Formula for segment sizes gluing up a circle?

[Matt Meiser]

I need to make a ring about 15" in diameter outside and 9-10" inside. Is there a formula out there to determine how many segments I should use and what the size of each segment needs to be?

[Bill Wyko]

Pie(3.14) x radius divided by number of segments will give you the length of the outside of each segment. 360 divided by # of segments divided by 2 will give you the angle of the cut. Hope this helps.

[Matt Meiser]

Yes it does. Is there any rule of thumb on how many segments to use?

[Myk Rian]

Here's a simple web page that calculates what you want. Very handy. I saved the page to my computer to use anytime.
http://www.cleavebooks.co.uk/scol/calpolyg.htm

[John Bush]

Hi Matt,
I am in the process of making a 6' dia. coopered tank and 33" dia wooden pipe and I went thru several hours of calculations to figure the right formula of stock width and length and miter angles. I finally figured out that it is easier and more logical to pick a convenient angle that is easy to reproduce and vary the stock sizes to get final dia. you want. I will make bevel cuts on the TS with 10' stock, so the bevel angle needs to be easy to dial in if I need make additional staves. For miter cuts, an accurate miter guage should do the trick. I used the same formula suggested above.

[David Duke]

Matt, I got this chart from Hartville tool http://www.hartvilletool.com/product/11041 it has various formulas including what you're wanting to do. I'm not in the shop now but if you need me to calculate it for you let me know.

[Robert McGowen]

12 sides with a 15 degree miter on each end and an outside length of
4 1/16" will give you what you are looking for. Use a board around 3" wide to get your inside diameter. You will need a board around 42" long to get all of the pieces you need.

[Dave Bureau]

I need to make a ring about 15" in diameter outside and 9-10" inside. Is there a formula out there to determine how many segments I should use and what the size of each segment needs to be?

it depends on how round you want it to be before you do any milling. the more segments you use the rounder it will be. 24 segments. 2" long, cut at 7.5*. use a 3-1/32 wide board. gives you 15 outside and 9 inside.

[Dave Bureau]

Pie(3.14) x radius divided by number of segments will give you the length of the outside of each segment. 360 divided by # of segments divided by 2 will give you the angle of the cut. Hope this helps.

should be 3.14 x diameter. No? Yes?

[Steve Jenkins]

Matt, I generally use no less than 8 segments. Fewer than that and you start getting a lot of endgrain showing on the edge. If I lay it out with 8 segments and the boards need to be too wide I will use more segments. The more you use the narrower they can be.

[Chris Padilla]

should be 3.14 x diameter. No? Yes?

Circumfrence of a circle:

2*pi*r OR
pi*d

...since obviously 2*r = d.

:)

[Ben Davis]

I need to make a ring about 15" in diameter outside and 9-10" inside. Is there a formula out there to determine how many segments I should use and what the size of each segment needs to be?
Matt, There is no easy answer to your questions because we don't know how much material you have to waste. If you had wide stock compared to the circle, you could easily just make a square and cut away. The closer the width of your stock is to the width of your circle/annulus, the more segements you will need.

[Matt Meiser]

Thanks for all the advise. Before seeing Steve's recommendation of 8 segments I headed out to the shop and cut 6 segments. The diameter wasn't critical but I started at 8-13/16" (or maybe it was 7-13/16", can't remember for sure) I had to recut the angles after adjusting my miter gauge because the circle didn't quite close :eek: but it looks good so far. The next step is to actually cut the circle.

The project is a trim ring for the bath fan/light in my daughter's bathroom. The old fan/light doesn't work right and is really ugly but it has a 13-3/4" circular cutout and we can't find a new one like that. So we bought a new fan which has a 13-3/4" outside diameter. The trim ring will be cherry to match the cabinetry and I'm going to attach a 13-3/4" plywood disk to the back which will fit into the existing opening and I'll caulk that to the existing drywall to seal things up. Anyway, its a good place for my first try at a circle because it will be harder to see my mistakes on the ceiling and I rarely use that bathroom so I won't see them ;) And for me its easier than patching the ceiling and repainting.

[Tom Veatch]

W = Width of each segment
L = Length of each segment at the outer edge
A = Miter angle of cut at end of each segment
N = Number of segments
Ro = Outer radius of hoop cut from glued up segments
Ri = Inner radius of hoop cut from glued up segments
----------------
A = 180/N
W = Ro-Ri*cos(A)
L = 2*Ro*tan(A)

[Cliff Rohrabacher]

http://www.educypedia.be/education/calculatorsmathadv.htm