Seasonal expansion

[petertay15]

Working on a coffee table for a friend. The top is divided into four panels of glass. She is uncomfortable with the kids banging things on the glass, and asked me to replace the glass with wood. I’ve made them of 3/4 inch walnut. End to end, I can make them kiss fit, but side to side must leave some room for expansion. If I cut some half inch deep kerfs on the underside of the walnut panels, would that absorb some of the expansion?

[jteneyck]

Working on a coffee table for a friend. The top is divided into four panels of glass. She is uncomfortable with the kids banging things on the glass, and asked me to replace the glass with wood. I’ve made them of 3/4 inch walnut. End to end, I can make them kiss fit, but side to side must leave some room for expansion. If I cut some half inch deep kerfs on the underside of the walnut panels, would that absorb some of the expansion?

Yes, but they will hump up when they expand and the kerfs try to close up.  Why not use 1/4 or 3/8" laminated or tempered glass.  It's really tough stuff and if they do actually break it they won't get hurt.  If it has to be wood, I would figure out how to let it float while still being full thickness.  Set them into a dado in the frame like you would a frame and panel door, for example.  

John

[AHill]

Black walnut expansion for full sawn boards is 0.00274 in/in/delta percent RH. If the panels are 24" wide, and the RH change throughout the year is 5% (I'm assuming inside a conditioned space), then the max difference will be 24 x 5 x 0.00274 = 0.3 inches. That's a lot. To be more pragmatic, I have a coffee table that has boards inside a frame and there are zero issues with expansion and contraction. You could make a subframe and then loose mount the walnut boards in that frame, similar to how breadboard ends are installed. You could tongue and groove each walnut board to add even more capability to accommodate the expansion and contraction. Many ways to solve the concern.

[ianab]

Black walnut expansion for full sawn boards is 0.00274 in/in/delta percent RH.  If the panels are 24" wide, and the RH change throughout the year is 5% (I'm assuming inside a conditioned space), then the  max difference will be 24 x 5 x 0.00274 = 0.3 inches.  That's a lot.  To be more pragmatic, I have a coffee table that has boards inside a frame and there are zero issues with expansion and contraction.  You could make a subframe and then loose mount the walnut boards in that frame, similar to how breadboard ends are installed.  You could tongue and groove each walnut board to add even more capability to accommodate the expansion and contraction.  Many ways to solve the concern.

Is that expansion for RH or MC? RH is the moisture in the air . MC is the moisture in the wood. Related off course, but it's a different curve.  5 % MC change is like going from 7% to 12%, which is a LOT. (and could move 0.3 of an inch). But to get that you have to move the RH from about 30 to about 70%. That could happen, but likely not in a climate controlled house. 

For the OP, plywood is your friend, it wont move enough to matter.

[barryvabeach]

As I realized the other day, while most houses are climate controlled most of the time , that is not always true on a few nice, though humid days, when someone turns off the AC to let the fresh air in the house   DAMHIKT.

[Willyou]

You might consider making the panels out of a good quality walnut veneer plywood. Unfortunately, a lot of the veneer plywoods these days have very thin veneers. If you can't find anything better, you might consider making your own using shop made or commercial veneer. By doing this, you won't have seasonal wood movement issues.

It is true that most homes are climate controlled these days and seasonal movement is not much of a concern under those conditions. However, when I am making something, I try to consider that someday the piece that I'm building might be involved in a family move and might be placed in non-climate controlled storage or on a moving truck for a while. Or, there could be an extended power outage due to storms, etc. So, I always assume that movement needs to be accommodated.

[petertay15]

Thanks. The discussion is appreciated. Especially the numbers for expansion. Seems as if the walnut veneer plywood would be the best solution. —Peter

[tablesawtom]

You might consider making the panels out of a good quality walnut veneer plywood. Unfortunately, a lot of the veneer plywoods these days have very thin veneers. If you can't find anything better, you might consider making your own using shop made or commercial veneer. By doing this, you won't have seasonal wood movement issues.

It is true that most homes are climate controlled these days and seasonal movement is not much of a concern under those conditions. However, when I am making something, I try to consider that someday the piece that I'm building might be involved in a family move and might be placed in non-climate controlled storage or on a moving truck for a while. Or, there could be an extended power outage due to storms, etc. So, I always assume that movement needs to be accommodated.

I have to agree, Wood will expand and contract plan on it.

Wood movement takes time and how the board is finished also affects wood movement, but allow for it. I would not put solid wood in a frame and panel situation for a top. In a door fine, but a top that allows for movement also has open spaces to dirt grime pinz and popcorn as well as spilled drinks to inter. I vote for a solid panel with two cleats slotted to allow for wood movement and not worry about how much the wood can expand or contract. No one will know or care if it ever takes place. I wouldn't do a bread board end either. the only time they will patch is the day they were sized.

[rwe2156]

How about either walnut ply or veneer?

But actually, I don't think there would be much worry with 1/4" tempered glass.  Properly supported, it really takes quite a hit to break it.  So maybe the solution is to  install a couple cross braces underneath?