TOLERANCES
Investment castings are designed to minimize the cost of producing close tolerance parts. Maintaining close dimensional tolerance in an investment casting is affected by many factors. Most of these factors can be controlled by the foundry, although minor “lot to lot” variations will occur. The tolerance bands provided for investment castings are determined by these uncontrollable factors.
With critical design review and minor expansion of tolerances, we can eliminate machining of your investment castings. If closer than cast tolerances are necessary, the machining required on an investment casting will still be substantially less than on conventional cast or fabricated pieces.
Factors affecting close dimensional tolerances in an investment casting include the following:
Variables
Casting size and shape determines the tolerance required to allow for process variables. These allowances will vary by foundry, depending on foundry experience or process control capabilities.
Many process factors affect investment casting tolerances, including:
| wax temperature | firing temperature |
| type of wax | pouring temperature |
| pressure of wax injection | rate of cool |
| die temperature | design configuration |
| mold or shell backup sand composition | number of coats applied |
Standard linear tolerances
As a general rule…normal linear tolerances on an investment casting can be +.010″ for up to 1″, and +.005″ for each additional inch thereafter.
Normal tolerances can be expected for production repeatability of all casting dimensions.