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I understand why larger router bits should be operated at lower speeds and there are a multitude of router bit speed charts out there to help you choose the correct speed for any given bit.

It all revolves around (haha, see what I did there?) the rim speed of the bit and, from what I've been able to work out, it looks like most recommended rotational speeds equate to a rim speed of something between 100 km/h (60 mph) and 180 km/h (110 mph).

But then every article you read about this will state that the size of the bit is only one of the factors to determine the speed at which you want to run the router. Another important factor is the hardness of the wood. But, while every article gives you a chart to illustrate how bit size affects the speed, I have yet to see something illustrating how hardness of the wood affects the speed.

Yes, I get it, harder wood = slower speed, but how much? If the bit size speed chart tells me that the max speed for a bit is 20,000 RPM for instance, I'm presuming that is if you're cutting a soft pine. But if you're cutting African Blackwood or Ipe, you should use a slower speed but how much slower?

Any rules of thumb or charts for this perhaps?

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    I believe that feed rate as grfrazee mentions is one of the key factors to consider. But depth of cut / amount of material removed in one pass is also a definite factor with harder, denser woods.
    – Graphus
    Jan 7, 2016 at 15:12

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Yes, I get it, harder wood = slower speed

I'm not sure this is really true. Router bits cut more efficiently at their top speeds.

What you might want to consider is using a slower feed rate for harder woods. For example, I know I can whip pine right through my router table with no issues, but maple I have to feed more slowly and more controlled.

Any rules of thumb or charts for this perhaps?

I'm not aware of any rule of thumb or any tables/charts giving you this data. It's more of a feel thing that you develop over time - or at least, that's how I did it. Basically, you should be able to use your hands to feel the vibrations and ears to hear how the bit is cutting to see if you're feeding too fast. Also, if you're getting too rough of a cut, that's a good visual/tactile indicator that you might be feeding too fast.

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    Generally, speed is decreased as radius of the bit increases, to keep the linear speed at which the outermost edge of the bit contacts the wood fairly constant. If you hear the machine struggling to maintain speed, you are exceeding the available cutting torque, and if you're already on the right speed range for that bit you probably should feed more slowly or take shallower cuts (progressively sneak up on the final depth) or both. "The engines cannae take it, Captain! "
    – keshlam
    Jan 8, 2016 at 15:10

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