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

Originally Posted by stevenjd

360 degrees in circle. Then an octagon which 360/6 equals 60 then your doing a half angle which 60/2 equals 30 degrees.

An octagon is 8, a hexagon is six, so an octagon is 360/8 = 45 div by 2 = 22.5deg

Originally Posted by stevenjd

360 degrees in circle. Then an octagon which 360/6 equals 60 then your doing a half angle which 60/2 equals 30 degrees.

Sorry, Steve, but an octagon has eight sides - like octopus has eight arms.

360 / 8 = 45
45 / 2 = 22.5

Rogerwilco actually wants to make a circle - more pieces means shape is closer to circle; less wastage and better grain alignment.

Yessem I acted before thinking..

Thanks everyone. I understand the geometry of creating a circle from 8 segments with 16 x 22.5 degree angles to complete the 360 degrees of a circle. The problem is execution.
I'm using stop blocks on the table saw, and my pieces are exactly the same lengths. I believe my saw and mitre gauge are correct and accurate. However, there's obviously some small error somewhere, and it's getting multiplied because of the repeated cuts. It could be because I'm not clamping the pieces (finger pressure only), and the angle of the workpiece into the blade is causing a tiny amount of deflection. Or, maybe there's some play somewhere in the sled. Or, one of the other things that Graeme mentioned.

None the less, I think I'll go the the shooting board as a way to resolve this.

So... what's a good and fast way to get perfect angles on a shooting board?
My first though is to set a bevel gauge using a precision protractor, then clamp a fence onto the shooting board referencing the bevel gauge.

Who's got a better way?!

If you think your going to set precise angles on a shooting board and end up with precise exact angles on your stock your going to end up with the same problem .
It doesn’t work that way , never has .

Drawing a plan of the frame on a board to follow and fitting each piece as you go around is the way to do it . Free hand adjustment of the stock to be planed on the SB to adjust angles after they have been roughed out with saw cuts is the way . It’s easy .
clamp down your first cut and planed piece then bring the second one up and hold joints together and see how much off the lines on the plan the second one is . Shoot join on a piece too long . Get the angle right . When it’s good mark and cut the second end for saw cut and then shoot that till it’s right . Clamp down or glue it to first piece and then go onto next piece .
You can just have a 90 degree fence on the SB and pivot the piece to be shot around to get it close or exact . Test and move it around to what you think is best and try again if you need to till it’s perfect .
Glue together as you go would be best .
Your safety net is leaving the pieces to long , getting the first angled join matching so the piece follows the plan . Shooting it’s end and fitting/ glueing . If the pieces are fitted down to a plan drawn on paper that is well taped to a board then the whole thing can be lifted off the board later and the paper cleaned off .

The easy way is to make a seg easy sled if you know someone with a 3d printer they can print a wedge with what ever angle you need some library's print them for a small charge.
I do a lot of segment work and have various printed wedges from a friend have cut 60 segments straight off saw and glue up full ring with tight joints. U-tube has videos how to make
a seg sled. Sled does not take long to make.

Originally Posted by auscab

Drawing a plan of the frame on a board to follow and fitting each piece as you go around is the way to do it . Free hand adjustment of the stock to be planed on the SB to adjust angles after they have been roughed out with saw cuts is the way . It’s easy .

Alright. You're about the 3rd or 4th person who's given similar advice... so, I'm gonna take it and try that first.
It finally makes sense to me now thanks to the way you've described it. It's not any individual angle that matters, instead it's how the mating pieces follow the outline of the template. The SB helps you cut clean and square to the face, and you manually pivot the edge to be cut according to the adjustment needed, and take fine shavings 'til the joint comes together as desired.

I'm somewhat puzzled about how to make the template. It's too big to print the entire thing to scale (450mm diameter). I guess I could print parts separately and stick it all together. That's how it's done with sewing patterns. Or, I could just get the drafting gear out and draw it directly onto a piece of MDF with a fine marker.

Printing is probably easier!

Thanks for the great explanation.

Originally Posted by Peter White

The easy way is to make a seg easy sled if you know someone with a 3d printer they can print a wedge with what ever angle you need some library's print them for a small charge.
I do a lot of segment work and have various printed wedges from a friend have cut 60 segments straight off saw and glue up full ring with tight joints. U-tube has videos how to make
a seg sled. Sled does not take long to make.

Thanks Peter.
I'm gonna check this out too. I don't anticipate doing lots of segmented rings, this is probably a bit of a one off. I'm curious none the less!

Originally Posted by rogerwilco

I'm somewhat puzzled about how to make the template. It's too big to print the entire thing to scale (450mm diameter).

Drawing it out is the only way I’d be doing it . 450 Diameter is nothing . I draw 3m x 1.2 M Parquetry top table tops for geometric sawn veneer fitting sometimes . Gives me something to stay on track as I’m going along in exactly the same way as you need . Everything is shot, fitted and glued down as I go along . If it’s an oval frame you can either mark it out with the string and two nails method or make an oval sliding marking jig for a true oval . Not sure of the right name for them . Or just draw out what you like and get one side of the oval good on a cardboard template and then flip it over .

Originally Posted by auscab

You can just have a 90 degree fence on the SB and pivot the piece to be shot around to get it close or exact . Test and move it around to what you think is best and try again if you need to till it’s perfect .
Glue together as you go would be best. Your safety net is leaving the pieces to long, getting the first angled join matching so the piece follows the plan . Shooting it’s end and fitting/ glueing . If the pieces are fitted down to a plan drawn on paper that is well taped to a board then the whole thing can be lifted off the board later and the paper cleaned off .

This is exactly how I'm going to do it. My shooting board is done, my plane iron super sharp and all tuned up, and template is ready to go. I can tell this is going to work perfectly. Thanks for the awesome advice.

One puzzle is how to keep pieces in the right spot place for the cut, when freehand pivoting. Without the support of the fence, they tend to shift when the blade comes through.
How do you avoid that? I'm thinking a bit of sandpaper stuck to the bed of the SB and/or some wedges between the piece and the fence. Wouldn't have to be exact, but might help.

Probably get away without it for small pieces, but I'll be plowing through 65x32 end grain on some heavy hardwood.

Originally Posted by rogerwilco

This is exactly how I'm going to do it. My shooting board is done, my plane iron super sharp and all tuned up, and template is ready to go. I can tell this is going to work perfectly. Thanks for the awesome advice.

One puzzle is how to keep pieces in the right spot place for the cut, when freehand pivoting. Without the support of the fence, they tend to shift when the blade comes through.
How do you avoid that? I'm thinking a bit of sandpaper stuck to the bed of the SB and/or some wedges between the piece and the fence. Wouldn't have to be exact, but might help.

Probably get away without it for small pieces, but I'll be plowing through 65x32 end grain on some heavy hardwood.

surely you could rig one of those screw on hold down clamps, they’re cheap and handy as, I use them all the time on spindle moulder jigs and such.

Yep, that'll work fine. Good idea!

Originally Posted by rogerwilco

One puzzle is how to keep pieces in the right spot place for the cut, when freehand pivoting. Without the support of the fence, they tend to shift when the blade comes through.
How do you avoid that?

The piece has to rest at some point against a fence of some sort . Its best resting up near the exit end of a cut or slice . I just rest it on the corner of the 90 degree fence. If its a soft wood that can leave an impression though . You could always nail in a short fence at roughly 22.5 degrees an inch long piece so you can still pivot the piece around .
You hold a piece down and shoot it straight . You've got to shoot out the center first to get a hollow then go right across till you get a clean one piece shaving . That should then be true and flat and square if your blade is adjusted right . Its all about being extremely sharp, waxing the two sides of the plane for slip and having a fine enough set so you get a whole shaving . It the set of the blade is stopping the plane your doing it wrong .

So once you have one piece down that is good and your shooting the second to fit and you find its out, which it will be, you now know where some has to come off .
What I do is get the plane up against its sliding position on the board and hold it still . Bring the piece needing a change up and rest it on the sole of the plane and on the stop /fence , rotate it so I see a small gap which is a guess at the new angle . Bring the plane back and shoot some of the end just touching at the new angle and then just micro move it in till you get a full swipe .
There’s a few ways of doing it that are close to this . Just play around with them .
I never clamp the piece down though . If the piece is to big for a shooting board then a clamping method like a Miter Jack would be used . That would give you two hands on the plane . Then you would be camping it and shooting . You could do a similar thing that gets held in your vice with the piece clamped to it . Miter jack - Google Search

OK Roger, It is a lot easier than you think.

First set the angle of of tilt on your table saw as close to 22.5° as possible. BTW-Your miter gauge should be set at the 90° setting. Next cut one end of the pieces of your octagon using the miter gauge on the out-fall side of the cut. Next cut the 22.5° other end of your octagon using the fence to get the precise dimension of the pieces.

Yes, I am right tile oriented, but this method will work for you left tilting wizards.