Shaping concave surfaces?

[Shawn Redfield]

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!

[Jamie Buxton]

Stand the slats on edge, and cut the curve with a bandsaw.

[Mike Henderson]

There's a method of cutting a concave surface into a board on your table saw. I've done it in the past. There are videos and articles describing it but I don't remember what it's called so that you can search for it. Maybe someone will posts some links.

In essence you push your wood across the top of the table saw blade. It requires some jigging and figuring out the angle of the board to the blade. The blade is not very high in this process, just above the table.

I'm assuming these slats are long, maybe 6 to 8 feet long, to span the length of the bench.

Mike

[Ron Citerone]

There's a method of cutting a concave surface into a board on your table saw. I've done it in the past. There are videos and articles describing it but I don't remember what it's called so that you can search for it. Maybe someone will posts some links.

In essence you push your wood across the top of the table saw blade. It requires some jigging and figuring out the angle of the board to the blade. The blade is not very high in this process, just above the table.

I'm assuming these slats are long, maybe 6 to 8 feet long, to span the length of the bench.

Mike
I have included a pic of a jig I made for a dewalt portable table saw.
Utube ogee feet and you should see this technique.

[Keith Westfall]

"Cutting coves on a table saw" will probably get you some results as well

[Ken Krawford]

I've cut coves on a table saw and it really works. There quite a bit of clean up and hand sanding required after to get a smooth result but it can be done.

[Charles Taylor]

There are limitations to the table saw approach, such as the need to clean up the surface, and also the fact that the shallowest curve possible is the diameter of the blade. That may be too much curve in the case of shaping a bench. Consider a coopering plane--with which I have no experience to share, but it is the tool designed for the job.

[Lee Schierer]

You can cut the concave surfaces by hand with a curved scraper. They also make a curved surform tool (https://www.hardwareandtools.com/stanley-tools-21-299-10-inch-1-2-round-surform-blade-efja-4521.html) that works like a hand plane (https://www.hardwareandtools.com/stanley-tools-21-296-11-1-4-inch-metal-alloy-plane-uljb-6630.html) that will cut a concave surface on your boards.
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[Edwin Santos]

I have a friend who makes coopered doors and large concave surfaces using a technique of multiple passes at the table saw. Make a pass, bump the fence over a blade kerf width, raise (or lower) the blade to the correct height, make another pass and so on. In this way you can hog out the curve with the series of saw kerfs and make a lot of sawdust in the process.

[Phil Mueller]

Of course a bit more labor intensive, and not knowing what tools you have, I’d rough them out with a coping or bow saw and fine tune with rasps, spoke shave, and sandpaper. A sanding drum on a drill press would also work well.

[David Utterback]

The cove cut can be cleaned rather quickly with the curved scraper that is included in many of the sets available. I have just been using mine in a chair seat.

Another option would be to use narrower boards but you may be limited by attachment to the cast iron frames.

[Jon Nuckles]

When I had to create a concave surface for a headboard, I made a hand plane to do it. I bought a radius edge Krenov-style blade from Hock Tools (http://www.hocktools.com/products/pi.html) and shaped the sole of the plane to match the radius of the blade. Took a little time, but it works great. You can find lots of tutorials online on how to make a Krenov-style plane.

[Mike Henderson]

There are limitations to the table saw approach, such as the need to clean up the surface, and also the fact that the shallowest curve possible is the diameter of the blade. That may be too much curve in the case of shaping a bench. Consider a coopering plane--with which I have no experience to share, but it is the tool designed for the job.

I don't believe that's correct. There are calculators that tell you what angle to address the board to the blade to give you various curves. Your comment assumes that you're addressing the board to the blade at 90 degrees.

Mike

[Edwin Santos]

There are limitations to the table saw approach, such as the need to clean up the surface, and also the fact that the shallowest curve possible is the diameter of the blade. That may be too much curve in the case of shaping a bench. Consider a coopering plane--with which I have no experience to share, but it is the tool designed for the job.


I don't believe that's correct. There are calculators that tell you what angle to address the board to the blade to give you various curves. Your comment assumes that you're addressing the board to the blade at 90 degrees.

Mike


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

[Zachary Hoyt]

I use a Kutzall disc in an angle grinder for concave (and some convex) shaping when building fiddles and mandolins and such, and it would work to rough out the shape for the bench if you have a steady hand.
Zach

[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 (https://www.finewoodworking.com/online-tools/cove-angle-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.428160

[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. ;)
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[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.

https://usercontent2.hubstatic.com/5013655.jpg

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.

[glenn bradley]

I think Derek has your answer but, if you really want to cove them.

428937 . 428936

[Philipp Jaindl]

I'll fully agree with Derek here, worked on a few Benches and Loungers (i hope thats the right word) over the years and always used flat slats. Fitting alongside the desired curvature is mostly done via Slat cross section, at most you'd want a convex upside for water to run off.
For outdoor Furniture you really dont wanna have any truely flat surfaces, which is usually not much of a problem with Flat slats due to Seat and Backrest having an angle to them anyways.

Regards Philipp

[Mike King]

Well, since Dennis posted a picture of his coopering plane and project, I feel the need to post mine. ;)
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You've got a ways to go! I also made a coopering plane that I used on my coopered chair backs.

Mike