Here's what I came up with, I only use one seam and a break for the other corners but you could cut it into pieces if you want.

Re: Need help with formula to figure dimensions

Posted: Fri Jan 12, 2018 8:25 pm
by djreiswig
Oops. Always remember to cut the bottom dimension in 1/2 when figuring right triangles like this. That must be why I have to cut things twice at times. Nice design rdj357.

Re: Need help with formula to figure dimensions

Posted: Fri Jan 12, 2018 11:08 pm
by rdj357
I try to learn something new every day DJ, hoping it will make up for the many mistakes I make! :-P We all cut some scrap metal now and again!

Re: Need help with formula to figure dimensions

Posted: Sat Jan 13, 2018 9:05 am
by mdwalker
Thanks for the help. Can you tell me the formula you use to calculate the size of the triangles to make it 2" high and 33" square?

Re: Need help with formula to figure dimensions

Posted: Sat Jan 13, 2018 9:38 am
by Bobkovacs
At 33" overall, the distance from an edge of the square to the center is 16.5", so the angle (viewed from the side) is the square root of 16.5x16.5 + 2x2, or 16.62" (16 5/8 will be close enough). So, measuring from your centerpoint out on the sheet 16 5/8" in each direction, create a square that's 33 1/4" in each dimension. From the center point of each side, measure out 16.5 on each side, and then connect those points back to the center. The wedges you're slicing out will be really small.

Re: Need help with formula to figure dimensions

Posted: Sat Jan 13, 2018 9:48 am
by mdwalker
Thanks for the explanation. That is the part I was having a hard time figuring out is how to measure the wedge to take out. I know in this case with it only being 2" tall that it is a minimum amount to worry about but I knew that overall it would have to be wider than 33" when flat to come to 33" when it is bent to shape. Thanks again for everyone's help on this.
Danny

Re: Need help with formula to figure dimensions

Posted: Sat Jan 13, 2018 9:52 am
by rdj357
Bob has it - to put it in generic terms, Take half of the desired width squared and the desired height squared and add them together then take the square roof of that. (16.5^2=272.25, 2^2=4, 272.25+4=276.25, sqrt(276.25)=16.621. This number is the HEIGHT of your side blanks so to draw each side you draw a triangle with a base the desired width and the height you just found. ASSUMING it's a square you can make 4 of them and hook them together any way you please. In the center so you have to weld each seam or all the sides except the last where you only have one seam.

Re: Need help with formula to figure dimensions

Posted: Sat Jan 13, 2018 10:46 am
by mdwalker
That is exactly the information I needed. Thanks guys!