Casting Tolerance Part 1: Linear Tolerances
We have found that many of our customers were initially hesitant to switch from aluminum billets to castings because they perceived the tolerances of sand castings were not tight enough. We’re here to say they absolutely are!
The biggest difference between casting tolerances, and billet tolerances is that casting tolerances are based on the size of the part, and where the feature is located. The aluminum association has published standards for sand castings. By considering these standards when designing your casting, or switching from billets or weldments, you’ll ensure success of your project. And like any other manufactured good, overly strict or unnecessary limits will drive the cost up.
Let’s start with the standard linear tolerance. A sand cast feature up to 6” long is nominally +/- 0.030”. As that feature increases in size, an additional +/-0.002” is added for each inchthat the feature is over 6”. This is represented in the figures below:
|
L (in) |
L Tolerance (in) |
|
Up to 6 |
± 0.030 |
|
Over 6 |
± 0.002 for every inch over 6 |
For Example
|
L (in) |
L Tolerance (in) |
|
6 |
+/- 0.030 |
|
L (in) |
L Tolerance (in) |
|
|
14 |
Tolerance for first 6”: |
± 0.030 |
|
Additional tolerance: |
± 0.016 |
|
|
Total tolerance: |
± 0.046 |
|
The next set of tolerances to consider is where the feature falls with respect to the parting line of the mold. Unlike the linear tolerance discussed above, tolerances across the parting line are based on the projected area of the feature that is crossing the parting line:
|
Projected Area = L x W (in2) |
H Tolerance (in) |
|
Up to 10 |
± 0.020 |
|
10 – 50 |
± 0.035 |
|
50 – 100 |
± 0.045 |
|
100 – 250 |
± 0.060 |
|
250 – 500 |
± 0.090 |
|
Over 500 |
Contact Palmer Foundry to discuss your needs! |
For Example
|
L x W (in2) |
H Tolerance (in) |
|
9 |
± 0.020 |
|
112 |
± 0.060 |
Finally, tolerances due to cores should be taken into account. Cores allow for a hole or undercut to be added to the casting that is perpendicular to the parting line. Since the cores are separate pieces of sand from the main mold, additional tolerances must be added to the part to allow for the positioning of the core, this tolerance is driven by the projected area of the core:
|
Projected Area = L x H (in2) |
W Tolerance (in) |
|
Up to 10 |
± 0.020 |
|
10 – 50 |
± 0.035 |
|
50 – 100 |
± 0.045 |
|
100 – 250 |
± 0.060 |
|
250 – 1000 |
± 0.090 |
|
Over 1000 |
Contact Palmer Foundry to discuss your needs! |
For Example
|
L x H (in2) |
H Tolerance (in) |
|
9 |
± 0.020 |
|
275 |
± 0.090 |
These are only three of the 21 engineering series standards provided by The Aluminum Association. To purchase a complete set of standards, please visit the Aluminum Association’s online bookstore.
How do Palmer Foundry’s castings compare to the AFS standards? Shown below is a process capability histogram of the thickness of a part that is over 500 in2 across the parting line. The data shows the results of over 1000 samples taken on parts produced throughout the year; and captures all natural variation in raw materials, process, procedures, and environment. The data show a process capability index (Cpk) over 2.0, and the standard deviation for this feature is +/- 0.014”!
Contact Palmer Foundry today to discuss how we can produce vacuum tight, dimensionally stable, tight tolerance castings for you.
