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alaninaustria

Question for aircraft techs

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Posted (edited)

I have often wondered during aircraft assembly, with all the aluminum parts being painted in zinc chromate (ZC) prior to being riveted together, does having the parts first painted in ZC increase the tolerances between parts when they are riveted together? Wouldn't it be best to first rivet the parts together and then paint in ZC? Any insight from people experienced with this would be appreciated! Thanks

Cheers

Alan

Edited by alaninaustria

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I'm not an engineer, but I've certainly been involved in a good bit of hands on airplane fixing.  I seriously doubt a layer of ZC would cause any issue with parts assembly.  Airplanes are pretty high precision items in most cases, but not *that* high precision.

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I assume that once panels were riveted together, much would then be inaccessible to paint, which sort of defeats the purpose, at least in those particular areas, but I'm not 100% sure. 

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Posted (edited)

I work on Gazelle and Bell 212. When panels are removed all of the panels are ZC sprayed, there are no exposed metal. If there is, something is amiss and an attention getter needing investigation.

Edited by Stevepd

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Interesting... I saw in a photo that parts were cut, trimmed, bent and shaped the deburred and then hung and sprayed with ZC... all prior to being riveted together. 

Does anybody here know the process that these aluminum parts would go through prior to being sprayed in ZC? Any special cleaning solution?

Cheers

Alan

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However, not all aluminum airplanes are chromated.  None of my Cessnas were, nor were my rag and tube airplanes.  Of course, my wooden homebuilt was varnished, not chromated.  And not everything you see that looks like ZC is.  Many aluminum parts go through a process called alodining where the parts are dipped in a solution that bonds to the aluminum, providing a degree of corrosion protection and surface prep.  Alodine is thin and watery and somewhat transparent when it dries, leaving the treated surface a sort of translucent bronzy/yellowy/greenish color.  Chromate is essentially paint and looks like it.  I would bet that some of the parts in the P-40 assembly pix above are alodined, not chromated.  Parts are usually alodined before assembly because this allows best coverage and the film, once dry, is usually so thin that it has little or no impact on assembly in most cases.  ZC offers another layer of corrosion protection, is usually sprayed like paint and can be applied before or after assembly, depending on what the contract says.  Probably more than you wanted to know.

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Having worked in an aircraft assembly plant, I can state that no, the paint on the parts is not an issue with respect to tolerances.  We just put the parts in the jig, drill the holes where they need to be drilled and shoot the rivets.  In some instances, we are required to touch up the rivets with primer (not necessarily with zinc chromate because it's environmentally unfriendly and unhealthy).  Zinc chromate primered parts are fast going the way of the do-do for those reasons. 

 

Where tolerances come into play is where multiple parts are assembled on top of each other (such as cyclic and collective sticks/control rods) and the final product is out of tolerance because all the component parts were at or near their maximum tolerances when manufactured.  If this happens (and it does), the entire assembly is either scrapped or disassembled and reassembled with component parts that are closer to zero.  Any assemblies that require very tight tolerances are usually marked as flight critical parts and require special assembly and inspection processes.

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Posted (edited)
26 minutes ago, Oldbaldguy said:

However, not all aluminum airplanes are chromated.  None of my Cessnas were, nor were my rag and tube airplanes.  Of course, my wooden homebuilt was varnished, not chromated.  And not everything you see that looks like ZC is.  Many aluminum parts go through a process called alodining where the parts are dipped in a solution that bonds to the aluminum, providing a degree of corrosion protection and surface prep.  Alodine is thin and watery and somewhat transparent when it dries, leaving the treated surface a sort of translucent bronzy/yellowy/greenish color.  Chromate is essentially paint and looks like it.  I would bet that some of the parts in the P-40 assembly pix above are alodined, not chromated.  Parts are usually alodined before assembly because this allows best coverage and the film, once dry, is usually so thin that it has little or no impact on assembly in most cases.  ZC offers another layer of corrosion protection, is usually sprayed like paint and can be applied before or after assembly, depending on what the contract says.  Probably more than you wanted to know.

 

Alodining is done in some instances on factory parts but by-and-large, it's an after treatment to preserve the aluminum being treated.  Alodine is a  chemical that creates that nice yellow/bronze shade when applied to pure aluminum (2024 Alclad has pure aluminum sprayed on both sides of the alloy so it corrodes instead of the base alloy).  Alodine doesn't react the same to all aluminum alloys.  I used alodine when installing antenna on aircraft to prevent skin corrosion should the fillet seal around the periphery of the antenna base be compromised; sometimes it came out looking great, other times not. There can be no paint between the antenna base plate and the surface of the aircraft skin due to the electrical bonding requirement (ground plane).  In manufacturing, anodizing is the preferred method of treating aluminum castings and such.  Anodizing imparts a layer of corrosion to the aluminum parts turning them grey in color.  The anodized surface is usually about 0.10-inch deep whereas an alodined surface is only a few thousands of an inch deep...anodizing is much hardier.

 

On our Gulfstreams, when we were installing satellite phones, we had to match drill the doubling plates and aircraft skin for the antenna, alodine the doubler and the holes in the aircraft skin.  Then we were required to shoot the rivets with wet primer and boy doesn't that make a mess if you use too much primer on the rivet going in.  Those were our installation instructions from Gulfstream Engineering.

 

Paint (including primer) adds a lot of weight and with some puddle jumper aircraft, that can be detrimental to the aircraft's performance so they do not paint interiors where they do not need to.

Edited by Juggernut

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Posted (edited)

In the context of WWII airplane construction, Zinc Chromate is not simply a "paint" to color or cover the underlying aluminum. It is applied as a form of Cathodic Protection where the Zinc Chromate is the sacrificial anodic metal to prevent corrosion of the underlying aluminum. 

 

As such, Zinc Chromate would have to be allied to all surfaces prior to assembly - including the edges of drilled holes, for example. 

 

HTH,

D

 

 

Edited by D Bellis

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Hi Al,  as has been said, primer wouldn't throw an assembly out of tolerance as assemblies

(and detail parts) have tolerance zones ( +/- ) so that some variance is engineered into the

process and is acceptable.  I can't speak to WWII era processes but painted parts I inspected

at Boeing were almost always under .002" (.5 mm).  Aluminum parts were always anodized first.

Alodining can accomplish the same goal and can be hand brushed or submerged. Anodizing

must be submerged in salt water while other chemicals are added to the salt water tank and

electrodes are inserted to create the reaction. Alodining is thinner and less expensive.

Anodizing penetrates deeper and also hardens the surface and imparts a resistance to static

electricity.   So the answer to your question is no it wouldn't cause a tolerance problem.

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