Replies

1. 
   rabs | Sep 23, 2011 04:21pm | #1

   sorry, not bevel, but what compound angle do I need and what is the formula to figure it out?

1. 
   Westchester | Sep 24, 2011 11:11am | #2

   Cut

   Are you doing this on a table saw or chop saw ?

   SA

2. 
   gdblake | Sep 28, 2011 04:59pm | #3

   No magic formula but I have a solution

   Rabs:

   The following is rough and fast, but this should give you a general idea how to solve your problem.

   The easiest way to approach this is just to build a mock up out of plywood.  Cut a 26" square, measure in 13" from each edge and draw in the 13" square on the 26" square.  Rip some 2 x 4 pieces down to 2" widths and a length of 13".  Cut 45 miters on the 13" long pieces so you can screw them to the 26" square to create a stand-off that is 13" square and rises 2" centered on the 26" square.  Your assembly will look something like the attached drawing.  Now it is just a matter of measuring the distance between the 26" square and the 13" square to know how wide to make the side pieces of your form.  You want these pieces to be an inch or so wide so you can attach a 13" square of plywood to them when you assembly your forms.   Cut them 26" long and lay a piece on the mockup so that you line up on the 26" square.  Now mark where the piece touches the corner of the 13" square formed by the stand-off (see drawing).  Strike a line between that mark and the corner of the side piece and repeat on the opposite side (see drawing).  This gives you a pattern for the form side pieces.  Assmble the sides and add a 13" inch square to form the bottom of the form.  Hope this helps.

   gdblake

   
3. 
   Mycosculpt | Oct 06, 2011 01:31am | #4

   Unsure of the math behind it all other than it's trig, but this is what I arrived at and how/where. First of all, you're dealing with a 5" run ([23 -13]/2) and a 2" rise. Since these are at right angles to each other we can use the calculator at http://www.easycalculation.com/trigonometry/triangle-angles.php . We now obtain a 5.385..inch width for your side and the angles 21.8 (outer) and 68.2 (inner/upper).

   We next go to http://www.woodworkersguildofga.org/ShopHelpers/MiterCalculator.htm and enter "4" for the number of sides and "22" for Slope of side to base. This gives us a Blade angle of 15.36 degrees and a Miter gauge angle of 47.16 degrees. Test cut on scrap for each corner setup and swap out left to right as needed.

   Since these are edge to edge joints, reinforcement bracing will most likely be required for your application. Good luck!

4. 
   bones | Oct 06, 2011 10:53am | #5

   sketchup answer

   If i understand you you start at 26' you have a rise of 2" coming in 6.5" from each side.   I drew that in sketcup and used the protractror and the angle is ~12.3 degrees, but I would not sweat getting it perfect there is no point.  Get it close on the inside (inverted) pour it on top let is set till solid and then remove the form and work it with a trowl.  You might get a better answer over at breaktime if there are still folks over there since that forum is more aplicable to building than FWW. 

5. 
   PSeverin | Oct 06, 2011 06:13pm | #6

   In sketch up lies the answer.

   It is tricky.  It depends on what angle you are after.   I assumed in the drawing attached that you were after what angle you'd have to set a saw to cut along the sloped side of the 45 degree joint for the pieces to have them fit together.  In my drawing the purplish side is perpendicular to the the yellow side face, not the bottom and the angle I come up shown by the purple circular piece is 78 degrees.  If you are looking for the angle between the sides and the top, that is a different matter.  

   I seem to be at variance with the previous response, but we may be talking about different joints.

   Peter