Valley Depth
Valley depth is the vertical distance from the mean line of a surface profile down to the deepest valley within the specified evaluation or sampling length, denoted as Rv in ISO surface texture language. Related measures include Rz and Rmax for peak-to-valley roughness limits. In CNC machining and millwork, it also informally means the depth of low spots in a contoured toolpath or carved surface.
On the shop floor, valley depth is the number machinists watch when a finish pass has to meet a roughness print callout. A long-reach end mill shaping a mold cavity, a face mill cutting a gasket land, or a router profiling veneer show surfaces can all leave periodic low spots that trap coolant, collect dust, or telegraph through paint. The deepest valley is not set by programmed Z-depth; it is dictated by tool nose radius, stepover, feed, spindle load, deflection, chatter, and material behaviour. For turning and fine finishing, smaller feeds and a larger nose radius reduce microscopic valley depth, while poor rigidity pushes it up. In millwork, low spots on profiled edges or sanded substrates cause glue-starved joints and uneven stain absorption. A consistent final pass with reduced stepover and verified roughness inspection keeps functional surfaces sealing and cosmetic surfaces looking clean.
- Chatter valleying on finishing passes: A long-reach end mill or under-rigid setup vibrates, leaving periodic deep grooves that exceed the finish spec even if the average surface looks acceptable.
- Toolpath scallops from excessive stepover: In 3D machining, too-large stepover leaves uncut cusps between adjacent passes, creating measurable valley depth that is obvious on mold surfaces, curved millwork, and V-carved signage.
- Misreading depth as geometry instead of roughness: Operators sometimes confuse programmed Z-depth, pocket depth, or flute valley depth with surface valley depth, leading to wrong process changes when the real fix is feed, stepover, tool radius, or a finishing pass.
Is valley depth the same as Rz?
Not exactly. Rv is the deepest single valley relative to the mean line, while Rz is an average of peak-to-valley heights across multiple sampling lengths.
What machining factors increase valley depth the most?
High stepover, poor tool rigidity, vibration/chatter, excessive feed for the finish condition, worn cutters, and unfavorable tool nose geometry all increase valley depth or the effective scallop left on the surface.
How do shops reduce valley depth without changing the part geometry?
They typically reduce stepover, add a finish pass, increase rigidity, optimize feed and spindle speed, use a sharper or larger-radius finishing tool, and verify the result with surface-roughness inspection rather than relying on visual appearance alone.