How do you make an octagonal shape

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Forum topic by TJ65 posted 01-19-2011 04:09 AM 8423 views 0 times favorited 24 replies Add to Favorites Watch
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1358 posts in 2467 days

01-19-2011 04:09 AM

Topic tags/keywords: question joining victorian clock

Ok, I am planning to do another project that requires a little more from me and once again have not done anything like it before.
Anyway as my husband is seriously getting into trains he thought it a good idea since I had finished with all the xmas presents that I should make him a clock. But he wanted it to resemble a station masters clock or regulator clock.
Now my problem is- how do I go about making the octagonal sides for the clock face. Is there a correct angle to cut them at? and how do I go about joining it all together once cut, is glue enough or is there something better that I should try/use

Similar to these-

-- Theresa,

24 replies so far

View Pete Jansen's profile

Pete Jansen

250 posts in 2338 days

#1 posted 01-19-2011 04:39 AM

You’re way too nice to him. That’s a lot of work, but a great learning experience. As Bentlyj said, it’s 22.5 degrees for the cuts and I would try to biscuit or dowel joint the parts to add strength. The frame around the clock face really isn’t going to have any stress on it as the clock is going to hang from the back panel.

Good luck and have fun with it. I’d look for a pattern to work from as well.

-- Lovin' sawdust in beautiful Fort Collins, Colorado

View bigike's profile


4048 posts in 2705 days

#2 posted 01-19-2011 04:45 AM

wow i got some info from this question posted, thanks bentlyj. I never thought of breaking it down like that.
I would have just said 22.5. ;)

-- Ike, Big Daddies Woodshop,

View Vintagetoni's profile


58 posts in 2110 days

#3 posted 01-19-2011 05:22 AM

At first I thought splines might be nice in the joints….but I was thinking the case itself was the octagon shape. It appears its just a face & it already has a profile to it….not such a good spline application. Sorry….can’t delete this comment.

-- toni --- SW WI...working on shop setup....wish I could say diligently. "Time is a healer, a friend & a maker of dreams."

View richgreer's profile


4541 posts in 2491 days

#4 posted 01-19-2011 05:57 AM

To get the angles right consider the following. Make 2 squares (hardboard, plywood, cardboard, whatever) of identical size. The distance from corner to corner across the middle should the same as the distance from opposite corners of the desired octagon. Put a nail or screw through the exact center of the 2 pieces. Rotate one board 45 degrees. The resulting position of the corners will be the corners of the octagon.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View ahohnstein's profile


2 posts in 2108 days

#5 posted 01-19-2011 06:02 AM

Tommymac had an episode on an octagonal lazy Susan that had a jig basically as Rich mentioned. Worth a watch as it may give some ideas.

View David175's profile


101 posts in 2106 days

#6 posted 01-19-2011 06:09 AM

Google this….. Segment Edge Length Estimation Table…... and it will give you everything you need

it will give you the angle and the length for any 360

-- Dave..Pensacola Fl.........In GOD we trust

View TJ65's profile


1358 posts in 2467 days

#7 posted 01-19-2011 07:47 AM

Wow thanks heaps guys.
Even I understand that lot! (My mathematical skills have a lot to be desired)
But that idea about working with the squares are great.

-- Theresa,

View Wes Giesbrecht's profile

Wes Giesbrecht

155 posts in 2228 days

#8 posted 01-19-2011 08:12 AM

My first question would be, what are you using to make the cuts?
Depending on what material you’re cutting and the quality and sharpness of the blade etc.,
I’ve often found it a challenge to do without any blade deflection, on a miter saw.

My choice would be to build a sled, to run in the table saw grooves
and create a 22.5ยบ cutting angle on both right and lefthand sides of the blade,
so that all cuts can be made thru the face side of the material.

Is that too vague? I’m sure we could clarify if you’re interested in going to the trouble.
Yes it’s a bit of a hassle, but if you go to the bother, you’ll have a jig that will always cut perfect octagons
for the rest of you’re woodworking career without having to give it a second thought.

-- Wes Giesbrecht

View Dan Lyke's profile

Dan Lyke

1509 posts in 3542 days

#9 posted 01-19-2011 08:16 AM

When I did my 12 sided plate frame, I went a step further: I cut the pieces out, glued them together to make two halves, and then clamped them solidly so they would hold together and ran a saw through the middle of the joint to make sure that the joint exactly matched up. (I use a track saw rather than a table saw, although with careful clamping you could do this on a cross-cut sled.)

This may actually be harder to do with the 8 sided shape rather than the 12 sided one, because that cut will be diagonal and take off just that much more off of one of the diagonals. But I figured I’d mention the technique.

-- Dan Lyke, Petaluma California,

View Wes Giesbrecht's profile

Wes Giesbrecht

155 posts in 2228 days

#10 posted 01-19-2011 08:49 AM

Cutting through to get a perfect joint has lots of good applications in woodwork.
Especially hand when the angle of the cut doesn’t need to be an exact division.

-- Wes Giesbrecht

View peteg's profile


3804 posts in 2240 days

#11 posted 01-19-2011 09:10 AM

Hey Theresa, my head is spinning also, I just want to see the end result, &, a big thank you from your man, youv’e got to have some brownie points with this one plus a free ticket on the next train. :) good luck from Pete

-- Pete G: If you always do what you always did you'll always get what you always got

View TJ65's profile


1358 posts in 2467 days

#12 posted 01-19-2011 10:30 AM

My choice at this stage for cutting is to use the bandsaw (as I dont have a tablesaw) . As I found with the mirror frames the blade does deflect a bit so I refined the angle on the sander and really made sure that the angle was perfect using the guide.
Can I cut through the joint without having a tablesaw? (you mean like adding a spline??)

-- Theresa,

View Wes Giesbrecht's profile

Wes Giesbrecht

155 posts in 2228 days

#13 posted 01-19-2011 10:34 AM

By cutting through the joint we mean making both sides of the cut in one pass.
Joint the pieces temporarily, cut straight through both of them at once so there’s an exact match.

Band saw and sander will work it’ll just be a little trickier. (I’d cut a few extra pieces just in case :-)

-- Wes Giesbrecht

View bobsmyuncle's profile


110 posts in 2108 days

#14 posted 01-19-2011 03:39 PM

The other question is “how long to make the sides?”

Here is a compass and straightedge method to determine an octagon:

- Draw a square the size that will enclose the octagon.
- Draw diagonal lines from the corners. The point of intersection is the center of the square.
- Set a compass at the distance from one corner to the center.
- Scribe the distance from each corner to both adjacent sides
- Those points of intersection will be the corners of the octagon.
- Measure the distance along any side.

There are more complicated ways if you don’t start with a square

View Steven Davis's profile

Steven Davis

118 posts in 2331 days

#15 posted 01-19-2011 07:33 PM

The real question is how do you want to build it? As noted above, the internal arcs are each 360/8 = 45. The other 2 angles for the triangle are the same, so they are each 45+2*x = 180 or x = 67.5.

If you were cutting an octagonal shape with a bandsaw or scroll saw, you would probably draw a circle of the appropriate size and use high school geometry to divide it into 8ths and cut carefully as noted above.

Once you’ve drawn the circle with the inscribed octagon, use a straight edge to check that all of the sides are the same length as you walk around the circle.

If you are using a table saw or miter saw to cut octagonal segments, measure carefully and cut the opposite segments “stacked” so that they are identical, if possible, and then adjust:

If you count the segments around the circle 1 through 8:

First make:
Segments 1, 5
and then
Segments 3, 7

Fill in to fit Segments 2,6 and Segments 4,8

The problem is definitely different if you are using molding, cutting a segment, or cutting out an entire octagon.

After all, the most important thing is a tight fit, not a perfect octagon.

-- Steven Davis - see me at

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