Typical Alloys Cast

The purpose of the investment casting is to produce a part which will be a near net shape as possible within the limitations of the process itself. These limitations can be greatly reduced when design engineers are well informed as to the design opportunities of the casting process. Although each casting application must be individually analyzed for control of dimensional tolerances, there are generally accepted standards which can facilitate the manufacturing process and reduce the cost to the customer.

• MATERIAL - Alloy specification and heat treatment.
• TOLERANCES - General and specific tolerances for linear tolerances, flatness and straightness, concentricity, angularity, parallelism and perpendicularity. (We recommend standard geometric tolerancing.)
• FILLET AND CORNER RADII- Specified as a maximum condition only, so tool makers can eliminate outside radii to facilitate tool construction and reduce cost.
• LOCATION AND AMOUNT OF MACHINE STOCK
• SURFACE FINISH
• CASTING IDENTIFICATION - DESCRIPTION AND LOCATION
  

The ultimate success of the casting design depends on the communication between the designer and foundry engineers. When the foundry is properly informed of part requirements, there is usually a compromise of specifications that will allow for maximum compliance to print specifications and finish part design.

LINEAR TOLERANCES

The following table should be used as a guideline for linear tolerancing. Normal tolerances are defined as those which can be controlled in casting. Premium tolerances are those which require secondary controls and processes to comply with print specifications.

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FLATNESS AND STRAIGHTNESS

Flatness or dish is the deviation from a given plane of reference and is measured by the distance between two parallel planes between which the entire surface must lie.

Flatness deviation is a function of shrinkage in the wax and metal in solidification. It is directly related to the thickness of the cross-section and although each casting will vary individually, a rough tolerance guide would be .003" T.I.R., (Total Indicator Reading), for each 1/4" thick section. This variation will be apparent in surfaces of 1" or more linear dimension.

Straightness is the deviation of a surface plane with respect to the centerline axis of the section. Straightness is a function of stresses developed in solidification of wax and metal. The control of tolerance usually requires mechanical means after casting. The mechanical properties of the alloy greatly affect the ability to meet print tolerances. Generally, a constant section will be axial bow of approximately .005" per linear inch under normal tolerancing and about .003" per inch under premium tolerancing.

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CONCENTRICITY AND ROUNDNESS

Out of round is the radial difference between a true circle and the cast surface diameter. The following table is a guide for out of round tolerance:

Concentricity is the relationship between cylindrical surfaces sharing a common axis. The dimensional difference in the location of one center to the other is the amount of eccentricity. Out of roundness does not affect concentricity, but straightness of the casting does. Concentricity tolerance varies according to straightness at about ±.005" per linear inch of difference between points of measure.

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ANGULARITY

Angular tolerance on investment castings should not be less than + 1/2 °

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PARALLELISM AND PERPENDICULARITY

Parallelism and perpendiculartiy are both relative planar measure with respect to a datum plane. They are affected by casting configuration and straightness. Tolerances less than within .010 T.I.R. may require secondary machining.

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