Investment casting tolerances
Fly foundry mainly custom investment castings according to customer's drawings.In the drawings,except main sizes,investment casting tolerances are also marked,these tolerances includes linear tolerances,flatness,straightness,parallelism,roundness,concentricity,hole tolerances,ect.From these tolerances,investment casting manufacturer will know whether the product is easy to make and whether machining is needed.
Linear tolerances
The general rule for linear tolerancing of investment castings is usually expressed as ± 0,5 % of the nominal dimension. Tighter tolerances can be made available for a few selected design characteristics. However, one should know that tighter tolerances usually require special procedures and/or secondary operations which can increase the production cost of the investment casting.
| Dimensions (mm) | Precision Grade | ||||
| MD1 | MD2 | ||||
| From | To | Deviation | Tolerance range | Deviation | Tolerance range |
| 0 | 6 | ±0.10 | 0.2 | ±0.08 | 0.16 |
| 6 | 10 | ±0.12 | 0.24 | ±0.10 | 0.2 |
| 10 | 14 | ±0.15 | 0.3 | ±0.12 | 0.24 |
| 14 | 18 | ±0.20 | 0.4 | ±0.14 | 0.28 |
| 18 | 24 | ±0.25 | 0.5 | ±0.17 | 0.34 |
| 24 | 30 | ±0.30 | 0.6 | ±0.20 | 0.4 |
| 30 | 40 | ±0.36 | 0.72 | ±0.25 | 0.5 |
| 40 | 50 | ±0.42 | 0.84 | ±0.30 | 0.6 |
| 50 | 65 | ±0.49 | 0.98 | ±0.35 | 0.7 |
| 65 | 80 | ±0.58 | 1.16 | ±0.42 | 0.84 |
| 80 | 100 | ±0.68 | 1.36 | ±0.48 | 0.96 |
Flatness tolerance
A flatness tolerance is the total deviation permitted from a plane and consists of the distance between two parallel planes between which the entire surface so toleranced must lie.
Degree of flatness in an investment casting is almost always determined by the volumetric shrinkage of wax and metal 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.
Moreover,the flatness error of an uninterrupted plane will be higher than the flatness error of a plane with holes or other things which interrupt its extension (basically, to calculate the tolerances, an interrupted plane must be considered not only one plane, but as many planes as those definited by the interruptions). The following tab reports a general indication:
| Areas under 2500 mm2 | Areas over 2500 mm2 | ||||
| Section thickness | Allowed dish | Allowed dish for every 3800 mm2 over 2500 mm2 | |||
| up to 6mm | No significant | Not significant | |||
| from 6 to 13mm | 0.25mm | 0.10mm | |||
| from 13 to 25mm | 0.25mm | 0.20mm | |||
| over 25mm | 0.25mm | 0.40mm | |||
Straightness
Measuring straightness is determining the maximum tolerable diameter of the part’s divergence from a perfectly straight line.Axial straightness of investment cast shafts or plates is determined by our rigorous quality verification process, weeding out propellers and other machine parts before any product is shipped to you.
Some castings are more prone to bending during the cooling process than others. Our investment casting professionals know when a design will bend,but the particular angle cannot be accurately predicted. Constant sections tend to experience an axial bow of approximately 0.024 inches per foot.
Parallelism
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.Parallelism is difficult to control in the as cast condition and may require a straightening operation. Parallelism is a feature that is a function of the complexity of the part. As the part is being designed,consult our investment casting foundry in Ningbo for tolerances.
Roundness
Roundness tolerance specifies a tolerance zone bounded by two concentric circles within which each circular element of the surface must lie. It is
the total indicator reading (TIR) when the part is rotated 360º, or calculated by taking half the difference between the maximum and minimum condition.The latter is usually preferred because it is quicker to determine.The actual inspection method,however,should be specified by the investment casting purchaser.
Concentricity
Concentricity of cylindrical surfaces is measured by the distance between their axes.Precise concentricity is essential in a wide variety of machines,often to reduce vibrations caused by rotating components. In these cases, above-tolerance eccentricity can be the root cause of gradual vibrational wear and tear to surrounding machine parts. R2 Quality Castings’ thorough quality verification process prevents defective parts from shipping with your order.
Shafts and tubes must not only be concentric, they must be extremely straight. Straightness correlates with concentricity; the ends of each piece may be true circles,however a lack of straightness will directly affect the concentricity. Our investment casting foundry ensures each part is within the bounds of acceptable concentricity, providing you with the best quality casting components.
Hole tolerances
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. Care is required to meet specified tolerances.
Hole tolerance ensures interior shrinkage, and clearance is +/- .003” for up to ½ inch diameter holes, to provide an accurate shape and fit.