Linear Tolerance

The dimensional tolerance achievable on investment casting are dependent on the following factors:

• Casting Material
• Casting dimention and shape

Investment Casting Tolerances means the dimension range that can be meet by investment casting. It can be classified 2 parts: Linear Tolerances and Geometric Tolerances

At Ronit Technomake we express Linear tolerance normally as ±.005″ per linear inch, of length (± .127 mm per 25 mm). It applied to the following features of investment castings: Length, Concentricity, Fillet radii, Holes, Flatness, Straightness, Corner Radii, and Curved Holes. And the linear tolerances of each casting process are not the same. Below is a list of tolerance standard for casting:

Geometric tolerance is normally applied to the following features of investment casting: Flatness, Straightness, Parallelism, Roundness, Hole Tolerance, Curved Holes, Internal Radii and Fillets, and so on.

Flatness tolerance is the total deviation accepted from a flat plane. The investment cast part’s surface must lie between the flattest plane and the plane which will affect the maximum deviation permitted. The amount of volumetric shrinkage that the wax and metal undergo during cooling will always determine the degree of flatness of investment castings. This shrinkage is usually in the centre of the mass and is referred to as a “dish”. This dish can be controlled by specialized techniques, but will always be present to some extent. Below are the general flatness tolerances.

People are always confused about the differences between straightness and flatness, as it seems so close. When it refers to the tolerance of straightness, the maximum tolerable diameter of the part’s divergence from a perfectly straight line will determine the straightness. Straightness may be a real problem for some types of investment castings. As it is easy to bend for some relatively thin, short parts. According to our casting experience, we may check if a designed part may bend, but it is difficult to say the bending extent before production. As a rough guide, constant sections tend to experience an axial bow of approximately 0.024 inches per foot.

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

We measure the roundness by the difference between the minimum and maximum radii of a circular part. Our inspection process will strictly control the roundness of a part and will pick out the parts that fall outside given tolerances.

Concentricity is checked by two cylindrical surfaces sharing a common point or axis as their centre. The extent of eccentricity is determined by the dimensional difference in the location of one centre with respect to the other. The centre of a bore to the outside diameter of a part will be concentric within .003″ for each 1/2″ of wall thickness.

When you see a blueprint that includes parallel prongs supported only at one end, it means a particular challenge for the completion of precise investment casting. Once there is a parallelism requirement for your part, our engineer in Ronit Technomake will discuss this issue with you prior to production.

 It is difficult to maintain the parallelism of these prongs because of the volumetric shrinkage of the supporting structure. Yoke holes can also deform substantially as they reach ambient temperature. Careful design adjustment for volumetric shrinkage is critical to achieving an accurate part. These work pieces benefit from more extensive tooling and finishing.

Hole tolerance is the necessary roundness of a cast hole. The surrounding metal of the hole must be symmetrical to provide proper fit and/or clearance. This is extremely important for cast parts which fit together, like wheel shafts, to ensure it will fit tightly without vibration. Longer holes can experience more internal concavity, such that the interior hole diameter is greater than that of the openings. We need to care more to meet specified hole tolerances.

Hole tolerance will be benefit for interior shrinkage, and for diameter up to 1/2 inch, hole clearance could be within +/- .003”, which provides an accurate shape and fit.

Preformed ceramic cores or soluble wax can create curved holes. The normal hole tolerance will be doubled to take the curvature into account and all dimensions’ tolerance will be multiplied by two. We cannot size these holes so we provide a diameter tolerance of +/- .005.” For internal radii and fillets, as it is hard to control and inspect, when designed, they should be as wide as possible.

The standard minimum wall thickness for investment casting lies between .040″ and .080″. Wall thickness as low as 0.020″ has been achieved within a small area. Overall wall thickness is dependent on the alloy used and the size of the surface area.

Tolerances may be affected by a number of factors.

Although there is a tolerance standard in investment casting industry, it may still vary from foundry to foundry, as some may be specialized in thin wall small parts, and others could be more professional in larger parts. Also, the equipment may also have a certain effect on the casting tolerances.