Anodize-helical Coil Inserts

[macduff]

Hey Folks,
I have a question about anodize and helical coil inserts.

I have a part that is black anodize (type II) and installing 4-40 helical coils into the part. What's your take on anodizing the finished part then installing the inserts with primer?

My aerospace background experience taught me:

Finish machine, chem film the entire part, mask the female threads (before inserts are installed), anodize, apply primer wet, and install the inserts. Sounds right to me!

My company wants:

Finish machine, anodize, apply primer wet, install inserts
or
Finish machine, mask, anodize, apply primer wet, install inserts

I do like the 2 mentioned above because....
1. anodize buildup will cause my insert to be a smaller thread and will cause the fastner to gall even more than it normally does.

2. I believe you would want to protect the hole from corriosion because the primer will not coat the entire hole.

The company wants to do this because of cost savings. Go figure....

Thanks,


Colin Fitzpatrick (aka Macduff)
Mechanical Designer
Solidworks 2008 SP 4.0
Dell 490 XP Pro SP 2
Xeon CPU 3.00 GHz 3.00 GB of RAM
nVida Quadro FX 3450 512 MB
3D Connexion-SpaceExplorer

 

[drawoh]

macduff,

On my parts, mostly aluminium 6061-T6, I specify ANODIZE BLACK, and we specify that inserts are to be installed after finishing. I do not specify masking, and I assume that the helicoil threads are mostly anodized. The process works fine.

Most anodizing results in a net removable of material because the anodizer etches the part. This is done for cleaning purposes, and you can tell them not to.

Are you trying to set your helicoils to chassis ground?

If I want a part to be grounded, I call up CHEMICAL FILM RoHS COMPLIANT. If I want the part to be black, I have it painted.

A long, long time ago, we had a vendor who did anodize silkscreens. This resulted in indestructible panel artworks. It would be really nifty if we could combine this with with a chemical film finish.

JHG

 

[KENAT]

It's been a while, but I think we used to insert helicoils into threads that had been anodized. However, these were larger threads, yours are pretty small.

So long as the drawing states thread dimensions apply after Annodize, then in theory you don't have a problem with annodize build up. In practice, for threads that small it might be an issue.

Could you mask the threads from anodize (or even machine them after anodizing) and then chem film the holes, rather than chem film the entire part?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[israelkk]

First of all helicoil inserts are made of stainless steel. Therefore, they can not be anodized!!. If anodize reduces the thread dimensions you have to use over sized taps, there is no work around. However, you can mask the threads (expensive manual work) and then install the helicoils using wet sacrificial primer such as wet zinc chromate paint primer. You can also use local MIL-C-5541 chromate conversion coatings for aluminum and its alloys on the uncoated threads which will not reduce thread dimensions (as KENAT suggested. However to be on the safe side for improved corrosion resistance use the wet zinc chromate paint primer too.

In the past someone in my work place tried to anodized a manifold where the helicoils were already installed and the anodize bath burned the threads with the helicoils due to a chemical reaction between the stainless steel helicoil and the aluminum manifold.

drawoh

"..Most anodizing results in a net removable of material because the anodizer etches the part.." is incorrect. Anodize usually embed half of its thickness into the base metal and half is added to the base metal (see MIL-A-8625).

 

[KENAT]

Drawoh, if by black anodize you mean hard anodize, the increase in size can be significant on precision parts.

The Anodize solution also has a tendancy to get stuck in small threads and bad things can happen.

Sorry Israel, when I said chem film later I didn't mean to necessarily forgo the primer.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[macduff]

isrealkk,
I never mentioned I was anodizing with the inserts already installed. I know the steps and procedures. I was asking about anodize buildup in the female thread then installing the insert. I feel that the buildup, the insert will be undersized when installed. The other concern I had was leaving the female thread un-chem filmed. I firmly believe that the part needs to be protected by the elements whether it's exterior or an interior feature.

I'm using Mil-A-8625, Type II, Class 2 for my design. Hard Coat is Type III and has more buildup in thickness.

Thank you all for your feedback!

Best,


Colin Fitzpatrick (aka Macduff)
Mechanical Designer
Solidworks 2008 SP 4.0
Dell 490 XP Pro SP 2
Xeon CPU 3.00 GHz 3.00 GB of RAM
nVida Quadro FX 3450 512 MB
3D Connexion-SpaceExplorer

 

[drawoh]

KENAT,

I have never called up hard black anodize, primarily for the reasons you mention. Most of our screws are small, ranging from 2-56UNC and M2.5 to 1/4-20UNC and M6. It has been a while since we have had problems with thread inserts. It is possible the vendor runs a tap into the hole after anodizing.

israelkk,

Aluminium parts usually are etched prior to anodize. This cleans all the oils and other crap off of them, and it provides a consistent surface finish. This usually removes more material than is added by the anodize process. You have options here...
[ol]
[li]You can tell the anodizer to not etch the part. This results in material added to the part.[/li]
[li]You can request matt black anodizing. The anodizer etches the part more than they would normally, resulting in a pitted, matte finish, removing more material than they normally would. This looks ugly, but if you want matte...[/li]
[/ol]

JHG

 

[israelkk]

drawoh

In the aerospace and military fields the practice of removing material by etching is not allowed to my best knowledge. I do not believe that parts machined to 0.01 mm accuracy can tolerate this practice and I have never heard of it. As MIL-A-8625 specifies, between 1/3 to 1/2 of the anodize layer embeds into the base material and the rest added to te base metals and the designer should take ihis into account. To my best knowledge the same MIL-A-8625 says nothing that some material is removed and it should be considered. In more than 30 years of experience I have never seen such practice as you describe. Usually the parts are machined and then anodized therefore, there is no need for etching, etc., just oil removal using solvent cleaning.

If rust or defects needs to be removed it should be done mechanically (blasting or machining) before final machining and not using strong etching chemicals which can adversly affect the alloy properties.