Conventional Wisdom - Slab Glueups

[RonB]

(Also Posted in abpw)

I would like to run this past the group. For years we have been told that the best way to glue up slabs, like table tops, was to alternate the direction of growth rings using 4"-5" boards. Occasionally this caused us to rip and join a perfectly good 18" to 24" slab of hardwood and end up with a slab with discontinuous grain pattern -- necessary for the integrity of the slab.

A couple of years ago I was building a 8/4 Oak table top for a coffee table and was unsure of the best rip widths for the glue-up assembly with this heavy material. I posted to forums of a couple of woodworking magazines and was surprised at a response. A magazine editor said that with heavy stock, with fairly large radius growth rings, it was ok to rip at 4" to 5" and glue them back together in their original position. He claimed the glue joints did an adequate job of relieving the stress and I could retain the original appearance of the slab.

The post came slowly -- after I had ripped and glued in the conventional (alternating) manner. The top looked fine but I still wonder.............????

ANY INPUT?

[Jay Pique]

Disclaimer: I really (really) am no expert, I'm just giving my input...

Say you rip the slab with a glue-line rip blade and the pieces don't need rejointing when you are done - there has been zero movement. It seems to me that with regard to internal stresses existing in the wood, you've either done nothing to relieve them or they didn't exist in the first place. Then you reglue the pieces back together, and you've lost a few kerf-widths of wood. I'm not sure you've accomplished anything. Does the fact that wood glue is stronger than the wood itself make the slab stronger as a whole? Maybe very, very slightly. If the pieces needed significant rejointing, my guess is that you have probably increased the structural integrity of the board.

However, it's my understanding that the reason you alternate flipping the boards over is to create an alternating warp pattern. Rather than risking the whole board being in one big cup, you end up with a series of little up and down cups across the width of the panel that tend to even each other out.

JP

********* Warped.

[Lenny]

There are certain factors to take into account : Will the slab be held in place (such as a tabletop or a countertop) which would help keep it from "cupping" , as opposed to.... say a lid for a blanket chest or such. Is the wood quartersawn, in which case the tendency to move or cup is greatly reduced. In our shop we generally go for "the best face", matching color and grain . If it IS for a tabletop, be sure to allow for movement by enlarging the holes for the screws that hold it down. (or by other methods)

[RonB]

[RonB]

As I noted in response to Jay above, I am having trouble with cupping not being a factor even when restrained as a table top. My thinking (right or wrong) is that an 8/4 slab of oak might be able to generate enough cupping force to damage itself as well as table structure.

Again - any one else have experience with this.

R>

[J T]

Sun, Aug 1, 2004, 1:07pm (EDT-1) [e-mail] (RonB) says: (Also Posted in abpw) I would like to run this past the group. For years we have been told that the best way to glue up slabs, like table tops,

Dunno. I suppose in part it'd depend somewhat on what your definition of "slab" is. I've seen coffee tables with a top cut dianogally from logs, they're one piece tops, and seem to be doing fine. Not sure how thiick they were. Some of them were some pretty damn bit tables too. Some had the bark left on (not my choice), and some off.

JOAT The highway of fear is the road to defeat.

- Bazooka Joe JERUSALEM RIDGE

[bridger]

absolutely. you have to use some common sense. the skirts of the table can help to hold the top flat, but it would be ridiculous to expect for instance a 3/4" x 3" stick to restrain an 8/4 top that really wanted to twist around.

you can estimate the capacities of the parts and what they will do in opposition to each other. set some blocks a distance apart and span them with the part in question. apply force in the center and note the deflection.

the thing that makes a table stay together is getting all of the wood to a decent moisture content and milled for stress relief before assembly, not forcing it to dry straight after assembly.

[George]

First, realize that it's not specifically the orientation or the absolute radius of the rings that count in cupping, but the difference between the smallest and the largest arc in the same piece. The science of shrinkage says the earlywood shrinks more than latewood. Where there is proportionally more latewood - near the heart, it shrinks hardly at all.

Look at a log left to dry on its own, and you'll see that it cracks radially to relieve this induced stress. Split the log green, and notice that it dries with no radial checks, but with a crowned center. Note also that the crown you see most often - on softwood, is the result of a large change in moisture content, beginning at the fiber saturation point, a condition the boards should never experience again.

The answer, to me, is to get rid of that center, something rarely a problem when hardwood lumber is sawn for grade. Only when it is sawn through-and-through as softwood commonly is, does it become a real problem. That's where and why I'd rip - the center of the tight arc to relieve stress, and then I'd probably find the best match not by reassembly of cut faces, but by mating sapwood sides.

As usual, Hoadley has a good secti>

[Larry Jaques]

On Sun, 1 Aug 2004 13:07:05 -0500, "RonB" calmly ranted:

I attended a 3-day seminar by Frank Klausz and he had a definite opinion on this. He said he uses the best face of each board on the top, regardless of growth direction. Use dry wood and seal all sides and you won't have a problem.

For those who alternate growth ring direction and do a poor finishing job, he said they'd get wavy slabs instead of cupped ones. Then he laughed with us. (I trust his word since I haven't built any large/thick/wide-slabbed furniture yet.)

Best bet: Use quartersawn wood for flatter slabs. ;)

Would you like to buy some -more- swamp land, sir?

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"Larry Jaques" wrote

You also get a panel in which the grain of each section runs in alternate directions - not much fun in planing along the area of the joint!

Jeff G