Reducing Tearout in Fir
From contributor A:
You did not mention if you have altered hook and grind angles on your moulder heads. Fir is a softwood and requires different angles than hardwoods. High speed steel is also probably going to be your best bet as it is sharper.
From contributor T:
I'm guessing your knives are running in 20° hook angle heads. Do you have any cutterheads with 10° or 12° hook angle? Use the lower hook angle with HSS corrugated knives ground at a high grinding angle like 27° or so. Lower hook angle = less tear out, but it takes more horsepower because there's less wedge effect separating the fibers ahead of the knife edge. For the same reason there's less tearout.
Also, if you run the stick in one direction and it tears out, save it, then just as a test, run it later in the other direction - I'll bet it doesn't tear out in the other direction. If you can determine which end is up before you feed them into the moulder, you may be able to minimize tearout.
From contributor S:
I realize you're running a moulder, but I've had very good results running Doug fir and WRC by climb-cutting on a shaper. May not be an option for you, but something to consider. The outside of the tree will cut more cleanly (less ridging) than the inside, though tear-out will persist.
From Dr. David Rankin, forum technical advisor:
Over the years I have tried many things to reduce tearout. Watching the direction of feed is one I recommend. I use the following tools.
High speed steel knives in a corrugated cutterhead with a 5 degree shear. I just recently completed testing at a large production shop that runs Douglas fir and we used a 20 degree hook with a 5 degree shear. We ran both 2 knife heads unjointed and 6 knife heads jointed. The result was the reduction of tearout by more than 80%. We used M2, T1 and DGK knives as part of the testing.
As a note, shear cutterheads do have limitations. Normally a 6" cut length is the maximum with 5 degree shear. You need to have a top head and a different bottom head as the shears go in different directions.
They are easy to grind with a special tool rest. I have been grinding profiles in shear for over 20 years and this has worked in many applications besides Doug fir.
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