Quick Answer
Casting surface finish measured in Ra — Ra (Roughness Average) is the most common surface texture measurement. Different casting processes produce very different Ra values: investment casting gives Ra 3.2–6.3 µm; sand casting gives Ra 12.5–25 µm; die casting gives Ra 1.6–3.2 µm. For OEM buyers, the key question is: does this surface need to be machined, coated, or left as-cast? Specifying the right Ra on the drawing prevents expensive disputes and ensures the part works in your application.
What Is Ra?
Ra (Roughness Average) is the arithmetic mean of the absolute values of the height of the surface profile deviations from the mean line, measured in micrometers (µm) or microinches (µin). Lower Ra means a smoother surface.
Other surface texture parameters you may see:
- Rz: Mean of the five highest peaks and five lowest valleys — gives a sense of peak-to-valley height
- Rmax: The largest single peak-to-valley measurement in the sampling length
- Rq (RMS): Root mean square of the deviations — more sensitive to peaks than Ra
Ra is the most widely used specification for machined and cast surfaces. Always specify Ra, not Rz or Rmax, unless you have a specific reason.
Ra Values by Casting Process
| Casting Process | Typical Ra (µm) | Typical Ra (µin) | As-Cast Usable? |
|---|---|---|---|
| Investment Casting | 3.2–6.3 | 125–250 | Often yes for appearance/non-critical |
| Precision Investment (high-gamma) | 1.6–3.2 | 63–125 | Often yes |
| Permanent Mold (Gravity Die) | 3.2–12.5 | 125–500 | Sometimes |
| Low Pressure Die Casting | 1.6–3.2 | 63–125 | Usually yes |
| High Pressure Die Casting | 0.8–3.2 | 32–125 | Usually yes |
| Shell Molding | 3.2–6.3 | 125–250 | Often yes |
| Resin Sand (No-Bake) | 6.3–12.5 | 250–500 | Sometimes (after shot blasting) |
| Green Sand | 12.5–25 | 500–1000 | Rarely — needs machining or coating |
What Affects Surface Finish in Castings
- Mold material: Metal molds (die casting, permanent mold) produce smoother surfaces than sand molds
- Pattern material: Metal patterns produce smoother molds than wood patterns
- Sand grain size: Finer sand = smoother surface (but higher cost and permeability issues)
- Metal melt temperature: Overheating increases gas porosity and surface oxidation, roughening the surface
- Pouring speed and turbulence: Turbulent pouring causes surface oxidation and entrapped gas; smooth filling gives cleaner surfaces
- Coating and wash: Mold coatings (refractory washes, zircon wash) improve surface finish
- Post-casting treatment: Shot blasting, grinding, and polishing all change Ra
When As-Cast Surface Finish Matters
In some applications, the as-cast surface finish is functionally or commercially important:
- Appearance parts: Architectural fittings, decorative hardware, consumer products where surface finish affects aesthetics and perceived quality
- Corrosion resistance: Smoother surfaces have less surface area exposed to corrosive environment; also easier to coat or plate
- Sealing surfaces: Gasket faces, O-ring grooves — smoother is better for sealing (but must still meet flatness and finish specs together)
- Fluid flow: Internal passages for oil, water, or air — smoother reduces friction losses and prevents turbulence
- Wear resistance: In some cases, a specific surface texture is desired for tribological properties (not always smoother = better)
- Cost reduction: If surface is acceptable as-cast, you avoid machining or finishing operations — significant cost savings
Ra and Functional Requirements
| Application | Typical Ra Requirement | Notes |
|---|---|---|
| As-cast appearance (visible) | Ra 6.3–12.5 µm | Investment, shell, or resin sand; may need light polishing |
| Gasket sealing face | Ra 1.6–3.2 µm | Machined or high-quality die/investment casting |
| O-ring groove | Ra 0.8–3.2 µm | Machined — not reliable as-cast |
| Bearing surface (sliding) | Ra 0.4–1.6 µm | Machined; some grinding |
| Threaded hole (tapped) | N/A (machined) | Thread always machined; surface finish of hole walls less critical |
| Press-fit surface | Ra 1.6–3.2 µm | Machined or fine investment casting |
| Paint or powder coat base | Ra 3.2–12.5 µm | Smoother is better for coating adhesion, but some texture helps |
| Electroplating base | Ra 0.8–3.2 µm | Smoother for uniform plating; heavy Ra causes uneven coverage |
How to Specify Surface Finish on a Casting Drawing
Use standard surface texture symbols per ISO 1302 or ASME Y14.36:
- Symbol with Ra value: Draw the surface texture symbol with Ra 3.2 (µm) or Ra 125 (µin) — always state units
- Measurement method: Specify “manufacturing attribute” (the foundry’s choice of measurement method) unless you have a requirement for a specific method
- Lay direction: Specify lay direction only if it matters for the application (rare for castings)
- Material condition: Use “maximum” (not “average”) unless your application genuinely requires average surface roughness
Example drawing note:
“Surface texture: 6.3 µmax unless otherwise specified. All gasket sealing faces and datum surfaces to be machined to Ra 3.2 µmax after casting.”
Surface Finish and Machining Allowance Interaction
Surface finish and machining allowance are related but different:
- Allowance: How much material is left for machining (typically 1.5–3 mm)
- Surface finish: The roughness of the as-cast surface before machining
A rough as-cast surface (Ra 20 µm green sand) does not mean high machining allowance is needed — allowance is about dimension, not surface texture. However, rough surfaces may have a harder skin or surface inclusions that accelerate tool wear during machining.
Finishing Operations to Improve Surface Finish
| Finishing Operation | Resulting Ra (µm) | Cost | Notes |
|---|---|---|---|
| Shot blasting/peening | 6.3–25 (changes texture) | $ | Removes sand, improves appearance; does not smoothen, changes profile |
| Tumbling | 3.2–12.5 | $ | Good for small parts, removes flash and sharp edges |
| Grinding | 0.4–3.2 | $$ | For specific surfaces; requires machined datum |
| Honing | 0.1–1.6 | $$$ | For bores and cylinders; requires specific geometry |
| Polishing | 0.025–0.8 | $$$ | Manual or automated; for appearance or seals |
| Electropolishing | 0.1–0.8 | $$$ | Electrochemical; good for complex shapes, stainless |
Buyer Checklist: Surface Finish
- Have I specified Ra (not Rz or Rmax) on all surfaces that need a finish requirement?
- Do all surfaces with Ra requirements actually need that smooth a finish?
- Are surfaces that need machining vs. as-cast finish clearly distinguished on the drawing?
- Has the foundry confirmed they can achieve the specified Ra on this part geometry?
- Is the Ra requirement on the drawing consistent with the casting process chosen?
- Have I accounted for the effect of surface finish on coating or plating adhesion?
- Is a finishing operation (shot blast, tumble) needed to achieve the desired appearance?
Before finalizing the sourcing decision, many OEM buyers also compare Casting Tolerances, Sand Casting Process, Investment Casting, and Gravity Die Casting to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
FAQ
Can sand casting achieve Ra 3.2 µm?
No — not as-cast. Sand casting Ra is typically 12.5–25 µm. If you need Ra 3.2 µm, those surfaces must be machined or you must use a different process (investment casting, die casting).
What is the difference between Ra 3.2 and Ra 6.3?
Ra 3.2 is smoother than Ra 6.3. For most functional applications, Ra 6.3 is acceptable. Ra 3.2 is needed for gasket sealing surfaces, bearing surfaces, and surfaces that will be plated or coated where uniformity matters.
Does smoother always mean better?
Not always. Some applications benefit from controlled surface texture: certain seals, friction surfaces, or paint adhesion may work better with moderate roughness. For general machined surfaces, smoother is almost always better — but there is a cost to achieving it.