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

I’ve not done an octagon so take my thoughts with a grain of salt, but I would think that any error in the angle is doubled at every join so that they have to be very precise as would the length of the pieces being exact. One thought would be to dry assemble seven pieces, put a straight edge across the outside points and draw/measure the last angles to fit.

In your octagon, the 'final' joint will always be out by 16x the error. Or, to look at it another way, when you try to fit the last side, both ends will be out by 8x the error.

When I've done similar, I glue it together in two halves then mount them to a scrap piece of ply (or similar) with double-sided tape, leaving a small gap between each of the final two joints. The gap is sized to be slightly smaller than my saw kerf.

I then run that assembly through the saw, so both ends of each joint are trimmed to match each other.

This is a fiddly process which, in your case, would be exacerbated by the two longer sides as you couldn't do just one cut to trim both joints in the same pass... but IMHO it's worth the effort for gapless joints.

Have you used the vernier on the mitre gauge to make fine adjustments? Also, I have found that when cutting angles other than 90 degrees that there is a tendency for the blade to drag the timber into the cut if the timber isn't clamped to the fence.
Darn frustrating, although the upside is a box of rather nice firewood.

Alan...

Incra vernier.png

Originally Posted by Uncle Al

Have you used the vernier on the mitre gauge to make fine adjustments? Also, I have found that when cutting angles other than 90 degrees that there is a tendency for the blade to drag the timber into the cut if the timber isn't clamped to the fence.
Darn frustrating, although the upside is a box of rather nice firewood.

Alan...

Incra vernier.png

Thanks Alan. Unfortunately, my v120 gauge doesn't have the vernier.
Good point about the blade drag, I've felt that, but have been careful to get clean and straight cuts.
The problem seems to be the compounding error.

Originally Posted by Skew ChiDAMN!!

In your octagon, the 'final' joint will always be out by 16x the error. Or, to look at it another way, when you try to fit the last side, both ends will be out by 8x the error.

When I've done similar, I glue it together in two halves then mount them to a scrap piece of ply (or similar) with double-sided tape, leaving a small gap between each of the final two joints. The gap is sized to be slightly smaller than my saw kerf.

I then run that assembly through the saw, so both ends of each joint are trimmed to match each other.

This is a fiddly process which, in your case, would be exacerbated by the two longer sides as you couldn't do just one cut to trim both joints in the same pass... but IMHO it's worth the effort for gapless joints.

That's a clever solution. I think I'll have to do similar.
I don't think I'll get away with doing 7 segments, then measuring the 8th to fit. If I do that, there's a pretty good chance the long side pieces will not be parallel. This might not be noticeable, but it's a big gamble.

When you do the kerf-trim method, how do you align the work piece for the cut? Do you just scribe a centre line on the scrap piece that is parallel to its edge, then stick the octagon halves on each side of the line with a small gap before pushing it through the saw?

Your right, my elongated octagon will require 2 separate cuts. I could probably still achieve that with one setup though. 2 scribe lines... Flip it over and set the blade height so that it cuts the work piece but only scores scrap backer. Then turn it and align for the second cut and go again.

Fiddly indeed, but it should work.

Thanks for a good idea!

Hi Roger,

It can be a little difficult cutting mitres on a circular saw. I regularly make 16 segment pieces and glue together straight from the saw, so it is possible. If you intend doing more segmenting, I highly recommend you have a look at Gerry Bennetts' web site, "Segeasy.com". If you are not going to de much more segmenting, stay with what you are doing.
What I suggest is cut the eight segments as usual. Assemble using a band clamp, or a large rubber ring but only apply glue four segments for each half together. Do the two halves but when you assemble the two halves do not apply glue to the joint. Slip a match vertically into the joint and apply the pressure. When the assembly is dry, separate the two halves and sand the joint until there is a good fit. Now apply glue and apply pressure. You will not get a perfect ring, but as you are going to make a mirror surround , nobody will notice.

Hope this is of some help.

Jim

Make an angled shooting board and do the final trim with a plane.

Originally Posted by riverbuilder

Make an angled shooting board and do the final trim with a plane.

I like that idea a lot better. I finally got my hands on a quality low angle jack plane and a spare plane iron to be setup for shooting. I can set one up with a fence for 22.5 and trim each piece off the saw.

This will be good practice for how to use a shooting board to perfectly dimension each part. Presumably there’s some thought involved to make sure that all parts are the exact same length to ensure a gap free glue up. How do people go about that? Cut slightly over size and use shooting board to pare back to a scribed line?

It's an easy fix on small projects. Fine line draw the job in full size on paper, mark in all angles required. Number all segments and mark orientation. Now you have a full size template for each piece required. Dimensions are correct and angles are correct.
For a large project you are somewhat handicapped for dimensional correctness because of the need to scale down the drawing but the angles are all still correct no matter the scale used.
Hope this helps.

It will never fit unless the angle is perfect and every piece is EXACTLY the same length, cut one, trim it, and then mark it to be the “tester”, and then use it to trim the other seven segments to be exactly the same as it is. Don’t bugger around trying to measure or whatever, use the actual piece you’re trying to replicate x7.
As rustynail has said, setting stuff like this out full-size on a piece of mdf board or whatever is the only way to go, then place your bits directly over the story set out, can’t go wrong.
Yes cut them on the saw to within half a mm then final fit with shooting board and plane. Sharp blades are essential.

I'm always wary of using mitres on the wide dimension of a board, because even if the angles are cut exactly correctly, they will change as soon as the wood expands or contracts across the grain with humidity change. With 22.5deg. for an octagon, the change will be less than for 45 deg. but it will still be there, and the change at each end will be multiplied by 16. If you're going to do it, you need to use biscuits or dowels to stop it opening up, and do the glue-up straight away, before the humidity changes.
As with all cutting, the machines need to be set up accurately for angles, and you should use a stop to get the lengths correct. Cut the angle on one end of each piece, set the stop and cut the angle on the other end. Don't forget to move the stop when you swap between the short and long pieces.

What tablesaw you got @ Rogerwilco?

Hi Rogerwilco; I used the same method to make some round stool tops with padded seats.

Stool Photo.jpg

I identified about five potential sources of error:

1. Mitre not precisely 22.5 degrees,
2. Mitre not perfectly straight - saw blade wanders a bit - light touch with shooting plane fixes,
3. Pieces not perfectly aligned in glue up - error can be horizontal, vertical or both,
4. Pieces not precisely the same length.

Thus I devised a method within my skill-set to cut as accurately as possible and to recover from any inaccuracies. Note: errors may be cumulative.

My technigue was as follows:

1. Jointed and thicknessed the timber,
2. Set saw mitre to 22.5 degrees and tested accuracy,
3. Put stop block on fence so that all pieces would be same length,
4. Cut needed pieces plus a couple of spares,
5. While saw mitre still set, cut a pile of 22.5 degree cauls from 2x1,
6. Lightly plane all mitres on shooting board,
7. With 2 pieces only, cut domino mortice into one mitre each side - this ensures vertical alignment,
8. With Hot glue gun, attach cauls on both sides of each mitre,
9. Apply glue, insert domino, align as carefully as possible and tighten clamps on cauls,
10. Repeat for all pairs of pieces,
11. Rap sharply to remove cauls,
12. Use same technique to glue two-pairs to two-pairs,
13. You should now have two half octagons - which probably have an alignment air gap,
14. Hot glue "half octagons" to sheets of MDF,
15. Very light pass on table saw to straighten both ends of "half octagons",
16. Repeat method; use cauls, dominos, to glue two halves together to complete the octagon.

Stool Clamping.jpg

Originally Posted by EagerBeaver71

What tablesaw you got @ Rogerwilco?

Taiwanese made 12 inch MBS300 variant. Hafco ST-12D.
I’m pretty with it. I find it sturdy, accurate, powerful, flat top, square fence, etc.