Rapid Dulling in Planer Knives: What's Wrong?

Trying to figure out why knives go dull so fast in a planing operation that handles both reclaimed and freshly sawn lumber. January 14, 2008

I have a cabinet and millwork shop in Georgia. We plane and rip lumber that customers bring to us. Some of it is reclaimed heart pine and some of it is new sawn pine lumber.

I have a Baxter Whitney No. 32 30" planer. It has a four knife cutterhead in it. This planer definitely does not have any trouble cutting the wood, but it does have a problem cutting the wood smoothly on a decent amount of lumber. I am running high speed steel straight knives in the planer. We have been trying different angles of grind, but I am not able to keep the knives cutting the lumber smooth. The last angle grind that we tried was 28.8 degrees. There are four knives in the cutterhead.

After I take the knives out and have them sharpened, they start to leave little ripples in the lumber after only a few boards have been run through the machine. For some reason, the edge keeps getting taken off of the knife right away.

I don't have a knife bar and grinder on my planer right now, but I just don't understand why the edge keeps getting taken off of the knives so fast. I would really love to be able to plane the lumber and have the lumber come out smooth. I don't know if a helical cutterhead would solve the problem or not. And I have considered using carbide knives, but I am cautious about them because there is always the possibility of hitting a nail in this old lumber.

Forum Responses
(Solid Wood Machining Forum)
From contributor D:
I know how you feel. The problem with your current setup is probably due to the knives not all being set to cut the same circle. I would immediately switch to a SHELIX head. If you nicked a carbide insert with a nail, you only change the affected inserts.

From contributor J:
To me this sounds like the nature of planing reclaimed lumber. Besides nails, there could be all sorts of abrasive particles embedded in the surface - dirt, paint, etc. - that would dull your knives almost instantly. You might do the first pass with a second junk planer, to get rid of the dirt and save the knives on your better machine.

From the original questioner:
Thanks. I am having the same problem with new sawn pine lumber. After I put the sharpened blades back in the planer, I start feeling ridges in the surface of the boards. If I rub my hand across the width of the boards, I can feel the ridges, and they run the length of the boards. Something is taking off the edge of the blades right away and causing the blades to leave small ridges in the boards. I am considering putting the spiral cutterhead on my planer.

From contributor A:
I think you may have a poor quality knife sharpening service that has drawn the temper out of your knives. New pine should plane very nicely with HSS. Perhaps the knives are some low grade steel, and it's not the service's fault, but they should be able to advise you. Try another service, consider replacing the knives with a good brand of steel, and make sure the sharpening people know what they are doing.

Do not let sand, dirt, grit or anything else contact your lumber. Even standing the boards on end on the concrete floor allows dirt to stick to the surface and nick the knives.

From contributor B:
Is it a round or square cutterhead? Are the knives parallel to the cutterhead axis or skewed for a slicing cut? Knife thickness and width?

From the original questioner:
I have a round cutterhead in my planer and the knives are not parallel to the cutterhead. They are skewed for more of a slicing action. There are four knives in the cutterhead. The knives that I have right now are 1 7/16" wide and 5/32" thick.

I have had two different places sharpen my knives numerous times. I know that one of the companies uses CNC machinery to grind the knives. The other person probably uses the same type of equipment. The last time, I had a guy sharpen my knives who has been sharpening knives for a company like Georgia Pacific. He has been sharpening a big lumber company's knives for about 30 or 40 years.

Thank you to those who have already contributed. I definitely am going to check into possibly getting a Shelix cutterhead for my planer.

From contributor Y:
It sounds like you're dealing with dirty lumber. Just the nature of the beast. Try blowing each board off with an air hose.

From contributor L:
There are many knife sharpening machines on the market today that sharpen knives across their cutting edges. They also do not use coolant while grinding. These grinders leave small nicks in the cutting edges of your knives and due to the lack of coolant, soften the knife's edge.

Try to find a grinding facility that grinds your knives parallel to the cutting length and under full flood coolant. You will be able to tell the difference by looking at the back of the cutting edges. The grind marks will go in the same direction as the length of the knives.

Some shops even use super abrasives like CBN to grind high speed knives under coolant, as CBN is standard with much finer grits.

From contributor V:
As a point of reference, could someone give a ballpark figure of how many feet you would expect to get out of your blades if you were planing some reclaimed boards? I avoid paint at all costs, but a lot of what I do use has dirt, etc. on it. I just brush it off and put it through. Hard to get rough lumber without dirt on it.

From contributor T:
I really do not believe you will be happy until you go with a spiral cutterhead. I have a wide planer as well (Woodmaster 25"), although not nearly the machine you have, but I know the difference between HSS knives even at their best and a spiral carbide cutterhead.

Byrd Manufacturing cannot make a cutterhead for any machine that has a tapered journal design, but fortunately for me Woodmaster sells the Shelix type (each insert is angled, not parallel to the wood) spiral head. I don't know if your machine has tapered journals, but if not, you should make the jump to a spiral head even if you can't get the Shelix type. It is night and day as to eliminating tearout and also how much easier your motor runs on wide stuff.

I have run 25" hardwood through the old HSS and the Shelix type head and the difference in how the motor sounds - it's like it works as easy on a 25" board as the HSS did on a 12" wide board or less. Just can't emphasize enough the difference. Just do it, you will never look back.

From contributor K:
When you grind to a razor sharp edge, you also get a razor thin edge. This results in small nicks, etc. after very little use. The sharpening service should be honing a secondary bevel or micro bevel on the ground edge so it strengthens that edge.

From contributor C:
It sounds like you may have too much hook in the head. Try another head with less hook. Also, try only two knives, put filler knives in the other slots.