Q: We are going to start making edge-glued panels that are both soft maple and hard maple pieces (1 to 2.5 inches wide) in the same panel. Is it OK to mix species? I wonder about shrinkage and glueability.
A: From an economic standpoint, it is usually better to use just soft maple. But if you can obtain low-cost hard maple strips, then it is probably worth doing. Here are some concerns and things to watch for.
If the wood panel never changes MC, then you will not have any shrinkage problems. If the panel changes MC only a percent or two, any shrinkage differences between hard and soft maple strips will be minimal. However, the big risk is that the two species will come to you at two different MCs. Mixing MCs in a panel, no matter what the species, is the best way to get into trouble - end splits, planking, telegraphing, warp and so on.
Soft maple is very easy to glue and is quite forgiving. Often, a little higher glue spread is used for soft maple to avoid a starved joint. Hard maple is much less tolerant of variations in gluing-surface prep, flatness and age. If you cut the strips on one day and then wait a few days before gluing, especially if the MC changes, most gluing failures will be in the hard maple. If your saws are slightly off of "true" or if the MC is slightly high or low, gluing failures will be in the hard maple before they show up in soft maple.
Are you planning to use a natural finish with the panels? If so, you will notice that even with color matching, the soft maple will have more color variations as you view the panel from different angles than will the hard maple. Differences in color can be rather dramatic at certain angles when hard and soft are mixed.
Machining after gluing
Another concern would be when you machine the panels after gluing. Hard maple requires more skill or perfection in machining, such as sharp knives, good knife angles and so on. Hard maple is prone to chipped grain in small pockets, as well as in general, so more sanding will often be required if machining isn't perfect.
After going through this listing of potential troublesome areas, I think I have talked myself into suggesting that you be very careful if you are mixing species within the same panel. When looking at the economics, make sure you include the cost of failures and problems that can result.
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