Finding a Radius - Router Forums
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post #1 of 20 (permalink) Old 12-21-2019, 11:09 AM Thread Starter
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Default Finding a Radius

First of all, I donít post much but I have gleaned much from reading here. But I havenít been able to find my answer to this question.

I have three different radiuses that I am trying to find without using the ďguessĒ method. I donít have a true diameter for any of these radiuses. I found a radius calculator that allows me to put in the width and a height and itíll give me a decimal radius but Iím having a hard time converting it to a usable measured fraction.

Typically, Iíd get my width (19í), find center and then measure up to my height (3í-6Ē), snap a right angled cross on the floor and then start measuring down until I got my arch to cross both points - guessing.

So with my example, my arch is 19í across and itís 3í-6Ē high. The radius calculator says my radius is 14.642857142857142. How can I convert this into something I can use? I donít have measurements for the other two. I am routing top trim pieces for all these.

Thanks in advance.
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post #2 of 20 (permalink) Old 12-21-2019, 11:30 AM
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...looks like it's 14' 7 11/16"...

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post #3 of 20 (permalink) Old 12-21-2019, 11:38 AM Thread Starter
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Thanks Nick! Would you care to share how you got that? Iíll need the info to find my other two radiuses.
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post #4 of 20 (permalink) Old 12-21-2019, 11:47 AM
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Originally Posted by rmark View Post
Thanks Nick! Would you care to share how you got that? Iíll need the info to find my other two radiuses.

...multiply everything to the right of the decimal point by 12...that will actually give you 7 5/7" but rounding to 16ths will make it 11/16ths...

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post #5 of 20 (permalink) Old 12-21-2019, 11:50 AM
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So really, your question is decimal to fractional with feet/inches thrown in for good measure? In this case you have a 14 and .6428 feet as a radius (ignore the rest of the fraction). Multiply the fractional part by 12 for 7.7136". So 14' 7.7136". To get the fractional inch (.7136), you do the same but with 16 instead of 12 for 11/16" ( .7136/16 - 11.4176), though I would just use 3/4" So, 14' 7 3/4"

I guess this is where the metric boys rightly explain how screwed up the imperial system is. I wouldn't argue that point but it is an extreme case.

[edit] I see nick beat me to it while I was composing. by the way, the difference between 11/16 and 3/4 is .02%.

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post #6 of 20 (permalink) Old 12-21-2019, 12:00 PM
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Depends on whether you're working with a true circle, or an arc. A circle is pretty easy, infact, here's a calculator where you put in the number for the radius in inches, and it calculates the rest. https://www.omnicalculator.com/math/circumference

If you are calculating an arc, you are only using a portion of the circumference, and it is more complicated. I found this calculator and explanation that may help. It is high school math, but I sure as heck don't recall this. https://handymath.com/cgi-bin/arc18.cgi?submit=Entry

Hope this is helpful.

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post #7 of 20 (permalink) Old 12-21-2019, 12:03 PM Thread Starter
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Thanks much guys! That’s what I wanted to learn! So when I come up with decimals, if feet is my whole number, I multiply the decimal by 12 to get it to inches. Then I can multiply the remaining decimal by 8, 16 or 32 depending how far I want to break it down (8ths and 16ths is typically good enough). This will be my first (2nd and 3rd) large radius to rout. I’m looking forward to it but want it to be perfect.
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post #8 of 20 (permalink) Old 12-21-2019, 12:05 PM
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Convert it to metric. Easy peasy then. No fractions. Your answer will be accurate to 1/25th of an inch.
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post #9 of 20 (permalink) Old 12-21-2019, 12:05 PM
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Quote:
Originally Posted by rmark View Post
Thanks much guys! Thatís what I wanted to learn! So when I come up with decimals, if feet is my whole number, I multiply the decimal by 12 to get it to inches. Then I can multiply the remaining decimal by 8, 16 or 32 depending how far I want to break it down (8ths and 16ths is typically good enough). This will be my first (2nd and 3rd) large radius to rout. Iím looking forward to it but want it to be perfect.

Exactly...whatever you want the denominator of the fraction to be...

Have fun...

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post #10 of 20 (permalink) Old 12-21-2019, 12:12 PM Thread Starter
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@des ertTom

This would be an arc because itís just the top part of a circle. If I had the diameter then the radius is half the diameter. Iím not sure the formula used to get the arc but the arc calculator does the work.

I donít recall much high school math either. I tried to forget as much as I could and then later realized I need some it.
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