O'Fallon Casting supports and encourages the use of Tooling Points and geometric tolerancing when defining investment casting configurations. Please refer to the American National Standard ANSI Y14.5M on Dimensioning and Tolerancing.
Three tooling points should be used to establish datum plane 'A', two for datum plane 'B' and one for datum plane 'C'. In all cases, a maximum of six tooling points, located as far apart as possible, are sufficient to locate a casting and establish the three datum planes.
These tooling points should be utilized when checking castings dimensionally and these same points should also be used to hold the casting during any secondary machining operations. If at all possible, tooling points should be located on surfaces which are not subsequently machined.
Cast surfaces, regardless of the process used, will have some irregularies. These can be caused by sink, shrinkage during solidification, mold lift or buckle, deflection, distortion during the cooling of the pattern and/or the metal and during heat treatment. The use of tooling points help minimize the affects of any casting distortion. Gating should avoid tool point areas whenever possible.
Tolerances applied can have a direct bearing on both the tooling and unit costs.
"Normal" tolerances should be applied wherever possible and "premium" tolerances should be applied only when necessary. Tolerances should be increased to as much as the casting function will permit.
Dimensional "Repeatability" of castings produced, from lot to lot, will normally be within 25% of the normal tolerances shown in the following chart. This can vary depending on the actual casting configuration.
The following tolerance guidelines are consistent with those published by the Investment Casting Institute.
| US (inches) | Tolerances + or - | Metric (mm) | Tolerance + or - | ||||
| Dimension | Normal | Premium | Dimension | Normal | Premium | ||
| Up to | .250 | .005 | .003 | Up to | 6.35 | .127 | .076 |
| Up to | .500 | .005 | .004 | Up to | 12.70 | .127 | .102 |
| Up to | 1.000 | .005 | .005 | Up to | 25.40 | .127 | .127 |
| Up to | 2.000 | .010 | .010 | Up to | 50.80 | .254 | .254 |
| Up to | 3.000 | .015 | .013 | Up to | 76.20 | .381 | .330 |
| Up to | 4.000 | .019 | .015 | Up to | 101.60 | .483 | .381 |
| Up to | 5.000 | .022 | .017 | Up to | 127.00 | .559 | .432 |
| Up to | 6.000 | .025 | .020 | Up to | 152.40 | .635 | .508 |
| Up to | 7.000 | .028 | .022 | Up to | 177.80 | .711 | .559 |
| Up to | 8.000 | .031 | .024 | Up to | 203.20 | .787 | .610 |
| Up to | 9.000 | .034 | .026 | Up to | 228.60 | .864 | .660 |
| Up to | 10.000 | .037 | .028 | Up to | 254.00 | .940 | .711 |
| Maximum variation ± .040 (1.016mm) | |||||||
| Angular Tolerance will be ± 0 degrees 30 minutes (use ± 2 degrees for chamfers and non-critical angles) | |||||||
| Conversion Facters: | ||
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| Inches to Millimeters | (inches x .03937 = millimeters) | |
| Ounces to Grams | (ounces x 28.35 = grams) | |
| 1 Cubic Inch | = | 1.57 oz Aluminum |
| 5.32 oz Be Copper | ||
| 5.17 oz Other Copper-base | ||
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