The square knives are not nearly as good as the vertical mounted 30×12 knives. The 15x15a are 4 sided but if you use the one side to long, the corners on each of the sides dulls out. So when they spin them 90˚, the corner often leaves a line in the product. And when changing the square knife, because they lie flat horizontal, you have to make sure not one piece of sawdust gets under the knife when you tighten them down. Or it will leave the knife proud and again leaves a line down the product. They are fine on 1st bottom moulders, because they typically have a last bottom straight knife head to clean up all the lines left by the square knife. But the vertical mounted 30×12 knives are far better.
The 30×12 knives are positioned by two pins and held buy a gib and screw. Very easy to change. There are fewer to change. And, because the knives are beveled on the ends, you get a far better blending of the individual inserts that result in a better finish.
And, because these are vertically mounted, it allows for greater depth of cut and still get a great finish.
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