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I laminated some 2x4s for workbench legs, and as you can see by the attached picture, there's a gap between two of the boards. I applied the glue in a zig zag line down the length of the boards and then a line along all 4 edges of the board's face. I clamped the 3' long glue-up with 5 clamps that I tightened pretty securely. I'm wondering why this gap exists, and what I can do better next time, since I still have to glue up the entire top. I did glue up all four legs at the same time, so will take the previous advice to do a pair at a time from this point forward. I don't think this is a problem structurally, but maybe next time I should brush on the glue smoothly? Maybe I should be planing these flatter to start with? Thanks.

glued boards glued boards 2 glued boards 3

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    The gap looks pretty obvious. Was it evident when you dry-fit and clamped? – Dan Wolfgang May 8 '17 at 1:40
  • I admit I was rushing to clamp everything because I clamped up four assemblies like this at the same time, so it was 12 boards together with the same clamps which I was manipulating in a very small space. I should've checked more carefully and taken my time. – neongreensticker May 8 '17 at 3:20
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    Not enough clamp pressure! If you're doing a full panel like this in the future, clamp each subsection one at a time. Even if it means you're glueing up three or four separate times it's important for solid glue joints that will be dependable in the long term. Oh and here, in case you don't realise you're going to need to separate the boards with the gaps (saw them apart), clean off the glue, then re-glue. – Graphus supports Monica May 8 '17 at 7:16
  • Plane the mating faces a bit to get rid of any very high points. You need a lot more clamps and a lot more pressure, maybe a clamp every 10 inches or so, alternating from above and below. A lot of people will say to only glue up 2-3 boards at a time but in my experience it is just as easy to glue up to 6 at a time. The glue is not going to set for around 10-15 minutes. Anyway gluing those up 2 at a time wouldn't solve the problem anyway, the gap is large enough to fit a quarter in, it needs a lot more clamps. – jbord39 May 12 '17 at 19:37
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I have several suggestions for gluing this type of slab.

  • The glue does not spread itself. Always distribute it evenly before making contact between pieces. You can do this using a brush or a roller. I keep a rubber ink roller to distribute glue in large areas and then simply rinse it to clean off the glue for its next use.
  • Glue on both contact faces will insure maximum contact better than on only one face.
  • Run the bottom of the boards through the jointer to insure the base of the slab is flat so that the pieces will set flat during and after gluing.
  • Do not glue and clamp all the pieces at the same time. These pieces are larger and it will take a lot of pressure and a lot of clamps to make full contact between all the pieces. For a 2x bench slab like this I would plane the contact faces of two boards, glue them together, let them set, plane the outside faces and then join the pairs together and then repeat for the 4 piece slabs until done.
  • Use plenty of clamps (1 per foot would be nice).
  • Once the slab is as wide as possible for your planer, runt the joined sections through the planer. (This will reduce the amount of hand planing needed to get the top surface flat and eliminate the ridges caused by the milled edges of the original 2xs
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Looking at the ends of the boards in the first photo you can see that they are tight at the bottom, but not at the top. It looks to me like the faces of the boards that you glued together are convex (i.e. the opposite of cupped, not sure what the correct term is). It might also be that they're twisted.

Either way I think the solutions is better milling beforehand. You can't really rely on construction lumber being straight or flat, and clamping it into submission isn't always possible.

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