Feeds & Speeds.
I cut a lot of exotic hardwoods, harder than rock maple or red oak. For many projects if I can use a 1/4" bit, that is what I go for. In the metal machining world the machinist lives by the cutting depth and speed ratios provided with the tool - in this case the end-mill. With wood routers bits there is no data, but you can get there with some thinking.
Number of flutes. The more flutes, the more cuts per revolution. However, the more flutes, the shallower the flute for removal of chips. If you are using a single flute cutter, the flute depth will be close to half the diameter of the bit itself - no more then 0.40 of the bit. Deeper flute means more material removed per cutting edge, and more material per revolution.
Revolutions = that is the cutting speed. Metal mills provide the depth of cut and the cutting speed. The cutting speed is always IPM - not RPM. So you must apply math to convert the IPM cutting speed into a proper RPM. The faster the rpm, the faster the cutting speed.
Feed rate - you can start playing with this once you have determined the speed and depth of cut. I am going to use some simple numbers here for the concept. Let’s assume that the mill cuts. .01" per revolution. For each revolution the cutter removes .01" of material. At 100 RPM the cutter removed .01" X 100 Revolutions = 1" of material removed = 1 IPM feed rate. Now - the Dewalt spindle can turn between 19,000 - 29,000 rpm. Let’s assume we have it set for the slowest speed and the same bit, .01 X 19,000 RPM = 190 IPM. If we speed up the spindle to 29,000 RPM the feed rate increases - .01 X 29,000 RMP = 290 IPM feed rate.
That is the theory. Now enters the reality. The X-Carve machine does not resist torsional forces very well, it is too light and made of flexible materials. High RPM and High Feed rates introduce more flex to the machine, creating chatter and reducing the quality of the cut.
While many of the end mill bits we can get allow us to have very high IPM feed rates, the stiffness of the machine limits how much stress we can put on the machine and get a high quality cut. Get the RPM speed wrong, the feed rate wrong, or the depth of cut wrong, and you will either have a rough cut or a broken bit.
Things get more exciting if you are doing a plunge and cutting a single pass grove. The cutter is cutting across at least 160 degrees of the bit, if not a full 180 degrees. In most milling operations (metal again) or in progressive passes, only about 90 degrees of the cuter is in full contact with the material. It is these plunge cuts where w place the most strain on the machine, and if we get depth or feed speed wrong end up breaking the bit.
As a few rules of thumb - when I am cutting a hard exotic wood, I use 2 flute cutters, set the spindle to the slowest RMP and start with the recommended feed speed to start. I can change the RPM up at any time independent of the program controls, and can change the feed speed in the program while it is running. I listen to the machine, and step up the feed and the rpm until the machine sounds the same cutting in any direction.
One last comment. The machine is the strongest cutting in the Y axis, as there are two motors driving the machine in that direction. Also, int he Y axis, the deflection in the gantry and the z axis is irrelevant - will not alter the quality of the cut. So I will speed up the cut speed manually for the Y axis cuts. In the z axis, the flex in the gantry and play in the both the Z and X axis will introduce chatter, si I will reduce the feed speed and increase the RPM to reduce this chatter.