What is the limiting factor for how deep to cut (per pass) with an endmill in a particular material (assuming sufficient flute length)?

• A. Cutting tool/ workpiece/ machine stiffness (which must resist the cutting forces and subsequent vibrations)
• B. Material hardness only
• C. Fluid temperature only
• D. Operator experience only

Answer: (A)

The main idea is that how deep you cut in one pass is limited by the stiffness and stability of the entire cutting system. When the endmill engages the material, cutting forces push on the tool, the holder, the workpiece, and even parts of the machine. If the depth of cut is too large, these forces cause deflection and can drive the system into unstable vibration or chatter. That instability degrades surface finish, accelerates tool wear, and can lead to tool breakage.

A very stiff setup—rigid toolholding, solid workholding, and a rigid machine structure—resists those deflections, so you can take a larger depth of cut per pass without losing stability. Conversely, even with a hard material, if the system is not stiff enough, you’ll hit the chatter limit early and can’t safely increase the depth of cut. Fluid temperature and operator experience influence how you tune speeds, feeds, and lubrication, but they don’t set the fundamental stability limit the way system stiffness does. Material hardness affects cutting forces to some extent and tool life, but it isn’t the primary factor that governs the maximum per-pass depth.