Rt
Rt, or total roughness, is the vertical distance between the highest peak and the lowest valley within the evaluation length of a machined surface. This surface texture parameter captures the maximum profile height, making it highly sensitive to isolated defects such as burrs, scratches, or chatter marks that average-based metrics like Ra tend to hide.
In a CNC shop, Rt acts as a guardrail for functional surfaces where a single extreme deviation is unacceptable. A machinist calls out Rt when inspecting gasket lands, hydraulic sealing faces, bearing seats, or precision slides, because a lone deep scratch or tall burr can create a leak path, point loading, or premature wear even when Ra is well within tolerance. On the floor, operators use Rt readings to verify that finishing toolpaths, tool wear, coolant delivery, and deburring processes are actually preventing extreme peaks and valleys rather than merely achieving a pleasing average. A worn insert or unstable workholding can produce a chatter spike that inflates Rt and scrap a part that looks acceptable. For millwork and edgebanding, the same outlier sensitivity catches chip-outs or glue ridges on finished edges. Therefore, Rt is interpreted alongside Ra, Rz, and Rq, and always with process history in mind.
Why is Rt more aggressive than Ra for acceptance testing?
Rt measures the full peak-to-valley span over the evaluation length, so one isolated defect can fail the part even when the average roughness is acceptable.
When is Rt the right parameter to specify?
Only when the functional risk comes from a single extreme defect rather than overall texture uniformity, such as sealing, wear, fatigue initiation, or precision mating surfaces.
What process controls most directly reduce Rt on CNC parts?
Tool sharpness, stable fixturing, correct feed and speed, controlled tool exit behavior, clean chip evacuation, and post-process deburring and handling discipline.