Trochoidal Milling: Going Deep...but, Not Too Deep

Trochoidal milling, in its many forms and names, is a very powerful tool path strategy. To describe it simply, it utilizes the full cutting length of the endmill with a light radial engagement or step-over (<25%) and fast tool path motions to generate high metal removal rates. 

There is a dynamics element to this. On a stability lobe diagram there is what we call the b-limit. This is the cutting depth, either axial or radial, that results in cutting forces light enough not to deflect the tool. High feed mills took advantage of the axial b-limit and trochoidal milling now the radial b-limit. You can run without chatter at any speed if you are under the b-limit. 

Since trochoidal milling works so well, naturally we want more of a good thing. In this case, it is by increasing the flute lengths up to four times the cutter diameter or more. But there are limits.

The tool assembly is not 100% rigid and the most flexible part of the assembly is the fluted portion of the endmill. On a typical 4-flute endmill, 30% of its mass is removed from grinding the flutes. So a 0.500” diameter endmill is actually the equivalent of a 0.350” diameter rod.

We tap tested three endmills of the same style in the same toolholder in the same machining center. The endmills had flute lengths of 1.5, 2 and 2.5 times the diameter. We overlaid the three stability lobe diagrams and as you can see, the performance declines as the flute length increases. Not just the stable lobes, but the b-limit decreases as well. You can expect that this trend would continue if we tested 3X and 4X endmills. To compensate for the declining stability, you will have to keep reducing the radial engagement and speed to avoid chatter. You may reach a point with such a small step-over (<2%) and slow spindle speed that a more conventional tool path strategy with a shorter flute length may remove more material in a shorter time. If you do want to go deeper with trochoidal milling you may have to increase the diameter of the endmill and/or increase the number of flutes with a greater core diameter to increase the system's stiffness.

Of course, different configurations (tool/toolholder/spindle/machine) could measure and behave very differently than the example above.