The word ‘laminate’ comes from the Latin ‘lamina’ meaning ‘thin slice, plate, leaf or layer’ which is pretty close to our modern meaning. Funny how that often works, eh?
During my career I have learned a fair amount about laminating wood. There are surely others out there who know far more about specific sorts of laminating than me and I do not pretend to know it all, but am happy to share what I have learned.
There are several reasons for laminating or glueing separate pieces of wood together, to wit:
1. To construct a curved shape in wood with a shorter radius bend or curve than a solid piece of wood can achieve without breaking;
2. To make a larger and bigger construction out of smaller pieces, as in a tabletop;
3. To make a stronger construction, as in a laminated beam;
4. To make a shape curved in three dimensions, as in boat hull or bowl;
5. To construct something more dimensionally stable than something made from a solid piece;
6. To apply a ‘finish’ layer to less attractive stock;
7. To economize on material use in curved parts by eliminating waste sawn from solid pieces.
Often a lamination is made for a combination of the above reasons. But generally speaking, many laminations are done for one of the first two reasons. That’s where we will start.
CONSTRUCTING A LAMINATED CURVE IN WOOD
A general rule of thumb: The more layers in a lamination, the better it will hold its shape, and no lamination with less than three layers can be depended upon to keep its shape (unless it is flat!). In other words, more layers are better for dimensional and shape stability. And, no matter how many laminations there are, there is always some amount of ‘spring back’ as the wood attempts to return to its original orientation. I sometimes glue in several hardwood dowels or pegs at right angles to the laminations after clamping the lamination to try to combat slippage and spring back.
Some allowance must usually be made for spring back, and unfortunately forecasting how much comes only through experience, trial and error and observation. Alternately, a lamination can be made oversized and trimmed back to the desired curve after the glue is set, but this can present problems with appearance and always seems very amateurish to me.
Bending form or jig: First, the curve is laid out graphically on a bench top, plank, or piece of plywood, making due allowance for anticipated spring back. (See Fig. 1)
The surface used must be sufficiently strong and dimensionally stable enough to withstand the considerable strains which will be imposed by the material being clamped. It is also wise to draw your curve somewhat beyond the length you need so that the ends go ‘fair’ when bent and do not end up with flat spots. Some sort of form is then constructed along this (new) line to hold the laminations to the desired curve(s) when glued. This must be on a flat surface (unless making a three-dimensional curve!) The ‘jig’ or ‘form’ can be as simple as nails or screws driven along a curve drawn on a bench top to which the layers are bent and/or clamped as they are glued up. This kind is useful only for very slight curves and generally not strong or secure enough for tighter curves. More often simple or elaborate clamping blocks are used. Sometimes a curved shape is sawn into a piece of blocking to serve as something to clamp to, especially in the case of multiple curves of changing radii. (See Fig. 2)
Sometimes sawn blocking is made for both the inside and outside of the desired curve, but generally not necessary unless the finished thickness of the part is critical. Whatever kind of form is used it must be strong enough to withstand the strain of pulling the layers to your curve(s) and then holding them in place until the glue sets. A considerable amount of force is usually needed to force multiple layers into place, especially on tighter curves. It is very frustrating to have a clamping block tear out as all your glued layers are being pulled to your desired curve!
I do a fair amount of laminating with different curves on my projects, so I have made myself semi-permanent clamping blocks which are securely screwed down to the bench top along any desired curve drawn there, and then removed after each use. (See Figures 3 thru 5). Note these have a PVC nose to allow the laminate to bend across the face without danger of getting flat spots. The nose is also tabbed through a larger bored hole to allow alignment to the line being curved to. They can be used on either inside or outside curves. These also have the advantage of holding the lamination clear of the bench while the glue is setting.
Use waxed paper or plastic sheet to catch glue drips and prevent your lamination from sticking to the bench or the clamping blocks. Hot, melted paraffin wax painted on the blocks or anything else which is in danger of glue sticking to works well too.
Layer thickness and length: Once the cross-sectional size of the desired part and the curve radius or radii are decided upon, the thickness of the individual layers is decided. This is normally done through trial and error, keeping in mind the three layer minimum rule. A trial piece is sawn to an estimated thickness, bent to the curve. If this can be made to bend without breaking or undue force, then all layers (of the same material) can be bent to the curve assuming there are no knots or hard spots in the individual layers leading to breakage. Layers should also be made somewhat longer than the finished part will be to allow for trimming to exact length.
Preparing the layers: Layers should be sawn slightly wider and thicker than their final dimension, typically by 1/8” to 1/4” or so. Individual layers are then planed back to their required thickness, but left wider than the planned finished width of the part. This is so you can dress the edges or your part after the glue sets as there will inevitably be some unevenness and glue drips. If the curved part is structural only and appearance is not a concern, the individual layers may simply be sawn to thickness and left un-planed if very carefully done on an accurate saw.
It is best to hold a ‘trial’ before you commit to glue. Place all the layers on your form and clamp them in place to make sure they can be bent to the form as an assembly. Try to orient the layers so that each layer’s grain offsets the grain of other layers to keep strain equalized as much as possible. This also gives you a chance to orient the layers to take advantage of appearance (best appearing pieces to the outsides!) and alternate by staggering any knots, weak or hard places in your laminates so they do not occur near each other. Once satisfied with this it is wise to number and label each layer in pencil to keep them in order and orientation when applying the glue.
Depending on the absorption characteristics of your wood, there is sometimes a risk of ‘glue starvation’ in some species. Spruce and other softwoods for example, will sometimes absorb some or most of the glue between layers, leaving little to hold them together when cured. To prevent this the layers may be pre-coated with thinned glue which is then allowed to dry before the actual glue-up. This forms a barrier which keeps the final laminating glue from being absorbed in the glue-up and does not affect the strength of the assembly.
Types of glue: Any suitable wood glue can be used for laminating wood. Obviously, waterproof glue should be used for anything exposed to the elements or moisture. Most glues like Titebond™ come in waterproof flavors, as do epoxies. It is sometimes desirable to use a glue which allows thinning (see ‘glue starvation’ above). Titebond™, and other yellow and white wood glues can usually be thinned with water within reason. Some epoxies may be thinned with denatured alcohol, which can effect its strength, but which does not really matter very much when the epoxy is being used only as a sealer. Heating epoxy also makes it thinner, and also greatly accelerates its drying time – which is actually desirable when used as a sealer. You should watch out when sealing to make sure you do not end up with glue lumps on the surface of the laminates which can cause problems unless removed before the glueing process. Another important consideration is working time for your glue and you should use a glue which allows you sufficient working time before it ‘grabs’ or sets.
Glueing the lamination assembly: This is the most crucial part of any lamination and must be done carefully and deliberately. It should go without saying the laminates should be clean and dry, and the environmental conditions suitable for the glue being used, i.e., not too cold or too hot. The parts are laid out in order and the glue is applied, covering all the joining surfaces (typically, just one side of each layer). The parts are stacked in order off the form as the glue is applied. Since the inside of any curve is shorter than the outside, I like to line up one end of the stack and use a small clamp or nail to hold the layers in alignment. The stack is then placed on the form or bending jig and the clamping begins – starting from the clamped end.
(NOTE: GET ALL YOUR CLAMPS READY BEFOREHAND, AND GET MORE THAN YOU THINK YOU MIGHT NEED! You should also have pre-positioned a suitable number of small wooden blocks to place under the clamps for the protection of your lamination)
The clamps are applied loosely at first, then tightened as you pull your assembly to the form or bending jig. Once everything is snug, but not too tight, use a heavy hammer with a block of wood for protection, to knock the edges of the laminations into alignment, and against the table top or blocking. Wood demonstrates a phenomenon when bent which causes it to ‘cup’ in a slight U shape on the outside of the curve (See Fig. 6).
To guard against this being an issue in the finished part, it is good to stagger the clamps up and down along the curve and/or place strong blocking under the clamp across the full width of the lamination. Cupping on wider laminates (over 3 or so inches) is harder to deal with and sometimes I resort to the use of ‘stretchers’ which can be clamped to the face of the last lamination to force it back into plumb. This is normally female (as illustrated in Figure 7), or male if needed on the opposite side of the curve.
The stretcher is merely a piece of stiff hardwood as wide or wider than the laminate, with appropriate ‘rocker’ cut into the profile to offset the cupping. It is OK to over-curve stretchers a little to allow the clamps to pull things right again. Stretcher spacing depends on the needs of the lamination being done. Occasionally, and especially in a repeat manufacturing type operation a long metal band the same width of the laminate is utilized to both combat cupping and protect the clamping surface of the lamination. Metal bands are normally not part of a one-off lamination operation however.
Now, the clamps are tightened and everything carefully examined for gaps, alignment, etc. When you are satisfied, go pour yourself a celebratory libation and allow the glue to set.
Finishing up: When the glue has set you may remove your part from the form. Just be sure to allow plenty of time for the glue to completely cure. Carefully note the amount of spring back you see as you remove the part. This will give you important empirical information to file alway in your mind for your next laminating project.
The part can now be cleaned up and planed back to its desired width and used or given whatever finish you need.
-- "Never let your dogma get run over by your karma!"