Tolerances depend on the geometry of each part, but the following tolerances are suggested as a guide to designers.
Lengths including hole diameters, held to:
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Flatness
Generally held to ±0.13mm (0.005") per 25 mm (1.00") square. Total Indicator Reading (T.I.R.) will, however, depend on configuration.
Straightness
Generally held to ±0.1 mm (0.004") max per linear 25 mm (1.00"). Total Indicator Reading (T.I.R.) will, however, depend on configuration.
Squareness and Angularity ±0.5 degree
Note: Angular tolerances are affected by irregular geometries.
When tolerancing, use of a central feature as a datum may allow detail to be cast that would otherwise carry too large a tolerance. Where a mechanical interface feature is required on a surface, it helps to use the centre of the feature as a datum.
Improved tolerances can generally be achieved by machining. This however introduces additional cost and lead time. The more machining operations that can be avoided, especially milling operations, the more cost effective the MM casting process becomes.
Drawings should indicate the essential tolerances and should relax those which are unimportant.
