Surface grinding across areas up to 500 × 1,000 mm, and wire EDM for complex contours and hardened parts up to 500 mm thick.
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, a single grade carries a figure confirmed by the machine’s data sheet: 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
Two very different processes both come after heat treatment, where cutting with carbide is no longer economical. But the choice between them follows the shape of the part, not its hardness.
The abrasive wheel removes a small allowance from a surface it can reach. It delivers flatness, parallelism and a fine finish, and hardness does not stop it. Its two conditions: the wheel must reach the surface, and the part must be held without distorting under the contact force — a thin part clamped tightly leaves the machine flat and bows once the vice is released.
The wire never touches the part: electrical discharges melt the material, and the cutting force is practically zero. That is why it cuts hardened steel as readily as soft, and why thin walls do not deflect under the tool. The wire follows a contour and cuts through the part, top to bottom — here, over a height of up to 500 mm.
Almost sharp internal corners. A cutter leaves its own radius in the corner, however small the tool; the wire leaves only its radius plus the spark gap — an order of magnitude less. That is why a slot with straight flanks and a flat bottom, or an internal keyway, is cut with the wire rather than milled.
The wire only cuts contours that pass through the part: a blind pocket with a floor cannot be made this way. The cut surface carries a thin recast layer, which on fatigue-loaded parts is removed by grinding or polishing. And for a large flat surface the wire is the long way round: there the wheel does the job faster and better.
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 such as “Rz is about 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.
Indicative figure, per catalogue documentation. Specifications from catalogue documentation; confirmed against the machine nameplate before quotation.
to be confirmed The exact model of the surface grinder: the client description says PSGS 50100AHR, the earlier data said PSGC-50100AHR, from a dealer listing. To be confirmed against the machine plate — one letter changes the model.
Wire EDM for complex contours, keyways and hardened parts that cannot be cut conventionally.
CNC wire EDM machine with reciprocating wire, for high-precision cutting of electrically conductive materials whatever their hardness. The process makes no mechanical contact, so complex contours, narrow slots, small radii and shapes hard to obtain by cutting become possible. It suits dies, punches, gauges, fixtures and precision components; CNC control holds path repeatability and keeps mechanical load on the part low.
Catalogue figure, confirmed on the machine. Specifications published by the manufacturer.
to be confirmed Confirmation that the machine on site is the AR65-MA variant.
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