Surface Finish Ra Explained for Machinists

What Ra Actually Measures

Ra is the arithmetic average of the absolute deviations of the surface profile from the mean line, measured over a sampling length. In plain language: if you could zoom in on a machined surface, you would see a series of peaks and valleys left by the cutting tool. Ra is the average height of those peaks and valleys, measured from a line drawn through the middle of the profile.

Ra is expressed in microinches (μin) in the US and micrometers (μm) internationally. 1 μm = 39.37 μin. Common callouts: 125 μin (3.2 μm) for rough machining, 63 μin (1.6 μm) for standard finish machining, 32 μin (0.8 μm) for fine finish, 16 μin (0.4 μm) for precision ground surfaces.

Ra is not the only roughness parameter, but it is by far the most commonly specified in US manufacturing. Other parameters include Rz (average peak-to-valley height over 5 sampling lengths), RMS (root mean square roughness, approximately 1.11 × Ra), and Rt (maximum peak-to-valley within the evaluation length). If a drawing does not specify which parameter, assume Ra.

The Formula: Feed Rate and Nose Radius Control Finish

For single-point turning (and by extension, single-point boring and face grooving), the theoretical surface roughness is determined by two parameters: the feed per revolution (f) and the tool nose radius (r). The formula is:

Ra (μin) = f² / (32 × r) × 1,000,000

Where f is in inches per revolution and r is in inches. This formula calculates the height of the scallop pattern left by the tool as it sweeps across the rotating workpiece. A finer feed produces shorter scallops (better finish). A larger nose radius produces shallower scallops (better finish).

For a 1/32" (0.031") nose radius insert at 0.005 IPR feed: Ra = 0.005² / (32 × 0.031) × 1,000,000 = 25 / 0.992 = 25.2 μin. To achieve 63 μin finish, this combination has substantial margin. To achieve 16 μin, you would need to reduce the feed to approximately 0.0025 IPR or increase the nose radius.

The formula gives the theoretical minimum — the best finish achievable with perfect conditions. Real-world finishes are always rougher due to vibration, tool wear, built-up edge, workpiece deflection, and other factors. Plan for your actual finish to be 1.5 to 3 times worse than the theoretical value.

Built-Up Edge and Material Effects

The theoretical formula assumes a perfectly sharp cutting edge with no material adhesion. In reality, many materials form a built-up edge (BUE) — a lump of workpiece material that welds to the cutting edge and periodically breaks off, tearing the surface. BUE is the single biggest cause of surface finish problems in practical machining.

Stainless steel (304, 316): Highly prone to BUE. The combination of high ductility and work hardening causes material to adhere to the cutting edge, creating an irregular, unpredictable surface. Expect actual finish to be 1.5 to 2.0 times worse than theoretical. Mitigation: use sharp positive-rake inserts with PVD coatings (TiAlN, AlTiN), increase speed to reduce adhesion tendency, and use high-pressure coolant.

Aluminum: Forms BUE at low and moderate speeds. At high speed (>800 SFM with carbide), BUE diminishes and aluminum finishes can actually be better than theoretical. Use polished, uncoated carbide inserts for best finish. Coolant prevents galling.

Cast iron: Minimal BUE. Chips are discontinuous (break into small pieces), which leaves a relatively clean surface. Cast iron finishes are often close to theoretical values. The challenge is abrasive wear from graphite and sand inclusions.

Titanium: Severe BUE tendency due to chemical reactivity with tool materials. Expect finishes 1.5 to 2.5 times worse than theoretical. Use sharp, coated inserts, moderate speeds, and heavy coolant. Surface finish in titanium is one of the most difficult machining challenges.

When to Machine vs. Grind vs. Hone

Turning/Milling (8-250 μin range): With a sharp insert, rigid setup, and controlled feed, turning can achieve 16-32 μin on steel and 8-16 μin on aluminum. Face milling with wiper inserts (extra-wide wiper flat behind the cutting edge) can reach 16-32 μin. For most prints calling 63 μin or rougher, single-point machining is the right process.

Grinding (4-32 μin range): Grinding is a different cutting mechanism — thousands of tiny abrasive grains each take a microscopic cut. The result is a very consistent, fine surface without the scallop pattern of single-point machining. Grinding is the standard process for finishes finer than 16 μin on hardened steel. It is also used when dimensional precision (roundness, straightness, size) must be tighter than turning can hold.

Honing (2-16 μin range): Honing uses abrasive stones with a reciprocating and rotating motion to produce a crosshatch pattern. The crosshatch retains oil, making honed surfaces ideal for bearing journals, cylinder bores, and seal surfaces. Honing can achieve 4-8 μin with fine stones and 2-4 μin with diamond or CBN stones.

Lapping (1-4 μin range): Lapping uses a charged flat plate or form to produce the finest finishes and the tightest flatness. Used for gauge blocks, optical surfaces, seal faces, and precision mating surfaces. Lapping is slow and expensive — reserve it for applications where no other process can meet the requirement.