22.5 deg octagon cuts on table saw doing my head in

**The Spin Doctor** · 8th January 2022, 12:35 AM

Originally Posted by rogerwilco

FFS! I'm having an embarrassing amount of trouble with this.

I'm making a framed mirror with a circular top and bottom. Intend to make this as the normal 8 piece octagon with 22.5 degree cuts. Two sides pieces will be longer, to create the dimensions I want. Then I'll use a router to cut the curves. This way I'll get continuous-ish grain, and it should look pretty good. I had the sense to do test cuts on some scrap to confirm angles and sizing before ripping into the good stuff.

It ain't cutting 22.5... I've done 7 or 8 test runs, and something just is not right.

I'm using an Incra mitre express on table saw. It's dialled right in. I have recalibrated, using an engineer's square to the blade. It's absolutely cutting 90 degrees, as confirmed by a trusted square. I've also checked the blade is parallel to the mitre slots (this shouldn't matter with mitre express, but I'm getting desperate). Dial indicator measures only 2 thousands of an inch variation across the blade. I'm tending to trust the Incra mitre gauge. It's indexed for goodness sake.

Anyway, check the pic. Attachment 505097
That shows the dry fit, and you can see on the right hand side, I've moved the piece over to see if it lines up with the dog holes in the MFT table. It's out, but about 2 mil. I haven't got a protractor, but if I do the maths, I know the angles are something like 22.2. Seems small, but I know this will result in gappy glue lines, and I think I should be able to do better than that.

So, question:
Is this normal, or should I really be expecting to hit perfect 22.5 cuts?
Maybe I should try the mitre gauge without the express... could be some error being introduced there.
Should I try something else? I've got nice wide boards, so at this point I'm considering edge laminated a couple boards to make a panel, then cut the arc from the panel. It would mean there would be vertical straight grain through the whole piece. Dunno, maybe that'd look naff...

This was meant to be quick project!

It's woodwork. Which means nothing will EVER be perfect. When doing such COMPOUNDING ERROR joinery I usually start with a mock up so I can fine tune the angles, as drop saws or mitre gauges are rarely 100% accurate. The last joint rarely ever lines up so you need to tweek it.

**rogerwilco** · 9th January 2022, 09:48 AM

For anyone following along, here's the process that's working well for me now. Mostly based on Auscab's ideas.

1:1 scale template, taped to a piece of masonite. I'm hot gluing each piece to the template before moving on to the next frame piece.
IMG_0596.jpeg

Using a cross cut sled for rough cutting close to correct angle. After 10x checking the sled and saw, i eventually worked out where the main source of error was coming from... The work pieces MUST be clamped to the fence. Hand holding the piece to the fence felt fine, but it was producing poorer quality cuts. Clamps help a lot.
IMG_0597.jpeg

Shooting the cuts to perfection. The shooting board has a temporary fence brad nailed to the bed. It's at 22.5 degrees. I can make small adjustments using shims. This method provides a very solid registration for the piece, with no slipping and sliding.

IMG_0591.jpeg

Routing mortices for loose tenons using a template routing jig. This frame will be hung on a wall and will hold a mirror inside. And it's made of White Mahogany, which is dense and heavy. I do not trust glue only on the end grain. I don't have a domino joiner. With careful placement, I could use small biscuits and make them invisible. Instead, I'm cutting my own shallow mortices with a 3/8 inch spiral up-cut bit using a guide bushing and template cutting jig.

IMG_0592.jpeg

Pretty confident this is all going to work out.
Will report back when it's all together.

**rogerwilco** · 15th January 2022, 01:20 PM

And, result! I give this a spray finish later and call it done.
That was far more work than anticipated, but at least I now know how to do it.

thanks all for the advice and tips!

031ACB62-C9A4-4D73-9F7B-55C452846470.jpg

**riverbuilder** · 15th January 2022, 03:41 PM

Originally Posted by rogerwilco

For anyone following along, here's the process that's working well for me now. Mostly based on Auscab's ideas.

1:1 scale template, taped to a piece of masonite. I'm hot gluing each piece to the template before moving on to the next frame piece.
IMG_0596.jpeg

Using a cross cut sled for rough cutting close to correct angle. After 10x checking the sled and saw, i eventually worked out where the main source of error was coming from... The work pieces MUST be clamped to the fence. Hand holding the piece to the fence felt fine, but it was producing poorer quality cuts. Clamps help a lot.
IMG_0597.jpeg

Shooting the cuts to perfection. The shooting board has a temporary fence brad nailed to the bed. It's at 22.5 degrees. I can make small adjustments using shims. This method provides a very solid registration for the piece, with no slipping and sliding.

IMG_0591.jpeg

Routing mortices for loose tenons using a template routing jig. This frame will be hung on a wall and will hold a mirror inside. And it's made of White Mahogany, which is dense and heavy. I do not trust glue only on the end grain. I don't have a domino joiner. With careful placement, I could use small biscuits and make them invisible. Instead, I'm cutting my own shallow mortices with a 3/8 inch spiral up-cut bit using a guide bushing and template cutting jig.

IMG_0592.jpeg

Pretty confident this is all going to work out.
Will report back when it's all together.

Shooting the ends to perfection? Who’s idea was that?

good job man, looks nice.

rrich · 15th January 2022, 04:12 PM

I'm sorry but I didn't explain completely.

You are trying to cut 22.5° FROM 90°. This means the angle is COMPLEMENTRARY. Or 90° - 22.5° = 67.5°. (Finally in my life of 79 years, I have found a use for something I was taught in High School Geometry.)

With the sides of your octagon flat on the table, set your saw at 67.5°. Then cut the ends. Do not change the blade setting between cutting the ends. When you assemble the joints will be surprisingly close.

**rogerwilco** · 23rd January 2022, 01:20 PM

Project complete.
Here's a couple pics of it. Finished with spray coats of Mirotone PC sealer and top coat. Mirror installed, and hung on the wall.

Very happy!
D109AFE0-9815-4BDA-A331-5AA17AB05F42.jpeg
82C62C7A-1113-449E-A3AF-63F37FAB25CC.jpeg

Thread: 22.5 deg octagon cuts on table saw doing my head in

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