What are my options when laminating wood with butt joints?

Question

I plan on starting to laminate soon. I was hoping to just do long pieces with each piece of wood would be as long as the the entire work. However, after looking at my stock, In order to get the best thickness I have to cut up the lengths to account for some warping.

I see on some lamination finger joints are used. I am just working with pine and don't have the means to do efficient fingers joints. I want to do just butt joints as the face of the wood should bear most of the burden.

Do I actually need to "glue" the butt joints together before laminating them to another piece? Do I work like I would a normal whole piece and just laminate the whole thing with clamps on either end?

Answer 1

One sure way to join end-to-end pieces in a lamination is to use scarf joints (also called scarph joints).

source

You can readily cut the joints by making tapered cuts on your table saw by using a commercial jig

or by using a homemade jig.

Of course, there are many different commercial offerings and, likewise,many different ways to construct your own. I like the homemade version shown here because of the clamping feature, which makes it easier and safer to use. It would be a good choice for your projects since it sounds like you are planning to make many cuts in the process of making your laminated members.

You can glue the scarf joints and the laminations in one fell swoop, but it will be cumbersome and tricky to hold everything in position, apply glue and set the clamps. Better would be to glue the scarfs, then do the laminations.

Ideally the face laminates would be one piece, but the scarf joints will work perfectly well. There is little concern with regard to the inner layers, especially if you stagger the scarfs so they do not overlap.

Answer 2

Rather than simply butting the joints, try to lap the ends of connecting boards by cutting the ends on a steep bevel or creating a lap joint. A table saw can get you up to 45 degrees where the angled length matches the board thickness. If the board is not wider than the height of your blade you can increase the length of the angled edge by using a jig. OF course a hand or band saw allows you to increase the length of contact between lapping boards. The more extreme the angle , the more contact surface is available to transfer stresses across the abutting boards. If you create a flat lap joint you can extend the lap as long as you determine is necessary to transfer the stress across the joints. Of course if there are multiple layers of laminated wood the alternate layers can also accommodate transferring the stress although if you are creating a top surface to support much weight its sagging performance will not match a simlar board that is continuous across its full thickness.

Answer 3

If you offset the places where each layer of laminate butts together -- like a "running bond" in brickwork -- you can distribute the weakness of the butt joints across the piece. Ideally no two of them would line up, but even if every other row lines up as with the bricks the much stronger long-grain to long-grain glue lines between layers will hold things together pretty darned well -- think of it as a micro-finger-joint where the fingers are created by shifting the butt junction back and forth each time you add a layer.

Answer 4

I was going to tack this on the end but I think I should start with it since it may supersede all the below.

Re. the warped boards, because of the great strength and stability of the completed glued-up panel it can be perfectly fine to have a few cupped or twisted boards in the mix.

^{Source: Woodsmith Magazine}

As long as you follow a few basic rules of construction (e.g. try not to mix quarter-sawn and plain-sawn wood, don't mix woods of different MCs, apply enough adhesive and bring the pieces together in sufficient time to fully wet all glue surfaces and last but not least clamp hard to squeeze out all excess glue) you shouldn't have any problems.

In reality I bet that rarely has anyone done a large assembly like this where every board has been absolutely flat and straight! Some of that is budgetary constraints, especially when working in hardwoods, but in large part it is because it just doesn't matter.

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I see on some lamination finger joints are used. I am just working with pine and don't have the means to do efficient fingers joints.

I'm theorising here but on some commercial stuff the finger-jointed ends are done merely because that's the way they always do it. There are times where finger-jointed ends may be more for convenience in handling the stock rather than as a means to improved strength, because the strength they do add is not usually required.

In something like an Ikea-type countertop where the panel is predominantly made from pretty short lengths of wood then finger-jointed ends could indeed add some needed strength. But where the top is largely made up of long (especially full-length) boards glued face to face then a few butt joints spaced around the glue-up won't affect strength in any way you'd notice.

the face of the wood should bear most of the burden.

This is correct, the joined faces will hold the piece together. Long-grain glue joints as you know are at least as strong as the wood if done well, so even without being glued together the end butt joints are no more disruptive than if you'd sawed through a piece of solid wood.

Do I actually need to "glue" the butt joints together before laminating them to another piece?

No, you might choose to but it's not necessary. It's fine to just glue them while doing the overall glue-up, as long as you're not losing time and working efficiently (you don't want to have glue partially set on the first few boards before bringing the whole assembly together and adding clamp pressure).

If you want to err on the side of caution size the end grain with diluted glue a few minutes before doing the glue-up, this will maximise the joint strength by preventing the full-strength glue from being over-absorbed.

Do I work like I would a normal whole piece and just laminate the whole thing with clamps on either end?

Depending on the total length, the width (hence the number of glue lines), as well as the type of glue and the temperature and humidity where you're working you may prefer to glue up in sections, thirds or halves of the total top. Once these are fully cured you can then do a very easy final glue-up of the two or three sections to make up the full top.