Q & A

I'm building some furniture for my son -- bed, headboard, bookshelves. We're going to use plywood. Is there a formula for determining what thickness of plywood I should use? I can guesstimate it by looking at the thickness and how much I can get the large sheet to bend, but is there a rule that determines what thickness is needed for given dimensions and expected load?- Chip McDaniel

Tim Inman: From your question, it sounds as though you are wanting to make curved or bent surfaces. Without knowing more, it is very difficult to give good advice. However, if you are wanting to make pieces with significant curves, I'd suggest using a product called “bending ply” for your lay-ups. For serious work, I'd laminate multiple pieces of 1/4 inch bending ply to get the curve, and the thickness, you'll need for your structure.

Chris Marshall: I'm going to approach my answer from the standpoint that you are only talking about load ratings for plywood and not about bending it to make curved rails and so forth. I'd use 3/4-in.-thick plywood for all of your furniture parts except for the back of the bookcase. You can use thinner plywood there, but that will depend on your bookcase design and how sturdy you build the rest of the case. In regard to the bed frame, I'd use 3/4-in. plywood to support the mattress and box spring, and I would consider how to stiffen that surface even more with cross rails. Keep it stiff, sturdy and, consequently, safe.

I have a Ryobi biscuit jointer that cuts slots for 0, 10, and #20 biscuits. I upgraded to the newer unit because I had a smaller unit that cut slots where the biscuit would literally flop around in the slot, making a good joint impossible (unless I did something wrong). Anyway, with this new jointer, the biscuits still flop around, only not as much. But a tight fit there isn't. When possible, I watch The New Yankee Workshop with Norm Abram. I noticed when he uses a biscuit jointer, the biscuits fit tightly, with no flop. So my question is this: Are there consistency standards in the biscuit manufacturing industry, or are the machines that cut the slots using different size cutters? I am annoyed when I purchase a unit like this and can't get a good outcome. Or am I doing something wrong? Please help. - Tom M.

Rob Johnstone: While there is certainly such a thing as too loose in the slot, biscuits should slide easily into the opening. The biscuits themselves are compressed and expand when they come in contact with white or yellow glue, thus fitting more tightly after glue-up than before. I don't think you are doing anything wrong as long as you are not trying to "free-hand" the cuts without supporting the biscuit joiner on a surface of some sort. It is true that the high quality biscuit jointers (for example, a Lamello) cut more accurately and have more exacting tolerances. It is possible your cutter is loose or bent, thus making a slot that is too wide for a typical biscuit, but I sure couldn't know that without looking at it. I hope this helps.

Tim Inman: Have you tried a different batch of biscuits? This sounds too simple, but it may not be the tool, but rather the insert biscuits that are undersized If you get a sloppy fit from more than one source of biscuits, then the tool - or the tool user - need to be questioned. My biscuit joiner leaves me with a nice, snug fit -- with multiple sources of biscuits. If I need a looser fit, I can manipulate the tool and make the groove sloppy. So from that, I can say the operator does have control over the fit. Maybe you're making multiple cuts on the same groove? Be sure the guide surfaces on the cutter are flat and seated before you plunge the cut.

Chris Marshall: The upside to biscuit jointers is that you can cut slots lickety-split. But speed can lead to sloppy cuts. As with the other answers here, I can't be sure if it's your machine or a bad batch of biscuits that could be to blame. However, make sure your technique is solid, and you can eliminate one variable from the equation. The tool's contact surfaces need to be planted firmly against the edge of your workpiece, and they can't move at all when plunging the cutter in. Jiggling the tool even the slightest bit will widen the slot more than what you want. Work slowly and carefully and see if that helps you achieve better results.

I just got a Box Joint sled from Rockler and started to make small drawers from Baltic birch plywood. Is there a trick to prevent the tearout from the spiral router blade on the plywood? I was thinking of either rolling glue onto the edge or sandwiching the plywood between two sacrificial pieces of wood. Any suggestions? - Abbott Fleur

Tim Inman: Perfection is always more difficult. If the little tearout is too much, then some sort of "sacrificial" surface is probably your best answer. Sometimes a little tape can do the trick. Adhere masking tape along the edge before you make your cuts. That jig makes it easy to lay another piece over the "real" one. I'd suggest something like 1/8-inch Masonite or hardboard to protect the finish wood, and let the bit cut clean. You may need to sandwich the workpiece between two sacrificial ones so both the front and the back cut clean.

Chris Marshall: Abbott, I second Tim's motion to use a thin "backer" board behind the plywood (shown in this photo)— and in front of it, if you are getting tearout there too. Make sure your spiral bit is razor sharp and clean. Here's another suggestion that once worked for me with Baltic birch. I was using a dado set for cutting box joints and was getting unacceptable blowout behind the workpiece. So, I brushed on a coat of Zinsser SealCoat (it's just dewaxed shellac) first to lock the face veneer fibers. It dries super fast and worked like a charm to help me achieve cleaner cuts. Then, I used more SealCoat as my final topcoat for the drawer boxes.