Shaping concave surfaces?

[Mike Henderson]

How could the radius of the cove cut ever be larger than the radius of the blade?

I can see where different approach angles and/or blade tilts could result in a smaller or asymmetrical radius, but I'm having trouble picturing how it could ever be larger. If it can, please share the technique. Thanks

Not larger, smaller.

Mike

[Here's a calculator for how to set up for different cove cuts.]

[Zachary Hoyt]

If you tilt the blade you could get a larger radius. If you leave the blade at 90 degrees to the table but run the boards at an angle to the plane of the blade you could get a smaller radius. I have not tried either method in practice, but in geometric theory both are possible.
Zach

[Dan Hulbert]

All of the previous methods seem focused on putting the curve along the full length of the slat. A knife and chisel can do the trick if you focus only on the attachment points. No one will see the bottom of the slats and leaving more wood will improve the strength of the slats.

[Mike Henderson]

All of the previous methods seem focused on putting the curve along the full length of the slat. A knife and chisel can do the trick if you focus only on the attachment points. No one will see the bottom of the slats and leaving more wood will improve the strength of the slats.

I think the OP asked for how to cut a concave curve along the length of the slat, which I took to be the top of the slat. It seems that the bottom of the slat would be convex to match the metal supports. That could be done as you suggest but is not what the OP was asking.

Mike

[Stephen Rosenthal]

Stanley #20 compass plane will form both concave and convex surfaces. Just make sure the blade is razor sharp. Follow-up with spokeshaves, scrapers and/or sandpaper to achieve the desired surface dimensions and finish. Sometimes (oftentimes?) hand tools are far more appropriate for the task than power tools.

[Shawn Redfield]

Thank you everyone for the great ideas and advice. It's getting time to try some of them.

[Dennis Yamamoto]

A couple of people suggested using a Krenov coopering plane. Here's a picture of my Krenov coopering plane where I'm planing the concave side of a coopered door. It's some effort to shape the sole and mouth, but once done it can make concave surfaces of various radii down to the radius of the coopered plane blade.P1020295.jpg

[William Hodge]

Newbie with a problem

Hello, I’ve lurked here off and on for some time, and it finally occurred to me to seek help with a long-standing problem here.

I’d like to replace the wood on a cast iron park bench. The metal frame is curved, and the slats ran lengthwise, of course, and fit the curved metal frame. So the slats have a concave surface and a convex surface. Some slats have quite the curve, maybe 5/8 inch depth over 3.5 inch width; some are almost flat.

The convex surface is not a problem, it can be shaped with a plane or whatever. But I’m stuck on how to shape the concave surfaces. There are 10 slats between ~3 and ~4 inches wide.

Any wisdom out there?

Thanks!

One way to approach this is to do what the original manufacturer did.

If the concave surfaces all have one radius, I would have a $125. knife made, and run it in a 4" tall molding head on a shaper. If there are different radii, I would do cove cuts by running the wood diagonally over a table saw blade. Given the various diameters of table saw blades, and the ability to change the angle of the blade and the feed angle, it's possible to make quite a wide variety of cove cuts with a saw.

Pro tip: Clamp the saw fence on the infeed side of the blade. I once saw someone clap the fence on the outfeed side of the blade, and find out what "surprise climb cutting while hand feeding a large table saw" means.

[Eric Schmid]

Lots of good methods already suggested, but...is that really how this bench was originally constructed? Was it shaped on the concave and convex surfaces or was it bent over the frame? I may be imagining something different than what you have.

[Jon Nuckles]

Well, since Dennis posted a picture of his coopering plane and project, I feel the need to post mine.
IMG_1743 (2).jpg

[Andy D Jones]

When cutting coves on a table saw, the only circular cove you can get is the radius of the saw blade, by feeding at 90 degrees to the plane of the blade.

Any other feed angle will produce an elliptical cove, portions of which can have an average radius greater than that of the blade. Whether that portion is wide enough for a given purpose is another question. The size of that portion may be increased somewhat by tilting the blade, which shifts the angle of symmetry of the cove.

The Mar/Apr 2020 issue of Fine Woodworking has a very good article on the technique.

The OP's application needs a curve across the grain, which is difficult to get by bending the wood.

-- Andy - Arlington TX

[Pat Barry]

Newbie with a problem

Hello, I’ve lurked here off and on for some time, and it finally occurred to me to seek help with a long-standing problem here.

I’d like to replace the wood on a cast iron park bench. The metal frame is curved, and the slats ran lengthwise, of course, and fit the curved metal frame. So the slats have a concave surface and a convex surface. Some slats have quite the curve, maybe 5/8 inch depth over 3.5 inch width; some are almost flat.

The convex surface is not a problem, it can be shaped with a plane or whatever. But I’m stuck on how to shape the concave surfaces. There are 10 slats between ~3 and ~4 inches wide.

Any wisdom out there?

Thanks!

If the slats run length wise as you say (which I interpret as left to right) I wouldn't bother to cut the concave at all. Just round over the adjoining edges as needed for comfort.

[Tom Bender]

Um,, did everybody forget about routers? A few passes on the router table will get you close without the excitement potential of doing it on a table saw.

[Derek Cohen]

Newbie with a problem

Hello, I’ve lurked here off and on for some time, and it finally occurred to me to seek help with a long-standing problem here.

I’d like to replace the wood on a cast iron park bench. The metal frame is curved, and the slats ran lengthwise, of course, and fit the curved metal frame. So the slats have a concave surface and a convex surface. Some slats have quite the curve, maybe 5/8 inch depth over 3.5 inch width; some are almost flat.

The convex surface is not a problem, it can be shaped with a plane or whatever. But I’m stuck on how to shape the concave surfaces. There are 10 slats between ~3 and ~4 inches wide.

Any wisdom out there?

Thanks!

Shawn, in complete contrast to my wise brethren, I would not bother about shaping any curves. Every bench I have ever seen, or sat on, which has a curved cast iron frame, uses flat slats. As they are fitted, they create their own curve. Perhaps, after several years, the flat slats take on a curve of their own.

I doubt you could tell the difference. I have done two at home this way, and I could not.



Regards from Perth

Derek

[Jeffrey Martel]

How could the radius of the cove cut ever be larger than the radius of the blade?

I can see where different approach angles and/or blade tilts could result in a smaller or asymmetrical radius, but I'm having trouble picturing how it could ever be larger. If it can, please share the technique. Thanks

Not in one pass, but if you offset the jig so the curve is off center, you can then flip the board around for a second pass. Clean it up with some scrapers.