Machining Surface Finish: What Ra to Expect From Each Process, and What It Costs to Go Finer

Q: What is the difference between Ra and Rz?

Ra is the average absolute deviation of the profile from its centerline; Rz is the average height between the highest peak and lowest valley over sampling lengths. Ra smooths over isolated defects, so a surface with one deep scratch can show good Ra while behaving badly as a seal seat. For critical sealing faces, Rz or Rmax is the more informative callout.

Q: Does EDM leave a rougher surface than milling?

Yes. Die-sinking EDM leaves a recast layer with a matte, dimpled texture, typically coarser than a finish-milled face at comparable settings. Where a part needs EDM geometry (sharp internal corners, hardened material) plus a fine surface, the EDM area is finish-ground or polished afterward.

Q: Will plating or coating hide a rough surface?

No. Electroplated layers such as nickel (5–25 µm) conform to the substrate and reproduce its texture; thin conversion layers like black oxide (~0.6–0.9 µm) change color, not roughness. If the coated part must look or seal better, the machining finish must improve first.

A buyer's guide from Boyang Hardware · 54 machine tools · Zhongshan, China

Direct answer Standard milled and turned surfaces land around 3.2 µm Ra (125 µin); finishing passes reach 1.6 µm Ra, grinding reaches 0.4–0.8 µm Ra, and EDM typically leaves 1.6–6.3 µm Ra with a matte recast texture. Each step finer adds a dedicated operation and cost — so specify fine Ra only on faces that seal, bear, or show.

3.2 µmstandard milled Ra
0.4–0.8 µmground Ra
6grinding machines
7EDM machines
100%inspection
1–2 dquotation

The three words that matter: Ra, Rz, lay

Ra — average roughness; the number most drawings carry. See the µin/µm conversion table on the 125 µin Ra page.
Rz — mean peak-to-valley height; more sensitive to isolated scratches, better for sealing faces.
Lay — the direction of the tool marks (parallel, perpendicular, circular). For a sliding shaft, lay should follow the motion; for a gasket face, concentric or random lay usually seals better than straight lines.

Roughness by process — what our machines actually leave

Table 1 — Typical surface roughness ranges by process, as produced at Boyang Hardware. Ranges depend on material, tooling and parameters; values are indicative production figures.
Process	Typical Ra (µm)	µin Ra	Notes
3-axis milling (standard)	1.6–3.2	63–125	Default finish on most faces
Milling (finishing pass)	0.8–1.6	32–63	Reduced feed, fresh tooling
CNC turning (standard)	1.6–3.2	63–125	Circular lay, good for rotating seals
Grinding	0.2–0.8	8–32	Bearing seats, sealing lands
Wire EDM	1.0–2.5	40–100	Skin recut improves finish where needed
Die-sinking EDM	1.6–6.3	63–250	Matte texture; polish or grind after if required
Die casting (as-cast)	1.6–6.3	63–250	Machined faces after casting are finer
Injection moulding (tool-finished)	0.1–1.6	4–63	Mirrors the mould cavity surface

What finer finish actually costs

A finer Ra is bought with time: lower feed, higher tool change frequency, an extra pass, sometimes a whole second operation (grinding or polishing). Practical rules from the shop floor:

125 → 63 µin Ra on selected faces: small cost, usually just a finishing pass on existing set-up.
63 → 32 µin Ra: tool wear management and tighter parameters; measurable but modest increase.
Below 32 µin Ra: expect grinding or honing as a separate operation with separate fixturing — the cost step is largest here.

Finish × coating interaction

Coatings sit on the roughness, they do not replace it. Plating (nickel 5–25 µm, chrome 0.5–2 µm decorative) mirrors the substrate texture; conversion layers such as black oxide (~0.6–0.9 µm) change color only. If the coated surface must seal or look premium, improve the machining finish first — see the nickel vs chrome comparison for coating thickness context.

Honest limits: quoted Ra figures apply to accessible flat and simple-curved faces. Deep pockets, small internal radii and complex 5-axis contours are harder to measure and to control to a stated Ra, and we flag those areas at quotation rather than promise a blanket number. Also, Ra alone does not describe waviness — very smooth Ra with long-wave waviness can still leak as a seal seat.

FAQs

What is the difference between Ra and Rz?

Ra averages profile deviation; Rz averages peak-to-valley height. Ra can mask a single deep scratch, so Rz or Rmax is the better callout for critical sealing faces.

Does EDM leave a rougher surface than milling?

Yes — die-sinking EDM leaves a matte recast texture, typically coarser than finish milling. EDM areas that need fine surfaces are ground or polished afterward; we run 7 EDM and 6 grinding machines for exactly this combination.

Will plating or coating hide a rough surface?

No. Plating conforms to and reproduces the substrate; conversion coatings change color, not roughness. Improve machining finish first if the coated face must seal or look smooth.