Curved bench rail and slats?


I am making an outdoor garden bench. Originally I was going to use a straight top rail for the back, and use separate mortises for the slats. I have decided to curve the back top rail, as I think it will look much better.
Does anyone have a suggestion as to how I could mortise the slots to accept the slats for the curved sections? I thought of routing a dado along the bottom and filling in the spaces with wood spacers between the slats, but I don't know how that would work in the curves. Wouldn't I need to take into account the curve angle for the spacers? I don't know how to do this.
Can anyone help?
Thanks.
Chap
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I'm doing the exact same thing, so I'm really curious to hear ideas. I was hoping I could route a dado too, but discarded that for exactly the reason you mention. So then I assumed I'd have to chop individual mortises. That way they can each be square. Of course, each slat would be flat, so would not exactly match the curve of the back, but I think that's not uncommon. Since slats are usually not very wide, it shouldn't be a problem. A *lot* more work, though.
However, if you're using seat slats that don't run the length of the bench, but rather the depth (I am), I think a more serious problem is how those meet the curved front and rear rails. If you're using mortise and tenons, usually the shoulders are square. But that won't work where they meet a curved surface. What do you do?
And lastly, I've decided not to fart around with steam bending and springback. This leaves coopering, laminating, and build-up (don't know what you call it) for creating the curved boards. Anyone know what's the best for that, given the extreme environment of a garden bench? I was thinking coopering, since it would only take 4 pieces of 5/4 stock to make a 51" bench, but I'd prefer that the grain ran the length of the bench, which would not be optimal, considering that it would mean gluing end grain.
-Tom
Chap wrote:

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It "depends" on how big the curve is... and how thick your back slats are.
For a back top rail that is maybe 1.5" thick, I would cut a slot using a router table about 1/2" thick and as deep as I could get with the slot cutter.
I would then create a mortise strip out of 3/4" stock by laying out a dado 2" wide, 1/4" deep and spaced out every 2" inches. After cutting the all those dados, rip the strip down to 1/2" wide and glue that in the slot you cut earlier.
You would do the same for the bottom back rail. You will then custom fit all your back slats one at a time.
This is gonna be very time consuming getting all those curved tops cut correctly.
I would dry assemble the back ane hold it together with clamps. It helps to "hot melt glue" the bottom of the slats into the lower rail.
This is very similar to using "filler" blocks that you asked about but makes the layout a "little" easier by creating two strips from the same board. They should then match top and bottom.
I also build garden benches and have not jumped on a curved back "yet".
To do it like the factory requires a slot mortising machine which ain't cheap($2400 for a starter).
http://www.atlanticmach.com/standard/callout_mortiser.html
Chap wrote:

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I had similar problem with the headboard of my mission style bed which had a curved rail. I cut a dado to the same width as the slats on the preformed curved rail with a router and then prefitted the slats into the rail (no tenon) together with spacers of fixed length. The spacers ( bits of slat material) were then glued in position leaving them proud of the rail surface. Ensure that the depth of the dado is less than the depth of the slats to ensure that they stand proud. The slats were left unglued and removed once glue had gone off. The rail could then be routed with a straight bearing bit to conform with the curve of the top rail. Refit and glue the slats back into position. Helps to number all the slats and put them back in the order they came out. Came out perfect
Regards Jamaro
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I am trying to follow what you did, but I got lost along the way... Got any pictures ???
Jamaro wrote:

How did you cut the dado and was the top rail already a curved piece at that point ???
The spacers ( bits of slat material) were

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I have posted a picture in alt.binaries.pictures.woodworking which illustrates. I do not have one of the top curved rail but this shows the straight bottom rail which uses the same approach.

Yes I cut the piece to shape first and then routed the dado. I thought initially that the dado would vary greatly in depth doing it this way but in practice there was not much varation.

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Regards Jamaro
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Thanks for all of your help.
What I decided to do was to rout the dado into the rails, cut longer spacers than normal, then trace the shape of the curved rail on to the spacers (set up with the slats in finished order). This helped with the spacers and the slats actually. A bit time consuming, but seems like the best way to deal with the curve to me.
Chap
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