Ra and Rz
Surface texture is described by parameters calculated from a measured profile.
- Ra, the arithmetical mean deviation, averages the absolute height of the profile about its mean line. It is robust and widely used but insensitive to isolated peaks and pits.
- Rz, the maximum height of the profile averaged over sampling lengths, reacts to single peaks and valleys and is often more relevant for sealing and coating.
There is no fixed conversion between Ra and Rz; the ratio depends on the character of the surface. Terms, parameters and drawing indication are defined in the ISO 21920 series, which replaced ISO 4287, ISO 4288 and ISO 1302. In North America surface roughness is commonly stated as Ra in microinches; 1 µm equals 39.37 µin.
What determines the as-cast surface
The cast surface is a replica of the first shell coat, which in turn replicates the wax. Influences are the finish of the die, the wax and injection conditions, the fineness and wetting of the prime slurry, stucco penetration, reactions between metal and shell at high temperature, and the alloy itself. Heavier sections, which stay hot longer, tend to be rougher than thin walls. Subsequent blasting changes the texture: it removes scale and evens out appearance, and may raise or lower measured roughness depending on the medium.
Typical surface conditions
| Surface condition | Ra (µm) | Ra (µin) | Remark |
|---|---|---|---|
| Investment casting, as cast (steel, typical) | 1.6 – 6.3 | 63 – 250 | Depends on alloy, prime coat, wax surface and section size |
| Shot or grit blasted | 3.2 – 12.5 | 125 – 500 | Uniform matt texture; the value depends on the blasting medium |
| Ground (gate removal, fettling) | 1.6 – 6.3 | 63 – 250 | Local areas only; lay direction differs from the cast surface |
| Turned or milled | 0.8 – 3.2 | 32 – 125 | Functional, sealing and bearing surfaces |
| Fine ground or polished | 0.1 – 0.8 | 4 – 32 | Only where function or hygiene requires it |
Typical indicative ranges from general industry practice, not values taken from a standard and not guaranteed manufacturing specifications. Achievable roughness is agreed per part, ideally with reference samples.
Specifying surface requirements
- State the parameter, the limit and the area to which it applies; a single global roughness value for the entire casting is rarely necessary and may be costly.
- Distinguish functional surfaces (sealing, bearing, flow, hygiene) from general surfaces.
- Remember that stylus measurement on a cast texture depends on measuring position and direction. For as-cast surfaces, comparison with agreed reference samples or tactile comparators is common practice and often more meaningful.
- Define the surface condition for delivery: as cast, blasted (with which medium), pickled and passivated, polished.
VDG P 690 contains surface classes specific to investment castings, which can be referenced on drawings instead of individual roughness values.
Surface discontinuities and visual acceptance
Roughness describes texture, not isolated imperfections. Pits, positive metal, small inclusions, gate and parting-line witnesses are assessed by visual examination against an acceptance standard:
- ISO 19959:2020 specifies acceptance criteria for the visual examination of steel, nickel-alloy and cobalt-alloy investment castings.
- ASTM A997 is the corresponding ASTM practice for investment castings.
- MSS SP-55 and ASTM A802 are widely used for valve, pump and general steel castings.
The acceptance level and the surfaces to which it applies should be agreed before production. Where no standard is cited, approved limit samples serve the same purpose.
Improving the surface
Where function requires a finer surface than as cast, the options are vibratory finishing, belt grinding and polishing, electropolishing of stainless steels, or machining. Each changes dimensions slightly, which must be considered in tolerancing; see finishing operations.