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Angle cuts for a 4 sided flared box

bwkwood's picture

I am building a new hopper for an antique cider mill. It is 4 sided with the sides flared at 30*. It requires an accurate inside dimension at the bottom of the hopper but the height and outside dimension at the top not critical. I have built a few skylight trim projects which flared out in both dimensions so this hopper is similar but not as difficult since the height is not critical. I have to admit that the skylight projects involved ALOT of trial and error but they eventually came out looking good. I compound mitred all the corners but for this hopper I planned to let 2 sides overlap for what I thought would be an easier butt joint corner. This turned out to be some more trial and error. I am somewhat competent at geometry but failed at calculating the required cuts. Has there ever been an article in Fine Homebuilding addressing this problem such as trimming out a skylight? Does anyone know of another resource that explains the calculations necessary for a 4 sided flared box? 

I can't recall what the (post #215538, reply #1 of 7)

I can't recall what the project was, but several years ago, for some minor woodworking project, I actuallly wrote a Java program to figure out the angles for me and double-check everything.  The joints still didn't come out right.

I think one problem is that most (home) woodworking tools are not accurate enough to get it right, even if you have the right numbers.  (I mean, it certainly couldn't have been my programming abilities!)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Easiest. (post #215538, reply #2 of 7)

The easiest way to do this would be to lean the boards against the fence of a chop saw at the angle of the flair and then make a simple 45 miter cut. You could make a jig to do this. No compound angle calculations necessary. 

Yes (post #215538, reply #3 of 7)

like cutting crown molding .

upside down and backwards.  Top of funnel on saw base, bottom of funnel against fence.

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Appreciate the feedback. Yes (post #215538, reply #4 of 7)

Appreciate the feedback. Yes the crown method would work for a smaller "hopper". My pieces are 10" boards and the skylight trimmers were wider than that. My mitre saw cannot handle these cuts. Trial and error does work especially when you have only one to do.

bwk (post #215538, reply #5 of 7)

take a look here and grab your framing square.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Make a test cut on a small board. (post #215538, reply #6 of 7)

Make a test cut on scraps using this method. When you get the angles right set a circular saw to the correct miter angle and scribe the taper.

15 degrees (post #215538, reply #7 of 7)

If you are not mitering the corners all cuts are 15 degrees.

If you are to miter everything you must use trig to determin the compound angles..  I recently, two years ago, did this same thing for soffit corners where teh soffit was not level but nailed to the underside of the rafters at X slope (one of two angles) .  The second relavent angle being the 45 degree but at an inside angle .  the result turned out somethign like 30 degrees and 22 degrees for a 10 in 12 roof..  Sorry not an easy answer unless you really like math and trig.  I do, but it still takes some time to figure out what oppsite is over hypotnus is and it is a double calcualtion.,