Compound Angles for Wood Range Hood

Joinery for a wood range hood gets into some tricky geometry. February 12, 2009

Question
After thirty years in the business I am finally building my own maple ply hood that has a slanted front and sides. How do I work out the compound angles so that the front, which is slanted back as well as narrowing from 48" to 30" on the top, will mitre to the sides that are also slanted to match the angle of the front panel?

Forum Responses
(Cabinetmaking Forum)
From contributor J:
We start with the front elevation detail which gives us the overall height with molding and mantel mold allowance. Subtracting the molding allowance we cut the top rectangle and bottom rectangular plywood pieces. We cut the vent insert hole in the bottom one. We then install temporary spreaders between the two pieces so they mimic what it will be hanging on the wall. This will give you top, bottom widths of your panels. A little trial and error on scrap pieces will give you the angles of front and sides.



From contributor R:
I just finished one. It was tapered on all four sides because it is suspended over an island.
I used SolidWorks to create a 3-D model of the hood and took all my measurements off that, including the angles. It was very accurate.

If you don't have access to a 3-D modeling system or someone locally who does, please feel free to get in touch with me. I'd be happy to do a model for you. There is a free downloadable viewer for SolidWorks that will allow you to take a model and rotate it any way you need to see it.



From contributor T:

I would just build a jig to use on the table saw kind of like a taper jig and tilt the blade at 45 degrees. Test it out on cheap MDF scrap.


From contributor J:
Good question! So far, I've been chicken and bought those hoods that taper like that. I'd really like to build one but wasn't sure about the angles myself. Would this work - cut the plywood to the proper taper needed vertically (not a compound cut)? Then once you have the basic angles correct, run the pieces that need to be mitered across a router table with a 45 degree champfer bit (bearing flush with the fence). Would that work or do I have my geometry all wrong?


From the original questioner:
I have also been chicken to try it, but at over $1,000 a hood plus transport, I have decided to try one. Less than a sheet of ply to build with 30 feet of moulding and I need to stretch my bucks these days. I am trying to avoid the trial and error system as there must be a mathematical answer. It will be painted anyway, so some body filler can fix errors.


From contributor J:
I just bought a 48" w x 48" h chimney style hood $532 delivered. I thought it was exceptionally well. I certainly wouldn't build one for that price. So will my idea with the router work?


From the original questioner:
The 45 degree angle worked great. I used my Festool saw with the guiderail set to 45 degrees. It took two minutes to cut the four angles. I angled the sides and front 65 degrees and was using 3/4 maple ply. Although the outside of the miter is perfect, the inside is a little open, but I will fill it with epoxy to strengthen the joint and it will never show at all as the hood liner hides the complete inside.


From contributor M:
For future projects, if you'd like to figure out the exact miter angle, it is easily calculated as a percentage of 45 degrees. For instance, if the panel (trapezoidal or square) that you are cutting is to be installed plumb, then the miter angle is 100% of 45 degrees (which is 45 degrees). If the panel is to be installed at a 45 degree angle itself, then the miter angle is 50% of 45 degrees (which is 22.5 degrees). This method of calculating will give you the angle as it appears on the table saw (a square cut is called 0 degrees). Calculate for each panel, as the front may be sloped more or less than the sides.