[OP]
Member
I am making a coffee table and have the same problem i always do when glueing boards together, they never come out perfectly flat. I bought the boards pre planned so they should all be the same thickness. When I glued the 4 boards for the top together they are all just slightly higher or lower than one another. How do I improve this to save time sanding and planning? I used some scraps as cauls but still didn't achieve a perfectly flat top.
Contributor
That’s usually considered glue creep, but could also be improper clamping techniques.
Adding biscuits to align the boards will help, but they have to be cut properly and be tight fitting to work their best. Dowels or Festool Dominoes work too.
A easy beginners mistake is too much clamping pressure which will make the boards want to be misaligned.
Gluing up 2 boards at a time can be helpful too, so you have time and can disused on one alignment at a time.
Lots of possibilities. Any chance you have a pic of what the panel looked like with the clamps on?
Contributor
You really can't glue two edges together without something holding them in place. Glue is slippery until it sets up and clamps generally do not exert force precisely in line with the boards. Any offset and the boards slide out of alignment.
If small enough, Ill just put a small clamp on the ends across each glue line to hold them parallel as I tighten the clamps across the width. If larger, I'll use cauls across the surface. The cauls can't really be just scraps; they need to be machined either dead flat or equally curved on each side.
If it has to be very close, I'll sometimes cut splines instead of using biscuits for alignment. Biscuits don't really fit well enough to ensure perfect alignment.
Dan
Contributor
Cauls are one way. A deadblow is also very handy for banging boards into submission after clamping. I typically like to skip flatten on side, square the edges, than glue up panels to my maximum planer width, then plane to thickness. This way I at least get fewer seams to flatten and the resulting panels are much less squirrely.
Friend of the Creek
Adam, you’re not alone. Happens to all of us. If I’m dealing with a glue up that can’t afford to be much thinner, I do as Matt suggests and start with just two boards. You have a good 5 mins before the glue sets up, so take your time to align them well when clamping. Start with a clamp in the center and tighten it while checking that the boards are aligned. Then add clamps from the center to the ends, tightening enough to hold and making sure the boards are aligned. I’ll then add clamps over the top in between the lower clamps. After an hour, I add another board and continue this way until all boards are glued up.
I also add clamps to the joint at the end just until the glue sets. Even after all this is done, I still may have a bit of misalignment, but at least its fairly minimal.
Member
Dowels will work if you have a good doweling jig. The self centering kind is not good enough. You need the kid that references the top surface instead. You really only need perfect alignment on the side that shows.
Member
If when you first snug the wood together in your clamps you would use a small sledge hammer and a block of wood you can hammer the boards flat. Unless they vary in thickness it should end up flush on both sides. Cauls just hold the overall panel flat if some of the boards are warped or not jointed square.
Moderator
There are many ways to get a better glue up and many have already been mentioned. Dowels/Dominos/Biscuits to help align one surface; using cauls (preferably with a slight curve to get pressure to to the center of the panel, clamps that help support the panel all at the same height, such as parallel clamps, etc. It's a dance...
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The most expensive tool is the one you buy "cheaply" and often...
Contributor
+1 on this.Originally Posted by johnny means
A variation on the same theme is to do your glue up in stages - i.e. instead of gluing up four boards, glue up two and two, and then glue the two pairs. In this way you are only wrestling with one seam per glue-up.
Member
I make them a bit over thickness, glue them together, and then plane them to thickness on my thickness planer.
I have good luck with this procedure.
Contributor
Do many of you ever use a glue line router or shaper bit to prepare the edges prior to glue up of boards to make a panel?
I've had all the same issues in the past, I've had fairly good success with Dominoes, but just wondered if my initial question might be a better approach. Although, I think, board thickness has to be exact for the glue line bits/cutters to be beneficial.
I would use these either on my router table or shaper.
Thoughts.
Contributor
I'd recommend cauls, also.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Contributor
It sounds like you bought S4S lumber. If so, you still have to ensure that the edges are 90 degrees to the face (and straight). If not, you'll have flatness issues during glue-up.
When I glue up boards, I alternate which face is against the jointer fence. This method allows a slight out of square error in the jointer fence to be "cancelled out" in the glue joint. Here's a great YouTube video that demonstrates this: https://www.youtube.com/watch?v=bbAYBtR7BYA. Even though I set my fence at 90 using a good engineer's square, I've seen small errors creep in.
The alternating jointing method, along with dominos, allows me to get dead flat glue-ups. With a couple of passes with a finish sander with 80 grit I get flat panels.
Last edited by Mark Hockenberg; 08-26-2019 at 4:56 PM.
Contributor
Adam
How are you preparing the edges? Any deviation from 90 degrees to the face is going to cause issues. A tried and true technique irsto run them through a jointer, and alternate the face of the board on the fence. This way any deviation from 90 degrees, will be nulled out.
I have always found that working alone always causes people a little trouble with panel glue ups. You just don't have enough hands.
I always use epoxy for panel glue ups. It cost quite a bit more, but you have a long, long, time to fuss about, and you only have to apply enough pressure to get the material to touch.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)
Member
+1 on alternating the faces on the jointer. I skip plane the board to ~15/16", rough out stock plenty wide, run all edges through the jointer alternating faces against the fence to cancel out errors, glue together using #20 biscuits for alignment, then run through the planer to final dimension and cut the panel to size.
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