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Hey Stan, I need to laminate up a vertical grain fir timber rafter, easy so far, right? Now, it is on a 9/12 pitch on one side and at ninety degrees to it the other rafter is a 12/12 octagonal face; now it gets difficult…..they have to blend into each other….
In other words, the rafter comes down at 9/12 due south and then turns due west and back up at 12/12. can you picture it? Kind of a twisted parabola(assymetrical). The rafter cross section is 5-1/2″ by 11-1/2″ high.
The math and the challenge has gotten my interest, and I’m wondering if you have any thoughts on how to build it up?
One idea is to do it with 3/4″ by 3/4″ laminations building up in both directions at once, or only on one plane at a time? Or….?
Any help would be appreciated.
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Phil--- WOW! what a twisty noodle. I can picture what you are wanting to do.
I assume the rafter is supported at the bottom of the 9/12--12/12 junction?
Another question that comes to mind is what is the radius of this rafters curvature as it turns west and back up at 12/12?
You will not be able to cut a continuous laminate for this because the stretch out of this laminate would look like a V---one side of the V is at a 9/12 pitch and the other side at a 12/12.
You will have to stagger the joints throughout the curved section. This will require the laminates to have exta width in this section to layout the transitional curves that will blend this 21/12 change in pitch.
A lot depends on how tight this transition radius is. The individual 11.5 inch laminates could have the same material glued on at the bottom to then have enough width for this to work.
I have drawn a laminate on my designcad, but am very ignorant how to post it correctly. If it doesnt come up, it is because I am not doing it right.
*Sorry about my drawing not coming up. Anyway, it was just a V with the bottom of the V's intersection having a curved transition. This is what the stretched out laminate would look like.
*Phil, when you get this noodle built please post a pic, what an interesting piece of work.
*i think i know what this looks like, but still not 100% sure. take a look and give me some guidance, i will modify as needed. the dark colored wood is the 9/12, the light colored is 12/12. two views at 90degrees to each other.
*Jason, that's close! For all who have posted,the section I have to do is the transitional curve which connects the two slopes; it will,in plan view, follow a circular arc segment over a circular tower for two spiral staircases, I get to do them too! I am getting the drawings next week and will post them and more info as well,AND if Santa brings me a digital camera, I will post pics during and after development.
*phil, if you know the radius of the circle, maybe i can incorporate that.
*Jason, I'll measure it tomorrow...thanks,Phil
*Jason, Stan et al......, the spec. has changed; the rafter is now to be a 9/12 sloped, elliptical "return" rafter over a 4'-4 3/4" radius turret into which a pair of spiral stairs will be built. So.... now it comes down parallel to the other rafters on the 9/12 slope and then does a u-turn back up at 9/12,parallel again to the other rafters, over the above radius(in plan view). The final section is to be 7-1/2" by 11-1/2" high, with the outside edge of the curve to be plumb throughout. I hope this clarifies and not mud-ifies.....
*P.S.....the material is still vertical grain fir ,glue to be UF-109, the beam will be interior/exterior...
*Phil: For this rafter to follow a path that is left by slicing this cylinder at a 9/12 pitch---then your laminates would have to be cut in an elliptical pattern. The minor axis would be 52.75 inches(which is your turret radius), and the major axis would be 65.9375 inches. This would be the ellipse that is formed when this turret is sliced at a 9/12 pitch.The wider the laminates, the less joints that will then be required to follow this elliptical path.Heres my jig that I would use to draw this ellipse. ^There are various other ways, string,pencil and nail method, square, etc. You just set the two pivoting dovetailed blocks at the major and minor axis lengths and draw away.
*Phil: Another view showing more of this ellipse drawing jig.
*Stan, is the inner radius just the outside minus the width section or is there some math change here?.....would the major (inner ) axis be related to the outside i.e., 52-3/4"is to 65.9375 as (52.75-7.5) is to the new inner axis? What is the formula for the elliptical section to begin with?
*Phil,
View Image
*Thanks Joe, I usually use my Machinery's Handbook for formulae,etc., but can't find it since moving.
*here is the next pass, based on the new info. is this close? when you said the outside of the return is "plumb", do you mean it stays perpendicular to the ground the whole way around? that is tricky (and not what i drew). (never mind the inacurate grain figure across the top of the curve, didn't put in the time to show the laminations).
*okay, this time with lamination lines
*Jason,
View Image
*joe: thanks for the feedback. walking to to lunch, i realized the "plumb" issue was much more simple than i thought. also, the curve is drawn as an ellipse, but i think the angle of view makes this hard to see. here is a version with plumb outside and inside faces.
*That's it exactly! Thanks for that.
*Damn.Where is this job taking place, and can I help?
*Deep Cove, Vancouver,B.C. near the marina and no thanks, I'm all over this one.Thanks for the offer though.
*PhiL;; I just got back online. My laptop started smoking. Sears gave me a brand new 850 mhz, no questions asked. It looks like your problem had some good graphics from Joe. Anyway, he showed exactly how I pictured it, and it looks like it is lamination time. I did incorrectly give the major and minor axis as one half their amount. I wasnt thinking. Thanks Joe for clearing that up.
*Stan,
View Image
*Anyway--that looks like one heck of a glue up!
*Hey Jason, what program did you use for your drawings?
*autocad r14, out of the box. no special plug ins or anything. 3d Studio is better/faster/more accurate with photo-realistic rendering. i can do small stuff much quicker on acad, but only out of habit, as that is what i draw on all day long.perhaps if stan and/or armin offers someone an informal discussion/tutorial/workshop on design/construction of curved stairs, i can follow along with some drawings.
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Hey Stan, I need to laminate up a vertical grain fir timber rafter, easy so far, right? Now, it is on a 9/12 pitch on one side and at ninety degrees to it the other rafter is a 12/12 octagonal face; now it gets difficult.....they have to blend into each other....
In other words, the rafter comes down at 9/12 due south and then turns due west and back up at 12/12. can you picture it? Kind of a twisted parabola(assymetrical). The rafter cross section is 5-1/2" by 11-1/2" high.
The math and the challenge has gotten my interest, and I'm wondering if you have any thoughts on how to build it up?
One idea is to do it with 3/4" by 3/4" laminations building up in both directions at once, or only on one plane at a time? Or....?
Any help would be appreciated.