M42 high speed steel

Information on M42 and jointer knives. April 2, 2002

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.

We have been using Esta HSS in our planer for the last year and are not really happy about how long they last. I ordered the M42 this time but have not tried them yet. The Esta people said the M42 will last longer but are prone to chipping from knotty wood. The HSS worked well in softwood and knotty wood but failed fast in hard maple and white oak. I would like to find the best combination for a shop that works with a lot of different species. We are also setting up a S4S machine and thinking about the M42 in that also. Just like to see how it works with knotty alder and knotty pine.

M42 is less prone to chipping or breakage than conventional M2 on tougher applications. Rockwell "C" scale hardness for M2 is 61 to 64; M42 is up to 68. M42 is much harder to machine than M2. M42 differs from M2 in the following general chemical analysis. Manufacturers vary these percentages within certain parameters.

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.

It's not how much the tool costs, but how much it costs to do the job. Sometimes your up-front costs may appear to be more, but your operating costs are less.

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.

All of the above info seems good to me. However, I have used the 'cheapest set' and have found satisfactory results. If you spend the time (setting the knives with a "v-block dial indicator"), and cut out knots before machining, set the feed rate -vs- hardness, keep an extra set (while the other set is being sharpened), you will always save money. Granted you will run it through a wide belt anyway! If you are sizing on end, the same still applies - it's all in the set up. Finely polished ss will work nearly as well as a higher cobalt content knife depending on tolerances on initial setup. If the planer is not set up within .03 it is redundant.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

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.