{"id":6958,"date":"2026-06-11T13:23:35","date_gmt":"2026-06-11T13:23:35","guid":{"rendered":"https:\/\/rapidcision.com\/?p=6958"},"modified":"2026-06-08T19:35:57","modified_gmt":"2026-06-08T19:35:57","slug":"surface-roughness-chart","status":"publish","type":"post","link":"https:\/\/rapidcision.com\/pt\/surface-roughness-chart\/","title":{"rendered":"Surface Roughness Chart: Ra, Rz, RMS, and N-Grade Conversion for CNC Machining"},"content":{"rendered":"

Revisado pela equipe de engenharia da Rapidcision | \u00daltima atualiza\u00e7\u00e3o: junho de 2026<\/span><\/i><\/p>\n

Surface roughness is the microscopic texture of peaks and valleys left on a part after machining, and Ra is the number used to specify it on most drawings. The practical anchor points are simple: Ra 3.2 \u00b5m is the standard, economical finish straight off a CNC mill or lathe, and Ra 1.6 \u00b5m is the usual threshold for precision mating surfaces. Anything smoother than that climbs in cost quickly, because it takes finer tooling, slower feeds, or a secondary process like grinding. The conversion chart below lets you move between Ra, Rz, RMS, and the older N-grade system, and the process table shows what each machining method can realistically achieve.<\/span><\/p>\n

Specifying the right finish is a balance of function and cost, and getting it wrong in either direction is expensive. We hold surface finish to drawing across our<\/span> Servi\u00e7os de usinagem CNC<\/span><\/a>, and the guidance here will help you call out a finish that performs without overpaying for smoothness you do not need.<\/span><\/p>\n

What Ra Actually Measures<\/b><\/h2>\n

Ra, the arithmetic average roughness, is the average absolute deviation of the surface profile from its mean line over a measured length. A lower Ra means a smoother surface. It is the most common parameter on engineering drawings because it gives a single, repeatable number that correlates well with how a surface performs for friction, wear, sealing, and fatigue.<\/span><\/p>\n

Ra has limits worth knowing. Because it is an average, a single deep scratch barely moves the Ra value even though that scratch could let fluid leak straight through a sealing surface. That is why some applications also specify Rz, which captures peak-to-valley height and is more sensitive to individual defects.<\/span><\/p>\n

Ra, Rz, RMS, and N-Grade Conversion Chart<\/b><\/h2>\n

The N-grade system follows ISO 1302, where N1 is the smoothest and N12 the roughest. Most CNC machined<\/a> features land in the N7 to N8 range, which is Ra 1.6 to 3.2 \u00b5m. Convert microns to microinches with 1 \u00b5m \u2248 39.37 \u00b5in. RMS runs about 11 percent higher than Ra, so multiply Ra by about 1.11. Rz shown here is a general approximation of roughly four times Ra and should be treated as a reference only.<\/span><\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
N-Grade<\/span><\/td>\nRa (\u00b5m)<\/span><\/td>\nRa (\u00b5in)<\/span><\/td>\nRz approx (\u00b5m)<\/span><\/td>\n<\/tr>\n
N1<\/span><\/td>\n0.025<\/span><\/td>\n1<\/span><\/td>\n0.1<\/span><\/td>\n<\/tr>\n
N2<\/span><\/td>\n0.05<\/span><\/td>\n2<\/span><\/td>\n0.2<\/span><\/td>\n<\/tr>\n
N3<\/span><\/td>\n0.1<\/span><\/td>\n4<\/span><\/td>\n0.4<\/span><\/td>\n<\/tr>\n
N4<\/span><\/td>\n0.2<\/span><\/td>\n8<\/span><\/td>\n0.8<\/span><\/td>\n<\/tr>\n
N5<\/span><\/td>\n0.4<\/span><\/td>\n16<\/span><\/td>\n1.6<\/span><\/td>\n<\/tr>\n
N6<\/span><\/td>\n0.8<\/span><\/td>\n32<\/span><\/td>\n3.2<\/span><\/td>\n<\/tr>\n
N7<\/span><\/td>\n1.6<\/span><\/td>\n63<\/span><\/td>\n6.3<\/span><\/td>\n<\/tr>\n
N8<\/span><\/td>\n3.2<\/span><\/td>\n125<\/span><\/td>\n12.5<\/span><\/td>\n<\/tr>\n
N9<\/span><\/td>\n6.3<\/span><\/td>\n250<\/span><\/td>\n25<\/span><\/td>\n<\/tr>\n
N10<\/span><\/td>\n12.5<\/span><\/td>\n500<\/span><\/td>\n50<\/span><\/td>\n<\/tr>\n
N11<\/span><\/td>\n25<\/span><\/td>\n1000<\/span><\/td>\n100<\/span><\/td>\n<\/tr>\n
N12<\/span><\/td>\n50<\/span><\/td>\n2000<\/span><\/td>\n200<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Actual finish depends on material, tooling, speed, feed, and coolant, so always verify a critical surface with a profilometer rather than assuming the nominal value.<\/span><\/p>\n

Achievable Ra by Machining Process<\/b><\/h2>\n

Each process has a realistic Ra range. Asking a process to go below its natural range means adding operations and cost.<\/span><\/p>\n\n\n\n\n\n\n\n\n
Process<\/span><\/td>\nTypical achievable Ra (\u00b5m)<\/span><\/td>\n<\/tr>\n
CNC milling<\/span><\/td>\n1.6 to 6.3<\/span><\/td>\n<\/tr>\n
CNC turning<\/span><\/td>\n0.8 to 3.2<\/span><\/td>\n<\/tr>\n
Grinding<\/span><\/td>\n0.2 to 1.6<\/span><\/td>\n<\/tr>\n
EDM de fio<\/span><\/td>\ndown to about 0.1<\/span><\/td>\n<\/tr>\n
Lapping \/ superfinishing<\/span><\/td>\nbelow 0.1<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

A common and costly mistake is specifying a very smooth finish, such as Ra 0.4 \u00b5m, on a part that will only see a standard 3-axis mill. Hitting that value forces grinding or polishing after machining, which can double or triple machining time. If your finish requirement sits below what milling or turning delivers, plan for the extra process and cost up front.<\/span><\/p>\n

Common Ra Values and What They Are For<\/b><\/h2>\n