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radius question, math guru's

larry merlau said:
would any plastic work, roger.. i got some abs plastic that i can try.. it bends easy and doesnt break like styrene does.. i also have some plexi glass but that will break if you go to far..
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ABS is better than styrene because of its rubber content. Polycarbonate is the best and you can find it reasonably priced at Mcmaster Carr. A 12"x24" 1/8 thick is just over $12. There are probably suppliers in your area that have "scrap" pieces for a few bucks.
 
Larry you are always better off using a set of trammels instead of bending a stick or a piece of plastic you never get a true radius otherwise. it will always have some flat spot or change in the radius that throws it off.

Here is the formula to figure a radius from an arc X=width Y=height at center


(X/2)2 +Y2 =R
2Y
the top formula is supposed to be x divided by 2 and square the result plus y squared.. over 2 times y. I couldnt get it to lay out properly.... sorry

A great book to pick up is "Circular Work in Carpentry and Joinery" by George Collings. Published London 1886 its still in print.
 
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You could use the old drafting trick Larry. Mark the end points on a piece of freezer paper, masonite, what ever you have on had. Find the midpoint of that line and plot the 3rd point up from that line--must be perpendicular to the first lone. Now connect the first two points to the 3rd point. Draw perpendicular bisectors to these two lines. The intersection of these tow lines will be the center point of the arc. If this interests you, PM your phone number to me and I'll try to walk you through it. You will need a fairly large compass or a string, pencil and pin.
 
hey ron take and make a small example and post a couple pics of the steps.. i tried what i thought would work and it came out wrong.. and your right rich the archs i got from the plastic werent symetrical.. but will work for what i had to do that night.. but will try to come up with a better way next time:) thanks folks
 
Larry, I'm late to the party but I'm wondering if you need to calculate the radius when you could do it graphically in a few seconds. I'd still bend a batten as a boat builder would but you won't get a true arc. It'll flatten a bit at the ends.
 

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hey dave i tried to do it in sketchup but couldnt get the radius to show up.. i can make the arch in sketchup but couldnt get the radius.. and is there a way geometrically to make it? with a trammel without math?
 
Larry, open sketchUp and go to Window>Preferences>Extensions. Tick the boxes you find there (except for Ocean Modeling). Then click OK. Close SketchUp and reopen it.Now, draw an arc. You can drag out the distance between endpoints and type the desired distance. Hit Enter and then start dragging in the direction of the bulge. Type the height of the arc and hit Enter. Right click on the arc and choose 'Point at Center' from the context menu. This will create a guide point at the center of the arc. It works on circles and polygons, too. Finally, if you want to draw the radius, get the line tool. For something like this where I just want to know the distance I would measure between the guide point and a vertex on the arc with the yellow Tape Measure tool. I only drew in the line for illustration purposes.So there you have it. No math required, you get a properly scaled drawing so you can visually check to make sure things look right, quick and easy.

There's another way in SketchUp to get the radius if you want. You can open the Entity Info box from the Window menu and select the arc. Then just read the radius.





Remember, if the arc you drew is the final radius subtract half the cutter diameter or saw kerf width for the length of the trammel.
 
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