Rz

Rz is a surface-roughness parameter measuring peak-to-valley height, representing the distance between the highest peak and deepest valley in a machined surface profile. In CNC practice, it is commonly reported as the average of maximum roughness heights over multiple sampling lengths. Rz responds strongly to isolated scratches, chatter, or burrs, making it more sensitive than Ra for surfaces where extremes affect function. The governing standard must be specified for exact calculation.

On CNC parts and millwork components, Rz is chosen when surface extremes matter more than average texture. Sealing faces, wear interfaces, load-bearing contacts, coating adhesion, and fatigue-sensitive areas can fail from one scratch, burr, or torn grain. In production, an Rz callout means the operator or quality technician runs a profilometer over the defined sampling length and compares the measured extremes to print. Because Rz reacts to isolated peaks and valleys, it can reject a part that Ra alone would pass. This matters after milling, turning, grinding, sanding, scraping, or edgebanding, where tool marks, chatter, knife-out, chip-out, or handling damage dominate function. Since Rz values are generally larger than Ra and not interchangeable without a standard-based conversion, the drawing must reference the governing standard and the shop must verify measured profiles against that same standard.