I have a radius fish tank job I will be bidding out but I'm very nervous about finding the radius of the convex tank. The tank is HUGE. probably 8 ft long 36" high, not sure how deep yet. I saw it when i wss working for another company and a lady im doing work for now...its her mom! Small world!
How do you guys propose i get the radius and i have no clue where they're going to put the fish.
Also, would you guys suggest i pick up the tank and bring it to my shop so i have it there or make a cardboard template if possible of the entire shape
Boat builders have neither the brains nor the time to do mathematics. In reality when scribing cabinets in boats with multiple curved and straight surfaces math is impractical.
The accurate way to do this is called spiling.
The way I like to do it is with a piece of cardboard and a stick. You rough cut cardboard within an inch of the object.
The math is also useless for not circle shapes.
Take a straight stick about 1" x 1" x 12".
Cut one end at 45 degrees. Butt the point of the stick up to the shape. Take a pencil and draw a line atleast halfway up the stick and continue across the butt end. You should have an L. Do multiple marks until you can accurately connect the dots. This technique works on straight, round and curved shapes.
Now take your template and you can reverse the marks on to a piece of ply and generate a very accurate shape.
You can take a thin batten 1/2" x 1/2" of straight grained redwood or cedar and connect the dots with it.
Its very easy. This is how you shape bulkheads for boats.
a) that the arc is a constant curve (i.e., circular and not elliptical);
b) the length of the arc; and
c) the height of the arc (what you call "depth" in your post),
then you can calculate the exact radius using a CAD program or a circular arc calculator you can easily find online.
With an exact radius you can swing a perfect template or have one CNC cut. The latter is extremely useful for very large radii -- every time I have to swing a 25 foot radius I find that I've just cut my last 25 foot stick ;8>).
Spiling is a great technique for irregular curves, but in this case math (along with a computer) is your friend.
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My initial thought, because the fishtank is already installed, is to find the center of the width (left to right) take a framing square at the center, measure the height of the chord, then transfer that into autocad
I would use the math method as in the Knowledge Base. There are also online calculators. You need the chord length and the altitude at the center point in the length of the chord for the calculators. There are also several good ones at the App store.
Once you have a good radius, put a router on a stick and swing the radius on some scrap 1/4" ply or similar. Take this out to the tank to insure 1. that the tanks a true radius, and 2. that your arc fits properly. Use the router on a stick to cut frame parts and such at the various radii. This will keep things concentric and true.
The radii in the attached photos were determined by interior measurements of chord and altitude, and by exterior measurements of chord and altitude at the ends of the chord (straight edge). Everything fit perfectly.
May seem obvious but dont you have access to the top of the tank? Cant you just have the owner remove any hardware/covers/filters, and lay a sheet of cardboard over the top and trace the top of the tank? If you were worried about deviation from top to bottom you could cut an external template from the traced patter and check it against the base and adjust accordingly?
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