woodworker (except SMITTY, of course) has ALL the answers. From time-to-time,
everyone hits a snag, trying to figure out some sort of in-shop problem.
Don't worry, SMITTY can help. Just use the special e-mail link to send your questions to SMITTY. He’ll do his best to get back to you soon, with the answers to those questions.
Here are the questions . . . and SMITTY’S answers for this issue!
If you're having a problem
setting-up, aligning or maintaining your Shopsmith equipment, you should
contact Shopsmith's Technical Support Staff (NOT Smitty).
veneer between layers of turning stock
This is a purely decorative approach and does not create a stronger bond between the woods. Assembling layers of contrasting woods merely looks snazzy.
tabletop has man baffled
Additionally, I was shown a dining tabletop which was built by creating a frame and strips of wood that were glued edge-to-edge and then fastened inside the frame. I am not sure how this was accomplished, but, once again, the strips are separating and the edges appear to be curling. What am I not understanding?
If the edges are curling up, one of two things are happening. Either the underside is taking on moisture (relative to the top), or the top is loosing moisture (relative to the bottom). Turning all the planks with the annual rings cupping down when you build your next table will hedge the bet toward flatness because, with equal moisture top and bottom, the annual rings tend to straighten. However, we think you have a situation with uneven moisture (obviously). Poly-U is a good sealer, but not even it will prevent cellular moisture loss. From research through the Forest Products Laboratory, no finish totally blocks moisture loss or gain except a thick coat (over 1/16") of paraffin wax.
Your problem has more to do with moisture loss rather than moisture gain, judging by your location. Even wood that is kiln dried to 6% moisture in a mill will loose moisture in the extremely dry desert air of Las Vegas. It may drop to 2% or even less.
Gluing planks edge to edge WITHOUT the plywood underlayment is your best bet. Also finishing the table surfaces equally top and bottom will help a great deal. Quarter-sawn wood is also much more stable (especially in oak) and it will only move half as much as flat or riff sawn boards.
You should add your thickness just around the edges, but your edging MUST follow the grain of the top. In other words, edge grain on the sides and end grain on the ends.
You must not frame the top with solid edge banding or restrict the movement of the wood in any way (such as with plywood). The top will still move, either splitting the top or the corners of the banding. You should also allow for the movement of the top with slotted or oversized screw holes in the table rails or use table-top clips in grooves in the table rails.
How much movement should you allow for? In your area (and only if the tables are going to stay in the desert environment), allow 1/16" per foot of width for quarter-sawn...and 1/8" per foot for flat sawn wood. If you ever plan to move these tables to a damper environment, these dimensions must be doubled.
How do furniture manufacturers get away with banding table tops, you might ask? They use veneers on mdf (or some other stable base)or some companies even vacuum impregnate their solid wood tops throughout with finish (like pressure treated lumber). When cured, all cells are either coated and/or filled with finish and in that way, the top is permanently stabilized. I hope all this helps.