Hard wood vs soft wood k factor?

I’ve been mostly cutting on soft woods so far. I just bought some Walnut and Maple because I want to play around with inlays and it got me wondering about feeds & speeds. I am using my Feeds & Speeds UI where I can enter in the K Factor for a given material. Looking at the SFPF sheet that I based the UI on I noticed that NYCNC has both soft wood and hard wood with a K factor of 10. I also see some incomplete sheets where people were measuring K Factor which I’m not sure I completely understand. Meanwhile I also found a sheet that specifies the Janka hardness for different species of wood.

A couple questions:

  • Is there a way to translate Janka hardness into K Factor?
  • Is there a good ballpark for hard wood K Factor if I am using 10 for soft wood in my calculations so far?
  • Presumably the K factor for hard wood and soft wood are not the same, in particular since I do recall Carbide Create has different feeds & speeds for hard wood and soft wood.

Thanks in advance :slight_smile:

A quick google seems to show that K Factor is a thermal conductivity value, not a hardness value. There would be no translation, they could be completely independent.


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Unfortunately I think you are thinking of a different K factor… K as a constant I can imagine is pretty common choice of variable. Are you familiar with the NYCNC feeds & speeds calculator that I believe @Julien and @gmack repurposed for Shapeoko?

the “K” factor (or its inverse value the Unit Power) is a constant that depends on the material’s hardness, and corresponds to how many cubic inches per minute (or cubic millimeters per minute) of material can be removed using 1 horsepower.


I don’t think so…you’d probably have to test each species.

The general consensus is to use a general chipload as your guide. Even though wood varies tremendously to a woodworker, you’ll rarely come across a wood as hard as aluminum. The hardness range for woods on the scale of material hardness is pretty narrow.
What hardwood species are you cutting?


Maple and walnut to start. Are you suggesting I just stick with the same feeds and speeds I would use for soft woods? Since after all, it’s not aluminum as you say…

For the C3D bits, they seem to run hardwoods at about 75% to 80% of the softwood feedrate. That was the case for the 201, the 102 and the 301 (1/4", 1/8", 90 deg V).

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I believe what you are looking for is “Janka Hardness” — see:

Right, so one of my questions was:

Is there a way to translate Janka hardness into K Factor?

The calculator explicitly expects a K Factor which typically is set at 10 for soft woods but figured for hard woods it should probably be higher since you should cut them slower?

Is the idea with hard wood to just target a lower chip load than soft wood?

Ok, I’m dumb. It actually talks about both in @Julien ebook. There’s a table where he recommends chiploads depending on cutter size and hard wood vs soft wood and then he also has this:

For example,

  • 6061 T6 aluminium has a K of 3.34 cubic inches per minute
  • it’s about 10 in³/min for hard woods and hard plastics
  • and up to 30 in³/min for soft woods, MDF, …

So apparently the settings I’ve been using have been for hard wood already!


When it comes to CNC wood work, I’ve found the E and P factors as most important.
E is experience. Learn as you go, write it down if you must. Make a lot of small trinkets before taking on a large project.
P is patience. Don’t be overly intimidated by a projected 2 hour 45 minute machine time estimate. Use fresh bits and go slow on hardwoods.
Sure, I’m being a little cute here, but you can hurt yourself and never get anything done if you spend too much time searching for an answer rather than working toward a solution.


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