Popular Woodworking 2009-10 № 178, страница 8

Popular Woodworking 2009-10 № 178, страница 8

- Letters ■


quartersawn orientation. My question is: How important is vertical grain orientation in a benchtop? I accidentally purchased a few boards that are more quartersawn, which would become flat-sawn in the lamination.

2) I have read that effective clamp pressure radiates out at a 45° angle on either side of the clamp face. If this is so, your recommended clamping strategy of one to two clamps every foot (no cauls), would not apply even pressure to the joints of the outer boards. A 2" caul would be required at the edge to even out the pressure with one clamp every 6". Should I be worried about this?

3) Lastly, I have also read that the required clamping force for woodworking joints is a function of the joint's surface area and the wood species and grain orientation:

Joint length x height x species (quartersawn or flat-sawn) factor = total clamping pressure required.

If this is true, the j oint of a massive Roubo lamination would require a Herculean amount of pressure:

4" x 96" x 150psi (approximation for Southern yellow pine) = 57,600 pounds.

Calculation of number of needed clamps: 57,600 pounds/600 pounds per parallel-j aw clamp = 96 parallel clamps.

This is impractical. However, are benchtop laminations typically way under-clamped?

Scott Vanzo Los Angeles, California

1.1 haven'tfound the quartersawn stragglers to be a problem. I don't find that they stick up or become depressed. Softwoods move so little in service anyway (once they are at equilibrium).

2. One or two clamps every foot does the trick if your mating surfaces are well-prepared. If your surfaces are flat you can get away with even less (though I rarely do). Yes, you want the radiating clamp pressure to overlap across the joint lines. But just because it doesn't overlap in some areas does not mean the joint will open up or fail. That's because that "undamped" area is co-planar to areas that are clamped. So the mating surfaces are brought close enough to form a bond. Quick historical note: Early joiners used one clamp (or no clamps — it's called a rub joint) for panel glue-ups. A lot of this modern engineering of laminations is overkill.

3. I've seen this formula and it flies in the face of everything I know from my life of wood

working. You do not need that many clamps. Promise! I wish I had an engineering degree to explain why, but I don't. All I have is my work and the historical record.

I hope this helps. And best of luck with your workbench.

— ChristopherSchwarz, editor

Dispelling Bubble Myths?

I've read articles where Bob Flexner likes to bust myths about finishing, including "How to Brush a Finish" (December 2008, #173). I've been waiting and waiting to read Bob bust his own myth about bubbles in finish. But time and again I see he still promotes the same incorrect theory: "There's no way to avoid them because they're mostly caused by the turbulence created by the brush movement (the same as underwater bubbles created by the turbulence from a propeller), not by shaking or stirring.''

Who puts on a finish coat while whipping their brush like a propeller? Nobody. I see bubbles even if I apply finish with a rag (the first coat or two, anyway). No propeller action there. I even drizzled unshaken finish on wood from a stick, and in each drop, bubbles arise. So saying that bubbles are "caused by the turbulence created by the brush movement (the same as underwater bubbles created by the turbulence from a propeller)" is a myth.

Bubbles must be the air in the wood pores being displaced by a viscous liquid; the displaced air rises through the liquid and as a result, we see bubbles. (It happens with water, too, but the bubbles go immediately to the surface and pop because water isn't viscous.) It is basic physics. It doesn't matter what the finish is or how it is applied; if air is being displaced by a viscous liquid, bubbles will rise.

Bob's theory makes it sound like a side effect of the brush application technique, or an operator error (what else would you call a "propeller effect?"), but it is neither. It is simply what happens when air inside the wood gets displaced by a viscous liquid. And it also is why a thinned finish sealer coat has less chance of trapping bubbles; it is less viscous.

What do you think? Have I burst his myth?

Spence Bloom via e-mail

My physicist son explained to me the principal cause of bubbles when brushing. It's a physical phenomenon called "cavitation," in this case turbulence caused by the brush movement. Take a look at the photo below of brushing varnish onto an aluminum surface: no pores, no shaking or stirring, and still there are bubbles.

You are right, however, that bubbles can appear above the pores of large-pored woods such as oak due to air displacement.

The one thing I think we agree on is that manufacturers are wrong when they assign the cause to shaking or stirring a finish. That's the myth to which I was referring. PW

— Bob Flexner, contributing editor

Question? Comment?

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Popular Woodworking welcomes comments from readers about the magazine or woodworking in general, as well as questions on all areas of woodworking. We are more than happy to share our woodworking experience with you by answering your questions or adding some clarity to whatever aspect of the craft you are unsure about, and if you have a complaint, we want to address it whenever possible.

though we receive a good deal of mail, we try to respond to all correspondence in a prompt manner. Published correspondence may be edited for length or style. AI l correspondence becomes the property of Popular Woodworking.

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14 ■ Popular Woodworking October 2009

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