American Welding Society Forum

I'm doing a project and the person wanted it done in an octagon. I've laid it out on my cad software and know my outside length. Doing the math and wanting to double check myself. I'm thinking that I need to cut 22.5 degree angles on all the pieces correct?? Looking at it though I guess I could cut off four pieces at 90* and four with 45* angles on both ends and get the same result. Geometry was not my strong point but doing this stuff now it's an interesting challenge. Think I got it figured out but would like to confirm before I go cutting up a bunch of steel. Thanks!

Shawn

Shawn

360 divided by 8 sides....gives you the number of degrees for each included angle. (45°)

Attachment: Octagon.pdf (9k)

If you are mitering the joints at each corner, like the base of a gazebo, then the joint will be 22.5°.

Attachment: o.jpg (0B)

That's basically it JMC, it's 1 1/4" pipe I'm fabbing up something for a lady and she wanted it octagon shape. I've come up with 135* total internal angle off the internet and then figured out the internal angle on the triangle was 67.5*.

I guess it comes down to the 22.5's give you the 45 degree angled triangle? At first I figured 16 cuts at 22.5 and it seemed way to easy to be true!!

I guess it comes down to the 22.5's give you the 45 degree angled triangle? At first I figured 16 cuts at 22.5 and it seemed way to easy to be true!!

If you have enough space, mark a pattern on the floor that's known to be perfectly square so you can line each piece up and tack it in place. Since you're useing round stock it will be hard to keep the angle on one end parallel with the angle on the other. One slight twist and the cut will be off. The pattern will allow you to fine tune the little inconsistencies and fill the gaps with your weld.

Jon

Jon

Clamp a short chunk of small channel to the pipe, the pipe should only touch on the two sides or legs inside of the channel. Then take a magnetic level and stick it on the channel. With the level on the top so that the axis of the work piece is reading level make one 22.5 Deg. cut on the end with your chop saw. DO NOT REMOVE THE CHUNK OF CHANNEL. Flip the piece end for end and with the chunk of channel and level now on the bottom so the axis is reading level make your second cut on the other end. Now you have two cuts that are indexed to each other perfectly. Continue with the remaining seven pieces and fourteen cuts. Post picture when done then cash check and smile!

Great idea

That angle you labled as 22.5° in your pic is actually 67.5°, if you look at the 45° you can see that the angle (that you labled 22.5°) is larger.

John I concur but it is time to head home hear.

The cut on the pipe will be a 22.5 x 8 pieces of pipe for 16 cuts, I double checked on CAD.

360° divided by 16 is 22.5°.

360° divided by 8 is 45°.

360° divided by 8 is 45°.

Cool. the main thing is you gave her what she wanted....LOL

That depends on how you look at the cut being made. If I were to cut that angle on the end of a piece of pipe with a chop saw, the saw will have to be set at 22.5°. In these types of problems you're basically dealing with a bunch of little right triangles. That means that the sum of the angles at each end of the hypotenuse always equals 90°. 22.5° + 67.5° = 90°.

Jon

Jon

Attachment: o001.jpg (0B)

Very true. Using right triangles always simplifies the math.

thats not a 45 degree angle there on the joints he will have to cut.

Thanks for the info on that. I've been explaining it to my wife, she's pretty slick with the math and she does not know where us "welders" are coming up with 22.5 degrees. There must be a formula or something right? What if I want to do a hectagon next time?! LoL!! Guess I should have paid attention in school during the whole geometry part!

In order to end up with the correct angles so that the tube joints fit properly, one must cut 22.5 degrees of scrap tubing off from the presumed 90 degree end of each tube... Leaving one with each end of the tube having a 67.5 degree angle which would then give one a 135 degree included angle once both members of the joint are fitted together... Therefore facilitating the proper joint angle to mate the joints accordingly to meet the main objective of achieving a true octagonal base shape - PLAIN and SIMPLE!!! ;) Btw, this was already mentioned with a different selection or words the more than one explanation but, I thought it was appropriate to simplify the explanation somewhat! ;) ;) ;)

The only question I have is how one could come up with a 22.5 degree angle represented & labeled in the "o.jpg" when it is clearly much larger in size visually without even measuring it??? :)

Respectfully,

Henry

I wondered the same, But I figured they meant 22.5 degree off of the 90, which is 67.5 :)

People get "spoiled" because a 45 degree cut is simple, in that it is 45 regardless of which side You measure from. Any other angle and You have to think a little harder.

I thought about it while driving yesterday and figured it out, well in my brain. I'm not cutting it at 22.5 I'm taking away 22.5 from 90 to give me 67.5 as Henry pointed out. I explained it over the phone to my wife and she came to me when I got in the door last night at 10:30. She had it laid out on graph paper(octagon) and had the pipe outside ring. She said if she extended one of the pipe pieces past the angle(22.5) and made it a 90 then it showed the 22.5 angle. More or less working with a right triangle. Interesting bit of schooling for both of us yesterday. Thanks for all the help and information.......education!!

"The only question I have is how one could come up with a 22.5 degree angle represented & labeled in the "o.jpg" when it is clearly much larger in size visually without even measuring it??? :-)"

Ans: (I think) Manual input/Whoopsie :-)

Ans: (I think) Manual input/Whoopsie :-)

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