Understanding Carbide Planer Knives

A question about planer knives that can hold an edge with exotic hardwoods leads to a detailed explanation about quality grades in the carbide knife market. June 9, 2007

Question
I have always used high speed steel planer knives in my shop for everything, but as of recently, I have begun cutting some exotic hardwoods. Some of them can wear out my knives in only a few board feet. Are there any better knives for these types of material?

Forum Responses
(Solid Wood Machining Forum)
From contributor T:
There are a couple of options that may be beneficial to you. One is carbide tipped and the other is solid carbide. The better of the two options is solid carbide, as the carbide grades used for these knives have much higher wear resistances. The price can be a bit high, but the edge quality and longevity are worth the investment.

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From contributor R:
I would agree with contributor T - carbide would be my choice. I would tend to lean toward the solid carbide as well.

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From Dave Rankin, forum technical advisor:
I agree also. The one thing to ask is what grade of carbide. The harder carbides can be more brittle and this causes edge breakage. The softer carbides may not last as long. When making that decision, be sure to advise the seller about the species that you run now and plan to run.

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From contributor A:
Anybody care to elaborate on why the solid carbide knives tend to be of higher wear? We use plenty of C4 on sawblades. Why not on carbide tipped knives? I personally wouldn't want that hard as they would chip easily on that occasional stone or staple I plane through. The cost difference is substantial. I currently run a set of M3 and a set of Amana carbide tipped.

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From contributor T:
Carbide is graded by hardness and toughness. Hardness translates to wear resistance and toughness translates to impact resistance. The C scale that most people know is a very generic measure of the carbide grades available today. Besides the hardness and toughness of these materials, you need to consider grain structure. Carbide is made up of small particles that are held together by a binder. The smaller the particles are and the tighter they are held together, the sharper the cutting edge can be. As a general rule, the higher the grade, the better the edge quality.

As an example, C1 carbide is one of the toughest grades, but it is not very hard. This makes it stay sharp for less time, but is less likely to chip or break under rough use. Now C4 carbide is one of the hardest carbides used in the woodworking industry. It is very hard, making it last much longer, however it is very brittle, making it a poor choice for rough applications.

Harder grade carbides do not work well in applications requiring large sections to be brazed into steel. This is due to the expansion and contraction that the steel goes through as it is heated up and cools. Harder carbides usually crack or chip while the product is cooling down. As a result most carbide tipped knives only use carbide grades of C1 or less.

Solid carbide knives can be made in any grade of carbide available as they are not heated during manufacturing.

The biggest thing to consider when choosing what type of knives to use is the application:
1. If you hit staples and rocks in your material, then stick with steel knives.
2. If you are rough planing materials that may contain sand and dirt, then you may find carbide tipped knives advantageous.
3. If you are looking for the finest finishes in hardwoods and exotics that are clean, then go with the solid carbide knives.
4. If you are cutting any type of manmade materials such as MDF, particleboard, or glued up woods, you should definitely consider the solid carbide knives.

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From contributor A:
Thank for the explanation. I was aware of most of your info except for the inability to braze the harder carbides. Makes perfect sense.