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Forum topic by stripit posted 06-20-2019 08:15 PM 594 views 0 times favorited 20 replies Add to Favorites Watch
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stripit

55 posts in 2604 days


06-20-2019 08:15 PM

Topic tags/keywords: oval octagon frame question

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

-- Joel, People ask what I make. I tell them I make sawdust, and now and then a nice box or frame,or clock, or lamp pops out.


20 replies so far

View bondogaposis's profile

bondogaposis

5570 posts in 2911 days


#1 posted 06-20-2019 09:31 PM

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

-- Bondo Gaposis

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ibewjon

1063 posts in 3353 days


#2 posted 06-20-2019 09:51 PM

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

View johnstoneb's profile

johnstoneb

3131 posts in 2733 days


#3 posted 06-20-2019 09:52 PM

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

-- Bruce, Boise, ID

View SMP's profile

SMP

1447 posts in 465 days


#4 posted 06-20-2019 09:59 PM



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

55 posts in 2604 days


#5 posted 06-20-2019 11:00 PM

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.

-- Joel, People ask what I make. I tell them I make sawdust, and now and then a nice box or frame,or clock, or lamp pops out.

View LesB's profile

LesB

2232 posts in 4003 days


#6 posted 06-21-2019 12:08 AM

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.

-- Les B, Oregon

View Lazyman's profile

Lazyman

4173 posts in 1947 days


#7 posted 06-21-2019 03:49 AM

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:
  1. Use the polygon tool to draw an octagon.
  2. Use the stretch tool to make it the right size—8×10 for example
  3. Use guides to Draw parallel lines of the desired frame width
  4. Connect the inside corners to the outside corners to define the 8 pieces
  5. 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.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View ibewjon's profile

ibewjon

1063 posts in 3353 days


#8 posted 06-21-2019 11:26 AM

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

5570 posts in 2911 days


#9 posted 06-21-2019 01:39 PM

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.

-- Bondo Gaposis

View Lazyman's profile

Lazyman

4173 posts in 1947 days


#10 posted 06-21-2019 02:40 PM

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.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Phil32's profile

Phil32

721 posts in 463 days


#11 posted 06-21-2019 03:21 PM



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

- stripit

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

-- Phil Allin - There are mountain climbers and people who talk about climbing mountains. The climbers have "selfies" at the summit! Likewise with woodworkers.

View LeeRoyMan's profile

LeeRoyMan

414 posts in 287 days


#12 posted 06-21-2019 03:49 PM

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)

View stripit's profile

stripit

55 posts in 2604 days


#13 posted 06-21-2019 10:58 PM

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

-- Joel, People ask what I make. I tell them I make sawdust, and now and then a nice box or frame,or clock, or lamp pops out.

View SMP's profile

SMP

1447 posts in 465 days


#14 posted 06-21-2019 11:33 PM


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

View Aj2's profile

Aj2

2562 posts in 2358 days


#15 posted 06-22-2019 03:14 AM

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

-- Aj

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