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How to Solve Various Investment Casting Tolerances

Compared with other casting processesprecision investment casting is a very precise method for producing near net shape casting products. This ability stems from the repeatability of the process without the need to add draft angles to the casting surface. As with all manufacturing processes, there are some differences between parts that need to be accommodated through dimensional tolerances.

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

Linear Tolerances

Linear tolerances usually apply 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 different. 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 accurate tolerance, CT5-CT6. The following is a list of casting tolerance standards:

Geometric Tolerance

Geometric tolerances are usually applied to the following characteristics of investment casting: flatness, straightness, parallelism, roundness, hole tolerance, curved holes, inner radius and rounded corners, etc.

Hole Tolerance

The hole tolerance is the necessary roundness of the cast hole. The metal surrounding the hole must be symmetrical to provide proper fit and/or clearance. This is very important for castings that are assembled together (for example, axles) to ensure that they are tightly assembled without vibration. Longer holes will have more internal concavity, making the diameter of the internal hole larger than the diameter of the opening. We need to pay more attention to achieve the specified hole tolerance.

Hole tolerances will facilitate internal shrinkage, and for diameters up to 1/2 inch, the hole gap can be within +/- .003 inches to provide precise shape and fit.

Parallelism

When you see a blueprint that includes parallel pins supported at only one end, it means that it is particularly difficult to complete accurate investment casting. Once you have parallel requirements, our CFS foundry engineers will discuss this issue with you before production.

Due to the volume shrinkage of the support structure, it is difficult to maintain the parallelism of these forks. The yoke hole may also deform significantly when it reaches ambient temperature. Careful design adjustments to volume shrinkage are essential to obtain accurate parts. These workpieces benefit from a wider range of tools and finishing.

Flatness

The flatness tolerance is the total deviation accepted from the plane. The surface of investment casting parts must be located between the flattest plane and the plane that will affect the maximum allowable deviation.

The volumetric shrinkage experienced by wax and metal during cooling will always determine the flatness of precision investment castings. This shrinkage is usually in the center of mass and is called a “disk”. This dish can be controlled by specialized technology, but it will always exist to a certain extent. Since the configuration and alloy of investment casting are different, general flatness tolerances cannot be quoted.

Roundness

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

We measure roundness by the difference between the minimum radius and the maximum radius of a circular investment lost wax casting part. Our inspection process will strictly control the roundness of parts and select parts that exceed the given tolerances.

Concentricity

Check the concentricity through two cylindrical surfaces centered on a common point or axis. The degree of eccentricity is determined by the size difference of the position of one center relative to the other center. For every 1/2 inch of wall thickness, the center of the hole to the center of the part outside diameter will be within 0.003 inches.

Straightness

People are always confused about the difference between straightness and flatness because it looks so close. When it comes to the straightness tolerance, the maximum allowable deviation diameter of the part from the perfect straight line will determine the straightness.

For certain types of precision investment castings, straightness can be a practical issue. Because some are thinner, shorter parts are easy to bend. According to our casting experience, we may check whether the design part will bend, but it is difficult to tell the degree of bending before production. As a rough guide, constant cross-sections tend to experience axial bending of approximately 0.024 inches per foot.

Curved holes, internal radius and rounded corners

Prefabricated ceramic cores or soluble wax will produce curved pores. Taking into account the curvature, the normal hole tolerance will be doubled, and the tolerances of all dimensions will be multiplied by 2. We cannot determine the size of these holes, so Ace Machinery provide a diameter tolerance of +/- .005. “As for the internal radius and rounded corners, they should be as wide as possible when designing because it is difficult to control and check.

For situations that require more stringent feature tolerances than industry standards in the design, be sure to contact the foundry to ensure that the required functions can be manufactured. Even if the foundry’s feedback is negative, or it may be found that additional tools are needed, it is always better to design a manufacturable design than to deal with the costs and consequences of an unwanted design.

ACE BEST is an ISO 9001 certified manufacturer for OEM casting and forging service, including China CNC machiningcustom die castingchina investment castingforging services, etc. Contact us for more custom casting services!

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If you would like to have a question then get in touch today, we are ready to assist 24/7.
We welcome your cooperation and we will develop with you.

 Factory-direct Connectivity Solutions.
 High quality, Competitive Price.
 Custom-tailored solutions.
 Problem-solving approach.