• Machining Allowances—Addi- tional stock added to the sur- face of a casting so it can be machined is called machine- fi nish allowance or machining allowance. T e thickness of this allowance will depend on the casting’s size and type, the required surface fi nish, how it is to be machined and how accurately the casting can be made. Large and/or low volume castings generally will have larger machining allowances than smaller and/ or high volume castings. With newly designed cast parts, a full machining allowance is usually added.
• Distortion Allowance—Distor- tion may occur when one area
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the part due to a more expen- sive pattern and higher labor costs in molding and core- making.
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Tolerances that are more rigid than necessary increase the costs and lengthen delivery schedules. Tolerances that are too loose lead to more extensive machining and usually have a heavier section size than required.”
• Tolerances—Casting re- quirements such as surface smoothness, dimensional tolerances or machining al- lowances cannot be stated in a general specification for all
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of a casting cools more rapidly than another. Designers should avoid sharp changes in adjacent sections. When this is not pos- sible, distortion often can be minimized by managing heat extraction or heat treatment.
castings. These requirements usually are established through mutual agreement between the customer and the metalcaster. The pattern equipment used directly affects dimensional tolerances, particularly when machine molding or coremak- ing methods are employed. Metal patterns and coreboxes that are machined to size often provide closer tolerances than cast-to-size metal or wooden patterns.
T e factors that most aff ect di-
mensional tolerances are: • T e shrink factor in pattern design, especially with larger castings.
• Variability in the pattern equip- ment and coreboxes.
• Variability in cleaning and fi nishing operations.
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• Mold and core variability due to inconsistencies among employ- ees, equipment and materials.
• Distortions due to the heat treatment.
Cast Surface Finish Cast components do not
have standard surface fi nish
specifi cations. Although root mean square values for cast surfaces are sometimes given on drawings, they are of questionable value because there is no dependable method of quantitative surface roughness mea- surement as-cast surfaces. However, sample castings of the type required provide a satisfactory way of des- ignating desired surface fi nish and observing what can be produced. In some casting applications,
the roughness of the cast surface is critical to successful use. Dimen-
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