Sunday, October 23, 2005

BUILDING JIGS TO ASSEMBLE THE RIBS
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SASKNELLIE: "For the sake of those of us who are novices, could you post a picture of the jig you used to make the arches?"
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There are two "classic" profiles for the gothic arch shape. One uses the full width of the floor as the radius to draw the arc (this makes a taller, narrower-looking shape). The other uses 2/3 of the floor width as the radius (this makes a shorter, wider-looking shape). For ease of reference, I call them "full-width arch" or "2/3-width arch." The 2/3 width arch (in the pictures) has five 3' facets on each side and they all join each other to form a 15 degree angle. This shape is 16' wide and about 10' high. The only exception is at the ridge which is a 45 degree angle. At the ridge, you remove 22-1/2 degrees of wood from the top end of the two pieces which meet each other at the ridge. The lower ends and ALL the other pieces have 7-1/2 degrees of wood removed from BOTH ends. An arch shape can be drawn (and built) using any combination of height and width. This particular one makes a good first choice because the angles are whole numbers and it's a reasonable size for a first effort.
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The simplest way to make a jig is to carefully cut and assemble four rib components together with the plywood plates on one side only. Lay 2-1/2 sheets of plywood down to form a shape 8'
wide and 10' tall and fasten them edge to edge with hinges or blocks of wood. In the lower right-hand corner, draw a rectangle 5-1/2" wide and 1-1/2" high -- this will represent your sill plate that the ribs will set on. Position the lower end of your mostly assembled rib on the line -
- just as it will be on top of the 2x6 sill and put a nail or screw (in a pre-drilled hole) close to that lower end so the rib assembly can pivot on that fastener. (This allows you to move
the top end of the rib while the bottom end stays in place.) Next take your fifth rib component (7-1/2 degree on the lower end; 22-1/2 degree on the upper end) and position it so it is touching the top end of the assembled four components AND is flush with the center line extending
down from the ridge (the left edge of your sheets of plywood). Then it's a simple matter of making sure nothing moves and carefully tracing both sides of your rib all the way from the
ridge to the sill plate. Then you can install 2x4 blocks -- two on one side, one on the other of EACH facet and presto -- your jig is done. When assembling the ribs, I start at the ridge and put a screw into that top piece to make sure it holds at the centerline, no matter what. Install all four plates (even the ridge plate can be installed just leaving half of it sticking out in mid-air) and then carefully remove the rib from the jig, flip it over (no jig required for this step) and immediately install the remaining four plates (but NO ridge plate on the second side -- that plate will be attached to the rib coming up from the other side). It's easier to leave the
ribs UNattached at the ridge until immediately prior to standing them up into final
position.

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SASKNELLIE: "In my case, not having a chop saw, I would probably make a jig for the cutting as well."
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That can work just fine -- use a sharp blade and keep your RPMs up -- go slow and let the saw do the work. Much of your FREE pallet wood will end up with nails embedded in it. I use
chisel-tip blades (go slow to not damage the blades) and swap 'em out as soon as they start
to get dull. It's cheaper to have them re-sharpened than it is to replace good quality blades.
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SASKNELLIE: "What angle are you cutting for the full sized version?"
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The full-width version has SEVEN 3' facets on each side and is 20' wide and about 17' 6" tall. On this larger model, the ridge angle is 50 degrees so you remove 25 degrees of wood from each
of the two pieces where they meet each other at the ridge. At the other end of those two pieces and BOTH ends of all other pieces, only 5 degrees of wood is removed from BOTH ends. This allows them to join each other and form the necessary 10 degree angles.
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SASKNELLIE: "Another question, what will the span be at the level of your loft? I would like to build this in the spring as a garage with lumber storage in the loft area. I'm wondering how much weight this can support before you have to use posts to support from below."
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I don't have the drawing in front of me (it's at work) but the span on the loft is considerable -- 14' or so -- too wide to support much of anything without support below it. Were you planning on the solid T&G (Tongue & Groove) 2x6 deck or something more like "collar beams" to store lumber on top of? As a cabin, one would want a wall under the loft to separate the kitchen area from the bath/utility area so making that span manageable with a wall underneath it (splitting the span in half or keeping it at 10' or less) is no problem. Depending on what you want to do, we'll have to get Laren involved to help calculate the anticipated load and what will be required to safely support it. We can only hang so much weight down from the arch ribs. The rest must be supported underneath the loft with either a wall or posts and a beam. So the big question at this point is: solid T&G loft or open framing -- like collar beams? And how long will your loft be -- assuming the width will be "fixed" by the 20' wide gothic arch?
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sail4free
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