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#### ThirtyWest

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Hi all,

I'm making a simple shelf and sports-medal display rod for the kids.

The leading edge of the top shelf will get a router above/below.

I'm interested in applying a curve to the triangles pieces shown--enough to make a curve but not enough to interfere with the rod.

I'll be using a router/jig, but I've never determined the radius of curve before.

For now, I'm just anticipating 10" top shelf, but I could change. The process should be the same though for any sets of numbers.

So here: 10" top shelf, and a 10" board down along the back (plus the 3/4" thickness of the top shelf).

Any help on this would be great.

I put some red marking here to show what I'm seeing in my mind.

thanks as always.

#### chandler48

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Lay the side flat on a work bench, take a large protractor and put the point on one end of your proposed arc. Extend the protractor to more than half across the proposed arc. Then bring the protractor down onto the table making an arc at the bottom. Do the same with the other side of the proposed arc. Where the two lower arcs intersect, you will use as an anchor point for your protractor and you can draw the arc on the side board from that. Gosh, what memory your brain has 55 years post Geometry

lenaitch

#### SeniorSitizen

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chandler 48 has forgotten more geometry than i ever knew so i'd be forced to do T&E , ( that's trial and error ) with my shop made tram point on cardboard off of a square corner.

EDIT: EDIT:

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#### Nealtw

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That is way to basic, give it some style.

#### ThirtyWest

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That’s sweet. I might shorten the back and overlap the top.

Do you have a picture of that protractor technique. ?

#### Nealtw

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That’s sweet. I might shorten the back and overlap the top.

Do you have a picture of that protractor technique. ?

Tie a string to a pencil and pin the other end with your finger. :wink2:

#### SeniorSitizen

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That is way to basic, give it some style.
That shelf is as old fashioned as i am.:vs_laugh:Stay with your original plan.

EDIT: EDIT:

#### DexterII

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Not picking at all because what he described is exactly the way I would do it, but I think Chandler meant compass, not protractor. How's that for 55 years?!?! And yes, if you don't have an actual compass that large a piece of string and a pencil will work, as Neal mentioned.

#### SPS-1

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Its whatever looks good to your eye.

If the top plate is 10" and the backplate is 10", and you wanted to make the radius tangent to the two, you would make that a 10" radius. But that would feather out the gusset to zero, and it wouldn't look right. Besides what the radius is, the other thing is where to put the center.

Just draw it, and if you like it, its good. Or you can draw it out on paper, either 1:1 or 1:2.

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#### Old Thomas

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Or find something with the radius you want and trace it.

ThirtyWest

#### 3onthetree

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If you want easy use a 1x12, so 11.25" rather than 10".

If you don't want to eye the curve but want a perfect arc, then you can draw the hypotenuse, then split this in half and connect that midpoint back to the corner. This result gives you the same center point of the circle as Chandler's method, but now you can extend this line as long as you want to adjust the radius of the circle (from a string/pencil) to get the arc that doesn't interfere with the rod.

Speaking of rod, if it's just for hanging ribbons, maybe go with 1/8" steel so your arc can be tighter like your drawing. Also I agree with SPS with wood you need returns on the end of the arc, not flush the arc to zero. You can hide this by capping the ends with these braces rather than putting them underneath.

#### ThirtyWest

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#### SeniorSitizen

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Or find something with the radius you want and trace it.
OH, now it understand why round Tupperware came in sets of several sizes. If that would have been taught that in 8th grade we could have remembered that rather than words like compass, tangents, pie, or was the pi, tram points etc.

EDIT: EDIT:

#### udraft

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An arc is a portion of a circle. A circle is generally worked from the radius, but you can use the chord for a few things.
What I see in your sketch is an arc that does not have a fixed radius. In other words in can be any radius. The method chandler describes will produce a radius point for an arc symmetrical to the 2 boards. If that's how you want it to look. I might set the ends of the brace back from the board edge, maybe an inch. And don't try to cut the curve to point, create a bit of a shoulder.

Carpentry math

#### overlandflyer

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why not go with plexiglass for the sides and rod?
you'd get a lot more visibility.

#### 3onthetree

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why not go with plexiglass for the sides and rod?
you'd get a lot more visibility.
At that point might as well go floating shelf, and hang the ribbons off of the trophies.

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