Hook Angles for Hardwood and Softwood

      Moulder pros discuss the reasons to prefer a 12 or a 20 hook angle in different situations, for minimizing tearout, getting a good finish, and maximizing tool life. May 28, 2008

Question
I have been working with moulders for years. The hook angle has always been lower for hardwoods and higher for soft. Hence the 12/20 degree pockets on Weinig heads. I have also read numerous test and study results defending this data, dating back from the 70s to the present. Lately I have been hearing of people getting better run times on hardwood with a 20 degree hook than the 12. My question is this - what happened? Is wood different today? Is the knife steel different? I guess I would like to hear what people are using for hook angles and sharpness angles for hardwood. Any other insight on the subject would be interesting as well.

Forum Responses
(Solid Wood Machining Forum)
From contributor A:
Good question. In general you're right - 12 for hardwoods, 20 or plus for soft woods. But not all hardwoods cut alike. Poplar cuts different from oak and oak cuts different from Mahogany, etc. For some hardwoods like poplar, I like to use a mix of 20 for the top head and 12 for side heads. Side grain on Poplar is way more likely to tear out than top (face) grain. Red Oak however cuts fine with 20 on the top and sides. Mahogany and Hickory cut better with 12 top and sides. For Cherry, Walnut, Hard Maple and Alder I like to run like the Poplar mix.

So why go through all this trouble? It is because a 20 hook will keep its edge sharp significantly longer (30-50%) than 12. Also, it takes more HP to cut with 12, so big deep cuts can be a problem. I like to tell the guys I work with to buy the 12, 20 heads and only use the 12 when they encounter tear out issues on top heads. In general I like to stick with the 12 for side cuts. There are always exceptions.



From contributor B:
The theory is correct but not perfect. The 12 degree hook will touch the lumber a longer time, with more scraping action and thus eliminates tearout. The trade off is less run time because of more friction on the knife during the cut. The 20 degree hook touches the lumber about directly under center of the spindle, less time on the lumber surface, less friction , longer run times. In general 20 degree hook for both hard and softwood unless you have a tearout problem, then the 12 degree hook will do a better job. That's just my opinion from the years I have been running lumber making mouldings. I am sure other will have much to add to this thread. 95% of the knives I make are made in the 20 degree hook with no problems and longer run times between sharpenings.


From the original questioner:
I ran 1600 lf of hard maple today s/4/s with a 12 on top and the 20 on last bottom (.020") taken off. The top head with the 12 degree - after the run - had a good finish with no nicks but the last bottom seemed to be more roughed up. I will do more tests but the 12 degree is winning so far in hard maple.


From contributor B:
The last bottom is taking .020". This is the minimum wood chip flow to keep the knife cool. This might be the reason why you are seeing this. But that's what it's all about - testing and trying things yourself. Whatever works best for you is the way you should run things.


From contributor C:
I have always used the 12 and never even tried the 20 because I was told by the moulder salesman that 12 is best overall. I guess I will have to try the 20 because I am having trouble with powering out on large deep mouldings. On this same topic how do you guys check the sharpening angle? I usually do it by eye. I am probably way off. And do you have to change it if you change hook angle on the head?


From contributor B:
It's not your fault, a lot of salesmen are trained this way. Many have never even used a moulder or ground even one set of knives. But let's not take away from the very good salesmen and women out there. I think you will find good results with the 20 deg hook. Try it and make your own conclusion, what ever works best for your application. On the big cuts powering out, try tightening your belt. I get this phone call about once a month.

When you switch from 12 to 20 degree hooks the knives must be resharpened in those slots. I still use 25 degree back and 20 degree finish grinds. Some steel manufacturers recommend other angles that work best with their products. See what your manufacturer recommends.



From contributor C:
How do you check your grinding angle? Do you have some kind of tiny protractor, or do you just eyeball it like I do?


From contributor B:
My machine has a radius slideway that tells me the angles based on the tool rest. A protractor would work.


From contributor D:
A hundred years ago they used to cut only winter cut logs, which meant tighter growth rings, and the hook angle of the cutter head was different for each type of wood species, depending on moisture content, either air or kiln dried. If you google Charles G. Monnett Jr., on page 78 of his book it depicts the cutting angle for different wood species pending on moisture content written in 1954.

Today, they cut winter to summer cut logs and manufacturing can be different. In the early 70s the tooling manufacturers got together and came up with a general purpose cutter head for the North American market to keep down costs to the consumer. For today's wood shop that is running white oak, white pine, regular maple, hard maple only, they purchase the specific hook angle cutter head that still refers back to the 1954 book, courtesy of the Forest Products Laboratory.

Do not get confused by the grinding angles of 25 degrees roughing and 20 degrees finishing. Just put a protractor on the knife itself and the bevel can be from 40 to 54 degrees. For red oak one can have a hook angle at 10 degrees up to 20 degrees and then one has to look at the defect free pieces at these angles. For example, a 10 degrees hook with a 40 degree bevel on the knife gives you a 96% defect free, 15 degrees 95%, and 20 degrees 93% defect free for winter cut logs at 5000 rpm.

The only golden rule still left to my knowledge is they still cut black spruce in the winter only, and use only the northern part of the log for manufacturing high end violins, guitars, and pianos with the same hook angle on the cutter heads from 1954.



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