Investment casting tolerances are the range of dimensions that can be met by investment casting. There are many investment casting processes, such as water glass lost wax casting, lost foam casting, sol casting and so on. . Each casting process has its own casting molding method, which is suitable for different types of products. Tolerances can be affected by a variety of factors. Things like mold temperature, wax or plastic temperature, injection pressure, mold or shell composition, support sand, firing temperature, cooling rate, position of the part on the "tree," and heat treatment temperature all directly affect the tolerances required in the investment casting industry. While the investment casting industry has a tolerance standard, it can vary from foundry to foundry, as some may specialize in thin-walled small parts, while others may be more specialized in producing large parts. Equipment may also have some effect on casting tolerances.

In general, we can divide investment casting tolerances into linear tolerances and geometric tolerances.

Linear Tolerance
Linear tolerances are typically applied to the following characteristics of investment castings: length, concentricity, fillet radius, hole, flatness, straightness, corner radius, and curved hole. And the linear tolerance of each casting process is not the same. For example, water glass lost wax casting can control the general linear tolerance between CT7-CT8, lost foam casting can meet CT8-CT9, and silica sol casting has the most precise tolerance, which is CT5-CT6.

geometric tolerance
Geometric tolerancing is commonly applied to the following characteristics of investment casting: flatness, straightness, parallelism, roundness, hole tolerance, curved holes, inside radii and fillets, etc.

Flatness

The flatness tolerance is the total deviation from flatness that is acceptable. The surface of an investment casting must lie between the flattest plane and the plane that affects the largest allowable deviation.

The amount of volumetric contraction experienced by the wax and metal during cooling will always determine the flatness of the investment casting. This shrinkage is usually at the center of mass and is called "digging". The dish can be controlled with specialized technique, but there will always be some way. Due to the different configurations and alloys of investment castings, general flatness tolerances cannot be given.

Straightness

People are always confused about the difference between a straight line and a plane because they seem close. When referring to straightness tolerances, the maximum allowable diameter of a part that deviates from a perfectly straight line will determine straightness.

For certain types of investment castings, straightness can be a real problem. Because it is easy to bend for some relatively thin and short parts. According to our casting experience, we may check whether the designed parts will be bent, but it is difficult to say the degree of bending before production. As a rough guide, constant section tends to experience axial bending of about 0.024 in/ft.

Parallelism

When you see blueprints that include parallel jaws supported at only one end, it means that precision investment casting is a unique challenge to accomplish. Once your part has a parallelism requirement, our engineers at CFS Foundry will discuss this with you prior to production.

It is difficult to maintain the parallelism of these prongs due to the volumetric contraction of the support structure. The yoke bore also deforms significantly when it reaches ambient temperature. Careful design adjustments for volumetric shrinkage are critical to obtaining accurate parts. These workpieces benefit from more extensive machining and finishing.

Roundness

The radial difference between a true circle and a given circumference will define roundness. We can test for roundness by reading the total indicator after rotating the part 360°. Another way to check for roundness is to take half the difference between the maximum and minimum values. The latter method is recommended as it takes less time and is more efficient.

We measure roundness by the difference between the smallest and largest radii of a circular part. Our inspection process strictly controls part roundness and singles out parts that fall outside a given tolerance.

Concentricity

Concentricity is checked by two cylindrical faces that share a common point or axis as their center. The degree of eccentricity is determined by the difference in size of one center relative to the other. The center of the hole is concentric to within 0.003 inch of the outside diameter of the part for every 1/2 inch of wall thickness.

Hole Tolerance

Hole tolerance is the necessary roundness of the cast hole. The metal around the hole must be symmetrical to provide proper fit and/or clearance. This is important for castings that fit together, such as axles, to ensure that they fit tightly without vibrating. Longer holes may experience more internal depression such that the internal hole diameter is larger than the diameter of the opening. We need to pay more attention to meeting the specified hole tolerances.

Hole tolerances will favor internal constriction, the hole can be within +/- .003” for diameters up to 1/2”, providing precise form and fit.

Curved Holes, Fillets, and Fillets

Prefabricated ceramic cores or soluble waxes can create curved pores. Normal hole tolerances will be doubled to account for curvature, and tolerances for all dimensions will be multiplied by two. We cannot determine the size of these holes so we offer a +/- .005 diameter tolerance. ” For fillets and rounded corners, design should be as wide as possible because it is difficult to control and check.