Tight Tolerances

Angles and Angularity
Angularity refers to the surface, axis, or center plane which is at a specified angle (other than 90º) from a datum plane or axis. The tolerances of angles are dependent upon their location in the casting. They range from ± ½º for well supported positions to ± 2º where inherent distortion could be expected. Inclusion of gussets and ribs often minimizes distortion and many sections can be mechanically straightened. Top

Concentricity
Concentricity is a condition in which two or more features (cylinders, cones, spheres, etc.) in any combination have a common axis. The measurement of the difference of one center and another center is the extent of eccentricity. Concentricity is dependent on measurements taken from the same plane and does not include out of roundness tolerance.

Concentricity of diameters in a shaft is also a function of straightness. Centers will be concentric within ±.005 in. per inch (.127 mm per 25 mm) of maximum separation. Top

Factors in Cost
The cost of any part increases in direct proportion to the precision of the part's tolerance requirements. This factor in determining a part's cost applies to castings, as well as fabricated and machined parts. Investment casting, however, is designed to minimize the cost of producing parts within a tight range of tolerence.

Maintaining a tight range of tolerence is affected by many factors. While careful machining can achieve a tighter range than available with investment casting, critically reviewing the design specifications of a part will often allow for an increase in the tolerence range so you can take advantage of the higher production yields and lower part costs of Investment Casting. If a tighter range of precision is required, the machining required on an investment casting will still be substantially less than on a conventional cast or fabricated pieces. Top

Flatness
Flatness, not to be confused with straightness, is measured as the distance between two parallel planes, within which the entire surface of the part must lie. The cooling behavior of the wax pattern and the part itself is influenced by the shape of the part as it solidifies. This tends to determine the process capability. Flatness is typically within ± .005" (.127 mm) per inch (25.4 mm). Top

Hole Tolerance
Though Standard Linear Tolerances apply, a cast hole's roundness is affected by the surrounding metal. If the mass adjacent to the hole is uneven, the hole will be pulled out of round, otherwise, holes up to ½” (1.3 cm) diameter can be held to a range of ±.005” (.013 cm).

Blind holes can be cast if the length does not exceed the diameter. In nonferrous materials, blind boles under 2” (6.5 mm) may be cast if length does not exceed twice the diameter. Top

Parallelism
The condition of a surface equidistant at all points from a plane or axis of the same length. In calculation, the control is between length and width of the elements being checked. Establishing a specific tolerance range for all combinations would be impossible. A general tolerance of .010” total indicator reading(TIR) per inch (.254 mm per 25 mm) may be used.
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Perpendicularity
Perpendicularity is when a surface, axis, or line is 90 degrees from a datum plane or datum axis. The longest plane should be used for reference and the datum plane should be established with three tooling points. The shortest plane will be perpendicular to the longest within .005” per inch (.13 cm per cm). Top

Positioning
Position tolerance is dependent upon the parent casting. Castings that are less symetrical yield greater deviation in position tolerance. More symetrical parts yield a tighter range of positioning tolerances. Generally speaking, consistent cross sections with gradual changes from heavy to light sections that leave the heavy sections open for gating yield the best casts and tightest position tolerance ranges. Top

Roundness and Out Of Roundess
Roundness tolerance, a function of normal shrinkage variations in the metal, is a tolerance zone for concentric circles within which each circular element of the surface must lie. This zone is the total indicator reading(TIR) when the part is rotated 360º. Another, quicker method is achieved by calulating half the difference between the maximum and minimum quantities of range. ±.005” per inch (.127 mm per 25 mm) is a general rule that may be applied.
Out of Roundness is the radial deviation from a true circle and a given circumfrence. Top

Standard Linear Tolerances
Generally speaking, normal linear tolerance on an investment casting is ± .005" (.127 mm) or better for each inch (25.4 mm) of length. With this, as with any rule, there can be and are exceptions. Top

Straightness
Straightness tolerance is an area within which the axis of a casting must lie. Experience has given the casting industry the knowledge of which design types will bow but it is almost impossible to predict to what extent the straightness will be compromised. A general range of acceptance for straighthness tolerance is ±.005” per inch (.127 mm per 25 mm) Top

Surface Finish
Surface finish refers to the level of flaws on the surface of the casting. These can be roughness, waviness, lay and other flaws. Lay of the surface is multi-directional except in ground areas. There are secondary operations that can be done to improve surface conditions to a level that is comparable to wrought alloys. Typical surface finish is 80-90 RMS. Top

Variables Affecting Tolerance
There are several factors that affect the tolerance of a particular part. The acceptable tolerance for any particular casting is determined by a foundries experience and specialty but generally they are determined by the size and shape of the casting. Beyond that, the following list has some of the other factors that affect casting tolerances: