Replies

1. 
   PeterJ | Jul 28, 2007 10:07pm | #1

   Do you have a compound miter saw? Table saw?

   Sounds sorta like a compound miter. Picture the louver mounted flat parralel to the floor...there would be a 45 on each end, long point on bottom. Now imagine rotating the louver down 45 degrees on front edge, the long point grows to meet the frame. Thats about as clear as mud, I suppose.

   Essentiailly you're cutting a 45 on the flat of the louver and a 45 bevel along that angled face.

   Gees, it's a lot harder to explain than to do...

   PJ

   Everything will be okay in the end.  If it's not okay, it's not the end. 

2. 
   Sphere | Jul 28, 2007 10:11pm | #2

   Its 55 I think..or was it 145? one of them two..it's been awhile since I did one.

    

1. 
   JoeBartok | Jul 28, 2007 10:48pm | #3

   Sounds interesting but I am not entirely clear on the givens. Can you post a rough sketch? I'll try to make it back tomorrow and take another look.Joe Bartok

1. 
   Woodhunter | Jul 29, 2007 01:15am | #4

   Well, I got it worked out, but I can't figure out why...and that bothers me.  I was using a table saw, but got out my compund miter saw and noticed the crown molding marks at 31.75?? and 34.5?  I set it to those marks and cut a test piece that fit very close to perfect.  It was probably off due to fact that my apex was not exactly 45 degrees.  It did not look like a CM cut, but it worked.  What I would really like to know is how to determine exactly what angle I needed.  Also, I thought that the CM marks were for typical crown that is not set at 45 degrees...

   Anyway, if anyone knows of a good resource for figuring out these angles, I'd appreciate it.

   Thanks for the resonses.

    

   Dan

1. 
   RedfordHenry | Jul 29, 2007 05:25am | #5

   Before you go insane, have you considered installing a manufactured louvered vent.  A small one costs less than $30 and will fit into a rectangular rough opening.  Those marks on your saw are for cutting CM on the flat.

1. 
   JoeBartok | Jul 29, 2007 06:12pm | #6

   Woodhunter, this online "cheat sheet" is one way to solve your miter-bevel angles:

   Roof Framing and Joinery Calculator

   What this program really solves are the various tetrahedra and right triangles in a complex roof - but what we are cutting doesn't have to be strictly a "roof". Think of whatever you are cutting as the slopes or pitches of a Hip-Valley roof intersecting at an angle in plan view (the "Total Deck Angle" field).

   If you already know the angles on the faces of the stick:

   Saw Blade Angle Calculator

   If you are interested in learning about the theory and mathematics behind the formulas in these calculators here are links to my blogs. They are not very well organised (actually they are all mixed up like a dog's breakfast) ...

   Roof Framing and Joinery Mathematics

   Online Math Notes and Sketches

   And some links to where an effort was made to explain compound joinery:

    

   Roof Math

   Backing Angle Formulas

   Geometry of Compound Joinery

   Joe Bartok

   Edited 7/29/2007 11:13 am ET by JoeBartok

   Edited 7/29/2007 11:26 am ET by JoeBartok

2. 
   Woodhunter | Jul 29, 2007 10:13pm | #7

   Thanks, this will help a lot in the future.  I appreciate all the suggestions.

   Dan

3. 
   DanH | Jul 30, 2007 12:34am | #8

   Frankly, the simplest way to handle this is to make yourself a jig to hold the workpiece at the proper angle.

   So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
1. 
   JoeBartok | Aug 08, 2007 03:53pm | #9

   Woodhunter, check out this Triangle Vent thread. I think the formulas and drawings might give you some insight as to why crown molding angles might work for your louvre compound angles.

   The angles on crown molding with respect to the plane of the ceiling are the same as the angles on purlins in a Hip-Valley roof (one face of a purlin is the reciprocal of the slope of the roof). And regardless of how we look at the angles on the vents ... we are in essence cutting purlins. :)

   This is looking at the intersecting planes as a "layover" purlin meeting the slope of the Main roof:View Image

   Or we can think of this a standard purlin intersection with the plumb plane of the side face of a Valley rafter:View Image<!----> 

   Joe Bartok

   Edited 8/8/2007 5:18 pm ET by JoeBartok

   Edited 8/9/2007 11:18 am ET by JoeBartok

   Edited 8/9/2007 11:27 am ET by JoeBartok

   Edited 8/9/2007 11:27 am ET by JoeBartok

1. 
   Woodhunter | Aug 09, 2007 08:21am | #10

   Thanks!  I'll have to get out some reference material, but this is just what I was looking for.

   Dan

1. 
   JoeBartok | Aug 11, 2007 04:17pm | #11

   This is another sketch comparing the lines, planes and angles in a louvered vent to the compound angles of a layover Hip roof ... the "math lite" version.

   View Image

   That's the theory and it can easily be adapted for use in CM or scientific calculators. There are all kinds of other ways to solve the angles ... by adjusting the values of entries any of the crown molding and square hung fascia calculators found on the Internet will do the trick.

   Layover Purlin intersects slope of the Main Roof Calculator ... this script will solve the angles from givens as specified on the diagram above. Set the calculator to "Pitch Angle" mode, make the angle at the foot of the frame the "Main Roof Rise" and the slope angle of the slat(s) the "Dormer Rise".

   The saw miter angle, angle on the stick and saw blade bevel to cut the slats are returned in the "Face set in Dormer Roof" fields.

   Joe Bartok

   Edited 8/11/2007 9:18 am ET by JoeBartok

1. 
   Sphere | Aug 11, 2007 05:01pm | #12

   Joe, I gotta thank you, your stuff has helped me immeasurably.

   I often "see" it in my mind, but cannot extrapolate it in real time, without a lot of trial and error, your work astounds me.

   Thank you ever so much. 

2. 
   JoeBartok | Aug 11, 2007 05:43pm | #13

   No probleema, I'm glad to hear that others find this stuff useful. Tackling the questions posed in Internet forums gives me some insight into things I wouldn't normally do and it's only fair to put something back.

   We don't generally get involved in the finish and turnkey end of things in my corner of the world; the small stuff is subbed out. Years ago we ordered some octagon vents and they were a hell of a mess. I don't know if the guy who made them was on drugs, had bad eyes or just guessed at the compound angles. Likely one of the first two possibilities since even the cuts where the slats met the flats of the octagon were crooked.

   We decided that henceforth we would do our own work ... but the need for decorative vents never arose again. I hadn't thought about how to approach cutting a compound angle for a louver for over twenty years until the threads started in this forum and over at Joe Fusco's.

   This is another way of taking advantage of the fact that, if the slopes or pitches are equal, crown molding = purlin = square hung fascia. I'm going to borrow Joseph Fusco's Crown Molding Calculator.

   Enter 2 × Frame Slope Angle in the Wall Angle field.

   Enter Slat Slope Angle in the Crown Angle field.

   The crown molding miter and bevel from plumb are the values needed to cut the slats.

   The same calcs can be executed on my Square Tail Fascia intersects Hip Rafter Foot Calculator.

   Set the "Entry Mode" to "Angle", enter the Slat Slope Angle in the "Main Pitch Rise" and "Adjacent Pitch Rise" fields, and 2 × Frame Slope Angle in the "Total Deck Angle" field.

   The "Fascia Miter" and "Fascia Bevel" will work perfectly well for the compound angle on the slats.

   Enjoy!Joe Bartok

3. 
   Sphere | Aug 11, 2007 05:51pm | #14

   Well , I did an orthographic projection to have a cylinder meet an inclined plane ( barrel dormer vent) and that is the hard way.

   I was doing alright, but the info given was a 12/12, the roof was a 10/12..ouch.  Site modified, was a shin skinner for sure.

   I am running out of desktop icons saving all of your links. 

4. 
   JoeBartok | Aug 11, 2007 06:50pm | #15

   Barrel meets inclined plane of the Main roof is another aspect of the "layover" roof.

   The compound angles at the intersection may be solved with the Layover Purlin intersects Main Roof Calculator. The Main roof slope remains fixed, the Dormer slope varies according to its position on the curve (diagram below). There are links further down the calculator page with a bit more info.

   View Image

   Formula for the slope of an Ellipse (or circle) at any point

   This is probably going off the deep end but in case anybody is interested this is how the ellipse slope formulas are derived from first principles.

   Joe Bartok

   Edited 8/11/2007 11:58 am ET by JoeBartok

5. 
   Sphere | Aug 11, 2007 07:26pm | #16

   Ok. I think I got it. a vanishing point from the focii (sp?)

   What I had done was project a 12/12 (45degee)  in PLAN view ( a 4x8 sheet in this case) and plot the radius in plan..then extend the random points ( well, not so random, as per se) and connect the dots on the slice of a half cylinder as it intersects the the roof plane.

   I got a headache..I just can't see wher the focus is shorted, in your diagram or why.

    I agree with "a" being equal angles, but the "major axis" points have me befuddled.

   Do not think I am in doubt, I am just unclear. 

6. 
   JoeBartok | Aug 11, 2007 08:31pm | #17

   The slope formula and geometry are general and work for both circles and ellipses, the circle being a special case of an ellipse. The foci move together to create the center of the circle; major and minor axes will be an equal radius. The major and minor axis terms cancel one another in the slope formula, which reduces to –x/y for the circle.

   This Dormer intersects slope of Main Roof might help (... or it might cause more confusion ...?). It's actually concerned with developing the curve (a sinusoid) on the surface of the roof but the same projection lines determine the stations or working points, and lengths of the purlins.

    

   Joe Bartok

   Edited 8/11/2007 1:34 pm ET by JoeBartok

   Edited 8/11/2007 1:46 pm ET by JoeBartok

   Edited 8/11/2007 1:46 pm ET by JoeBartok

7. 
   Sphere | Aug 11, 2007 08:59pm | #18

   Could I plot an interface where the metal meets the shingles? An arc/tangent?

   never mind, the radius changes as we gView Imageo....... 

8. 
   JoeBartok | Aug 11, 2007 09:35pm | #19

   "Could I plot an interface where the metal meets the shingles?"

   You sure can. That process outlined in the link will plot a curve for any dormer profile in cross-section.

   I wrote a mimi-program for my Sharp scientific calculator to plot this Development of Flashing intersection with the Main Roof. Same curve as for the barrel dormer, just rotate your perspective. But you don't really need a formula and most curves are pretty much intractable when it comes to expressing them algebraically.

   Joe Bartok

   Edited 8/11/2007 2:37 pm ET by JoeBartok

9. 
   Sphere | Aug 11, 2007 10:02pm | #20

   I'm headed in the shop and try that..that is wayyyy. Cool.

   Thanks man, I am stupified.