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I make limited production hardwood lamps. One lamp is essentially a milled block of hard maple 7 1/2"W x 2"D x 11"high. A 1/4" thick piece of walnut is glued to the top of the block as a decorative cap. The grain of the 7 1/2" face of the maple block runs up and down. The cap's face grain runs left to right, i.e. perpendicular relative to the maple grain. Since wood movement occurs mostly from edge to edge I am worried that the walnut cap will eventually be split by the movement of the maple left to right. Could a solution be to saw the walnut in half prior to gluing? A clean cut could be hidden quite well by careful gluing and clamping. My theory is that each half of the walnut would then be free move with the maple as the maple expands and contracts from it's center. I'd appreciate any ideas you have.
The shrinkage calculator on this site can help you calculate the amount of movement one would expect in a block of maple that wide.
There are a variety of design approaches that would eliminate this issue - saw the cap out of a piece with the grain oriented the same direction.
Anchor the cap only in the center, leaving the ends free to float.
Mill a recess in the cap so that it fits over the maple, then glue it down in the center.
Build a test example then cycle it through humidity changes to see whether this is even an issue.
A glue seam does not reduce the movement in half. Not gluing the seam will provide the half movement you want, but not the look. Huang has given good options.
The stress will be on the maple block, not the cap. Rather, the cap won't split across the grain, but a long-grain crack might show up in the middle of the maple piece, or more likely, the glue will fail prematurely at the cap ends. Look up "bread board ends" for a traditional solution to a similar problem, then design a joint that accomplishes the same function while working within the limitations of your lamp design.
Thanks for all the input. I'd never considered that the block might crack instead of the cap! I like Huang's idea of anchoring the cap only in the center but unfortunately that wouldn't give the permanently tight (no gaps) interface I need between the flush-trimmed edges of the cap and the face and sides of the block. I'll continue to monitor the effect of seasonal changes on the lamp and alter the design if necessary.
I have seen objects built with classic cross grain construction - they have remained sound for years. There are a lot of variables involved in whether or not any particular design or joint fails.
The thing is, you won't have flush edges between the cap and the block very long. Something is going to give with the seasonal change, and they won't be flush.
You're right Rich. Actually it's the reason I submitted the question. I've had a finished version of this lamp for about a year now and noticed this summer that the sides of the block had moved 1/32 or 3/64 past the edge of the cape. The glue seal still looks fine (the glue moved?) and no cracks have developed. I'm assuming with dryer winter air they will move flush again. Visually it's OK because it's only perceptible when you rub a finger across the edges. Time will reveal the real consequences of this joint.
Dan, time has revealed the real consequences of this joint. You're getting some good advise above from folks who have put this knowledge into practice.
Observations on cross grain construction:
I have made cross grain construction items for 40 years with very few problems. The most extreme example are many bread boards (for making bread) with breadboard ends. Most of these are about 18" wide, with quartersawn hard maple in 1-3/4" wide strips. Most had 3/16" wide Walnut strips between the Maple. This board is surfaced to about 7/16" and then tenoned on each end (about 3/16" thick by 5/8" long) and a Walnut end cap is fitted, about 1" x 1", plowed for the tenon.
These have been made with resorcinol, TB3, TB1 and TB2 as well as epoxy. They get a mineral oil finish, and little maintenance over the years. That means these are very prone to environmental changes in humidity, therefore wood movement.
I have had no joint failures, or even movement, period. 18" wide. I attribute this to the Q/S Maple and the surface area of the tenon. As well as the fact that these are almost never soaked in a sink or get more water than the occasional rinsing and drying.
So....if you were to increase the surface area of the joint on your two parts - splines, tenons, even rabbets - this should reduce or eliminate the movement, or the appearance of movement. A step between the two parts will also disguise any movement.
If absolute flush is required, then man-made core and veneers should be used.
Thanks David. So it looks like the easiest solution may be to use quartersawn or rifsawn boards for the maple block. I've been using 6/4 plainsawn wood milled into two pieces each 1 3/16 thick. Those pieces are then glued (face to face) creating a block 2 3/8" thick. But plainsawn gives me maximum movement across the 7 1/2" width. Qtrsawn or rifsawn will reduce and maybe eliminate noticeable movement. The question is whether I can find those boards in the retail barn where I purchase my stock. I'm not into large volume wholesale purchases or custom orders yet.