I built a pair of infeed/outfeed tables for my planer - 48x15". They ended up not being as flat as I had hoped...I have a theory as to the problem but am seeking alternative explanations.
I used 3/4" MDF for the sides with 1/4" MDF for the grid and top/bottom surfaces. I started by gluing up the 4 sides. I positioned the sawhorses to minimize sag and used a 48" Starret straightedge to check for any bow. I used winding sticks to judge twist - wedging between the parts and the sawhorses to compensate. I then glued and pinned the bottom surface (1/4" MDF), again checking for bow and twist. They had a day for the glue to dry. I then flipped them over and adjusted for bow and twist again (they were still pretty flexible at this point). I then glued in the grids. They fit amazingly well (I dare say perfect) - there were 2 running the length and 5 running the width. I then added glue to the top of the grid, placed the top and clamped (moderate pressure) with cauls. I checked for twist again, though this was tricky with all the clamps in the way. I could not check for bow at this point with all the clamps in place but had no reason to believe it had moved since the last check (10 minutes prior). BTW, the grids are 2.5" in height, plus 2x1/4" for the top/bottom makes a total box height of 3".
Both boxes seemed straight and flat when I took the clamps off a day later, though I can't recall how closely I checked. I set them aside to work on some other things. Measuring today (2-3 weeks later?), they both have an identical 3/64" sag when supported at the ends - which over 48" is considerably more than I had hoped for. If I flip them upside down, the direction and magnitude of the sag does not change - i.e. they are "sagging" up. There is no twist. Moderate pressure does not change the sag - they seem to be VERY stiff - at least within the expected use load.
My shop is cool (~65F) and there is a dehumidifer. The thicker MDF had been in the shop for years. Some of the 1/4" had been in the shop for a few weeks, some for just a few days.
The one thing I haven't mentioned yet - the top skin has plastic laminate applied to the top. It had been applied to the MDF a few days prior to assembling the box. Maybe related? My only explanation is that some of the MDF was pretty dry and that as moisture has entered the material it expanded a bit. The laminate on the top prevented absorption...therefore not expanding and resulting in the bow. It seems unlikely, as the MDF was purchased from Home Depot. I would not expect it to any drier than my shop...though I have not taken a humidity reading at either location. Maybe moisture from the contact cement used to apply the laminate has left the MDF, shrinking it? That seems highly unlikely - as the laminate itself is not going to shrink...not even a little.
Any ideas? I'm stumped. I was pretty meticulous with this and had been expecting no measurable sag when not loaded. THat both sagged the exact same amount in the same direction implies this was a design/technique problrm, rather than the variability of accuracty here or there.
TIA,
Chris
I used 3/4" MDF for the sides with 1/4" MDF for the grid and top/bottom surfaces. I started by gluing up the 4 sides. I positioned the sawhorses to minimize sag and used a 48" Starret straightedge to check for any bow. I used winding sticks to judge twist - wedging between the parts and the sawhorses to compensate. I then glued and pinned the bottom surface (1/4" MDF), again checking for bow and twist. They had a day for the glue to dry. I then flipped them over and adjusted for bow and twist again (they were still pretty flexible at this point). I then glued in the grids. They fit amazingly well (I dare say perfect) - there were 2 running the length and 5 running the width. I then added glue to the top of the grid, placed the top and clamped (moderate pressure) with cauls. I checked for twist again, though this was tricky with all the clamps in the way. I could not check for bow at this point with all the clamps in place but had no reason to believe it had moved since the last check (10 minutes prior). BTW, the grids are 2.5" in height, plus 2x1/4" for the top/bottom makes a total box height of 3".
Both boxes seemed straight and flat when I took the clamps off a day later, though I can't recall how closely I checked. I set them aside to work on some other things. Measuring today (2-3 weeks later?), they both have an identical 3/64" sag when supported at the ends - which over 48" is considerably more than I had hoped for. If I flip them upside down, the direction and magnitude of the sag does not change - i.e. they are "sagging" up. There is no twist. Moderate pressure does not change the sag - they seem to be VERY stiff - at least within the expected use load.
My shop is cool (~65F) and there is a dehumidifer. The thicker MDF had been in the shop for years. Some of the 1/4" had been in the shop for a few weeks, some for just a few days.
The one thing I haven't mentioned yet - the top skin has plastic laminate applied to the top. It had been applied to the MDF a few days prior to assembling the box. Maybe related? My only explanation is that some of the MDF was pretty dry and that as moisture has entered the material it expanded a bit. The laminate on the top prevented absorption...therefore not expanding and resulting in the bow. It seems unlikely, as the MDF was purchased from Home Depot. I would not expect it to any drier than my shop...though I have not taken a humidity reading at either location. Maybe moisture from the contact cement used to apply the laminate has left the MDF, shrinking it? That seems highly unlikely - as the laminate itself is not going to shrink...not even a little.
Any ideas? I'm stumped. I was pretty meticulous with this and had been expecting no measurable sag when not loaded. THat both sagged the exact same amount in the same direction implies this was a design/technique problrm, rather than the variability of accuracty here or there.
TIA,
Chris
