Drawing says Ra 0.8 micrometer and your profilometer reads microinches. Type in one, get every other unit. Plus which processes can actually hit your target finish.
Surface finish is the texture left on the part after machining. The drawing calls out a number, usually Ra in microinches, wherever two surfaces have to seal, slide, or press-fit together. The profilometer measures it, you either hit the spec or you send it back to the grinder.
Ra is the one you see on most US prints. It averages all the peaks and valleys across the trace length into one number. When somebody says “32 finish” without any other qualifier, they mean Ra 32 microinch. Every profilometer on the market reads Ra by default.
The catch with Ra is that it averages everything. One deep tool mark in an otherwise smooth surface can pass an Ra check because the average looks fine. That is why critical sealing surfaces sometimes call out Rz or Rmax in addition to Ra.
Rz looks at the five biggest peak-to-valley swings in the sample and averages them. It catches the deep scratches that Ra smooths over. Ground surfaces run about 4 times Ra. Turned surfaces about 5 times. EDM can be 7 times or more because the craters are deep and irregular.
European and Asian drawings use Rz more than US shops are used to seeing. If the print says Rz and your profilometer only shows Ra, multiply Ra by 4 to 6 for a rough estimate, but talk to the customer if the tolerance is tight.
RMS is about 1.11 times Ra for a typical machined surface. It shows up on older US military specs and legacy aerospace prints. In practice, the terms Ra and RMS got used interchangeably for years. A drawing from the 1980s that says “32 RMS” almost always means 32 microinch Ra.
The ISO system boils roughness down to 12 grades. N1 is a mirror polish at 0.025 micrometer. N12 is a rough saw cut at 50 micrometers. Each step is double the previous one. If you see a triangle symbol with a number on a European drawing, that is the N-grade. N6 (0.8 µm, 32 µin) is the most common callout for ground surfaces.
Sawing, flame cutting: Ra 250 to 1000 microinch. Nobody cares about finish on a saw cut. It is getting machined next.
Turning, milling, boring: Ra 63 to 125 is typical without trying hard. A finish pass at high RPM with a sharp insert or wiper can get you to Ra 16 to 32. This is where most shop work lives.
Grinding, reaming, broaching: Ra 8 to 32. The go-to when the print calls out a finish tighter than what a lathe or mill can hold reliably. O-ring grooves, bearing journals, hydraulic bores.
Honing, lapping, polishing: Ra 1 to 16. Slow and expensive. You do this for precision bearing races, gauge blocks, hydraulic valve spools, or anything where a few microinches matter.
O-ring grooves: Ra 32 on the sealing surfaces. The bottom of the groove can be 63. High-pressure hydraulics, go to 16 on the bore.
Hydraulic cylinder bores: Ra 8 to 16, honed with a crosshatch pattern. The crosshatch holds oil. A polished bore with no crosshatch actually seals worse because the wiper has nothing to ride on.
Bearing journals: Ra 16 to 32 for most plain bearings. Precision spindle bearings can need Ra 4 to 8.
Gasketed flanges: Ra 63 is fine with a gasket. Metal-to-metal sealing needs 16 to 32.
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