Shelf Design Questions...

[ptt49er]

After I got the crib built back in July, the wife started in on wanting a changing table. Personally I don't see what's wrong with using the folding table we have now but SWMBO wants one bad, so I guess there is no alternative.

I am building the table in a design to mirror the crib and it's gonna end up being pretty much a set of book shelves.

I saw a sofa/window table someone built a while back and they ran the shelves with the grain perpendicular to the length of the shelf. Kind of opposite the normal direction. I really liked the look of this, not to mention it's different.

I am wondering if running the grain this way will provide too much wood movement or is there someway to allow for this movement? I'm using white oak if that helps with the amount of space needed for proper movement.

WOW, that's long winded...I'm off to bed, hopefully tomorrow I'll have some WIP pics to share.

TIA!

 

[DavidF]

My first thoughts were that the perpendicular grain top would be fine if it was on top of a window seat box construction type of thing so that there would be very little deflection. The cross grain would be a very week point, as would the glue lines. If the "shelf" is unsupported I wouldn't do it that way.

 

[Ray Martin]

I agree with David on the strength of the shelf. If you have grain running perpendicular to the width of the shelf, it has to be supported. The challenge would be in providing that support because it might set up a cross grain joint that would crack as the shelf moved over time. If you can provide loose / unattached support that might stop the shelf deflection that would result in a crack. Perhaps a heavier (1/2" or greater) backing on the shelves, with a dado into which the shelf fits (no glue, of course) might offer that support. A face frame could hide the space between the shelf and the sides of the case, with that space allowing for wood movement.

It can be done, but you will have to add some components to make up for the structural integrity that is compromised by the grain direction of the shelf.

Ray

 

[ptt49er]

I had already decided to run a piece the length of the shelf w/ a dado (or groove?) for the shelf to sit in.

How much space should I allow for movement on the ends? Would an 1/8" on each end be enough?

 

[Ray Martin]

Phil,

Since the movement of wood (with changes in moisture) is mostly across the grain, the total amount of movement is dependent on the width of the shelf. The type of wood will affect the amount of movement; some are just more susceptible to moisture changes than others. The type of cut also plays a part. For example, quarter-sawn wood will move less than flat-sawn.

What kind of wood? What are the shelf dimensions? What kind of a load do you expect to support with the shelf?

If you do use a dado in the back panel of the unit, remember to not use glue in the dado.

Ray

 

[ptt49er]

I am using white oak, I think I have some qs left in the stack. If so I'll use that.

The shelves are going to be 42" long and 20" deep.

The load is a good question...right now not much, but it could end up holding books and other do dads.

Here's a rough sketch of what I'm thinking of doing in the front and back to support the shelf.

 

[cpowell]

I am using white oak, I think I have some qs left in the stack. If so I'll use that.

The shelves are going to be 42" long and 20" deep.

The load is a good question...right now not much, but it could end up holding books and other do dads.

Here's a rough sketch of what I'm thinking of doing in the front and back to support the shelf.

I used "the sagulator" (link) http://www.woodbin.com/calcs/shrinkulator.htm

Select white oak. For KD white oak, assume initial moisture content (MC) at 7 percent and final MC at 13 percent, at a width of 20 inches, you would expect a maximum of 1/2 inch tangential growth. That would be absolute worst case for flat sawn stock. Radial movement would be 1/4 inch, and that would reflect perfect QS white oak. If pressed for space I would personally plan on 3/8 total movement, allowing for 3/16th inch on each side.

Given your support detail, you can easily hide the shelf edges so you could give yourself +/- 1/4 inch play on either side to be safe with flat or QS white oak! I like the foam backer rod for spacers. You won't need any glue.

The selection of 7 to 13 percent MC is arbitrary. I don't think you will have a CHANGE in MC over 5 percent in an indoors environment, especially with finished wood. Better to plan for the worst case though.

Chuck