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Thread: Coopering angles
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3rd June 2005, 09:14 AM #1
Coopering angles
Hello all,
Seeing Ryan's blanket chest with the curved lid has prompted me to ask "wouldn't it be good if there was a standard set of calculations to use in determining the coopering angles for any size curve?"
Is there a guru around who can answer that?
Cheers,
Keith
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3rd June 2005 09:14 AM # ADSGoogle Adsense Advertisement
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3rd June 2005, 10:18 AM #2
Keith,
The angles are a very simple calculation, provided you are creating a true arc.
Unfortunately describing it at this time of the morning is not!!
Sooooo....http://www.woodworkforums.ubeaut.com...ead.php?t=1127
Originally Posted by jacko
The angle of cut can now be calculated by by dividing the angle subtended (108°) by the number of pieces (20) = 5.5°, then subtracting the result from 180° and dividing by two. (in this case the angle of cut would be 87.5°)
Cheers,
P(who failed maths at every attempt, but can get by, by drawing stuff and nutting it out)
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3rd June 2005, 11:23 AM #3
It's a lot simpler if you just know the angle of the arc. If you draw it out to scale then you can measure it with a protractor. But then if you are going to do that, you can just mark off two staves, scribe back to the centre point, and measure the angle straight off the drawing, which is what I do.
"I don't practice what I preach because I'm not the kind of person I'm preaching to."
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3rd June 2005, 11:27 AM #4
Thanks Peter, I should be able to nut it out from here...
K
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3rd June 2005, 11:57 AM #5
There is an article on making a coopered cabinet in FW #163. The author describes making a coopering jig with arc-shaped ribs, from which it is possible to directly measure the bevel angle of the staves using a sliding bevel tool and a protractor. It seemed to me that the method is rather prone to error, so I used TurboCad to draw the staves and accurately determine the bevel angle. My version of the cabinet is here:
http://www.woodworkforums.ubeaut.com...ead.php?t=5667
Rocker
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3rd June 2005, 11:58 AM #6Senior Member
- Join Date
- Nov 2003
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Even simpler - well in fact it is the mathematical formula for the length of an arc
The length of the arc is:
Length = radius * angle (but angle must be in radian which is the angle unit in trigonometry)
To convert in radian: angle in rad = (pi * angle in degree)/180
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3rd June 2005, 11:58 AM #7
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3rd June 2005, 04:35 PM #8Originally Posted by Rocker
Rocker,
I got "no matches" for your link. :confused:
Thanks for asking keith!
-Ryan
there's no school like the old school.
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3rd June 2005, 05:54 PM #9Originally Posted by ryanarcher
Rocker
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4th June 2005, 11:02 PM #10
A tip when coopering.
Cut one test stave the width and angles that you reckon are correct, then cut it into the number of pieces that you are going to use in the final piece. These only need to be short. Join them up and check that they give the shape that you want. If so, cut the real pieces, if not, adjust the angles and/or width.
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5th June 2005, 11:39 AM #11
My maths has never been that great and I have always relied on the kindness of strangers to work out anything complicated.
However, it strikes me that for someone who does know maths it wouldn't be too hard to make up a small spreadsheet wherein a few dimensions could be entered to give the angles needed to properly cooper a drawer front etc.
Given:
The length of the drawer, say 462mm
The depth from the front of the bow to the back, say 95mm
Material thickness,say 19mm
Number of pieces required to be coopered together, say 5
Given all that, surely it is possible to work out the angles required?
Somebody??? Somebody???Bob Willson
The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.
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5th June 2005, 09:40 PM #12
I have attached to this a zip file that contains my excel spread sheet I use to calculate the information you want.
You need to enter the number of staves, the chord of the arc, the height above the arc above the chord and the rest is calculated for you. If these terms are not clear you can see the illustrations in the sheet that graphically depicts them for you.
You will also note that I have also provided a way to construct the exact angle using measurements. You need to enter the base of the triangle to use and the height is calculated. I used metric measurements but cubits or imperial measurements would work too. I used a dial caliper to get to the nearest hundredth of a millimetre as I was working to precise measurements. You may not need that precision.
If you have any questions I would be happy to assist you.
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6th June 2005, 05:04 AM #13
That is great Mark Well done.
Now, just to make sure that I understand it properly.
c= chord. I entered 540 here
The number of staved I put at 10 and this made the data at E2 change to 56.7 which I assumed was the width of the staves that I needed to cut.
h=height which I had entered as 75mm
This gave me the number 6.2 at E6 which I believe is the angle that I need to cut on the side of the stave. Do I need to divide this by 2 to get the individual angles for each stave?
Why is the radius r smaller than the chord c?
What do I do with the theta and the alpha?
As I said before, my maths is just VERY basic stuff indeed. I c'n spell good but.Bob Willson
The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.
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6th June 2005, 11:05 PM #14Originally Posted by Bob Willson
Originally Posted by Bob Willson
Originally Posted by Bob Willson
Originally Posted by Bob Willson
Some tips on coopering.
1. Construct the coopered panel first and build around it.
2. When assembling coopered panel, ensure that grain direction is running the same way. It is very painful if you don't! DAMHIKT...
3. More staves give you a smoother look, assuming that is what you are after and it's not actually that much harder.
4. To glue up a coopered panel first construct a jig. Make two semi-circle cutouts with the correct radius from your calculation and assemble the panel. I used epoxy glue and only light pressure to glue up hence avoiding a moderately complicated clamping solution.
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7th June 2005, 04:55 AM #15Originally Posted by markharrison
Anyway, thanks again Mark. I will use this formula next time I make a coopered panel, which should be within the next month.Bob Willson
The term 'grammar nazi' was invented to make people, who don't know their grammar, feel OK about being uneducated.