Investment Casting Tolerances
Ningbo Investment Casting Co., Ltd strictly casts and produces the investment castings according to our customers’ drawings. Except the dimensions marked with tolerance, we’ll do the casting as per general cast tolerance. These tolerances including linear tolerances, flatness, straightness, parallelism, roundness, concentricity, hole tolerances, ect.
One of the many advantages of investment casting is that tight tolerances can be maintained. In fact, tolerances may be affected by a number of factors, such as pre-heated shell temperature, wax or plastic temperature, injection pressure, mold or shell composition, firing temperature, rate of cooling, riser and gate location, position in the assembled tree, and heat treat temperature all bear directly on tolerances required in the investment casting industry. Although there is a tolerance standard in investment casting industry, different foundry has its ability and advantage, as some may be specialized in thin wall small parts, and others could be more professional in larger parts.
The linear tolerances of each casting process are not the same. For example, water glass lost wax casting can control general linear tolerances between CT7-CT8; Lost foam casting can meet CT8-CT9, and silica sol casting is the most precise, it is CT5-CT6.
The following tables contain information relating to the tolerances that we can maintain when manufacturing our precision investment castings.
|Casting Tolerances ISO 8062
(Linear dimension tolerance)
|Ningbo Investment Casting Co., Ltd|
|Raw Casting Basic Dimension
|Total Casting Tolerance1)
|Over||Up To and Including||Casting Tolerance Grade CT2)3)|
|The dimension tolerances for lost wax investment casting are according Casting Tolerances ISO 8062 CT4 – CT9 depand on the casting size of the parts.|
Flatness tolerance is the total deviation permitted from a plane. The investment cast part’s surface must lie between the most flat plane and the plane which will affect the maximum deviation permitted.
Degree of flatness in an investment casting is almost always determined by the amount of volumetric shrinkage that the wax and metal undergo during cooling. This problem is typical of the casting process, and can be controlled but not avoided.
General flatness tolerances cannot be quoted because they vary with configuration and alloy used. Wall thickness of the plane is, for instance, a significant parameter: with the same surface extension, the higher the wall thickness the larger the flatness error.
Usually, most of new workers are confused at the differences between flatness and straightness.
The maximum tolerable diameter of the part’s divergence from a perfectly straight line will determine the measuring of straightness.
Some castings are easy to bend during the cooling process for some relatively thin and long parts. According to our investment casting experience and professional knowledge, our professionals will check when a designed part may bend, but the particular angle cannot be accurately predicted, what’s more, it’s difficult to confirm the bending extent before sampling. For reference, constant sections tend to experience an axial bow of approximately 0.024 inches per foot.
Parallelism is the condition of a surface equidistant at all points from a datum plane or an axis equidistant along its length to a datum axis. It is difficult to maintain parallelism of these prongs because of volumetric shrinkage of the supporting structure. We may require a straightening operation, and sometime, we’ll do machining process if the drawing required.
When specifying perpendicularity, use the longest plane for reference: the allowed deviation to be specified will be the deviation of the shortest plane against the longest one.
Perpendicularity tolerance of a plane is generally 0.8% the plane length. Basically, when A is the dimension of the shortest plane and B the dimension of the longest one, plane A will be perpendicular to surface B with a tolerance of 0,008 x A.
Roundness tolerance specifies a tolerance zone bounded by two concentric circles within which each circular element of the surface must lie. The roundness can be tested by reading the total indicator after rotating the part 360°, or calculated by taking half the difference between the maximum and minimum condition. The latter method is usually preferred as it takes less time to determine and is more efficient.
Our QA person will strictly control the roundness to ensure the parts are qualified, and we will reject the parts which are out of given tolerances.
Concentricity of cylindrical surfaces is measured by the distance between their axes, sharing a common point or axis as their center. Precise concentricity is essential in a wide variety of machines, often to reduce vibrations caused by rotating components. Shafts and tubes must not only be concentric, they must be extremely straight. Ningbo Investment Casting foundry ensures each part is within the bounds of acceptable concentricity as per drawing requirement.
Hole tolerance refers to the necessary roundness of a cast hole. The surrounding metal 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.
Hole tolerance ensures interior shrinkage, and hole clearance could be within +/- .003” for up to ½ inch diameter holes, to provide an accurate shape and fit.
Curved Holes, Internal Radii and Fillets
Preformed ceramic cores or a 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.
Ningbo Investment Casting Co., Ltd always do the best to provide products with superior tolerances, and confirm each provided cast components within required tolerance ranges. We are always follow strict tolerance standards so that all investment castings from m our foundry are accurate and try our best to avoid or reduce further machining, thus to save costs for our customers. Our business core is to deliver high quality metal castings with most accurate dimensions in the best cost.
Our machining engineers will review and study the drawings if the given tolerances are not available for casting finish, then we will offer machining cost during quoting, and our machining capability will help to achieve the required tolerance.
Please contact Ningbo Investment Casting foundry, we will study and discuss in detail about the tolerance issue for your casting parts to offer our best cost, deliver high quality steel castings with most accurate dimensions.