Help explain wood movement

  • Advertise with us

« back to Woodworking Skill Share forum

Forum topic by jcwalleye posted 12-31-2012 08:00 PM 1224 views 0 times favorited 7 replies Add to Favorites Watch
View jcwalleye's profile


306 posts in 3313 days

12-31-2012 08:00 PM

Topic tags/keywords: question mdf movement warping

Can some of you help explain some material movement that has been bedeviling me?

I started out about a week ago to build a table saw mitre sled. I was very precise with the alignment but when I started making test cuts the miters were coming out awful, and fence adjustments weren’t fixing the problem like I’d expect. Finally traced the problem to warpage of the sled’s base material.

The first try was out of 1/2” mdf. The material came from the garage a couple days ahead of use which was below 32 degrees and around 70 pct humididty. To the shop which is kept around 65 degrees and normally 25 pct humidity, which is typical for this part of Montana. Here is a picture showing the amount of movement. You can see a large bow from the front to the back. This piece started out dead flat.

Thinking the problem was the 1/2” material, I added runners to a 3/4” piece and allowed it to sit overnight. It bowed left to right as you can see in this second picture.

Thinking it must be the glue, I cut a third piece and attached the runners without glue. I let it sit for a couple hours and it too had bowed though opposite the way the 2nd piece had.

Since then every time I’ve gone into the shop the mdf has bowed up on the ends. If I turn it over, it becomes flat in an hour or two and then continues bowing until it is bowed up again. Has anyone else had this experience and how long before it stabilizes and will it stabilize flat?

The outdoor humidity seems high, but not for the low temperatures. I thought one of mdf’s enduring characters was that it was flat and stable. But apparently not this time when I need it.


-- Trees, a wonderful gift --Joe--

7 replies so far

View dustbunny's profile


1149 posts in 3535 days

#1 posted 12-31-2012 08:27 PM

Cabinet plywood is all I use for sleds.
The multi direction of grains holds it from moving.
There is no MDF in my shop.
New England humidity fluctuations are brutal on wood, especially composites.
I also try to keep wax on my sleds to keep the moisture out.

Good luck,


-- Imagination rules the world. ~ Napoleon Bonaparte ~

View Cosmicsniper's profile


2202 posts in 3398 days

#2 posted 12-31-2012 08:34 PM

Makers of radius dishes for guitars, if they opt for MDF, will make them 2” thick. You’ve just discovered why.

Even plywood can bow, some more than others. For this application in your conditions, I’d use Baltic birch plywood…the more layers and the less voids, the better.

-- jay,

View Grandpa's profile


3261 posts in 2915 days

#3 posted 12-31-2012 08:42 PM

The surface that is on top or up has the air moving over it. This drys and shrinks. When it is smaller than the other side it bows up on the ends. Turn it over and you will shrink the other side and it will eventually curl the other direction. This will coninue until the moisture content is equal all through the MDF. There is nothing to prevent this in MDF. The more layers the more table like sniper states. If you had this in the shop a couple of weeks before you cut it…..maybe this wouldn’t happen. Try the Baltic Birch as suggested. Store it in the shop for a couple of weeks before you begin and see what happens.

View gfadvm's profile


14940 posts in 2930 days

#4 posted 01-01-2013 02:51 AM

I had the same experience with 1/2” Baltic Birch ply so that ain’t the answer either! Post the solution when you get it please.

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View Grandpa's profile


3261 posts in 2915 days

#5 posted 01-01-2013 03:29 AM

I don’t have that problem with the plywoods. Not noticable. I think it is moisture content in the product.

We don’t have enough moisture in SW Oklahoma to use lick and stick postage stamps. How would I know?

View jcwalleye's profile


306 posts in 3313 days

#6 posted 01-01-2013 03:43 AM

I think you guys are right. Grandpa has the best explanation. I’ll try some plywood, but it often doesn’t stay flat either.

Maybe I’ll try to catch it while it is flat and wax it????

Thanks for the posts.

-- Trees, a wonderful gift --Joe--

View dustbunny's profile


1149 posts in 3535 days

#7 posted 01-01-2013 03:55 AM

Also you need to add a hardwood front and rear rail to help keep it stable.
Like this one

-- Imagination rules the world. ~ Napoleon Bonaparte ~

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics