Surface grinding on a magnetic table, after CNC machining: flatness, parallelism and a fine finish on functional faces, hardened parts included.
A dimension says how big the part is. Ra says how smooth its surface is. They are two different things, and the second one shows up only in assembly: a seal that holds, a press fit that grips, a sliding face that does not seize.
Ra is the arithmetic mean of the profile’s deviations from the mean line, taken as absolute values, over the evaluation length. The absolute value is not a detail: without it the mean would be zero, because the area above the mean line equals the area below it. Ra is the height of the rectangle that would have the same area.
The grades are those of the standard roughness series. The drawn profile is a teaching representation of the Ra definition, not the measurement of a specific part.
For a sinusoidal profile, the arithmetic mean of the absolute deviations is 2/π of the amplitude - about 0.64. To show a profile at Ra 0.8 µm, the drawn amplitude is therefore 1.26 µm, not 0.8. Confusing the two is one of the most common misreadings of a drawing.
The evaluation length is 4 mm, while the deviations are measured in micrometres. At the same scale on both axes, the profile of a ground surface would be thinner than the line drawing it. The exaggeration factor is calculated and shown: it is not a hidden graphic licence.
From our own machine park: the Novick AR65-MA wire EDM gives Ra 0.8 µm. The other grades are the standard ladder, shown here so a drawing can be read - not as a commitment.
Novick AR65-MA
Ra averages the deviations of the profile; Rz looks at the largest of them. That is why two surfaces with the same Ra can look - and behave - quite differently. All three profiles below have an Ra of one micron.
| Profile | Ra µm | Rz µm |
|---|---|---|
| Sinusoidal, amplitude 1.57 µm | 1.0 | 3.1 |
| Triangular, amplitude 2 µm | 1.0 | 4.0 |
| Flat, with a single groove over 2 % of the length | 1.0 | 25.5 |
Calculated, not estimated: for a sinusoidal profile Ra = 2a/π and Rz = 2a; for a triangular one Ra = a/2 and Rz = 2a; for a plateau with a groove of depth D over a fraction f of the length, Ra = 2·f·(1−f)·D while Rz is D itself. With f = 2 %, one micron of Ra means a groove 25.5 µm deep.
Ra adds up every deviation and divides by the evaluation length. A single scratch, however deep, occupies little of that length and therefore moves the average little - exactly the third row. Rz does not average across the whole length: it takes the largest peak-to-valley height in each sampling length and averages those, so it sees what Ra smooths away.
The Rz/Ra ratio is not a constant of the material but of the profile shape: 3.1 for the sine, 4.0 for the triangle, 25.5 for the plateau with a groove. Rules of thumb putting Rz at roughly four times Ra hold only between surfaces made the same way - two turned surfaces with the same nose radius. Between a ground surface and a turned one the same rule gives a wrong answer, and across processes the standard defines no conversion at all.
For a sealing face what matters is the depth of the worst scratch, because that is where the oil passes: there Rz is the requirement. For a surface that slides or carries a bearing, what matters is how much of it is load-bearing plateau, not the maximum depth. For every other machined surface Ra says enough and is the cheapest to measure.
The parameter written beside the value, not the number alone: “Ra 0.8” and “Rz 0.8” are two very different requirements, and the second is far tighter. If the surface has a particular job - sealing, sliding, bonding - it is worth saying so, because the measuring direction then follows: across the marks left by the tool.
Surface grinder with magnetic chuck, for precision surfaces and hardened parts.
Surface grinding machine with hydraulic control and feeds, for precision finishing of flat surfaces. A work table in the 500 × 1000 mm class takes medium and large parts. It is used for close dimensional tolerances, flatness, parallelism and low roughness: plates, guideways, die components, fixtures and other parts that need precise finishing after milling or heat treatment.
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