Sometimes in woodworking you need a shape that just won’t work because of short grain issues, stock availability, design concerns etc. It comes to a choice of steam bending or bent lamination. When choosing bent lamination there are some concerns to consider.
The First consideration is the form you wish to use to use for the glue-up process. I normally hate MDF with a passion, but for this application it works well. I have used cdx and a/c plywood, solid wood, and phenolic plastic to make forms and I like the smoothness of the form you can get with MDF, but they are very delicate and need to be stored safely. Laminating many sheets of MDF and using a pattern follow bit you can create a form of almost unlimited width, but they get heavy quick. When you use two piece bent lamination forms there is an inherent spring-back to the glue up that needs to be addressed so your piece will work out how you want.
Unfortunately we have to go back to math class before we can build our form. Working with your plan you need to discern your arc or chord depth, meaning the deflection the curve is to make from a straight plane. There are several formulas that can be used to approximate the final shape of the material after being removed from the form.
Fortunately we don’t need to read “Composite Materials: Mechanical Behavior and Structural Analysis” to figure this out. There are some simplified formulas to be found from woodworkers who have experimented for years. Now we can move forward and feel safe using our good materials to produce the bend. The simplest formula is as follows;
Y=Distance the arc will change
X=Height of arc from base
N=The number of layers of material
Using this equation if you have a curve with a chord height of 3” and you are laminating 4 strips of wood to make the curve you will have y=3/42 which will yield Y=.1875 or 3/16” difference.
Knowing this you can change your pattern to give you the curve result you are looking for.
The next thing to consider is the thickness of the material you are laminating. Again if you are laminating 4 strips each 1/8” thick your lamination will be 1/2” thick. Your form must be made with a positive and negative part with a 1/2” hollow between them. This will result in two different arcs.
After I have figured out the math and cut the layers out and glued them together it helps to run some bolts through the form to keep the layers from de-laminating under clamping pressure. I like to add a piece of plywood to disperse the clamp pressure and protect the form corners form being damaged.
A good coat of finish on the form helps when you need to get the glue cleaned up. I sheet the form with a clear drawer liner as added glue protection. You have put a lot of time and material into building this form, anything you can do to keep it from getting coated in glue that can telegraph into the piece is good.
As you can see from my photos I have added strips of wood down the middle of the form, this helps keep the form even as it is pulled together.
So now you have a form and have milled up your wood and you are ready to glue it up, dry test everything to make sure you have all the tools you need at hand, have fun and good luck.
-- Brian Noel