What "surface finish" actually measures
When a drawing calls out a surface finish, it is specifying how smooth the machined surface must be. The standard measure is Ra (roughness average) — the average height of the tiny peaks and valleys left by the cutting tool, measured in micrometres (µm) or microinches (µin). A lower Ra means a smoother surface. It is one of the most misunderstood and over-specified numbers on an engineering drawing, and getting it right saves real money.
The Ra scale for machined parts
| Ra (µm) | Ra (µin) | What it looks/feels like | Typical use |
|---|---|---|---|
| 3.2 | 125 | Visible tool marks, standard as-machined | General surfaces, non-critical faces |
| 1.6 | 63 | Smoother, light tool marks | Mating faces, moderate appearance |
| 0.8 | 32 | Fine, marks hard to feel | Bearing seats, seal faces, sliding fits |
| 0.4 | 16 | Very smooth | Precision seals, hydraulic surfaces |
| 0.2 and below | 8 and below | Mirror-like (needs grinding/polishing/lapping) | Optical, high-precision sealing |
Ra 3.2µm is the standard as-machined finish most Irish shops produce without special effort — it is the sensible default. Each step smoother (1.6, 0.8, 0.4) adds cost because it needs slower finishing passes, sharper tooling, or a secondary operation like grinding or polishing.
Ra is not the whole story
Ra is an average, so two surfaces with the same Ra can behave differently. For demanding applications, engineers may also call out Rz (average peak-to-valley height, more sensitive to isolated defects) or a maximum Ra rather than an average. For most parts, a single Ra callout on the faces that matter is enough.
How the finish is achieved
Smoother finishes come from a combination of: a finishing pass with a light cut, the right tool and a sharp edge, higher spindle speed with the correct feed, and rigid workholding. Below about Ra 0.4µm, machining alone usually is not enough and you move to grinding, polishing or lapping — separate operations with their own cost and lead time. Some surface treatments (bead blast, anodise, powder coat) change the texture again after machining.
Spec it without over-paying
Default your drawing to as-machined (Ra 3.2µm) and tighten the finish only on the specific faces that need it — a seal groove, a bearing bore, a sliding interface. Calling out Ra 0.8µm on every surface tells the shop you have not identified which faces are functional, and you will pay for polish on faces nobody will ever touch. Pair this with the right tolerance using the tolerances and finishes guide, and see how both move your quote.
Frequently asked questions
What is Ra in surface finish?
Ra (roughness average) is the average height of the microscopic peaks and valleys left by machining, measured in micrometres (µm) or microinches (µin). A lower Ra means a smoother surface.
What is the standard surface finish for CNC machining?
Ra 3.2µm (125 µin) is the standard as-machined finish most shops produce without extra cost. Smoother finishes such as 1.6, 0.8 or 0.4µm add cost through finishing passes or secondary operations.
What surface finish is needed for a seal or bearing?
Sealing and bearing surfaces typically call for around Ra 0.8µm or finer so they slide and seal reliably; very high-precision sealing may need Ra 0.4µm or below.
How do you get a surface finish below Ra 0.4?
Below roughly Ra 0.4µm, machining alone usually is not enough — the part moves to grinding, polishing or lapping as a separate finishing operation with its own cost and lead time.
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