Why is my table top glue up cupping?? Please help.
Hello all – newish woodworker and member here..
Here is the breakdown for my situation:
Yesterday, I purchased (5) 10 foot long 5/4 S3S kiln dried red oak boards ranging between 6”-7.5” wide for a tabletop with breadboard ends. This will be for a trestle table I’m building. I purchased the lumber from a local lumber yard (about a 30 minute drive) from my work shop. I live in Southern California (within a desert climate) and both my workshop and the barn where the lumber is stored are very similar leading me to believe the environment is similar too (temperature and humidity).
I had each of the 5 boards cut in half for easier transport and brought them back in my truck. We’re in a bit of a heat wave with no end in sight (approximately mid 90’s with 20-30% humidity). The red oak was in full sun on that 30 minute drive home. I put it in the shop on some sawhorses and left it for a couple hours. The wood was warm to the touch from the sun.
Before purchasing the wood, I spoke to a Rockler employee, explained the similar environments between my shop and the lumber yard I was buying from and he advised that I don’t let it acclimate. He suggested I mill and glue up the same day. There is also another person who’s been working in my shop for the last 40 years and suggested the same thing. I’ve read it’s best to let the wood acclimate but since I don’t have much experience I decided to take their advice.
After letting the wood cool off a bit (waited approximately 2.5 hours), I milled the wood to its final thickness (1 1/8”) only removing 1/8” of material thickness. I jointed the boards very well so when I butt the edges together, the seam disappears. I then used a biscuit joiner for the first time (Dewalt joiner with dewalt #10 biscuits) using 6 biscuits across the width to join together the 60” long boards. Also used titebond 3. This was now in the evening and the temperature was in the mid 80’s/ humidity 29%.
This was also my first time making a hardwood edge to edge glue up panel. I’ve read and heard many different methods to do so.
This was my chosen route:
I used a mallet to seat the biscuits into one of the board edges as the biscuits fit was quite tight. I did not put glue in the slot beforehand. I then used a glue bottle with a roller head to apply the glue to the other mating piece, making sure that the one edge I applied glue to was completely covered. I then used the mallet to drive the boards together as much as I could (I left about a 1/4-1/2” gap. Once I completed this process across all 8 boards, I then used (5) parallel clamps to clamp the boards together until they were snug. I did not alternate the clamps on bottom and top. I just had them clamp from the bottom and made sure to not over tighten for fear of inducing cupping. This process took about 15-20 minutes for the glue up.
One thing I realized after clamping is that I had very little squeeze out. This was one of the mistakes I’m assuming I made. Upon this realization, I took a glue syringe, diluted some titebond 3, and did my best to squeeze some glue in the seams. It did work some – but definitely should’ve applied glue to both boards.
Afterwards, I visually inspected the flatness of the panel. For the most part, it was fairly flat except one of the end boards (on the corner and about 20” long where it was tilting up.
This morning, I went to check the glue up panel and noticed that it was cupping in the middle. There was about a 3/32” dip in the middle that was not there last night.
Later in the afternoon, the gentlemen who also works out of this shop suggested to use a belt sander to get it flat and to attach the breadboard ends as I mentioned in the beginning and is still my plan. After about 14 hours of clamping, I released the clamps. I then came back a couple hours later to look and noticed the panel is cupping even more now! About 1/8” at the worst part!
I found someone online who suggested that excessive heat and humidity will cup panels. Although it’s not humid here, it is hot. They said you can turn the panel over and sometimes the cupping will correct itself. So I did that. Upon flipping it over, I noticed the bottom somehow seems flatter than the top which doesn’t make sense to me. I will see how it looks in the morning.
My plan is to either add the breadboards tomorrow after trying to flatten the top some or cut the table top back into the 8 individual boards and start over. Can I add the breadboards via mortise and tenon first and then flatten the entire top?
Any insight here? I’ve read so many different ways to approach this and would love to salvage this glue up if possible without losing more thickness!
Thank you so much for your time reading this!