# Oval octagon



## stripit (Oct 21, 2012)

Hi everyone. I want to make an oval mirror frame. I need to make an octagon with 2 long sides and 2 short sides first.
Anyone know where I can find a calulator for this type of octagon.

Thanks Joel


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## bondogaposis (Dec 18, 2011)

What do you need a calculator for? It's an octagon, angles are 22.5° and opposite sides must be the same length.


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## ibewjon (Oct 2, 2010)

Draw a rectangle with the long and short dimensions you need, then clip the corners with the 22.5 corners where you like them.


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## johnstoneb (Jun 14, 2012)

Why not just make an octogan first then stretch the straight sides the amount needed and go from there.


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## SMP (Aug 29, 2018)

> What do you need a calculator for? It s an octagon, angles are 22.5° and opposite sides must be the same length.
> 
> - bondogaposis


You are assuming a regular octagon is the only type of octagon that exists.

OP, for the type I think you mean, only the 2 long sides need to be longer, but the octagon is still equiangular, meaning the cuts are all the same angle, as long as the 2 long sides are equivalent in length you should be good.


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## stripit (Oct 21, 2012)

Hi. Thank you all, your all right. What I am looking for is something hingmy frame calulator to tell me the frame size for a given size (8×10, 11×14)photo or in this case a mirror.
Thanks again for all your help.
Joel.


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## LesB (Dec 21, 2008)

Sometimes when I'm trying to work out a size problem you are looking at I use a CAD program or the free version of Sketchup to draw it out. It is easier than making a lot of trial cuts to get the dimensions correct. You could also just draw it out by hand on paper.


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## Lazyman (Aug 8, 2014)

If I understand what you want, you are trying to cut an octagon that is elongated on one axis (North/South for example)? You will definitely have to figure out the combination of angles and lengths. The angles will vary depending upon the length of the sides and they will be different for different width to length ratios. Also, the angles on the adjoining ends will be different. I think that it will also vary depending upon whether the corners are at the longest and narrowest point or if you have a flat side centered at those points. I doubt that you will find an online calculator because there are so many variables so the easiest method is to model it in sketchup or some other CAD program. Here's how I would do it in Sketchup:

Use the polygon tool to draw an octagon. 
Use the stretch tool to make it the right size-8×10 for example
Use guides to Draw parallel lines of the desired frame width
Connect the inside corners to the outside corners to define the 8 pieces
Use the protractor tool and the dimension tool to determine the side lengths and the angles.

Or if you don't have or know how to use a CAD program just tell me what dimensions (size and frame width) you need and I can do it for you and post a diagram.


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## ibewjon (Oct 2, 2010)

Sorry to disagree, but you can stretch or shorten any 2 parallel or opposite sides without changing the angles. It is also possible to shorten 1 side without changing angles, just shorten one side while adding to the two adjacent sides equally. No computer needed, at most a piece of paper.


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## bondogaposis (Dec 18, 2011)

The angles will vary depending upon the length of the sides and they will be different for different width to length ratios.

I have to disagree with that. The angles will be 22.5° all the way around as long as opposite sides are the same length. It makes no difference what those lengths are.


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## Lazyman (Aug 8, 2014)

You're both right, that works as long you only want to make 2 sides longer and 2 sides shorter, for example the horizontal and vertical sides. If you adjust the diagonals as well, all 4 diagonals have to be adjusted to the same length, at least if you are trying to cover a rectangular mirror anyway. My approach was under the assumption that he wants to maximize how much of a rectangular mirror was exposed within the frame which may require you to adjust the size of all 8 sides which will change the angles. It should be possible to find a combination that will yield a near optimal result while maintaining 22.5° angles but without some sort of calculator, it will require some trial and error with a pencil and paper or sketchup. Maybe someone has a better intuition about how to achieve that than I do about how to come up with that optimal size.

Below is an example for a 8×10" mirror. The one on the right was scaled using the method I outlined above. The one on the right was scaled by stretching only the the horizontal and vertical sides to maintain the 22.5 degree angles. The one on the right required a wider frame to cover the mirror. You may be able to start with a different size (I started with 5" sides) to achieve a more optimal result. Maintaining the 22.5 angle will certainly be easier to cut if you can find the optimal length for the diagonals.


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## Phil32 (Aug 31, 2018)

> Hi everyone. I want to make an oval mirror frame. I need to make an octagon with 2 long sides and 2 short sides first.
> Anyone know where I can find a calulator for this type of octagon.
> 
> Thanks Joel
> ...


First, it should be noted that an octogon has eight sides, not four.


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## LeeRoyMan (Feb 23, 2019)

First you have to draw the oval, then you have to place the octagon around it, stretching as needed.
Make the inside come as close to the edges as you can, 
then adjust the width of the octagon to cover the outside edges
To get a better fit you may have to use more than 8 sides, try 12 sides.
(Sample drawing is an 8" x 10" x 1 1/2" oval)


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## stripit (Oct 21, 2012)

Thanks again to all who took the time to write.

Yes do know an octagon has 8 sides. Thank you Phil.
LeeRoy, I think you have helped me see the light, no short cuts just do it. I have a mirror I will use as a pattern (I wass never good witha string and 3 nails). Thank you.

I'm having trouble getting a nice tight fit with the 22.5 deg. angles. I am using a Incra miter gague, I have checked it for square with 2 squares,then I cut piece and it came out perfect. When I shift it 22.5 and make 16 cuts I end up with a gap of 1/32 or more. Does anyone have any way of meassurng the 22.5 deg (actully 67.5) after i make 1 cut not 16.

Thanks again Joel


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## SMP (Aug 29, 2018)

> I m having trouble getting a nice tight fit with the 22.5 deg. angles. I am using a Incra miter gague, I have checked it for square with 2 squares,then I cut piece and it came out perfect. When I shift it 22.5 and make 16 cuts I end up with a gap of 1/32 or more. Does anyone have any way of meassurng the 22.5 deg (actully 67.5) after i make 1 cut not 16.
> 
> Thanks again Joel
> 
> - stripit


Protractor, even a basic one from big box store

https://www.homedepot.com/p/Empire-Stainless-Steel-Protractor-27912/202035326?MERCH=REC-_-PIPHorizontal2_rr-_-100349259-_-202035326-_-N


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## Unknowncraftsman (Jun 23, 2013)

Your probably experiencing accumulative error. 
I have a digital protractor it's my go to for compound angles.
Not only do the length need to be very very close your angles need to be spot on.
Keep trying you'll get 
Good Luck


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## Lazyman (Aug 8, 2014)

If the octagonal has equal length sides, I would glue up 2 halves and then sand the joints flat with a piece of sand paper on a flat surface before gluing the halves together to correct for the cumulative error. With unequal lengths, that won't work. An alternative is to cut 2 pieces on opposite sides exactly in half at 90°. Then you can glue the pieces into 2 halves and sand the 90° joints flat before gluing them up.


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## LesB (Dec 21, 2008)

I have made a lot of School Clock faces that are octagons and the best way I have found to get that tight fit is to assemble two halves, then dry fit them and using a sander (I use a disk sander) make small adjustments on the ends of both halves until the fit is tight. Be careful that all the pieces lay dead flat during the glue up.

Also you will want to reinforce the joints on the octagon and I have used two methods. One is with splines in the joint or biscuits and the other is with an inlayed butterfly on the back side where it won't be seen.


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## sras (Oct 31, 2009)

> I m having trouble getting a nice tight fit with the 22.5 deg. angles. I am using a Incra miter gague, I have checked it for square with 2 squares,then I cut piece and it came out perfect. When I shift it 22.5 and make 16 cuts I end up with a gap of 1/32 or more. Does anyone have any way of meassurng the 22.5 deg (actully 67.5) after i make 1 cut not 16.
> 
> - stripit


One possibility is that your work piece is shifting slightly during your cut. I have found that clamping the piece to the fence helped a lot.


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## stripit (Oct 21, 2012)

Hi. Just wanted to thank all you who took time to write. I did all you suggested and then some. I even put a sharp blade on the TS, made a differance. Thanks to aj, the digital protractor was abig help. This where I would put in a photo, but it seems I'm lacking in that skill(I have aphotobucket acct.) just can't get to work. Guess I'll have to find a 6 yr. old to help the old man out. lol.

Joel


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## MrRon (Jul 9, 2009)

I know of no machine other than a CNC that will cut at a precise angle. The angle only has to be off by a fraction of a degree (44.99°) to result in a gap in the joint. The eye won't pick up the off angle, but the joint will know it.


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