8 Sides, 16 Miters, and Zero Margin for Error: Building an Octagonal Mirror Frame from Walnut
Have you ever looked at a mirror and thought, “This needs more geometry?”
That was pretty much the idea behind my latest project: an octagonal mirror frame made from my favorite wood species, walnut. This wasn’t just about making a cool shape, it was about precision. When you’re working with 8 sides and 16 miters, you’re not just building a frame. You’re solving a woodworking puzzle where every angle counts.
Let’s dive into how I tackled the design, made those angled cuts work like a charm, and managed not to lose my mind in the process.
Why an Octagon?
Square and rectangular frames are great, don’t get me wrong, but an octagon has that extra bit of wow factor. It plays nicely with mirrors, adds visual interest, and is just complicated enough to be a fun challenge.
Plus, let’s face it: if it were easy, where’s the challenge in that!
And as a side note, my wife has been wanting an octagonal mirror for MONTHS now, so I didn’t really have a choice.
Cutting the Frame Segments
I started by cutting down the walnut boards into eight equal-length segments. Since the goal was to create an octagon, each piece would eventually need to be mitered at 22.5° on both ends, but for now, I left them square and focused on getting uniform lengths.
Precision here is everything. A single off-cut can throw off the whole layout.
Cutting the Rabbets
Before jumping into angles and bevels, I headed to the router table to cut the rabbet that would hold the mirror in place. I find it’s much easier to do this step while the pieces are still square. It gives you a stable reference edge, and you can batch the cuts cleanly.
Multiple light passes with the rabbeting bit gave me a clean, even groove around the inside edge of each segment. This saves a lot of trouble later when fitting the mirror and securing the back.
Beveling the Face
Next up: bevels.
I tilted the table saw blade to about 75°, and used my crosscut sled to run the front face of each piece across the blade. This creates a clean sloped edge that adds depth and dimension once the frame is assembled.
The bevel not only looks slick, it adds a professional touch that plays well with the beveled mirror itself. Just be sure to keep track of orientation so you don’t accidentally bevel the wrong side of your freshly rabbeted pieces.
The Math of the Octagon
An octagon has 8 sides, which means each corner needs to be cut at 22.5°. But because each joint is made up of two 22.5° cuts, every miter needs to be precisely that. One degree off, and you’re gonna have gaps, or worse, a frame that spirals instead of closes.
I started by calculating my frame size based on the mirror diameter and backtracked the inner and outer measurements of each segment. A little trigonometry, a few scribbles, and a whole lot of triple-checking.
Cutting the Miters
This is where the real fun began.
I rough cut each piece slightly long, then snuck up on the final angle using a stop block on the miter saw. I highly recommend using a digital angle gauge to set your blade perfectly. I don’t trust my eyeballs for 22.5°, and you shouldn’t either.
Pro tip: Cut one side of each segment first, then set up a stop block for the second side. That way, all your lengths are identical and symmetrical, which is the only way this thing is going to fit together.
Dry Fit Like Your Life Depends On It
Before you reach for the glue, do a full dry fit.
Lay out the pieces in a circle, check every joint, and make sure the angles come together cleanly. I had to adjust a couple of cuts by a hair to close up the gaps. This is one of those “measure twice, cut once, then measure three more times” kinda builds.
Glue-Up Strategy
Octagonal glue-ups are notoriously tricky. You’ve got joints trying to pull apart in every direction.
I used a combination of:
Strap clamp
Cauls to spread pressure evenly
Titebond III and a little prayer
Working fast is key here. I applied glue to all the miters, assembled the frame like a jigsaw puzzle, and cinched it all up in one go.
Let it sit. Walk away. Don’t even peek at it until the glue’s dry.
Finishing Touches
After a light sanding (don’t round over those crisp outer edges), I applied a hardwax oil finish to bring out the grain and warmth of the walnut. The beveled mirror dropped in perfectly, and I secured it with some construction adhesive designed for hanging mirrors.
I added hanging hardware to the back, and boom, done.
Final Thoughts
Would I build another octagonal frame? Absolutely.
Would I build a hexadecagon? Let’s not get ahead of ourselves.
This project served as a poignant reminder of the importance of precision. When every cut has to work in harmony with the others, there’s no room for shortcuts. But when it comes together, it really comes together.
If you want to challenge yourself with angles and miters, give the octagon a shot. And if you want to see how I pulled it off (and maybe pick up a few tips), check out the full build video here.
