I get asked fairly often why there are so few wood choices in G-Wizard Calculator’s Material Database, or how to convert wood hardness to Brinell to tell G-Wizard about the differences in hardness for various kinds of wood. After all, there are probably a couple of hundred different choices for Steel including all the alloys and conditions, but only 5 for Wood (I added Melamine because it’s a hybrid composite):
Just 4 wood types? Why not more?
People wonder why we don’t have a lot more categories and sub-categories. After all, there are so many categories for steel, why not for wood too?
The answer has to do with the size of the “sweet spot” that relates feeds and speeds to good results in a material, together with the relative hardness of the woods (or other materials) versus the cutters (HSS or Carbide). The Sweet Spot determines how the relationship between speed (spindle rpm) and feedrate need to come together for good cutting results. A typical Sweet Spot looks like this schematically:
The green areas reflect ideal Sweet Spot matches for a material and cutting conditions. You can optimize MRR (Material Removal Rate), Surface Finish, and to an extent a blend of all three. Red reflects danger zones. It’s important to note you can get into just as much trouble running too slowly (due to rubbing) as running too quickly.
Tools usually fail for a couple of main reasons:
– They get too hot, which softens the tool and results in a dull edge. Tools have a certain maximum speed, called the “Surface Speed” that governs how fast they can run against a given material. Running too slowly relative to feeds also generates a lot of heat due to Rubbing. This destroys a tool gradually, though gradual can mean a matter of minutes if the tool is hot enough.
– Tools can break because the flutes get too full, the chips have no place to go, and the tool jams and snaps off. This destroys a tool suddenly and is more commonly what beginners see.
A lot else can shorten a tools life–chipped edges in some materials, materials like stainless steel can “work harden” and suddenly become much harder during the cutting process, and some materials like aluminum try to weld themselves to the cutting edge if lubricant or appropriate tool coatings are not used. But, over heating and chip loading are the two main tool life enemies that are directly feeds and speeds related.
You can learn all this and much more from our free Feeds and Speeds Tutorial, but for now, let’s stay with this idea of a Sweet Spot. In general, the sweet spots for woods are much larger than for metals. For harder steels and difficult materials like Titanium, the sweet spot gets very small indeed. Imagine there are actual numbers on our Sweet Spot diagram. “Size” of Sweet Spot refers to how wide a range of numbers are safe.
What determines the “Size” of a Sweet Spot?
A lot of this is all up to that relative difference in hardness between material and cutter. Even Carbide is not all that much harder than hardened steel whereas it is phenomenally harder than even the hardest woods. Red Oak, for example, has a Brinell Hardness of about 3.7. Mild steel is 120, hardened steel is 900, and tungsten carbide is 4000!
It turns out that the characteristics that affect the feeds and speeds for wood are different than hardness. They have to do for example with the behavior of the sap relative to the sawdust and how the chips are made. Plastics, similarly, can behave differently–hard plastics as a name is a bit of a misnomer (though it is the industry standard in tooling catalogs). It refers to the chipping characteristics of the plastic. Hard Plastic turns to dust–it shatters. Soft Plastics allow a nice clean chip to be sliced off, almost like you slice cheese with a knife. Of course, the same is true for woods. “Hard” Wood versus “Soft” Wood has more to do with the seeds of the Wood than the actual hardness. Balsa seems soft but is technically a Hardwood. Hardwood seeds have a covering–shell or fruit, while Softwood does not.
Another factor influencing all this has to do with manufacturing process for the material. In the case of materials like MDF and Plywood, their manufacture can result in abrasive grit being embedded in the layers and that grit matters more than the wood in feeds and speeds. This is why so much router work prefers Carbide tooling even though wood is relatively soft. HSS just doesn’t stand up to that kind of grit very well.
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