Q. How do you compare or rate carbide and steel knives, especially for planers?
A: This is indeed a difficult question. Perhaps the answer involves an understanding of how good the wood’s surface must be after planing. Stated another way, carbide has its place, especially when additional machining or sanding afterwards is part of the picture.
Any potential problems with a carbide surface finish can easily be fixed with sanding. Carbide also wears so well that it is preferred for planing rough, air-dried lumber that likely will have small amounts of dust, sand or grit imbedded in the surface.
Special note: What is carbide? A carbide knife, or any cutting tool, refers to just the tip material. It is too expensive to make the entire cutting tool of solid carbide. Carbide is a tough material made of carbon and tungsten metal (or sometimes titanium) that are cemented together with a special binder (6 to 10 percent) and pressure. It is often classified as C2. Not all carbide is the same. There can be low quality foreign-made material, so be careful.
With HSS, we also almost always sand the surface to achieve the desired smoothness, but the amount of sanding (or maybe the amount of putty used to fix defects) is often less with HSS.
Why the difference between HSS (high speed steel) and carbide? The traditional carbide tip on a knife has to be rather large to prevent the knife from chipping or cracking, especially when the knife is machining a knot (which is often dry, very dense and very tough, compared to clear wood). We cannot get a really sharp edge on carbide as it will break off quickly. So, we can think of all carbides being a bit dull…not a lot, but a little bit. This sounds bad, but the good news is that this sharpness stays the same for day after day and week after week. The extremely long life between sharpenings is worth a lot indeed.
Now, let’s be realistic about HSS. Certainly, the freshly sharpened HSS knife is wonderful and the quality surfaces it produces are awesome (if everything else is perfect, such as MC, feed rate, etc.). But how long will this HSS knife stay in this excellent condition? In my experience, many HSS knives are run way past their useful life before being resharpened. That is, although the initial surface from a freshly sharpened knife is better than carbide, it is not too long before the HSS surface, even the knives are not sharpened soon enough, to be worse than a carbide surface.
Over the recent decade, we are processing more knotty wood (or should I say “character” wood?). That means a lot of shock on the blades. That is tough on HSS and causes rapid dulling. Good carbide (new formulations of carbide are being used) would handle this knotty wood with ease, especially with the small cutters instead of one long knife on a planer. So, my preference for HSS is overall a mild preference generally, but many times carbide is indeed the top choice.
Have something to say? Share your thoughts with us in the comments below.