Question
I just received a set of insert tersa knives for my jointer. We ordinarily use high speed steel for the sharpness. Our vendor recommended M42 for longevity. Can anybody tell me more about this classification?
Forum Responses
M42 is a grade of high speed steel with a high content of cobalt in the alloy. It is less brittle than carbide steels. Similar to nickel in hardness and durability, cobalt alloy steel is likely to be fairly expensive. Those in the machine tool business can tell you more as this type of tool steel is common in machine shops.
M42 versus AISI.M2
Tungsten 1.5% vs. 6.76%
Molybdenum 9.5% vs. 4.83%
Colbalt 8.0% vs. 0.00%
The other chemical components are very close in percentage between the two.
Tungsten: increases hardness, edge life and heat resistance.
Molybdenum: produces toughness and neutralizes brittleness of chromium. Also helps increase edge life.
One thing to bear in mind in comparing these percentages is that Molybdenum usually has twice the value of Tungsten in the final analysis of the steel being used. Since Tungsten is very expensive, you can see why some manufacturers try to use more Moly to make up for less Tungsten.
For most woodworking applications, M2 is still the most preferred steel.
You also need to consider the hook/rake angle, clearance angle, cutting edge to gib distance, thickness of the knife, the grade of the knife steel, feed rates, number of wings and depth of cut. Too much to consider? Well, they all play a roll in your operational costs.
Many planers have the rpm's set, the number of wings set. The only variables are in the feed rate and depth of cut. Our simple rule of thumb is to "run the feed rate as fast as possible to achieve the quality of finish you require." This will not only increase your tool life, but improve your finish.
The slower feed rates heat up the knives, causing them to dull prematurely, and can burnish your material making it more difficult to sand and finish. In some cases, you can actually remove knives by inserting filler knives to achieve the same affect if you cannot increase your feed rate. This works great on four wing cutterheads, but cannot be done on three wing cutterheads.
Comment from contributor A:
We machine a wide variety of hard and softwoods in our window and door shop - oak (red and white), cherry, maple, teak, walnut, afromosia, Douglas fir, yellow cedar to name a few. We have been running Tersa heads (4 wing) in our joiners, planers, and moulders for a couple of years now. I have found that, for the most part, the M42 doesn't produce a better finish, but it does last a lot longer when machining hardwoods. For woods such as teak, walnut, or afromosia, a combination of 2 M42 and 2 solid carbide seem to work well and last a lot longer than just the M42. The difference between the M42 and the regular HSS Tersa knives is most noticeable in the footage obtained from a set before they are worn out. As well, the M42 is ground differently than the HSS; it has a 'micro-bevel' (has been ground at 2 different angles, 2nd angle being about 3 degrees less). We usually run 2 knives with 2 'dummy' knives (used Tersa that I grind down so there is no edge, then re-balance), this not only cuts usage in half while keeping same finish quality, when running laminated product, 2 M42 and 2 HSS seem to hold up the best.
