spring angles,irregular pyramid - Woodworking Talk - Woodworkers Forum
 
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post #1 of 9 Old 11-08-2010, 10:28 PM Thread Starter
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spring angles,irregular pyramid

Hello

I have an interesting problem where I have to construct several small irregular pyramids using 3/4" baltic birch ply. Each side of a pyramid has a different slope. My question is how do I determine the miter cut and bevel cut for each corner?

Imagine a 3/4 ply base, each corner is 90 degrees but each edge has been sent through the table saw at a different angle(spring angle).
So how does one miter/bevel each corner.

Thanks
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post #2 of 9 Old 11-09-2010, 12:27 AM
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Have you seen these charts?

http://www.compoundmiter.com/chart.html bill
or this:
http://jansson.us/jcompound.html

http://www.installcrown.com/Miter_sa...cute_angl.html

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 11-09-2010 at 12:31 AM.
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post #3 of 9 Old 11-09-2010, 08:34 AM
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WELCOME TO THE FORUM

Using the charts can be a big help. Before there was the internet, and "Google", and charts, us oldtimers had to figure out what and how to do it with minimal measuring devices.

One of my ways is to set up the base, and take another piece (a straight edged section) and lay it on the base and draw a line across the bottom of the tilted piece. This will get you very close to the angle of tilt (you could call it the spring angle, but I'm more used to using that term in reference to crown moulding).

As for the bevel, set up two pieces from the base at the angle of tilt, Take a strip of wood and lay it across where the two tops meet, and draw a line on both sides on the tops of both pieces. Again, this will get you very close.

To know what you drew, you could use a common drafting protractor to find what angle you have. With the lines drawn, take the piece to the saw of choice, and line up the blade with the line. If you measured the angle, you could just set the saw and try the results for a fit. You may have to make slight adjustments.










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post #4 of 9 Old 11-09-2010, 09:25 PM
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trial and error. im not smart enough to figure it out on paper. you can try a adjustible square. thats what i start with.
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post #5 of 9 Old 11-12-2010, 03:24 PM Thread Starter
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Thank you everyone for your input.
I've used the trail and error method lots of times but its slow.
The charts and calculators are fine when the slope or the spring angle or the pyramid sides are the same. Then its just like when your installing crown in a coffered ceiling or on top of kitchen cabinets.
I am specifically looking for the math to build irregular pyramids out of 3/4 ply. Built up from a rectangular base in which each side of the base slopes back at a different angle.
Maybe Im stuck with mocking the angles up first but Id love to find the clean math formula.
Thanks again, Enjoy the weekend.
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post #6 of 9 Old 11-12-2010, 05:12 PM
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Google "true length"....its an age old sheet metal technique.BW
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post #7 of 9 Old 11-12-2010, 08:33 PM
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Make a 45 shooting board for a skill saw and you're good to go.
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post #8 of 9 Old 11-14-2010, 09:56 PM
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I would start with my base cut to finished size. Then locate where the point of the pyrimid is on the base and with a sharpened 3/4x3/4 stick cut to the hight of the point secure it to your base. Now with the point in correct reference string strings from each corner up to the point. Now you can see the shape and what you have to do to " get er done". With a bevel square get the angles for each corner, you have the length of each corner also.You also have each bottom angle cut at the base. I'm asuming you only need to see the outside of the piece so backcut each angle real good so the exterior is really all the touches each other. With nailers on the inside assemble, deliver and collect a check.

Last edited by Wood Dave; 11-14-2010 at 10:01 PM. Reason: additional idea
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post #9 of 9 Old 12-02-2010, 11:12 PM
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Quote:
Originally Posted by Wood Dave
I would start with my base cut to finished size. Then locate where the point of the pyrimid is on the base and with a sharpened 3/4x3/4 stick cut to the hight of the point secure it to your base. Now with the point in correct reference string strings from each corner up to the point. Now you can see the shape and what you have to do to " get er done". With a bevel square get the angles for each corner, you have the length of each corner also.You also have each bottom angle cut at the base. I'm asuming you only need to see the outside of the piece so backcut each angle real good so the exterior is really all the touches each other. With nailers on the inside assemble, deliver and collect a check.
Bosch makes a digital angle finder which takes the spring and automatically calculates the miter and bevel more than pays for itself
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