Before I even get started, I want to emphasize that this shortcut is NOT even close to being a good substitute to running a proper sweet-spot test to determine the best combination of feed and speed for your application. However, since you have decided to ignore that anyway, the following technique will USUALLY get you close enough to be able to make clean, reliable cuts.
This technique assumes that, at some previous time, you have run the sweet-spot test to determine the optimum feed & speed when machining a piece of wood on your CNC router. Now you want to use the SAME bit (or at least a very similar bit) to machine a different species of wood and you are wondering how fast (feed-rate) you can cut the new material and still end up with a good edge finish without breaking your cutter.
To calculate the new feed rate, we will use a very handy value, called the Janka number or Janka force, to compare the cutting properties of the two species of wood. The process proceeds as follows:
- Look up the Janka number of the wood evaluated in the previous sweet-spot test. Call this value Janka(sweet wood).
- Look up the Janka number of the new wood. Call this value Janka(new wood).
- Divide Janka(sweet wood) by Janka(new wood).
- Multiply the result by the feed-rate that you determined from the sweet-spot test.
- The result is the feed rate that you should use to carve the new piece of wood.
In other words:
As stated above, this shortcut is not a substitute for a full sweet-spot test. While the sweet-spot test can be applied to any combination of cutter and material, the "guesstimate" will only yield useful results if:
- a similar bit ( same dia. & number of flutes) is being used
- the two woods have a similar grain structure and differ in Janks number by no more than 400
- the depth per pass is the same
- the RPM is the same (±10%)
We have found that Janka ratio guesstimating seems to be more accurate when going from a softer to a harder wood (low Janka to higher Janka). Going from harder to softer more often than not, yields a feed-rate that is too low. We have also found that trying to "stack" guesstimates is not a good idea since the deviation from the true sweet-spot increases the more removed you are from the original test. Where we have found the best value in this technique is using it to fill in gaps in our in-house feeds/spped logs, especially when we need to make some chips and do not have time to set up a full test.
In spite of these limitations, Janka guesstimating is a very valuable technique and can be used in virtually any shop.