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STEEL

CASTINGS

HANDBOOK

SUPPLEMENT 3

2003

DIMENSIONAL CAPABILITIES

OF STEEL CASTINGS

DIMENSIONAL CAPABILITIES OF STEEL CASTINGS

1. Introduction

Dimensional tolerances are selected by the designer or purchaser to make sure that the part can

perform its function reliably and fit into its designed location. Assigning dimensions to a part

requires identifying the desired feature size. Tolerances communicate how much variation from

the desired size can be tolerated. Overly stringent tolerances are costly and do not add value.

They require added work to meet tolerances that may be beyond the process capability.

Inadequate tolerances are a problem because parts may be able to meet the tolerance but fail to

either fit or function in accordance with the design.

To assign dimensions and tolerances to a part that is produced as a casting involves

consideration of function and fit of the finished part, allowances for machining operations involved

in producing the finished part, and production requirements such as draft and taper. Allowances

for castings and the major tolerance considerations in the production of parts as steel castings

are presented below. Along with this information a set of tolerance grades is introduced to

facilitate communication on tolerances.

2. Allowances

The shapes of cast steel components reflect not only the functional requirements of the

component, but also manufacturability requirements dictated by the casting process. Castings

shapes must incorporate the proper use of draft allowances for successful mold making and

machining allowances for surfaces requiring more precision and better surface finishes than can

be achieved in the as-cast conditions. Draft and machine finish allowance guidelines and

practices are presented to assist in the specification of draft and machining allowances for

castings.

Similarly, size or pattern allowances must be incorporated into the production of patterns and

coreboxes from which steel castings are made. These pattern allowances (sometimes call shrink

rules) must also be correctly applied to ensure that final castings can meet customer dimensional

tolerance requirements without extra pattern dimension adjustment cycles. Other castability

guidelines that influence the recommended geometry of steel castings are discussed in “Steel

Casting Design”.1

2.1 Draft (Taper) Allowances

Draft should be designated on the casting drawing in consultation with the casting producer—

typically in a drawing note. The draft angle selected should be no less than can be tolerated in

the design. Figure 2.1 illustrates the use of draft on a typical pattern and corebox.