How do I make an arc that is part of a HUGE circle?

[LDO]

How the heck do I do that? I need to make what is basically a styrene former, like in an r/c plane wing.

[Harry P.]

If you have a French curve you can just fake it. Otherwise, geometrically speaking, there's no way. The radius of the circle (or the arc–the part of the circle you want) is what it is.

[Longbox55]

If you know what the raduis is of a full circle, you can do it with a pin/nail, a length of string, and marking/cutting devise of your choice. Simply tie the string to the pin/nail, firmly plant it at the centerpoint, measure out the radius and attach the marking/cutting tool, pull the slack out of the string, and use that as your compass. It's an old trick my Dad, a retired sheetmetal worker, taught me for doing layouts. Works everytime.

Edited June 1, 2011 by Longbox55

[MicroNitro]

I have used a string tied around a pencil on one end and a nail on the other then tack the nail the distance you need from were you need the arc. Or a batten also is good for large arcs.

[diymirage]

if you know the diameter of the circle heres what you do

put a nail in something solid, a piece of wood, the vice, you name it

then tie a peice of string to that nail

at a distance that is exactly half the diameter you tie a pen

put your styrene under the pen and make a circle keeping the string tight

there you have it...make shift protractor

[Longbox55]

Is it just me, or is there an echo in here?

[diymirage]

darned bill, you beat me to it

[Harry P.]

Yeah, that's fine if you happen to know the radius! But judging by his post I assumed he doesn't... so you fake it with a French curve.

[LDO]

I don't know the diameter of the circle, but I can use trial and error to figure it out. I'll go outside tomorrow with a pencil, nail, and string.

Thanks.

[Kit Basher]

If you know any of the dimensions of your arc, This may be helpful. If not, do your thing with that string.

Edited June 1, 2011 by basher

[diymirage]

If you know any of the dimensions of your arc, This may be helpful. If not, do your thing with that string.

from the looks of it those are the blueprints to a baseball field

[MikeMc]

measure the width at the base......

Then the height........

then plug in http://www.handymath.com/cgi-bin/arc18.cgi?submit=Entry

Edited June 1, 2011 by Stasch

[Kit Basher]

from the looks of it those are the blueprints to a baseball field

Type in the right numbers and it would be!

[Jim Gibbons]

Geez, it's been so long since I had to do this. I was a lighting applications engineer, and we used to have curved track lighting, with huge radii. Unfortunately, all my old geometry books are in storage, and I forget the method we used to QC it. You could make a jig something like this quick sketch:

However, Harry's idea is probably the way to go.

Edit: the internet is wonderful. He's the math if you what to calculate it:

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

Edit again: you can get flexible french curves at a drafting supply store. They're a soft plastic with a "lead" core that allows you to form a curve to a needed shape. I used to have one of my own when I had to do road or contour map drawings when I was an Army Enginner.

Edited June 1, 2011 by Jim Gibbons

[Mike Kucaba]

It sounds (or reads) to me that you want to make a bulkhead? You could use one of those contour devices. They have fine diameter wires and you push this against the object where you want a shape.One side gives a positive and one gives a negative.

[diymirage]

It sounds (or reads) to me that you want to make a bulkhead? You could use one of those contour devices. They have fine diameter wires and you push this against the object where you want a shape.One side gives a positive and one gives a negative.

like those thingamabobbers with the little pins you can put your face in and somehow your lips always look huge