Perpendicularity


When perpendicularity is specified, the reference plane should be the longer of the two planes, the datum plane to be established by 3 tooling points.


Longer Dim


1 A Datum Plane B


Shorter Dim


In drawing of the casting shown above, surface B will be perpendicular to surface A within .008" per l" of length of surface B.


Example: Length of B = 3" .008" x 3" = .024"


Therefore: surface B should be perpendicular to surface A within .024" TIR.


Some improvement on tolerance can be effected by straightening. Surface Texture


PER ASA B 46.1.1962 "Surface Texture Includes Roughness, Waviness, Lay and Flaws':


In general, industry uses the term RMS (Root Mean Square) and AA (Arithmetic Average) interchangeably. RMS gives about 11 % higher values than AA and the difference is not significant.


The investment casting industry will measure to either standard. In order to correspond with the published literature of ICI member companies, the values expressed in this section are shown in terms of both RMS and AA.


Both RMS and AA are measured by an electric profilometer.


Another system of measurement of surface texture which is gaining wide approval is the use of a visual comparator. This requires the use of samples of surface finish shown on a graduated scale from fine to rough surfaces. A good example is the wheelcraft comparator.


The surface texture standards for investment castings are as follows: 125 RMS Max. 115 AA Max. 60 Wheelcraft Max.


All lay is multi-directional except in ground areas. Secondary operations will improve the surface texture. Such secondary operations will give results corresponding to the equivalent wrought alloy.


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