Oak Planter

[Pintglass]

Hello

i'm trying to make an oak planter from some offcuts of oak i've got (Yeah i know it would be easier to buy one but still)
I wanted to use two different size staves to form the circle. For example i have a diameter of 620mm and want to use 24 pieces,16 at one size then 8 at another to form the circle, if i do 3.14 x 620 i get 1946.8 then divide this by 20 i get 97.34, then to give me 24 pieces i have divided the 97.34 by 2 to give me 48.67 thinking i could have 16 pieces at 97.34 and 8 pieces at 48.67.
The bit i am not sure on is the angles for the pisces if i was using all the same size pieces i would just divide 360 by the number of pieces, so for example if i was using 24 pieces i would divide 360 by 24 but as i'm using different sizes i'm not sure how to calculate the angles.
Hope this makes sense for someone to help me out.

Thanks Pintglass

 

[Davon]

Pintglass, I am trying to get my head around your problem as it may be of use to be at some point, but I keep coming back to the fact that 360 divided by 24 is 15 no matter what width the pieces are so should they not all be 15degs. I'm sure a definitive answer will be along shortly.

Davon

 

[Pintglass]

I agree with you regarding the angle but do think that the angle could change depending on the size of the piece. I suppose it depends on how true the circle will be.

What did you use to make the drawing?

Thanks Pintglass

 

[Tom K]

The 16 pieces occupy the same area as the 8? so calculate the angles based on 16 pieces to start. I think you have to account for 2 edges on each section so that would be 360 divided by 32 or 11.25 degrees.
Do check though because the cider is kicking in :lol:

 

[Johnboy]

Like this?

John

 

[Lofty]

Johnboy's drawing has the answer to this question although the number of parts is not right. You want to make a circle out of 16 large pieces and 8 half the size. Two of the smaller pieces are equivalent to the larger ones so 8 are equivalent to 4 of the large pieces. This means that effectively you are making a circle out of 16 + 4 = 20 of the larger pieces. These pieces will require an angle of 360/20 = 18 deg at the centre of the circle. The smaller pieces will require an angle of half of this ,9 deg. Just as a check 16 x 18 + 8 x 9 = 360! Hope that is clear.

Mike

 

[Tom K]

Johnboy's drawing has the answer to this question although the number of parts is not right. You want to make a circle out of 16 large pieces and 8 half the size.

Mike

No he doesn't :lol:

 

[Pintglass]

Thanks for the replys

I think everyones been right with what there saying, but i don't really want to put the smaller pieces together, i do want to have 16 of the larger pieces and 8 of the smaller ones. I wanted to put the smaller pieces on there own, so for example to have a large piece then a small piece then a large again then small then perhaps a couple of large ones, so to follow the circle i'm sure 3 different angles would be involved.

Thanks Pintglass

 

[Lofty]

Pintglass,

The order in which you put the pieces around the circle does not make any difference to the angles. Each of the large pieces will have an angle of 18 deg at the centre of the circle and the smaller pieces 9 deg. Try drawing it out on a piece of paper or MDF, the actual size of the segments does not matter as long as one is twice the size of the other. Having drawn it out you can then cut out the segments (easier with paper!) and put them back together in any order. One thing I forgot to say is the angle between the staves and a radius of the circle will be 81 deg for the large piece and 85.5 deg for the small piece. These are the angles that you would need to cut on the sides of the staves.

Mike

 

[9fingers]

Pintglass,

You need to think about each sized piece in terms of the angular contribution it makes to your circle.

You have two sizes of wood so that means two angles. To make life easy, if we assume your larger pieces have double the angular contribution, we get 16 pieces each giving 2 alpha degrees and 8 pieces each of 1 alpha degrees.

A total of 40 times alpha which has to add up to 360 degrees so alpha = 9 degrees

The two angles in this case are 9 and 18 degrees at the centre of the planter.

Drawing a triangle out from the centre to each flat segment of the planter you can work out the chamfer angle for each size of segment as the sum of the angle in the triangle must equal 180 degrees.

So small segment are cut at (180 -9)/2= 85.5 degrees and the wider ones are cut at (180-18 )/2 = 81 degrees


Width of segments are given by 2 x tangent(half centre angle) x radius of the planter.
If you dont have tan tables to hand or a scientific calculator,
Tangent 4.5 degree = 0.0787
and tan 9 degrees = 0.1584

So measured at the widest point small segments are 48.8mm wide and wide ones are 98.2mm for a 620mm outside diameter

Hope this helps

Bob

 

[Anonymous]

don`t forget to half the angle when cutting. I found out the hard way.

Koolwabbit

 

[9fingers]

don`t forget to half the angle when cutting. I found out the hard way.

Koolwabbit

I've allowed for that in my calculation above KW so should be OK!

 

[markymark12]

Talk about making hard work of a job, you don't work in Government or a local Authority do you?? :lol: :lol:

It has all made me brain hurt. :? :?

Buy a bigger bit of wood. Job done.

 

[MikeG.]

I'm tending towards Markymarks sentiments!

I would do this as coopers used to. I would just offer one piece up to the next with a circular template/ guide cut out of cheapo waste to guide me. Take half the gap off each edge with a plane, draw knife, spokeshave whatever and get it pretty close. Clamp it all tight then run a crosscut saw down along the dry joint, then a final tiffle up with a plane and you've got a joint made. Work you way around the planter doing this as at each joint, then your last one is a "special", which will have to be ripped to width.

It's taken longer to write about it than it would have done to do the job! The danger of "engineering in wood" using machinery for everything is illustrated by this problem.

Don't forget, this is a planter......not part of a pair of matching coopered doors on the front of a bow-fronted bookcase!

Mike

 

[Davon]

What did you use to make the drawing?

Thanks Pintglass

Sketchup 7

Davon

 

[Pintglass]

Thanks for all the help on this and i know we have done too much talking about this so this will be my last question.

So the angles are 18 and 9 degrees which when halved gives us 4.5 and 9. Am i right in saying that to have the same angle on the small piece and large piece when they meet i can add 4.5+9= 13.5 divided by 2 which gives me 6.75 degrees, if i don't do this the length of the angle would be different, not a true bisect.

Thanks Alot Pintglass

 

[MikeG.]

You're planning to work to a quarter of a degree???!!!

As I said, I would be taking a polar opposite approach..........and I doubt if anyone would be able to tell the difference with the finished article.

Mike

 

[9fingers]

if i don't do this the length of the angle would be different, not a true bisect.

Thanks Alot Pintglass

Strictly true but the difference in the two halves of the joint will be minimal.
Once it is glued up a light sanding or a couple of strokes with a spokeshave will even things up.

Draw it out scale up and you might just spot the variation.

Stop worrying and go make some sawdust :lol:

Bob