Question re: Cutting cove on table saw

[Art Greenberg]

Merry Christmas, all!

I'm about to attempt cutting a cove on the table saw, for a picture frame moulding. This will be my first attempt at this.

I have a nice hunk of 8/4 poplar 8 inches wide that I'll be cutting. Once the cove is done, I will rip the board down the middle to yield two lengths of moulding. The board is flat sawn, and the arc of the growth rings is fairly symmetric. Should I cut the cove with the arc up (so the cove will follow the rings) or with the arc down (so the cove will cut through the rings)?

I imagine that cutting so the cove follows the growth rings would result in a stronger moulding. But I'm not sure about that and other issues, such as wood movement.

TIA.

[Swingman]

"Art Greenberg" wrote

Easy to do, a bit fussy to set up, but safe, fun, and satisfying ... providing you think it all out carefully beforehand and have enough stock on hand for some test passes.

IME, probably the only thing you'll gain by grain orientation with the flat sawn stock on a TS cove cut is maybe a bit more smoothness, and slightly less clean up, by cutting "downhill" instead of "uphill" (IOW, looking at the edge of your board, I would generally tend orient it to make a right to left pass over the blade with the grain sloping to the right).

... but in actual practice I really don't think you gain too much either way, including the issue of "strength" in the final product. At least that's been my experience.

How much "strength" to you need in a molding anyway? I would go with a couple of test cuts to see which makes the best/most pleasing grain pattern for your project, and not worry too much about strength.

As a side note, the TS cove cut can end in some unusual grain effects ... take a look at the corner cabinet project on the following link and notice the "half moon" effect of the medullary rays in the cove cut on the quarter sawn crown molding in the last couple of pictures:

[DonkeyHody]

Art, I built a fireplace mantle with a 5" radius 90 degree cove. If you place the arc of the growth rings up so the cove follows the growth rings, you will get only a few grain rays spaced far apart, sort of like rotary cut plywood. If you put the arc of the growth rings down so you cut through more rings, you'll get a grain pattern more like dimensioned lumber. If you can find a way to hide it, glue a piece of plywood to the back surface of your cove. That will stabilize the cove where it gets thin. Be prepared to do a lot of hand sanding to smooth out the cove surface. Even with a fairly thick blade, it tends to chatter a little. It just wasn't made to take that side load. I took a scrap piece of the cove I had cut, dammed up each end and poured it full of plaster of paris. Then the plaster is used as a custom-made sanding block to smooth out the cove.

Good Luck with your project, and Merry Christmas.

DonkeyHody "Every man is my superior in that I can learn from him." - Thomas Carlyle

[J. Clarke]

I'm curious--did you try cutting to maybe 1/16 of the finish dimension then make a finishing pass? Or does even that little bit of stock produce enough lateral resistance to cause chatter?

[DonkeyHody]

None of the passes cut more than 1/8 or so of material. All of the last 10 or so passes were at 1/16" or less. The final pass was probably 1/64" or so, then a cleanup pass without moving the blade at all.

But you have to understand that I had several things working against me. I was moving the wood directly in line with the axis of rotation to get the widest cove possible from a ten inch blade. Narrower coves would move at an angle to the blade, which should cause less chatter. The piece being shaped was over 8 feet long and it had a very slight bow in it that had to be pressed flat to get a consistent depth of cut. And the cove was so deep that at the final cut, a full quarter of the blade's circumference was in contact with the wood. A good deal of feed pressure was required. After the operation on the table saw, the cove surface was about as rough as rough-sawn lumber from the sawmill. I spent about an hour with each sandpaper grit, starting with 60 grit and ending with 220. In the end, it turned out beautiful, but it was a lot of work. I'd do it again.

DonkeyHody "Even an old blind hog finds an acorn every now and then."

[J. Clarke]

Did you experiment with tooth profiles?

[DonkeyHody]

No, my best blade is a Freud Glue Line Rip Blade. I use it for everything. I considered trying one of several other blades I have, but I would have had to move my guides I had clamped down in order to access the blade. I didn't want to risk losing the alignment I already had started, so I kept going.

DonkeyHody "There's a difference between doing things right and doing the right things."

[dpb]

For cove cuts initially (that is the roughout) I have found that a regular 20-something crosscut steel blade w/ good set works best.

--

[toolman946 via CraftKB.com]

How do you intend to finish the molding? Poplar is a very stable wood but it doesn't offer much in the way of "grain". If you're planning to paint the molding, grain orientation is a moot point.

The earlier suggestion about gluing a plywood backer board for added strength may seem like a good idea but you'd likely experience problems due to the differential in how the plywood and poplar would react to seasonal changes in humidity. If you feel the need to add a backer board for additional strength, use a piece of wood with the same response to seasonal changes.

[Art Greenberg]

Yep, I did say poplar. I probably should have mentioned that I'm going to paint it. I didn't think anyone would start discussing grain effects. I am concerned with proper technique, and strength of the finished product.

I'll probably have enough material behind the cove (at least 3/4 inch) to assure it will be strong enough. This isn't for a crown moulding, its for a picture frame. As I mentioned in my original post, I'm going to rip the piece down the center once the cove is cut, to create a picture frame. After ripping, the thinnest part will be the inside of the frame.

Thanks to all for your suggestions!

[Art Greenberg]

I think whatever happens (short of not cutting it straight, or the whole thing turning to dust after its cut), the result will be OK. This ain't furniture I'm making (not yet, anyway). I figure a picture frame will be a great learning exercise.

OK, thanks. I hadn't thought about the grain going lengthwise. So you're suggesting I look at this in the same way I'd approach jointing, going in the direction that will minimize tearout?

Perhaps I didn't state my question clearly. Looking at the end of the board, I can see the growth rings. The rings are symmetric. (i.e., if I come off the centerline of the board at a 90 degree angle, that line would go through the center of the tree.) This means the arc of the cove will either be concentric with the growth rings if I have the "open" face of the board down when I make the cut, or it will intersect them if I have the "open" face pointing up.

Something to consider when I'm making something out of a nicer wood than poplar. I intend to paint this piece. But once I've got a little experience with this under my belt, I can see myself making some mouldings out of figured maple.

the same style (I lean heavily toward modern/contemporary), but you provide plenty of inspiration nonetheless.

Thanks for your input, very much appreciated.

[Art Greenberg]

I'm not really concerned with the appearance of the grain, as this is poplar. I'm going to paint it. Sorry that I wasn't explicit about that.

It won't be very thin, its going to be a relatively shallow cove in 8/4 material. After I rip it, I need to cut a rabbet in the back on the thinner edge to mount the picture. So I'll need enough thickness there to do the mounting.

Yeah, that part I was wondering about. I have a curved scraper that I was planning to use. The custom sanding block is a neat idea, too.

Thanks, and same to you!