Coopering a box lid

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Forum topic by James Gallo posted 08-20-2015 12:05 PM 1595 views 3 times favorited 14 replies Add to Favorites Watch
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James Gallo

56 posts in 1527 days

08-20-2015 12:05 PM

I am building a wedding memory box, and am trying to make a curved top. I’ve done a lot of research on how to determine the angle of the staves, but I’ve done 2 test runs, and I cannot get the curve to match the curved ends of the lid. The curve that I’m trying to match is 7 1/2” wide x 3.78125” (25/32) high. The outside dimension of the box is 8.84375” (27/32) I know I should have made the lid first, then build the box to match, but I realized that after I built the box. The box is made from purpleheart, so I don’t want to scrap the box, and make another one. Is there someone out there that can figure out the angle I need for my staves, and the width of the staves widest and narrowest dimensions? This really has me stumped. Thanks for any direction that you might offer. Jim

-- Jg, Pittsburgh

14 replies so far

View Jim Finn's profile

Jim Finn

2390 posts in 2339 days

#1 posted 08-20-2015 02:20 PM

I have made some round top trunks and some round waste baskets by making staves like you are looking to do. I make the ends and measure the distance along the curve. Divide that distance by the number of staves I plan to make and that will give me the width of each stave. You are wanting to span 180 degrees so you divide 180 by the number of staves and then divide that number by two. (each cut will give you 1/2 of the angle required on each side of the stave) I cut the staves on my table saw and always make an extra stave. In case I screw one up. Dry fit them all and trim any that may be required to fit.

-- "You may have your PHD but I have my GED and my DD 214"

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James Gallo

56 posts in 1527 days

#2 posted 08-20-2015 02:47 PM

Thanks Jim. I used that method, however, they just won’t come out right. I think the stave thickness has a lot to do with it but I’m not sur how that fits into the equation.

-- Jg, Pittsburgh

View ClammyBallz's profile


297 posts in 553 days

#3 posted 08-20-2015 04:15 PM

The picture may be deceiving, but it looks like the top is taller than the radius of the circle.

View jdh122's profile


878 posts in 2234 days

#4 posted 08-20-2015 04:39 PM

To me it doesn’t look like the curve you have to match is exactly circular, although I might be wrong. If it’s not a segment of a circle you won’t be able to match it with equal width staves cut all at the same angle. But I would think you would be able to adjust slightly the pieces that are furthest from the top, either on your jointer or with a plane.
Even if the curve is round, I would think that when you set your angle on the saw, any deviation by even a fraction of a degree (or width of board), being multiplied by 2x the number of staves, will almost inevitably need some adjusting. It’s why coopers never measured angles and fit by trial and error. Of course, they had a lifetime of muscle memory to help them.

I should add that I’ve never done any coopering, so take what I say with a grain of salt.

-- Jeremy, in the Acadian forests

View HornedWoodwork's profile


222 posts in 631 days

#5 posted 08-20-2015 04:59 PM

I think you aren’t taking into account the thickness of the wood, or that the wood has many facets, that is, it is not smooth on the underside. Removing the waste wood will help you to nail the arch. Your measurements are from (I suspect) inside edge to inside edge. But these edges are going to be fared away. You need to address the arc of the circle by measuring the width of staves at the thinnest part (middle) of the stave after it is shaped. Draw your arc segment on the edge of the stave, starting from one point arcing to the other point on the edge near the thin face of the stave. The point where your arc peaks (it will be very subtle) is the point where you want to measure the width of the staves. this means the interior of the arc is slightly less than you have measured for right now, because you are taking the measurement on a piece of waste wood. You are assuming the arc terminates at the points where they staves join, but that is not so, the arc terminates slightly above that point. Essentially you got the math right to get to 180, but each stave is just slightly too wide at it’s thinnest part, to make the arc you want.

Incidentally based on the numbers you gave if you had 10 staves (11 pictured but 10 makes math easier) then my math says that the width of each stave should be 1.1734 inches wide. Also the arc isn’t perfect because at 7.5 inches wide it should be 3.75 inches high. Not being a perfect circle (or half circle) isn’t going to help you get a perfect fit..

-- Talent, brilliance, and humility are my virtues.

View mahdee's profile


3454 posts in 1184 days

#6 posted 08-20-2015 05:17 PM

I can think of two options:
Easy option: Use a piece of bendable plywood or regular plywood and cut a bunch of curf at the bottom and bend it to match the curve.
Hard option: Place a piece of cardboard on the side of the box and cut it to shape of the top. Measure equal amounts of “pie pieces” on the cardboard and secure to the side. Cut staves according to the the angels drawn on cardboard using a ruler.


View Hammerthumb's profile


2511 posts in 1392 days

#7 posted 08-20-2015 10:42 PM

This may be the picture, but it looks like each stave has a bow to it. Are they made of BB ply? Looks to me that that would be your issue.

But I might be wrong. Cant tell from just one pic.

-- Paul, Las Vegas

View BadJoints's profile


103 posts in 506 days

#8 posted 08-21-2015 06:48 AM

Here is how I would tackle that problem, keeping in mind I’ve not actually built a curved top yet.

I would start with a single stave cut at your calculated angle, and then adjusted to fit perfectly on the front. Then I would cut and finesse the next with a plane(or your tool of choice) until it fit perfectly with both the curve and the previous piece. Repeat cutting and fitting each piece individually just seems like a lot less stress to me.

It will certainly take more time, but will be much less stressful I think. Attempting to ‘mass produce’ identical parts has only ever led to frustration for me. Yes, you’re last piece might be a bit oddly sized compared to the rest, but I’m betting not much. This is why I suggest using a correctly sized piece on the front first, and working backward. Only you will ever know.

-- Producing furniture grade firewood since 1984

View CharleyL's profile


190 posts in 2781 days

#9 posted 08-22-2015 02:42 PM

I would not work to width dimensions, but to degrees of arc. Divide your curved end in equal degrees depending on how many staves that you want. This will also give you the angle that each stave edge will need to be cut. Then carefully trim each stave to it’s width and angle to match the radius increments that you laid out on the end piece.


View Ghidrah's profile


667 posts in 639 days

#10 posted 08-22-2015 06:40 PM

The curve that I m trying to match is 7 1/2” wide x 3.78125” (25/32) high. Jim
- James Gallo

You don’t have a half circle, (hgt rad longer than width rad) you have a distorted oval, so, you can’t cut all facets at the same degree. If you keep what you have, like previously mentioned by a member, cut and finesse each piece. Odds are you’ll end up with a short or long piece at the opposite side.

Without knowing the width of a facet one can’t get anything close to an acceptable degree of angle. If you make the existing a true circle. You could use 18 degrees as the bevel for 10 pieces with a bottom cut of 1.17809724536517” = 1 23/128” I’d go for 1 3/16”.

For future reference make the top 1st then scribe the sides to it.

-- I meant to do that!

View Daruc's profile


459 posts in 549 days

#11 posted 08-22-2015 07:22 PM

Here is what I came up with sketchup. Using your measurements of 7 1/2 wide x 3.781 high

In case you can’t read the measurements
Top of each stave 1.302
Bottom of each stave 1.086
Stave thickness .750
Angle cut on stave ends 8.2
10 full size staves
2 – 1/2 size staves at the 2 ends

You also need a jig to do the glue up with, because even if you cut all the angles exact you will lose the accuracy with the glue. I would make some kind of jig to glue them up something like this.

Another idea, if you keep having trouble, is to make your lid a tad smaller than the existing sides then alter your sides to fit it.

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View Logan Windram's profile

Logan Windram

289 posts in 1879 days

#12 posted 08-22-2015 07:55 PM

Coopering is not a great way to match that top to that radius, you will go crazy with the slight variability of angles and that inside surface will be hell to match to that curve. I’d make a MDF form to match up to that bottom radius, cut veneers and laminate that top. You issue will be needing a vacuum press, but perhaps a shop in your locale might have one.

I have seen people try to use sketch-up to figure out the angles, and get darn close, but to me coopered parts are finicky and often require a little handplane fudging.

I’d also experiment with poplar not ply, all that glue is making your machine blades very unhappy, and I’m not sure how well ply machines with precision.

View sras's profile


4362 posts in 2546 days

#13 posted 08-22-2015 08:09 PM

If it were me, I would use the sides as a form and build up the curve one piece at a time. That way you can adjust the fit as you go.

-- Steve - Impatience is Expensive

View Daruc's profile


459 posts in 549 days

#14 posted 08-22-2015 08:23 PM


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