How to find a Radius with chords?

[neuralsnafu]

Okay, I've posted this on another board as well to see how many different ways to accomplish this goal exist.

Okay, so we had this problem show up at work.

Company wanted a new panel for their RV/Coach, had a radius-ed corner.
All we have is a 2" and 6" od pipe die (8'long for bending some of our light boxes)

2" and 6" didn't work.

I know the chord theories can make this problem go away (part has already gone away, we declined the job)

I think the section I could measure was about 3" (the part of the radius).

I've been scouring youtube and google, but every equation I can find lists the chord length, and the distance TO the CENTER of the circle. So that equation does me no good.

So The only two numbers i would have are the length of my chord and the distance from the chord to the inner/outer radius of the piece. How would I solve this puzzle?

 

[bsaint]

If you can find the width of the arc and the height, you can find radius of an arc. Then multiply that by 2 to get the diameter.

http://mathopenref.com/arcradius.html

Then you can use the arc radius to get the arc length if you know the angle is 90 degrees.

S = r . θ

 

[royce]

If you could show a sketch of what you are explaining and the known numbers, it may help.
I like these sort of puzzles.

Royce

 

[FMC]

Use dividers to define the radius? the only string operation i know of is for making ovals

 

[shepherd]

Go to Home Depot and get a contour tool...a bunch of needles that you push against a surface to get a traceable pattern.

 

[WNYflyer]

Another site that also shows the logic.

http://www.had2know.com/academics/arc-radius-angle-formula-calculator.html

 

[neuralsnafu]

Thanks for the info.

Royce, here is a quick sketch



what is a Divider?

 

[FMC]

Thanks for the info.

Royce, here is a quick sketch



what is a Divider?

pointy sticks for people with no math skills i guess lol

 

[BadSpellar]

Online calculator http://www.mathopenref.com/arcradius.html

Radius = 1 7/8

 

[PelicanPines]

thanks for the info.

Royce, here is a quick sketch



what is a divider?

1.875

 

[royce]

Now then, if I was to do this mathematically I would use the formula that WNYflyer posted.

If I was to do it practically, I would lay out two lines perpendicular to each other and measure out the cord length from center and the arc height from center.
Then I would set my trammels or dividers to what I think is close and try to hit both the cord ends and arc height.
Now, adjust the trammels and try again.
It will take few tries at adjusting it, but it is easy and foolproof.
I changed your inches to feet and laid this out to illustrate.
Hope it helps

Royce

 

[MW.]

So this doesn't have to be tangent to the panels joining it?

 

[neuralsnafu]

It was 'supposed' to match the other panels, but judging from this one, it wouldnt have been possible. It was a luggage compartment door, that was trashed. They sent us the damaged panel but no reference for a good one.

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[MW.]

Then with a 3" chord length and assuming the panels on either side are 90 degrees to each other the radius would be 2.121".

http://www.calculator.net/triangle-...x=0&vy=0&va=45&vz=3&vb=&angleunits=d&x=72&y=8

Much easier to break it down into a triangle to work with if possible.

I hope I understood correctly what your after.

 

[neuralsnafu]

I guess i dont get what you mean. The panels are side by side on the long side of the coach. The radius would have been on the bottom side, ie under the bus.

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[MW.]

This is what I thought you were saying. The blue line would be the bottom side of the bus.

Edit
Not sure why the picture is so small.

 

[neuralsnafu]

Yes. The blue side would be the undercarriage.
The panel ended where you start the blue line.

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[MW.]

Try this

That would work out to about 5/8" (0.621") instead of 3/4" though.

 

[mike13u]

I have to find radius of parts often to draw things in CAD for waterjet/laser/plasma cutting. If the item is small I use a set of cheap machinist radius gauges. For larger items I have a set of Arc Master radius gauges. Great tools. If you find yourself in this situation more often, I would highly recommend a set of Arc Master gauges.

 

[larry_g]

One other issue here in real life is that the part you were trying to duplicate may not have been a constant radius but ovaloid or elliptical.

lg
no neat sig line