Dishing a 2"x8" board - How to?

[Kyle Kaldor]

I'm thinking about a project that I want to "dish" a board along its length with a large concave radius. Any ideas on how to do this? And don't say "buy a cupped board and plane one side" that would be too easy ;)

[Steve Meliza]

I just saw a jig for making crown molding cuts similar to this in the Rockler catalog that landed in my mailbox yesterday. The jig holds the board at and angle to the table saw blade, the angle determining the shape of the cut. Here it is: http://www.rockler.com/product.cfm?page=17468&filter=molding%20jig

[Conrad Fiore]

Do a cove cut on your table saw using a wider board. After the cove is established, turn the board over and rip it to width.

[Van Huskey]

Since you have a 20" R in your plan I am not sure a simple cove cut on the table saw will suffice.

[Kyle Kaldor]

Since you have a 20" R in your plan I am not sure a simple cove cut on the table saw will suffice.

Right, I thought of that, but I'd only be able to get a 5" R with the table saw.

[Sam Murdoch]

I think that the table saw cove option is limited to the diameter of the portion of the blade sticking up. You can get a big cove if you are willing to raise your blade high enough. Caution - your board needs to start out thick and it becomes thin and fragile as you run the cove - potential to push through as your board collapses on the spinning table saw blade.

The only way that I can think of to cove an 8" wide board without hand tools is to make up a curved sled that your router can slide on across the width and incrementally down the length of the board. BUT, stand by, I'm sure someone here in The Creek has done it another way. I look forward to getting smarter :D.

[Kyle Kaldor]

I do have one idea of making a jig that uses a router on a curved "track", but I wanted to know if there was an easier way.

[Richard Wolf]

No, you don't understand the concept of making the angled cut, you are not limited to 5". Check out this calculator; http://dknudson.unixcab.org/covecut.html

[Kyle Kaldor]

The only way that I can think of to cove an 8" wide board without hand tools is to make up a curved sled that your router can slide on across the width and incrementally down the length of the board. BUT, stand by, I'm sure someone here in The Creek has done it another way. I look forward to getting smarter :D.

Sam, we were thinking on the same page, you just beat me to it. My idea came from Nick Offerman's Router Jig to flatten large slabs (http://www.finewoodworking.com/SkillsAndTechniques/SkillsAndTechniquesArticle.aspx?id=34272). But I would use a concave arched sled, instead of a flat one.

[Van Huskey]

No, you don't understand the concept of making the angled cut, you are not limited to 5". Check out this calculator; http://dknudson.unixcab.org/covecut.html

I didn't run the numbers but I don't think you can get a 20" radius cut on a 8" board with a nominal 10" saw blade no matter how what the skew angle is, but I could be wrong!

[Kyle Kaldor]

No, you don't understand the concept of making the angled cut, you are not limited to 5". Check out this calculator; http://dknudson.unixcab.org/covecut.html

Richard, I'm pretty sure I understand the concept. Correct me if I'm wrong, however. The largest radius/cove that you can achieve is by running the piece perpendicular to the blade, right? Would this not be the diameter/radius of the blade? If I have a 10" diameter blade, I can only get a 10" diameter cove (5" radius).

[Joe Hillmann]

If you have a hand plane you could sharpen the blade with a slight curve and it shouldn't take too long to get a cup in a board.

[Sam Murdoch]

Kyle, using something less gargantuan than that shown in the article :). I have not made such a sled but I'm confident that it can be done. I am the kind of guy that would have to make one up through trial and error though. 2 rails to saddle the board and a couple of properly curved sled sides on which the router would glide into the curve and out again. You might need to remove the router base and fit it with a narrow board that would allow the travel in the curve. As for Richard's post above with the calculator - there are limiting factors that I don't think will allow the table saw to produce the size cove you are intending.

[Jason Roehl]

If you're stuck on a strict, circular 20" radius, that is not possible with a 10" blade making a cove cut. However, if you're okay with an elliptical curve, it MIGHT be possible. By tiliting the saw blade, and running the board perpendicular to the blade (I'd do it about 1/32"-1/16" at a time), you might get an acceptable cove.

[Richard Wolf]

Your right, I stand corrected.

[Chris Ambroson]

http://books.google.com/books?id=4Xf9FSm6oqUC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
Page 28 of this PDF is the first thing that comes to my mind.

[Wes Grass]

A 10" blade tipped over 45° won't get you anywhere close to a 20" radius. Not even over a very narrow range. *IF* you could tip it 77°, it'd look awesome at 6" wide. Or, only 75° with a 12" blade looks really good at the full 8" width ;-)

I think the only reasonable way to do this on a table saw is to draw the outline of the curve on the end of the board and notch it out by eyeball with a normal coving setup. Or set each pass off a template.

BTW, you need to cut .404 deep over an 8" width to get a 20" radius.

[Myk Rian]

I just saw a jig for making crown molding cuts similar to this in the Rockler catalog that landed in my mailbox yesterday. The jig holds the board at and angle to the table saw blade, the angle determining the shape of the cut. Here it is: http://www.rockler.com/product.cfm?page=17468&filter=molding jig
Don't need to buy a jig. just clamp 2 boards across the table saw to run the board between.

[David Posey]

What kind of project is this for?

If it just has to look good, I'd use one of the above suggestions and use a handplane with a cambered blade. If it needs to be the full thickness at the ends, use one with the shortest body you can find. You could even take an old woodie smother and reshape the bottom to fit your purposes. Lots of boatbuilders do this. Other hand tool options would be a travisher, convex spokeshave, or scorp, although that last one is going to leave a pretty rough surface.

If, on the other hand, it needs to be machinist-accurate or you have to do a whole bunch of them, you might have to do the router jig thing. Just keep in mind how much time that setup might take you.

[Edward A Miller]

224916A router sled jig might work for you. Quite accurate and simple to build. Remember that the radius of the cut is to the bottom of your router bit, so the radius on the jig will be less by the extension of the bit from the router base; setting the depth can be simplified by setting the depth to the top edge of the fence sides (you'll discover this as you set the router bit depth). This was crudely build to illustrate a point, but if you want a clean project, put some plastic face plates on the sled fence with double sided tape.

Take your time routing because you're asking the bit to cut quite a bit toward the bottom of the arc. Cutting with 1/2 - 3/4 of the bit width will be less demanding than trying to go full width at a pass.

If this was going to be a full time job (?), Two routers, one a hog and one with a fresh cutter would produce superb finish. Also, the longer the sled fences, the smoother the sliding action.

Hope this helps!

[Gary Kman]

I'd rip a dado down the center of the board at the final depth, move the fence and make shallower cuts on each side of the first etc. Clean up with a plane or coarse sandpaper on a curved block.

[John Coloccia]

I'm thinking about a project that I want to "dish" a board along its length with a large concave radius. Any ideas on how to do this? And don't say "buy a cupped board and plane one side" that would be too easy ;)

Could I ask what you're using it for? Anyhow, if it doesn't need to be precisely 20" and dead straight, and somehow I get the sense it does, a simple method would be:

1) draw the radius you want on the end grain
2) grab a cove bit and stick it in your router table
3) run the board over the bit, moving it over little by little and adjusting the depth to match the pattern you scribbled
4) sand/scrape to get rid of inevitable bumps and imperfections, and you're done.

It shouldn't take but 10 minutes to knock this out without any fancy schmancy jigs or calculations. :rolleyes:

:p

The reason I like a cove bit for this is that it lets you take as small a bite as your patience will tolerate. A straight bit will not let you get to the bottom of the cove without leaving an ugly flat that will be very hard to get rid of later.

[Ted Calver]

Depending on how many you want, you could build a router lathe like the one Seth Dolcourt uses to make his drum shells. Look for some of his build posts in the musical instrument forum. Build a staved shell using 2 x 8 material tacked together at ends and middle, radius to 20 inches and take it apart. Voila!

[Sam Murdoch]

Could I ask what you're using it for? Anyhow, if it doesn't need to be precisely 20" and dead straight, and somehow I get the sense it does, a simple method would be:

1) draw the radius you want on the end grain
2) grab a cove bit and stick it in your router table
3) run the board over the bit, moving it over little by little and adjusting the depth to match the pattern you scribbled
4) sand/scrape to get rid of inevitable bumps and imperfections, and you're done.

It shouldn't take but 10 minutes to knock this out without any fancy schmancy jigs or calculations. :rolleyes:

:p

The reason I like a cove bit for this is that it lets you take as small a bite as your patience will tolerate. A straight bit will not let you get to the bottom of the cove without leaving an ugly flat that will be very hard to get rid of later.


If you have a router table - this is a GREAT and SIMPLE solution! Work from the outside edges- each edge to the fence before raising the bit - 2 passes again - raise the bit - 2 more passes , etc, etc. done!

[Kyle Kaldor]

224916A router sled jig might work for you. Quite accurate and simple to build. Remember that the radius of the cut is to the bottom of your router bit, so the radius on the jig will be less by the extension of the bit from the router base; setting the depth can be simplified by setting the depth to the top edge of the fence sides (you'll discover this as you set the router bit depth). This was crudely build to illustrate a point, but if you want a clean project, put some plastic face plates on the sled fence with double sided tape.

Take your time routing because you're asking the bit to cut quite a bit toward the bottom of the arc. Cutting with 1/2 - 3/4 of the bit width will be less demanding than trying to go full width at a pass.

If this was going to be a full time job (?), Two routers, one a hog and one with a fresh cutter would produce superb finish. Also, the longer the sled fences, the smoother the sliding action.

Hope this helps!

Ed, this is similar to what I had in mind, but my idea wasn't quite as simple. Thanks!

[Kyle Kaldor]

Could I ask what you're using it for? Anyhow, if it doesn't need to be precisely 20" and dead straight, and somehow I get the sense it does, a simple method would be:

1) draw the radius you want on the end grain
2) grab a cove bit and stick it in your router table
3) run the board over the bit, moving it over little by little and adjusting the depth to match the pattern you scribbled
4) sand/scrape to get rid of inevitable bumps and imperfections, and you're done.

It shouldn't take but 10 minutes to knock this out without any fancy schmancy jigs or calculations. :rolleyes:

:p

The reason I like a cove bit for this is that it lets you take as small a bite as your patience will tolerate. A straight bit will not let you get to the bottom of the cove without leaving an ugly flat that will be very hard to get rid of later.

John, this is a great idea, too! Probably a little more sanding and finishing than the jig method, but still an option.

It does not need to be exactly 20" radius, and to be honest, I don't even know what the radius will be yet, but it will be larger than I can accomplish with a cove setup on a table saw. Straight, as always, would be greatly preferred ;)

I am going to use this for the stretchers of a trestle table. I want the convex shape to match the convex shape of the legs that they will attach to. Make sense?

[Edward A Miller]

Kyle, be certain to use a flat work surface on which to place both the workpiece & jig and that the jig is slightly higher off that work surface than the upper face of the workpiece. Otherwise, the jig might start out gliding directly on the surface of the workpiece, then tilt sideways as the workpiece surface is routed. IE: The jig should slightly clear the top of the workpiece like a bridge. The guiding jig rails could also be slightly wider than the (3/4" each) test jig used. Enjoy.