Hi Nick,

It’s great to hear from someone with an interest in the intricacies of CNC manufacturing, particularly when it comes to nuanced areas like determining tolerances for surface treatments such as anodizing. With over 16 years of experience in managing the complexities of manufacturing processes, I appreciate the importance of ensuring each part meets the required specifications even after finishing processes.

Considering Anodizing in the Design Stage: It is crucial to anticipate the effect of anodizing on part dimensions during the early stages of the design process.
This means:
– Understanding the thickness typically added by your anodizing process.
– Adjusting the initial dimensions of your parts to accommodate this thickness.

Determining Tolerances:
– Pre-Process Tolerances: Some choose to specify the dimensions and tolerances of the part prior to anodizing, knowing that the process will add a predictable amount of material.
– Post-Process Tolerances: Others prefer to specify tolerances for finished, anodized parts to ensure the final product is within the desired features, regardless of the added layer.

Alternatives:
– Designing with Anodizing in Mind: Specify part dimensions and tolerances prior to anodizing but adjusted for expected film thickness. This method requires a good understanding of how much material the anodizing process will add.
– Specifying Final Dimensions: Specify dimensions and tolerances for finished, anodized parts. This approach can make the fit of the final assembly more predictable, but it requires a proper understanding and control of the anodizing process.

Conclusion:
Each method has its own advantages and challenges, and the choice often depends on the specific requirements of your project, the required precision, and the nature of the assemblies in which the parts will be used. Don’t hesitate to ask further questions or seek clarification. I’m here to assist and make the information as accessible as possible.

Best Regards,

Attila

Hello Nick and Attila
As a Production support engineer at Xometry, I could add that by default, when a drawing is submitted to us and an anodizing or other post-treatment is ordered, we consider that the tolerances are indicated AFTER treatment, as it would be difficult for us to know exactly the thickness obtained on all surfaces by our manufacturing partners and their anodizing partners. Our partners have then the responsibility to adapt the machining parameters accordingly.
Happy to help
Pierre-Yves

Hello there, so let me know if I understand correctly with this example. We consider making hard anodizing and indicate 15μ thickness on our drawing ; and determine our tolerance in accordance. Can we be sure that the manufacturer will adapt the machining parameters, to match with the anodizing thickness and after-processing tolerance ? 

Ciao Nick,
for my experience I can only tell you: it depends.
We must admit that normally the treatment must be performed on finished product after all the working and therefore on precise dimensions.
With reference to the MIL-A-8625, I can tell you that the anodic oxidation type I and type II change the dimensions of the finished product by a few microns, but the anodic oxidation type III (known as Hard Anodizing) changes the finished size of the part and even modifies the roughness worsening it.

I believe it is necessary to define tolerances before and after this anodizing treatment, even the final roughness must be defined.

Keep in touch,

Mario Coppola