Mantle - Wood expansion question

[Jason Hanna]

Hi, all. I am working on the design for a mahagony mantle to go over the fire place.

It's rough dimensions are 8"x8"x48". It is basically face framed panels with a top and a bottom and some trim. My problem is that I don't want to buy a whole sheet of veneered plywood for such a small project and I don't have a whole ton of room for scrap storage. My thought is to make it all out of solid hardwood since it's not that big.

This makes for some contrasting grain directions that I am afraid might develop problems in the future:


Overall design so far:
72926

Colors showing grain direction:
Pink = horizontal "Y" axis/direction
Green = horizontal "X" axis/direction
Blue = vertical "Z" axis/direction
72927

Problem area #1:
Trim mitered around corners to hide end grain.
72928

Problem area #2:
Panels (normally would be ply) and the vertical stiles.
72929

Sorry for the lengthy post. You guys/gals see any problem with this or have any advice?

[Bob Aquino]

If your panels are floating in the mortice's, then you shouldn't have an issue with expansion. The end panels, you are not making a separate face frame for those? As for the end grain on the top, that could be a problem if you cover it and try and trim around it. You could just leave it exposed or if you build up the top using some plywood as a core and veneer it with the finish wood, then you probably wont have a problem.

I am working on finishing up a mantel project of my own. Its about 7 feet long, wraps around the brickwork and is supported internally by 3 large angle irons that fit in slots. It is made up of 2 pieces of 1/2" ply with mortice's for the angle irons and then a top and bottom veneer of 1/4" Jatoba. The edges are also veneered in jatoba. I don't think I should have a problem with expansion since the plywood should minimize that. Time will tell.

[Dick Strauss]

Jason,
The maximum shrinkage factors for drying mahogany are only 2%(r) and 3.6%(t) (2.4 and 3.3% for true mahogany). Mahogany is considered a very stable wood and is often used in pattern making according to my neighbor. The longitudinal shrinkage (along the 8' dimension) is far smaller than the radial (r) or tangential (t). Other common woods like cherry, walnut, and maple have much more shrinkage (usually about 2x) that of mahogany.

If you figure a worst case of 2% expansion where the wood went from 6% moisture to 20% moisture, you'll be looking at 0.16" expansion across the 8" flatsawn board. More than likely you'll be looking at half this amount if the board is closer to quartersawn instead of flatsawn.

Here is some info to help you understand (refer to pages 36-37):
http://www.fpl.fs.fed.us/documnts/usda/ah188/chapter01.pdf


Good luck,
Dick

[Jason Hanna]

Awesome information Dick, all the more reason to use mahagony.

[Richard Magbanua]

Jason,
As long as those are floating panels you shouldn't have much of a problem. The edging at the ends may be the only problem as far as I can tell. What you could do is just glue the first few inches of the edging starting at the corner and then, if the edging is thick enough, use a pocket screw underneath the top where it will be hidden to attach it without glue. Back off the pocket screw enough to keep a tight joint but loose enough to allow for movement. Kreg recommends this method when attaching tabletops to aprons with pocket screws. If you don't want to use fasteners, you can attach the the back part of the edging with a sliding dovetail. Either way, the front part is attached to the corner looking all nice and pretty while the back half or so is free to allow the mantle top to move as it wishes. Good luck.

[Jason Hanna]

Richard, I had originally thought the panels to be fixed, but if you think it might be a problem I could rethink that. As far as the edge banding goes, would I be better off using brads with glue only on the front couple of inches to hold the mitter true?

[Richard Magbanua]

Jason,
Yeah, If you're gonna use solid wood panels they should be floating panels. Brads on the back portion of the edging would be fine, too. And much easier. Especially if you're using thin edging. I was assuming you wouldn't want any nails or putty marks to show. It seems whenever I use my cheap brad nailer on nice wood it makes a mess and I end up having to doctor up the holes with wood filler. Good luck!

[Jason Hanna]

Richard, I think I can make the back panels floating without much problem. The brads in the back of the edging should be easy to because the second part of the project is putting rock on the wall around the mantle. This will hide back 1" to 1/2" of the mantle and would conceal nail holes.

Dick, so wood expansion effects the width more than the length of a given board?

[Dick Strauss]

Jason,
Longitudinal shrinkage is typically less than 0.1% for mahogany (typically less than 0.2% for all woods). This translates to 48"x0.001=0.048" in your case.

This is the reason why old glue joints let loose along the longitudinal glue joints and not on the end caps (they expand and contract so much across the grain). As you can see, there is 10-30X the expansion across the board than along the length. Put another way, if your piece was 20' long and 1' wide (assuming a 20x ratio), you'd expect the same amount of expansion/contraction in each direction. We usually don't make items in these 20X proportions, so the calculators don't even mention the longitudinal shrinkage because it's usually not an issue.

Here are a few tools for future shrinkage calculations.
http://www.diadot.com/links/shrink.html

http://www.woodbin.com/calcs/shrinkulator.htm

http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=shrinkage

http://justwoodworking.com/software/wood_move.php