Quick Answer
Casting surface finish — surface finish is measured in Ra (roughness average) and varies by casting process from Ra 0.4 µm (die casting) to Ra 25 µm (sand casting). Better surface finish costs more. The right surface finish is the one that meets functional requirements at minimum cost — do not over-specify surface finish where it is not needed.
What Is Surface Finish (Ra)?
Surface finish describes the micro-roughness of a surface. It is measured as Ra (Roughness Average) in micrometers (µm):
- Ra 25 µm: Very rough, like coarse sandpaper
- Ra 6.3 µm: Rough machined surface
- Ra 3.2 µm: Standard machined surface
- Ra 1.6 µm: Fine machined surface
- Ra 0.8 µm: Very fine machined surface
- Ra 0.4 µm: Ground or super-finished surface
Surface Finish by Casting Process
| Process | Typical Ra (As-Cast) | Best Ra (As-Cast) | After Machining | Cost Impact |
|---|---|---|---|---|
| Sand Casting | Ra 12.5–25 µm | Ra 6.3–12.5 µm | Ra 0.8–3.2 µm | Lowest cost |
| Investment Casting | Ra 3.2–6.3 µm | Ra 1.6–3.2 µm | Ra 0.8–1.6 µm | Moderate cost |
| Gravity Die Casting | Ra 3.2–12.5 µm | Ra 1.6–3.2 µm | Ra 0.8–1.6 µm | Moderate-high |
| Low Pressure Die Casting | Ra 1.6–6.3 µm | Ra 0.8–1.6 µm | Ra 0.4–0.8 µm | High |
| High Pressure Die Casting | Ra 0.8–3.2 µm | Ra 0.4–0.8 µm | Ra 0.4 µm or better | Highest |
Why Surface Finish Matters
Functional Reasons
- Sealing surfaces: Better finish = better seal (lower leakage rate)
- Bearing surfaces: Too rough = excessive wear; too smooth = oil wipe-off (optimal: Ra 0.8–1.6 µm)
- Corrosion resistance: Rough surfaces trap moisture and corrode faster; smoother surfaces resist corrosion better
- Fatigue strength: Very rough surfaces create stress concentrations that reduce fatigue life
- Coating adhesion: Powder coating, painting, plating all adhere better to smoother surfaces
- Fluid flow: Friction losses in pipes and channels are affected by surface roughness
Appearance Reasons
- Consumer products: Visible surfaces need good appearance; Ra affects paint/coating quality
- Architectural: Decorative castings benefit from smooth surface
- Medical devices: Regulatory requirements for surface cleanliness and finish
Surface Finish Requirements by Application
| Application | Required Ra (As-Cast) | Notes |
|---|---|---|
| Hidden structural surfaces | Ra 12.5–25 µm acceptable | No surface finish needed; sand casting OK |
| General machined surfaces | Ra 6.3–12.5 µm as-cast | Machine after casting |
| Painted/Coated surfaces | Ra 3.2–6.3 µm as-cast | Investment casting or machined sand |
| Bearing surfaces | Ra 0.8–1.6 µm machined | Must be machined; too smooth = oil wipe-off |
| Sealing surfaces | Ra 1.6–3.2 µm machined | Machine to flatness + finish spec | Ra 1.6–3.2 µm | Smooth = better corrosion resistance |
| Food/Medical surfaces | Ra 0.4–1.6 µm | Regulatory requirements; polished finish |
How to Specify Surface Finish on Drawings
Method 1: Ra Value Only
Example: “Surface finish: Ra 3.2 µm unless otherwise noted.”
Method 2: Manufacturing Process Indicator
Indicate the manufacturing process that produces the required finish:
- “Surface to be as-cast” (accept natural casting finish)
- “Surface to be ground” (specify Ra 0.8–3.2 µm)
- “Surface to be machined” (specify Ra 1.6–6.3 µm depending on operation)
- “Surface to be polished” (specify Ra 0.4–0.8 µm)
Method 3: Machining Allowance + Finish Note
Specify the allowance for surfaces that will be machined:
Example: “Surface A: 2.0 mm machining allowance. Finish to Ra 1.6 µm after machining.”
Improving Surface Finish After Casting
Post-Casting Operations
| Operation | Resulting Ra | Cost | Best For |
|---|---|---|---|
| Shot blasting/peening | Ra 6.3–12.5 µm (removes soft sand) | Low | Clean up sand casting, stress relief |
| Grinding | Ra 0.8–3.2 µm | Moderate | Bearing seats, flat surfaces |
| CNC machining | Ra 0.8–3.2 µm | Moderate | Precision features, bores, faces |
| Honing | Ra 0.2–0.8 µm | High | Cylinder bores, precision bores |
| Polishing | Ra 0.1–0.4 µm | High | Medical, food, decorative |
| Electropolishing | Ra 0.1–0.4 µm | Very high | Medical, food, corrosion resistance |
The Cost of Over-Specifying Surface Finish
Every improvement in surface finish costs money:
- Switching process: Sand → Investment casting: +30–60% tooling, +20–40% per-part
- Adding machining: As-cast Ra 12.5 → Machined Ra 1.6: adds $0.50–$5.00/part
- Adding polishing: Machined Ra 1.6 → Polished Ra 0.4: adds $2–$20/part
- Adding electropolishing: For medical/food: adds $5–$50/part
Buyer action: Specify surface finish only where it matters functionally. Over-specifying surface finish wastes money.
When to Specify Better Surface Finish
- Where function requires it (bearing, sealing, fatigue, corrosion)
- Where appearance matters (consumer products, architectural)
- Where coating/painting requires it (adhesion)
- Where regulatory requirements apply (medical, food)
When to Accept As-Cast Surface
- Hidden structural surfaces
- Where the surface will be machined anyway
- Where the part will be painted or coated thickly
- Where cost reduction is a priority and function permits
Measuring Surface Finish
How is surface finish verified?
- Surface roughness meter: Portable instrument that measures Ra directly; used for production inspection
- Surface comparator: Set of reference surfaces for visual comparison; quick but subjective
- Profilometer: Laboratory instrument for detailed surface profile measurement
Buyer tip: Specify the measurement method and sample frequency on your drawing or quality plan. “Ra measured per ASME B46.1, sample frequency 1 per lot.”
Before finalizing the sourcing decision, many OEM buyers also compare Casting Tolerances, Casting Defects Prevention, Machining Allowance, and Quality Assurance 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.
Buyer Checklist
- Have you specified Ra value on the drawing for each surface that needs a specific finish?
- Have you distinguished between surfaces that need post-casting machining and those that can be as-cast?
- Is the specified surface finish justified by function, or is it over-specified?
- Have you specified the measurement method and sampling plan for surface finish?
- Are sealing and bearing surfaces clearly identified with both finish and flatness/geometry tolerances?
- Does the supplier have the capability to measure and certify surface finish?
FAQ
Can sand casting produce good surface finish?
Standard sand casting produces Ra 12.5–25 µm, which is rough. For better surface finish, use investment casting (Ra 3.2–6.3 µm) or specify machining after sand casting.
What is the difference between Ra and Rz?
Ra is the average roughness over the measured length. Rz is the average of the five highest peaks plus five lowest valleys. Ra is the standard for most engineering applications; Rz is sometimes used for very rough surfaces. Always specify Ra for precision work.
Is smoother always better for fatigue strength?
Not necessarily. Extremely smooth surfaces (Ra < 0.2 µm) can have lower fatigue strength due to micro-crack propagation. For fatigue-critical applications, an Ra of 0.8–1.6 µm is often optimal.