1. ## radius question, math guru's

ok how do you folks determine your radius's for a preselected arch,,

example need a arch that is 13.375 long and 1.75 high.. i am sure there is a formula to do it out there but this old noggen has forgotten it and the string trick is getting old.. help also need the radius for a 15.375 arch as well

Why do math when someone else will do it for you? Sigh. I swear the internet will rot your brain (I to used to know how to do this.. at one time..).

Practically speaking I probably wouldn't measure the arc at all, but would just lay it out with a flexible stick pinned at the middle and edges and mark to the curve.

3. Do you know the arc angle? I'm not sure that table will solve the problem unless another variable is known, i.e. the arc angle. In Larry's example I think he's referring to the arc length rather than the chord length, but.......?
Last edited by Al Launier; 12-10-2012 at 11:22 PM.

4. Originally Posted by Al Launier
Do you know the arc angle? I'm not sure that table will solve the problem unless another variable is known, i.e. the arc angle. In Larry's example I think he's referring to the arc length rather than the chord length, but.......?
Yeah good point, a more complete calculator that tries to solve all of the cases is at: http://www.handymath.com/cgi-bin/arc18.cgi?submit=Entry
it will solve either the arc length or the chord length given one or the other and the height.

5. thanks guys after i saw the arc swing marks my dusty brain woke up will make me a example and hang it up for later dates as for laying the spring stick down aqnd tracing it ryan my spring stick doesnt flex equally along its length so that didnt work for me i tried that first.. so that stick can become kindling..

6. Originally Posted by larry merlau
thanks guys after i saw the arc swing marks my dusty brain woke up will make me a example and hang it up for later dates as for laying the spring stick down aqnd tracing it ryan my spring stick doesnt flex equally along its length so that didnt work for me i tried that first.. so that stick can become kindling..

Try the following. I have not tested it out.

H = height of arch

L = length of arch at the base, not arc length

7. Larry,

When I get home, I will check to see if I have a better geometric solution.

8. Or:

L=Arc Length = 13.375
A=Arc Angle = 12.47481
P=Pi=3.1416

L=(R*A*P)/180 or R=(180*L)/A*P

R=(180*13.375)/(61.43030*3.1416)=12.475

9. Originally Posted by Bill Satko
Larry,

When I get home, I will check to see if I have a better geometric solution.
bill, swinging a couple arch's and using the intersection is fine with me.. i got a outfeed table that works well for large arch's just needed to see the picture for me to wake up

10. Ryan, thanks for the handy tables. Didn't know they existed. I saved as a favorite place.

Any others that are as useful? I had to did into my old Machinist Handbook that I bought new in 1960 (16th Edition) to find the formula.

Al
Last edited by Al Launier; 12-10-2012 at 11:55 PM.

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