Octagonal Tower

[shinobi]

Hi

I'm new to this forum, so thanks for any of your help. I'd like to build a bedside light for my son. I am trying to build a wooden lighthouse. The core part of the lighthouse will be an ocatgonal (i.e. eight sided) tower that starts with a wide base, but tapers the further up the tower you go. Any tips on:

a) how to get eight sides to perfectly fit together
b) how to allow for the narrowing of the tower

I am guessing that there are some key tools/techniques that would make this task easier - and make it look more expertly done!

Initial research indicates that a 'birds-mouth' joint is what is needed - but I don't seem to be able to find a router bit like this in the UK.

Thanks again for any help you can provide.

 

[Steve Maskery]

Hi Shinobi
Welcome.

Do you have a tablesaw? If so, you need to make a jig to cut a taper. There are several good designs about: I'd make a sliding baseboard to fit in the slot in the table, set a fence at a suitable angle and clamp the work to the fence and to a backstop.

Whe you make the taper, the blade must be tilted to 22.5 degrees, but getting this exactly right is tricky. The most accurate way is to use a SIne Bar. I wrote an article in GW about this technique a while ago, if you have a GW archive.

I'd glue the column up in two halves, four in each, then joint the two halves so that they fit each other. That way, even if the segments are not all 22.5 degrees, nevertheless the total will be 360, which is what you want for no gaps.

HTH
Steve

 

[jasonB]

I don't think that there is a router cutter that will do the job.

If it were parallel sided then the cuts would be 22.5 degrees but as the angle of the taper increases you need to adjust the bevel.

eg for a 5degree slope each side will need to taper by 2 degrees each side and the bevel cut will be 22.25 degrees.

Probably best to plot it out on a drawing package to get the correct angles then cut your eight tapered sides with square edges. Next make a false base for the router table to hold the pieces at the correct bevel angle then run them against a straight bit.

And welcome to the forum BTW.

Jason

 

[Woody Alan]

Hi Shinobi

Welcome to the forum,
An accurate tablesaw/routertable set to the correct angle to cut your angles I would be tempted to make a sled as a jig to hold each piece exactly so they are all perfectly the same. The angle of the blade will accomodate the 22.5 degree cut required along each edge (as this is a constant) then just pass each edge of the wood past the blade at the angle you require for the taper.
I am sure someone here can give a better answer but it starts (EDIT beaten by two minutes )the ball rolling
Cheers Alan

 

[DaveL]

Hi Shinobi,

Welcome to the forum.

How big is the base going to be?

I have a Trend Router Lathe and if the base size would fit then I think it could be done using this.

Note Trend have discontinued this useful bit of kit.

 

[jasonB]

Steve & woody, am I right in my posting as I tought we were dealing with compound cuts as its a tapered multi sided shape therefore 22.5 degrees won't work? or have I got it wrong??

Jason

 

[shinobi]

Thanks guys. Am just reading through the responses and figuring out what makes sense. JasonB, if you are correct, it sounds fairly tricky.

Having seen the below website, I assumed that this would be fairly straightforward - I just can't find anything like this is in the UK...:

http://www.leevalley.com/wood/page.aspx ... at=1,46168

Any thoughts?

 

[Woody Alan]

Funny you should say that Jason I wasn't quite sure whether you meant how it sounded, I was going to comment that all angles still have to add up to 360 degrees at the bottom and the top of the tower don't they ? The tower taper is a red herring if you made a tower constant width all the way up with 22.5 angles you could then machine the taper on afterwards too, does that help?

EDIT http://www.leevalley.com/wood/page.aspx ... at=1,46168
Your link caught in spam trap don't worry it goes away after a number of posts

Alan

 

[Steve Maskery]

Steve & woody, am I right in my posting as I tought we were dealing with compound cuts as its a tapered multi sided shape therefore 22.5 degrees won't work? or have I got it wrong??

Jason

No, Jason, you don't have it wrong, 22.5deg is not completely right, my apologies. However, I had it right in my head, I was just incomplete in my description:

As well as the 22.5deg setting, all the 8 segments then have to be tapered in thickness according to the desired taper. So for a table lamp which was, say 75mm in dia at the bottom rising to 50mm dia at the top, using stock 38mm thick, each piece would be tapered from 38mm at the bottom to 25mm at the top.

Without this tapering thickness, Jason is right about needing compound mitres. Over to someone whose maths is better than mine...

S

 

[Woody Alan]

Looking at your link I would say it depends on what your end desired result is if you want nice sharp external angles where the wood joins I don't reckon that bit will help because the only piece which looks in the picture to represent what you want is deliberately photo'd from the inside, I'm guessing because it's a pigs ear on the outside...but I have been wrong before if you look at the other piece on the left is that the appearance you want?.
If you want stability, when trying to glue up machine some slots and insert some tongues, it might no be like trying to glue together a bag of eels then

Alan

 

[Steve Maskery]

What on earth are well al pratting about at? It's a bedside lamp, so why don't we just glue up a solid block and, as WA says, cut the angles afterwards? A bandsaw and plane are all that are required! Cut one face, then tape it back on to keep the block square. Do four faces this way, then tilt the table to do the other four faces. If the block stays properly taped together you should get plenty of support. A quick whizz with aq handplane and Robert is your father's brother.

Forget everything I said about tablesaw jigs, way over the top!
S

 

[shinobi]

Woody

Good spot - I didn't notice how shoddy the finish was on the outside. Am looking for flush edges.

Like the solid block idea - but the light is to go inside the tower (probably on the base) - not on top of it. So baiscally, the tower needs to be hollow all the way through.

Need to think about how to cut windows into each panel also....(guessing you'd drill a hole and use a jig to cut a square window out?)

 

[Inspector]

I don't think that there is a router cutter that will do the job.

If it were parallel sided then the cuts would be 22.5 degrees but as the angle of the taper increases you need to adjust the bevel.

eg for a 5degree slope each side will need to taper by 2 degrees each side and the bevel cut will be 22.25 degrees.

Probably best to plot it out on a drawing package to get the correct angles then cut your eight tapered sides with square edges. Next make a false base for the router table to hold the pieces at the correct bevel angle then run them against a straight bit.

And welcome to the forum BTW.

Jason

If you look at Lee Valley's bits and click on the "instr" it will tell you how to make tapers with them. The slight change of angle you mentioned will get filled with the glue and won't show after the light house gets its traditional coat of white paint.

http://www.leevalley.com/wood/page.aspx ... at=1,46168

Welcome to the party shinobi! ccasion5:

You can try this calculator to get the cutting angles for the light house.

http://www.josephfusco.org/Calculators/ ... utter.html

You don't need a birds mouth bit to make those cuts, it just makes it easier. The blade in your table saw can be tilted and a pass made with the board vertical to the table against the fence. A second cut with the board flat to the table completes the birds mouth.

An alternative if the lamp isn't going to be too big would be to make a thick square box section and saw or plane the tapered octagon you need.

 

[jasonB]

I was thinking making the tower out of thinner stock like a tapered 8 seded box then the thickness of each piece would not matter. As the taper is only likely to be a few degrees thsi will only alter the bevel by about 1/10th degree so nor really worth bothering about, its as the taper steepend the bevel decreases by a greeater amount per degree.

A sled would work if the tower was made from solid and could be indexed round 45 degrees at a time, seem to remember Norm doing it on some 6 or eight sided bed posts.

JAson

 

[Sgian Dubh]

I was thinking making the tower out of thinner stock like a tapered 8 seded box then the thickness of each piece would not matter.
JAson

Jason, if shinobi is making an eight sided polyhedron with eight pieces of wood then he needs to know the dihedral angle, and the angle the two rising edges of each piece describe to that pieces bottom edge.

Here are some numbers as an example.

1. Assume that there are eight sides.
2. Assume that each side leans in from the vertical by 5º ie, each side's faces form an angle of 85º from the base line.

The dihedral angle of the structure is 135.18º. To mitre the eight sides together divide the dihedral angle by 2 = 67.6º. To cut the correct angle on a table saw set the blade tilt (bevel) to the complementary angle, ie, 22.5%º to the nearest 0.5º. This setting never changes once locked.

Next the mitre gauge angle needs calculating. This is 88º. To cut the edge of each of the eight pieces the mitre gauge should be set to 88º, ie, 2º less than 90º.

The base of each piece has to be held on the saw's mitre gauge set to 88º one way and a cut made by the bevelled saw blade. Next the mitre gauge is set to tilt the other way to 88º. To finish off all the cuts the top edge of each piece is held against the mitre gauge and the piece passes by the bevel angle set on the saw blade.

The pieces of wood all require a 5º angle cut at the bottom so that the assembled polyhedron sits neatly on a flat surface. The top needs the same if it doesn't go to a point.

Change the number of sides, and change the angle that the sides lean in from vertical and new calculations are needed.

Pinpoint accuracy is required for perfect results. It's fiddly to do even with large bits of wood or man made board, but special holding devices are usually required for small pieces to keep fingers well away from the saw blade.

That was all probably far too much information, but I'm pretty darned certain the numbers are correct-- I've made quite a few things like this. Slainte.

 

[Woody Alan]

Just to get back to Jason's point, after a constitutional walk in the rain from which I returned soaked I was mulling it over and I reasoned that if the tower was to be squashed flatter (like an umbrella can't think why that came into my head) the angles would gradually change until when flat they would be right angles (or 0 depending how you look at it) so yes I agree the angle would gradually get smaller but how you would calculate it I have no idea

Like the solid block idea - but the light is to go inside the tower (probably on the base) - not on top of it. So baiscally, the tower needs to be hollow all the way through.

It doesn't have to be solid the octagonal tube only needs to be made thick enough to accomodate the machining of the taper afterwards.

Alan

Probably will think how at 03.00AM :roll:

 

[Nick W]

The way I'd do it is to model it in SketchUp and then print out bits of the model at large scale. Then use the prints to set, either directly, or using a sliding bevel, the angle of the saw blade. At least that's what I did for the legs on this table

 

[Woody Alan]

Ok woke up with the answer. No maths required

I'll try and take this in stages and it is in my head at the moment so feel free to shoot me down.
Make a sled for the mitre slot (one capable of being turned around either way)
Imagine you are going to cut a piece of wood square on the sled.
set the blade at 22.5 degrees.
Raise the far end of the piece of wood by the amount you want to taper (safely on jig) the tower (might be half the taper haven't got that straight in my head yet)
Pass the wood through the blade the required compound angle will automatically be cut.
reverse the jig so that the raised end is toward you and repeat for the other side of the wood to desired width.
What do you think?
Alan

 

[NickWelford]

I think you should settle for a round lighthouse and make it on the lathe from a solid block ......

 

[Sgian Dubh]

Make a sled for the mitre slot (one capable of being turned around either way)...set the blade at 22.5...
Raise the far end of the piece of wood by the amount you want to taper...Pass the wood through the blade the required compound angle will automatically be cut.

What do you think? Alan

It looks like you used the sample numbers I provided Alan, so I'll respond.

With modifications to your sled that would work. Raising the wood in the sled wouldn't actually do the trick because that doesn't take care of the second element of the compound cut that I calculated, ie, the 2º angle between the base line and the edge of each piece.

The bevel angle is taken care of by tilting the sawblade, in this case at 22.5º. You would still need to make the sled with an appropriate mitre setting 2º off square, ie, 88º.

Simply raising the board in a sled that's set at 90º wouldn't give that requisite 2º mitre cut. All it would do is change the 22.5º bevel angle to something else. I'm not sure what it would change it to without some number crunching, but I'm pretty sure it would change it.

Certainly a custom or bespoke sled along the lines you suggested is very do-able, but it would need to take into account that vital 2º mitre cut, as well as the 22.5º bevel, and the material would have to stay flat on the sled. Slainte.