# COMMUNITY FORUM

gusbali

09:18PM | 04/18/04
Member Since: 04/18/04
How do you draw a perfect arc? I need 9.5 ft. long and 17 in. high at the center. Since plywood (or anything else) is only 8ft. length, do I need to cut it in two halfs?

Thanks.

treebeard

11:00AM | 04/20/04
Member Since: 01/14/03
If you're going to use plywood, I'd butt 2 sheets together, and from the corner where they butt, measure out 4'-9" on each sheet (one helf of your 9'-6" dimension). From each of those points, using a long string and a pencil, swing 2 arcs of 8'-8 1/16". The point where the 2 arcs intersect is the centerpoint of your arc. Again, using the pencil and string, you can now swing an arc of that very same 8'-8 1/16" on the plywood. This new arc should connect the 2 original points you marked at 4'-9", and should cross the butt joint of the 2 sheets at 1'-5" (17") up from the corner where the 2 sheets butt.

Cutting the arc can be accomplished with a jig saw.

Mark Hammond

05:42PM | 04/20/04
Member Since: 05/09/01
Hi Gus,

You can also use a tool called trammel points that attach to a long strip of wood metal or plastic, or even a dowel. They can be adjusted to whatever radiuus you need.....MJH

gusbali

08:50PM | 04/20/04
Member Since: 04/18/04
Thank you TREEBEARD...!!!

It sure sounds complicated, but I will try to follow your wonderful instructions step-by-step.

...and I'll let you know.

Thanks again.

gusbali

08:56PM | 04/20/04
Member Since: 04/18/04
Hi Mark,

Thanks for the suggestion. I've never heard of this tool; "trammel point", but I will check it out. But first i have to try TREEBEARD's suggestion (seems cheaper)

Thank you,

Altereagle

11:42PM | 04/20/04
Member Since: 12/27/02
Nope you have an ellipse...

The radius for the 9'6 points is 6'8 5/8 which leaves you 23 5/8" at the center of the arc.

A sq + B sq = C sq the hypontenus theory.

4'9 being A & B leaving 6'8 5/8 for C

6'8 5/8 minus 4'9 at the center leaves 23 5/8" not 17" I think anyway... I could be wrong?

Try putting 2 nails 9'6 apart and let a string droop down to 17" that may be your best way to get your answer.

By the way the tramel Mark speaks of can be made with a 1x4 with a nail for the compass point and another nail to mark out the arc. I do that for arc top windows & even laying out high rise entrance circles... works great for the trim to put two nail on the making end. Gives you both diameters.

http://www.altereagle.com/

Alter Eagle Construction & Design

Altereagle

11:53PM | 04/20/04
Member Since: 12/27/02
Found a site for you:

http://www.geocities.com/Heartland/8108/ellipse.html

that's a good way to draw it exact.

http://www.altereagle.com/

Alter Eagle Construction & Design

treebeard

03:43AM | 04/21/04
Member Since: 01/14/03
The OP asked about an arc...not an ellipse. Using the 9'-6" as the chord length, and the 17" as the segment height, the radius is 8'-8 1/16". An ellipse is quite a different animal.

whit.millwork

07:17PM | 04/21/04
Member Since: 04/10/03
I'm with treebeard. (length squared) plus (chord height squared) divided by four divided by (chord height times two) = 104.058. take a 1"X4"X10'board and drill a hole the size of a pencil.measure down 104.058" and put a screw. butt your plywood together and measure up 17". thats where your pencil goes. follow your plywood center line down and screw your homemade trammel jig down so that it can swing around. draw your line. thats it.

Altereagle

11:25AM | 04/22/04
Member Since: 12/27/02
AH.. I see where I went wrong. I didn't drop the arc down to 17" properly I should have used 2 x A x R = A squared + B squared

2x17xR=17sqrd + 57sqrd

34xR=3538

r=3538 div. by 34

r=8' 8 1/16

I got it now...

Nice going Treebeard!

http://www.altereagle.com/

Alter Eagle Construction & Design

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