Knife Characteristics for Best Moulder Finish Quality

This informative thread includes some expert discussion of the old-school method of back-beveling cutter knives, and also describes other knife characteristics that can improve the quality of machined wood straight off the moulder. August 13, 2014

Question (WOODWEB Member) :
I would like to achieve the very best possible finish coming straight off the moulder as possible. The raw material is red oak. Most every article I've read on the subject refers to knife marks per inch as the reference for optimum finish quality. However, the most in depth literature comes from Charles G. Monnet Jr. in the Knife Grinding and Woodworking Manual. I ran one paragraph that I can't seem to wrap my brain around. I'm hoping that one of the knife grinding gurus will chime in.

"Front bevels are generally ground to provide the correct cutting angle for a particular kind of wood. However, front bevels can be used to act as chipbreakers. When using a front bevel it is usually advantageous to use a reduced grinding angle, otherwise, the sharpness angle may be too blunt. Many operators do not realize that there is not a better or safer chipbreaker made than a knife properly ground on the face side. When a knife is ground with a front bevel there is no danger of shavings being forced under the knife and breaking it off."

I'm curious as to how a front bevel on the knife would cause the knife itself to act as a chipbreaker. Maybe this was only necessary before different hook angles were milled into cutterheads.

Forum Responses
(Solid Wood Machining Forum)
From contributor S:
It is possible that he is referring to this technique, which I can say does allow planing of problem woods with very little tearout. It requires a bit more horsepower, but may help your efforts for a better finish. Effectively one is reducing the hook angle, as I recall.

From Gene Wengert, forum technical advisor:
The so called front bevel is more commonly called the clearance angle. The idea is that we grind away this part of the knife using an angle of about ten-twelve degrees. That assures that the heel of the knife will not rub on the wood or pound the wood. However, if this angle is small enough, the heel will touch the wood and this will give the effect of a chip breaker, although feed speed will give different results for this action, so what works at one speed will not work at another. Further, with rubbing comes burnishing, heat, and high power. In fact, it is possible that the wood will not feed into the machine, as the force from the heel wants to push the piece back out of the machine. Oftentimes, a machine does not have anti-kickback levers, so this is quite dangerous especially if an operator is standing right behind the feed. It is a good idea to stand to the side in all cases and also have a leather apron to help deflect any pieces flying out of the machine.

So, in my opinion, using the clearance angle reduction to create a chip breaker effect is too dangerous to consider in today's machines and is not totally effective anyway. It might work in a perfect world, but not in a production situation. Note that clearance angle plus knife angle plus rake equal 90 degrees, so when the clearance is reduced, the knife angle would increase. In fact, when chipped grain is an issue, often we reduce the rake to make the knife act more like a plow rather than a wedge. Small cuts per knife also reduce shipped grain as well as avoiding overly dry wood and deep cuts.

From the original questioner:
Gene, what you are describing sounds more like the bevel created by jointing the knives. The angle described in the Charles G. Monnet manual refers to a secondary bevel on the actual face of the knife. We do joint the heads on our moulder but I remove most of the heel from the cutting bevel created by the grinder.

From Gene Wengert, forum technical advisor:
When we grind the front of the knife, which we were initially taught that we should never do, we are decreasing the rake angle. As a planer has a fixed rake angle determined by the angle of the knife slot in the head, it is not possible to decrease the rake except by grinding the front, sometimes called a back bevel. I have drawings in my book THE WOOD DOCTOR'S Rx. What we are actually doing is creating a plowing action (type III chip) rather than a chisel, splitting action (type I chip). I would argue that the knife itself does not act as a chip breaker with a smaller rake angle, but acts more as a plow. Feeding becomes more difficult and heating is often increased. When there is a large land from jointing there is a small clearance angle indeed.

From Dave Rankin, forum technical advisor:
The grind link you refer to is one that was common on hand ground knives. After a profile was ground into the back clearance, a face grind was done at 15-25 degrees. The main reason this was done was to change the attack angle on the tool. This was most common on slotted knives that were run in square heads. With the new corrugated round heads, which have been used for the last 40 plus years, the face grind has gone away. The ability to make heads with different hook angles has made changing attack angle of tools fairly easy to do. To your main question, the best way to improve the finish quality on red oak is to use a good quality HSS and grind it accurately in a shear angle head. I use a 12 hook head with a 5 shear. I grind these on my Weinig profile grinder. I grind both straight and profile knives in shear and can see an improved finish.

From Contributor D:
We use a front bevel on our planer because the hook angle is so sharp that we get horrendous tearout on hardwoods if we don't. However, I don't see that being feasible on a profile knife, and as has been mentioned, the availability of cutterheads with various hook angles is a much more effective method. Going back to your original post, you may have already found this information in the book you mention, or by trial and error. In my experience there are four machining factors that affect the quality of the finish.

1. Proper hook angle. Generally, higher (number) for softer woods, lower (number) for harder woods.

2. Good knife material and proper sharpening. Note that while carbide will give an acceptable level of finish far, far longer than HSS, HSS will give a much better finish initially.

3. Knife marks per inch. There is a sweet spot. Too many and you will start getting burns (mostly because the chip is so small that it can't take the heat away effectively), too few and you will not only have very noticeable marks, but start having problems with tearout.

4. Cutter diameter. The only reason a 1/2" diameter router bit can do a decent job is because it's spinning at 20,000 RPM. The larger the diameter the shallower and less noticeable the cutter depth mark. In point of contrast with a router bit, a super surfacer can get away with 0 RPM because it essentially has a diameter of infinity.