Surface Texture of Investment Castings

The Investment Casting Institute’s “The Investment Casting Handbook” states that a 125 RMS surface texture is typical (for steel castings) and that no other casting process produces a finer surface finish than does investment casting.  Although “The Investment Casting Handbook” says Typical, a 125 RMS is commonly cited by the industry and a 125 RMS Max has become a common callout on many investment casting drawings.  As the surface texture provided by O’Fallon Casting is generally much better than a 125 RMS Max requirement it is not considered to be an issue at Contract Review.  However, every few years a profilometer inspection of an as-cast surface texture will find and reject some area of a part for being in excess of a 125 RMS Max drawing requirement.

Because of these occasional incidents I argue against placing a surface texture requirement on a casting drawing unless the texture is important to the function of the part.  My thoughts are echoed in ANSI/ASME B46.1 Appendix B on the Control & Production of Surface Texture.  Appendix B1 states that:  “Surface characteristics should not be controlled on a drawing…” as “Unnecessary restrictions may increase production costs…”.  Since the surface texture of a casting is a function of a given foundry process, the options available to improve the native surface texture will entail some nature of secondary operation that will add expense to the cost of the product. 

ANSI/ASME B46.1 Appendix B reinforces this notion by further stating that “Surface texture is the result of the processing method…”  In effect a Sand Casting will have a sand cast surface texture while a Die Casting will have a die cast surface texture and an Investment Casting an investment casting surface texture.
An accompanying Figure (B1) in ANSI/ASME B46.1 shows the normal ranges of surface texture by processing method which for Investment Castings is 60 – 200 RMS.  Designers should consider 60 – 200RMS as an appropriate range of variation when specifying a surface texture requirement and also deciding if a given surface should be machined to a finer finish or remain as-cast.

The occasional rejection of castings to a 125 RMS Max requirement is also in part explained by ANSI/ASME B46.1 where it states that “Castings are characterized by random distribution of nondirectional deviations from the nominal surface.” 
 ANSI/ASME B46.1 further states that “Surface characteristics of castings should never be considered on the same basis as machined surfaces.”  An inspection of an as-cast surface texture with a profilometer is in itself an inappropriate method for a foundry product.  The inspection of Surface texture as-cast surfaces should instead be performed by visual comparison to a standard.  GAR Electoforming (http://garelectroforming.com) manufactures a “C-9 Cast Microfinish Comparator” that can be used as a standard for visual and tactual comparison to a cast surface consistent with ASNI/ASME B46.1.  The use of this type of Comparator is highly recommended for the evaluation of an as-cast surface texture. 

Overall it is recommended that a Surface Texture should only be specified on a drawing when it is important to the function of the end product.  The surface texture of a casting will be representative of its manufacturing process and it should be inspected visually to an appropriate surface texture gage.