Anodizing Effects on Threads

My company is having an issue with stripping anodized parts and then re-anodizing them. After the strip and re-anodize process the pitch diameter of the parts are acceptable. The thread diameter is acceptable. When the parts are measured using a Go and No-Go gage, at the stripped parts level the parts pass both the Go and No-Go gages. The problem is after the re-anodizing is completed the parts pass the Go gage but fail the No-Go gage. With the pitch diameter and thread diameters within specification at each step we cannot determine why the parts fail the No-Go gage. Has anyone seen this problem in the past or have an answer.

I am positive there is no Malware with the pdf file.

We understand the anodizing process just not the fact that the parts dimensionally are acceptable but the No-Go gage fails on after the re-anodizing process. The parts pass the No-Go after they are stripped of the old anodizing.

Thank you,

thats going to happen. Every time you anodize, the first step is a stripping process (the caustic bath(not the acid bath in the next step)) to open up the pores of the aluminium, this removes a slight amount . when you strip the anodizing, you are using the same base chemical, and removing more material using the caustic bath. When you re-anodize the part for the second time, thats a total of 3 times you remove the surface using a caustic base chemical. the time that the part spends in the stripping bath on the removal step is probably the cause, but all 3 steps are removing material. you can observe the effect by just leaving a part in the caustic soda and watching it disappear. (suggest reading material the chemical reaction of a caustic/alkaline base on aluminium, such as lye(toilet declogger crystals/ZEP)
I understand they are passing after the stripping, but that is probably leaving them very minimal depending on how long they spend in the stripping tank. I will suggest you do a few trials to see how long the part needs to be in the stripping bath, and how long it takes to ruin the threads, smaller threads will obviously be ruined faster.
If the stripping and re anodizing is being done as a rework effort, I suggest masking off the threads for the stripping process/re anodizing process. If the threads are the critical part and the ano is being removed for assuming cosmetic reasons, masking off the threads during the stripping/reano process is probably the best course of action.
if it is a 2 stage/multi colour anodizing, perhaps mask off the threads for the first anodizing/stripping process and only remove the masking for the third process.

I do not work in the anodizing industry. these are speculations based on what I know about anodizing and the results I have observed based on re anodizing parts with critical dimensions and observing the effects on said critical dimensions after multiple re-anodizing . check with your manager/supervisor before doing anything and wear safety glasses and proper ppe when working with chemicals.

edit, am i reading this pdf(hopefully virus free) correctly? those numbers are imperial? or just really tiny metric?? because that is a lot of material removal, looks like a whole 1mm after the process is complete. Im having a hard time understanding the shart, but if you are seeing .020" after the first stripping and .020'' after the second, thats really a lot. is the stripping process static, or is the base being circulated/agitated in some way?