Ever have a glue up turn into a taco? There’s no magic to making a flat table top, you just have to avoid these common mistakes and use the right techniques and you’ll get repeatable happy results.

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Most mistakes that turn your glue up into a horrible experience with a not flat table top fall into one of four categories. If you scroll down, there’s a handy infographic you can save to your phone or print out to have in the shop with you.

1. Not Starting with flat lumber / Milling improperly

   It sounds obvious, but needs to be pointed out. There’s simply no way to make a flat table top if the wood you are working with isn’t flat to begin with.

   If you’re using pre-milled (S4S or surfaced four sides) material, then take the time at the lumber yard to pick out dead flat boards. If the supplier does milling on site, don’t be afraid to ask them to re-mill something flat again if it’s gone out of flat while sitting in the racks.

   Which ties right into milling your own rough lumber into flat boards - which is my preference. I did another video that goes more in depth. But the short version is that lumber moves for lots of reasons.

   I stay ahead of the movement by letting lumber acclimate to my shop climate for at least a week before milling it, and I mill it over three sessions. Sneaking up on the final dimensions I want. And if I’m going to be gluing those into a panel, I leave them a tad thick so I can mill or sand the panel flat after gluing it up.

2. Not offsetting jointing error

   It doesn’t seem to matter how long I slave over dialing in my machines, they’re never perfect. Don’t get my wrong, they’re pretty dawg-gon close. But depending on how I feed my stock through my machine to joint it before glue up, it’s possible that I’m doubling any error in my set up.

   Which means instead of my glue joint being 180˚ (flat), an error of .5˚ compounds to a 1˚ error. That doesn’t sound like much, but with my 9” boards that 1˚ turns into a 5/64” error with each seam and would’ve meant a 15/32” difference between the center of my table and the edges, that’s almost 1/4”!

   Correcting this is pretty easy, you just use supplementary angles. It’s best to understand this by watching the video but I’ll try to explain. You mark each side of the joint and then feed them through the machine opposite ways so when you lay them back down beside each other, one board is jointer at 90.5˚ and the other is at 89.5˚, which adds up to 180˚, or perfectly flat.

3. Not using alignment aids

   I like to use cauls, but there are other methods. I’ve tried biscuits, but didn’t haven’t much luck with then holding my boards flat. I have several friends that swear by their domino, but that’s not something in my tool kit.

   A caul is basically a piece of hard wood scrap that you clamp perpendicular to your table top to hold it flat. There’s some nuance to it though. If you just clamped any old board you’d probably just bend it and not actually deliver pressure at the glue seam to pull the joint flat. The answer to this is to use a firm hardwood and taper your caul so it’s thicker where the glue seam is, that way that is where the pressure will be delivered.

4. Improper clamping pressure

   When I first started I read all about how much clamping pressure to get a good glue joint, and some of the numbers where crazy. I was terrified of not getting enough pressure for this yellow liquid to bond the wood. And to be honest, sometimes I over clamped because I didn’t know how to joint a straight edge and was trying to clamp crook out of a board.

   Well the reality is you don’t actually need a lot of pressure for glue to do its job so long as your edges are straight and the boards actually mate well. As you keep cranking up the pressure one of two things will happen, the fibers of the wood at the point of the clamp will compress and you’ll damage your lumber. That’s likely with soft wood. With harder wood, the force will take the path of least resistance and the panel will start to fold at the glue joint. And no, once you take the pressure off, the panel won’t go back flat, you just glued a cup into your panel and the only way to fix it is to cut the panel at the joint and start over (something I have some experience with).

   So how much pressure is right? Well, for my clamps I know that if I tighten them until they’re just snug against the pieces, it only takes another 1/8 to 1/4 turn to get enough pressure if I’ve done my job getting straight edges.

   How did I find that out? By testing. To learn your clamps do a test. Clamp up a panel, you don’t even have to glue it, then get your clamps snug. Start tightening them just a bit a time, say 1/8 of a turn, and sight across the panel. Keep counting and tightening until you see the panel start to deflect. Then count and back off until it’s flat again. Next time, you want to tighten mid way between “just snug” and “flat again.” For my clamps, it takes 1/2 turn to see deflection, and 1/8 turn back (3/8 turn from “snug”) gets it flat again. Half of 3/8, is 3/16, but I just go by feel for around an 1/8 but no more than 1/4 past snug depending on what I’m gluing and that works for me.

And that’s it. I hope these help! If you’ve learned anything I didn’t share, be sure to let me know!