Aluminum Investment Castings

1. Drawing Notes.

In the drawing notes shown below, specify either note 2A or 2B.
• 1. This casting was designed for the Investment Casting Process.
• 2A. Premium quality casting material to be A356-T6. High strength aluminum alloy per MIL-A-21180 Class 11. Mechancial properties to be determined from separately cast test bars.
• 2B. Normal quality casting material to be 356-T6 per MIL-C-601.
• 3. Quality: Non-destructive testing per MIL-C-6021. Radiographic criteria to be in critical designated areas Class 3 grade C. All other areas to be per Class 4.
• 4. This casting may be impregnated per MIL-STD-276.
• 5. Welding permissible per MIL-W-8604.
• 6. Unless otherwise specified, all corner radii to be 0.03 maximum and fillet radii to be 0.06 maximum.
• 7. Unless otherwise specified tolerance on .xx to be +/-.03 and .xxx to be +/-.01.

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2. Dimensioning of Castings.

As shown in the linear tolerance table, the larger the distance the greater the tolerance range. The shortest distance to any given point of the casting feature is from the theoretical center point of the casting. Therefore it is recommended that castings be dimensioned from centerline datums as illustrated here.
• Tooling Points and Datum Planes. Tooling points are specific locations on accessible areas of the casting, which serve as contact points for inspection and subsequent machining operations. If the casting design does not have centerline features on which to locate the tooling points, it is suggested that tooling lugs or other features be added. However, it is preferred that these additions remain part of the finished casting.
• Datum Planes. Six tooling points define three datum planes. These are reference planes from which the casting features are dimensioned. These same planes are used to establish the 3 machining start datum planes.

1. Linear Tolerances:

   • Functional or critical dimensions should be controlled to within the limits specified in the table below. Increasing tolerances for less critical applications will result in an appreciable cost savings.

   Tolerances are plus or minus.

   Inches		Milimeters
   Dimensions	Tolerance	Dimensions	Tolerance
   Up to .250	.005	Up to 6	.12
   .250 to .500	.006	6 to 12	.15
   .500 to .750	.007	12 to 19	.18
   .750 to 1.250	.008	19 to 31	.20
   1.250 to 1.750	.010	31 to 44	.25
   1.750 to 2.500	.012	44 to 62	.30
   2.500 to 3.500	.015	62 to 87	.35
   3.500 to 5.000	.018	87 to 125	.45
   5.000 to 7.000	.020	125 to 175	.50
   7.000 to 10.000	.024	175 to 250	.60
   Max. Var.	.030	Max. Var.	.75

   • Slots, grooves, holes, printed wiring board guides, etc. can be controlled to within the limits specified in the table below.

   Tolerances are plus or minus.

   Inches		Milimeters
   Dimensions	Tolerance	Dimensions	Tolerance
   Up to .250	.003	Up to 6	.075
   .250 to .500	.004	6 to 12	.10
   .500 to 1.000	.005	12 to 25	.125

   • Wall Thickness Tolerances.
     • An exception to the suggested linear tolerances are wall thickness tolerances. A minimum of +/-.010" (.25 mm) should be specified. Walls with larger areas i.e. 10" x 15" (250 mm x 375 mm) should carry a minimum tolerance of +/-.015 (.50 mm).
     • Maintaining wall thickness as shown below will yield the most economical castings. The use of .04 walls should be avoided. Consult your casting vendor if you need .04 or less.

     Tolerances are plus or minus.

     Inches
     Size of Casting	Wall Thickness
     1.00 to 10.00	.06 +/-.02
     11.00 to 20.00	.09 +/-.02
     21.00 to 25.00	.12 +/-.12

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2. Geometrical Tolerances:

   The geometrical tolerances (feature form variations) must be added to the linear variations.
   • Flatness and Straightness. After the solution treatment (T4), castings are mechanically straightened. Depending on alloy and casting configuration the recommended tolerance ranges from .002" to .005 (.05 to .12 mm) total indicator reading.
   • Parallelism and Perpendicularity. These tolerances vary, depending mainly on the configuration and alloy of the casting. This variation is corrected by straightening to a recommended range of .003" to .005 (.08 to .12 mm) per linear inch (25 mm).

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3. Other Tolerances:

   • Angles. Generally acceptable angular tolerance is +/-1/2 degree.
   • Draft. In general, draft allowance for electronic enclosures can be disregarded.
   • Radii.
   • On small radii a minimum of +/-.015 (.4 mm) is recommended. The linear tolerance table applies on larger dimensions.
   • As a general rule, a 0.03 maximum corner and a 0.06 maximum fillet are recommended.
   • Roundness. Out of round tolerance is approximately 50% of the linear variation.
   • Surface Finish. NAS 823 visual comparison. Normal-125. Premium-63.

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4. Size Limitations. Parts can generally be cast in sizes up to 15"x20"x45" (375mm x 1125mm). Consult your Foundry for their size limitations.

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