HELP my benchtop cupped after flatening

[dan sherman]

Tuesday night I flattened one side of a benchtop I'm working on. I checked it after I finished, and it was dead flat. Tonight a friend came over and helped me flip it, so I could do the other side. I happened to look underneath at the saw horses, and I noticed that it was no longer flat. I broke out the feeler gauges, strait ege, cralled underneath and started measuring. It has a consistent cup of about .040" over its entire length.

What could have caused this in a little under 48 hours where the EMC% barely changed? And a better question is what do I do to fix it?

for reference the top is face glued SYP 30" by 72" & 3" thick. The moisture content of the wood is 12% or less.

[dan sherman]

I Think I figured out what caused the problem.

I used the router sled method to flatten the top, and I was using pipe clamps
to hold the guide rails fast to the top. When I measured the cup about an hour ago, I had already re-clamped the rails so I could flatten the bottom. On a hunch I removed the clamps & remeasured the cup, it had reduced to .026" (light bulb time).

Thus I'm pretty sure the issue is that I'm a gorilla, and as usual over tightened the claps. This caused the center to bow up before flattening. When I released the clamps after flattening, the center relaxed and cupped.

Anyone have another theory?

[Corey Wilcox]

Dan,

How much material did you remove when you routed the top flat? Also, is your pine kiln dried to 12% moisture or did you let it acclimate to that level in the house? If your wood isn't kiln dried to 12% and you removed a fair amount of stock when you milled the surface flat I would have expected it to bow. The fresh side would be able to give up moisture faster than the bottom. I would flatten the bottom and see if the cup in the top disappears.

[Paul Murphy]

Dan, @ 12% wood moisture (plus residual glue moisture), you may very well be loosing moisture from your top to the surrounding air. Wood in my shop for many weeks tests around 8% right now.

I have learned the hard way to bring wood into my shop and sticker it for a couple weeks, mill it about 1/8" oversize and wait another week, and then mill it to size. With all that, it is still wood, and will still move a little with the climate changes.

Twelve years ago I made my first workbench out of beech I cut on my property, and air dried without acclimating to my shop. I had the top dead flat when I first finished, but it moved quite a bit by the time it dried to the EMC of my shop!

[dan sherman]

Dan,

How much material did you remove when you routed the top flat? Also, is your pine kiln dried to 12% moisture or did you let it acclimate to that level in the house?

Hi Corey,

I removed less than a 1/16" from the top. The wood I purchased was kiln dried (2" x 12" x 14'). I purchased it a little over a month ago. I ripped the boards in half, and then stickered them for about 10 days. After 10 days, the MC of the wettest board was down to 14% (one was down at 6%). Next I ripped the boards to rough width, and let them set for another week. Then 2 weeks ago I started the joint/plane/glue-up process (it took some time as I'm making two utility benches).