Sharpness of High Speed Steel Versus Carbide Tooling

      Here's a long, technically detailed, and authoritative thread that explains why high speed steel can be sharpened to a finer edge than carbide, but will lose that edge quickly in hard or abrasive materials. (And more.) February 22, 2011

Question
Can anyone explain how high speed steel is sharper than carbide? How is one material sharper than another? Sharpness, I've always believed, was directly related to the grit used to grind the edge. The sharpest chisels and knives I've ever seen had a final buff with jeweler's rouge. I use polished carbide blanks for inserts in CNC tooling, which give the best finish and hold an edge much longer than standard ones. Can anyone really convince me to switch to steel?

Forum Responses
(Solid Wood Machining Forum)
From contributor M:
In a nutshell, carbide is a lot harder and as a result, more brittle. That final hone crumbles the edge, whereas HSS burrs rather than crumbles, ultimately leaving a sharper edge. However, as you noted, the life of carbide is far superior to HSS steel, so your slightly duller tool is going to be slightly duller for a long time. The comparison of carbide to HSS is more common in moulder and planer work than CNC applications. Stick with your carbide inserts on the CNC.



From contributor R:
Well, there are some materials that you just don't want to use steel on, like MDF or plywood. They will dull steel fairly quickly. Carbide tips and inserts are composed of very fine particles that are compressed or sintered into a whole, so the finest edge or corner that can be achieved is relative to the particle size. Steel particles or grains are more homogenous, and can be shaped or polished to a finer edge. All this is on a microscopic level, so it really doesn't apply much to the person doing standard wood machining and molding, except that the carbide tooling will last much longer before getting dull. If you are doing carving or fine joinery, the polished, clean cut of steel tools is more obvious and you are not as concerned about tool edge life, because you are continually touching up the edge. I use carbide tools and inserts almost exclusively on CNC equipment, and steel tools for handwork.


From contributor A:
The technical answers to your questions above are right on the money. For a real demonstration, you need to use a HSS bit in a hand router. I used to make my own custom bits. The blanks are just big steel paddles. The first time I went to use one, I was a bit terrified. No anti-kickback body, just a big wing.

That first cut was an eye opener! The router swept into the cut just as smooth and easy as you please. A carbide bit of similar proportion cannot compare.

Carbide will certainly last longer under most conditions, but I believe we have lost much by abandoning HSS. HSS is also vastly preferable to carbide in high heat situations. For deep mortising cuts, a HSS bit will outlast a carbide every time. Excessive heat breaks down the bonding agent that holds the carbide particles together, causing the particles at the cutting edge (thinnest part) to fly off, blunting the edge.



From contributor J:
Carbide starts out dull compared to HSS, but just stays that way for a long time. And I'm learning (again) with the moulder that it's best for the harder woods. We profile sand everything anyway, but with ash, walnut, mahogany, etc. it does not do as good of a milling job. But you get into the hard maple and you can run for weeks without resharpening. Carbide is a godsend if you have an application that can benefit from it.


From contributor B:
The non-scientific explanation... Steel can be made very, very sharp because it is steel (metal/iron). It has a long-grain molecular structure that lends itself to the production of a long, sharp edge.

Carbide (carbon) on the other hand, is not a metal at all, but only very hard little rocks. These individual rocks (atoms) are crystalline in structure and do not lend themselves to any kind of a long sharp edge. They're just very, very hard. Diamond, for example, is the hardest of all and is pure carbon.

When alchemists and wizards eventually learned to mix the two together, they came up with an iron that is as hard as rock, but unfortunately not as sharp as steel. Why is that? The little carbon atoms are suspended within the greater iron matrix, making it very hard, but it's still the edge of steel that does the cutting. It's a compromise. They traded some of the inherent sharpen-ability of iron for the hardness of rock.

HSS is simply a special recipe for steel that is very heat resistant. It works best in almost all rotating machinery, but it ain't as hard as rock. Sometimes you need those little rocks to support the edge of the steel because the steel often comes in contact with stuff (silicates/dust) which is harder than itself.

Carbide steel, though, is dull compared to pure (say, surgical) steel. Steel still requires a little more sharpening to maintain its razor edge. Fortunately it sharpens easily with regular stones (and not diamonds).

Unfortunately the high speed of machinery also translates into high heat. Heat vs steel is like Kryptonite to Superman. Heat erodes and destroys the sharp edge of steel but leaves the carbon sticking up. Now the razor blade knife edge is blunted, but not dead. This is the compromised edge of carbide steel that seems to work and last longer (but not necessarily better).

Experience dictates which is truly better, but don't fall into the mindset that everything now needs to be carbide. Diamond is the newest hyperbole that we all definitely must have. I am not buying it.



From Dr. David Rankin, forum technical advisor:
This is a very good question, and many good answers. HSS will sharpen sharper than carbide. I explain it this way. Carbide is a powderized metal made up of a bunch of ball bearings. The size of the bearing determines the sharpness of the carbide. As soon as you run the carbide, the first row or two of the balls break away, but then holds up well for a long time (in comparison to normal HSS).

As stated in an earlier post, heat is the enemy. Heat can be reduced on a cutting tool by several things. You can add a lubricant to the tool and this can help. This is done on saw blades for many applications. The lubricant acts as a coolant. I have seen this increase the tool life several times the normal expected. Lubricants work well on HSS and carbide. It is important that the lubricant used on carbide is designed for carbide so that you do not break down the cobalt binder in the carbide.

I use special coatings for tools that reflect heat. Many years ago I patented a coating that has proven to extend tool life 6-10 times for HSS and 3-4 times for carbide. This coating does not eliminate the balls breaking off of the carbide but does help carbide hold that running edge for an extra time. In the case of HSS, the coating reflects heat of 2200F and is harder than carbide. We have run moulder runs of 6 - 8,000 lineal feet in many MDFs. This does not replace carbide but is a good middle range tool between normal HSS and carbide.

Another thing to consider is the type of grinding wheel you are using to sharpen your HSS and carbide. Carbide being a man-made material requires the correct diamond wheel. Many grindermen use a wheel that is not fine enough to produce the quality of finish desired. Also the coolant used is a major concern. Carbide requires a coolant that does not leach the carbide. (This is where the binder is damaged.) Clean coolant is critical in both HSS and carbide.



From contributor B:
Dr. Rankin, I've always considered spray coatings to be kind of a gimmick, especially for router bits that are rotating at 20,000 rpm! I do understand how some coatings can reduce the buildup of pitch and resins on cutting tools, but I am skeptical of any coating's ability to actually adhere to the cutting edge (while it's cutting). I am also skeptical of your claim of a coating which is harder than the carbide itself (diamond mist?).

I do understand how cutting coolants and lubricants are essential in machine tool applications, but liquid lubricants are not practical in woodworking applications. So I am a doubting Thomas here, but I hope you can convince me that there really is such a thing as a magical mist that I can believe in.



From Dr. David Rankin, forum technical advisor:
The coating that I use is not a spray, it is a vacuum process. We take the knife steel or router bit to 275F in a vacuum. We then explode a series of targets onto the steel. These targets are made of several materials (I am not at liberty to release this info). The coating I use is made up of several layers. Without giving out trade secrets, I will try to provide you some info.

One layer increases the lubricity of the substrate, in this case reducing the buildup of pitch or other similar byproducts. One layer reflects heat, much as a heat sink in electronics. One layer is called a diamond like coating and in comparison to carbide is about 20% harder. The total coating is only 4 microns thick, so flacking of the coating has not been a problem.

I can totally understand the concern about coatings and tool treatments. When I started the research in the late 1990's, most of the coating processes produced a thick coating that would flack. As for treatments or sprays, the results depended on the material being cut, the amount of spray and several other factors.

In the last 10 years, I have used a couple of good sprays and systems that got the spray to the correct placement. However, sprays are not useable in all applications.

When I first patented the DGK coating, we found that in solid woods we could increase the life of the tool by over 6 times. I did the original tests in Mississippi at a long time customer. During the test we ran at 150 feet per minute jointed on hardwoods. I stopped the feed and let the knives (6 wings) run onto maple. After 5 minutes the wood was very close to burning, while the tool's temperature was under 110F. When the feed was restarted, we had not lost joint and the tool ran the remainder of the day. This customer has run DGK since 2000 and has had consistent results.

A couple of secrets:
Grind with a ceramic or CBN wheel
Do not hone the face of the tool
In the case of router bits, recoat after sharpening.
When moulder, planer and shaper knives, you do not need to recoat as the coating in on the face of the tool.



From contributor V:
One point that has not been noted is that the wedge angle must be larger for carbides than steels. Thus we can grind to a keener edge in steels - if we did this in carbide the edges would break off, as it is too brittle. When machining solid woods, steels will always have an advantage in the edge produced. We go to carbides where tool lifetimes are more important than finish qualities - especially in man-made materials. They both have uses - good suppliers can steer you to the right knife materials based on the intended uses.


From contributor A:
It is well worth noting that quality and grade of carbide varies. The micrograin carbide from CMT is sharpened to close to HSS sharpness, and if used on MDF, there is no comparison after even a little use, as HSS is very poor cutting anything slightly abrasive. Good quality micrograin carbide, kept clean (which helps the blade to eject waste better and consequently keep cool) holds its edge far longer than standard carbide.

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