|Forum topic by jcwalleye||posted 12-31-2012 08:00 PM||939 views||0 times favorited||7 replies|
12-31-2012 08:00 PM
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--