Tolerance & Surface Finish Guide
Specifying the right tolerances and surface finishes is critical to both component performance and cost-effectiveness. Over-specification increases cost without functional benefit. Under-specification leads to performance problems.
Understanding Surface Finish — Ra Values
- 500 Ra: Very Rough — rough sawing or torch cutting
- 250 Ra: Rough — rough turning or milling
- 125 Ra: Machined — standard turning or milling
- 63 Ra: Smooth Machined — fine turning or milling
- 32 Ra: Ground — standard surface grinding
- 16 Ra: Precision Ground — precision surface grinding
- 8 Ra: Fine Ground — fine grinding
- 4 Ra: Super Finished — super-finishing or lapping
Surface Finish for Common Valve Applications
- Raised face flanges (spiral wound gasket): 125–250 Ra — serrated/phonographic finish
- Raised face flanges (ring joint): 63–125 Ra — smooth finish
- RTJ ring grooves: 63 Ra — API 6A requirement
- Metal-to-metal seat faces: 16–32 Ra — depends on seating mechanism
- Soft-seat body bores: 32–63 Ra — for O-ring or elastomer seating
- Guide bores (stem guides): 16–32 Ra — for sliding contact
- Gate and disc faces: 16–32 Ra — metal seat contact
- Backseat surfaces: 32–63 Ra — metal-to-metal contact
Surface Grinding Tolerances
- Flatness — Standard: ±0.0005″
- Flatness — Precision: ±0.0002″
- Flatness — Super Precision: ±0.0001″
- Parallelism — Standard: ±0.0005″/in
- Parallelism — Precision: ±0.0002″/in
- Surface Finish — Standard: 16–32 Ra
- Surface Finish — Precision: 8–16 Ra
- Surface Finish — Super Precision: 4–8 Ra
Vertical Grinding Tolerances
- Diameter (ID or OD) — Standard: ±0.002″
- Diameter (ID or OD) — Precision: ±0.001″
- Cylindricity — Standard: ±0.001″
- Cylindricity — Precision: ±0.0005″
- Face Flatness — Standard: ±0.001″
- Face Flatness — Precision: ±0.0005″
ID Grinding Tolerances
- Bore Diameter — Standard: ±0.001″
- Bore Diameter — Precision: ±0.0005″
- Cylindricity — Standard: ±0.0005″
- Straightness — Standard: ±0.0002″/in
- Surface Finish — Standard: 8–16 Ra
- Surface Finish — Precision: 4–8 Ra
OD Grinding Tolerances
- Diameter — Standard: ±0.0005″
- Diameter — Precision: ±0.0002″
- Roundness — Standard: ±0.0002″
- Roundness — Precision: ±0.0001″
- Concentricity — Standard: ±0.0002″
- Surface Finish — Standard: 4–8 Ra
- Surface Finish — Precision: 2–4 Ra
Tolerance Selection Guidelines
The most important rule: specify the tolerance your application actually requires, not the tightest tolerance your grinding shop can achieve. Unnecessarily tight tolerances increase grinding time, raise cost disproportionately, and provide no functional benefit.
Cost Implications of Tolerance (approximate)
- ±0.005″: 1x baseline
- ±0.002″: 1.5x
- ±0.001″: 2x
- ±0.0005″: 3–4x
- ±0.0002″: 5–8x
- ±0.0001″: 8–15x
Common Specification Mistakes to Avoid
- Specifying surface finish without function in mind — use Ra values appropriate to the function
- Applying tight tolerances uniformly — identify critical dimensions and apply appropriate tolerances
- Ignoring material effects — some materials are difficult to grind to extremely tight tolerances
- Confusing Ra and RMS — clarify which standard applies, especially on older drawings
- Forgetting the inspection method — make sure specified tolerances are measurable with available equipment
