Truncated pyramid - Woodworking Talk - Woodworkers Forum
 
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post #1 of 9 Old 02-14-2016, 03:53 PM Thread Starter
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Truncated pyramid

Yet another seemingly simple shape is impossibly difficult to cut. What I need is a box where all 4 corners are mitered, so you don't see any end grain. I also want it to taper big to small, (pyramid), and to be flat on the bottom. (truncated)

So, it's pretty east to cut the first 45, with a taper. But to get the mirror on the other side, and get it accurate, so the corners look good, I don't know. I've already discarded making the tapered pyramid shape as a pattern, and using a 45 degree router bit to get the bevel. The bits and equipment aren't big enough to do the thickness I want.

Ideas?
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post #2 of 9 Old 02-14-2016, 04:28 PM
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Should be able to do on a compound miter saw if the piece isn't too big.

I'm not young enough to know everything!
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post #3 of 9 Old 02-14-2016, 04:34 PM Thread Starter
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Yeah, it kind of is. Plus, I've never had much success with these kinds of cuts on a CMS.
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post #4 of 9 Old 02-14-2016, 04:56 PM
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You can make the cut on your table saw. Adjust your miter gauge to the angle of the pyramid wall and the table saw blade to the angle for the 45. The important thing to remember is that the correct angle to get a 45 degree joint is not 45 degrees. I don't remember the math for figuring out the correct angle, but it is only 45 when the walls are vertical. You could probably use google to find the formula for a compound angle.
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post #5 of 9 Old 02-15-2016, 07:10 AM Thread Starter
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Yes, I considered that, too. But, you would have to tilt the mitre gage the opposite way to get the other side. Which you'd have to do perfectly in order for the joints to come out right. And, if the piece slid AT ALL on the gage, it'd also ruin the cut.

Crap, I forgot about the angle thing. I do recall now, though. I have little to no experience with compound miters.
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post #6 of 9 Old 02-15-2016, 10:38 AM
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right, it is not 45 degrees, but a compound angle. you can cut them on the ts with a miter.


my trick is to cut and assemble 2 halves. then, if needed, I run each half over the jointer to flatten the bevels so the 2 halves meet perfectly.


here is compound angle calculator:
http://www.pdxtex.com/canoe/compound.htm
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post #7 of 9 Old 02-15-2016, 12:23 PM Thread Starter
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Yeah, I was looking at that calculator earlier. Turns out the angle needed for my application is very near 45, but not quite.

I have thought about your jointer method before; that really works, huh?
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post #8 of 9 Old 02-15-2016, 12:31 PM
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Quote:
Originally Posted by TooPicky View Post
Yeah, I was looking at that calculator earlier. Turns out the angle needed for my application is very near 45, but not quite. yeah, the slope of the side has to drop below 80 degrees before the miter angle drops below 44 degrees.

I have thought about your jointer method before; that really works, huh?
works fine
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post #9 of 9 Old 02-15-2016, 12:34 PM Thread Starter
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Hmmm, will see what I can do...
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