Aluminum Thread Galling , Type II vs Type III Anodizing 2

[Linghunt]

Not sure if I'm over thinking this.

Material is 6061-T6 Aluminum for both parts. The Female thread is anodized Type II (MIL-A-8625) and I was considering having the Male thread anodized in Type III (hard coat).

Does the Type III buy me anything as far as galling is concerned for the added processing cost.

Is it a non factor as galling is concerned if anodizing is dyed or clear?

 

[MikeHalloran]

I always specify dyed anodize because of a bad experience when I specified clear anodize, the parts weren't anodized at all, and nobody noticed until they corroded in storage.

Anodizing both parts converts the mating surfaces from aluminum on aluminum to ceramic on ceramic. ... what you will get is not strictly galling, but you won't like it anyway.

How about a tiny squirt of graphite?


Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Anodize does increase the chance for fatigue cracking. Maybe anodize one part and use stainless for the other?

 

[MikeHalloran]

3DDave, that sounds like a good idea.

Linghunt, have you got room for a stainless Keensert in the female part?



Mike Halloran
Pembroke Pines, FL, USA

 

[Linghunt]

Been using a Stainless Male threaded part currently.

On a new design that is smaller size I was considering a reduction in weight. Less strength and galling are draw backs.

Connection is used in a saltwater environment.

attempted to attach Picture of Current design.

 

[tbuelna]

From the picture the male fitting does not look very large, so the best option would seem to be making both parts from a stainless alloy like A286 or Nitronic that is less prone to galling.

If you really want to use anodized aluminum, then take a look at the MIL-A-63576 type I process. This is MIL-A-8625 class III (hard anodize) with PTFE resin impregnation. The PTFE treatment improves corrosion resistance by sealing the anodize coating, and it helps reduce galling by serving as a surface film lubricant. For your threaded parts specify the min anodize coating thickness of .0005"+/-.0002". The PTFE treatment will add less than .0001" to the anodize build-up. You will need to adjust the machined dimensions of the threads to compensate for the coating build-up. The increase in dimension from the coating is approximately half the coating thickness. So for a finished coating that is .0006" thick you would need to adjust the machined dimension .0003" per surface. With hard anodize it is also beneficial to round sharp corners and use the largest fillet size practical.

 

[israelkk]

tbuelna

You recommend MIL-A-63576A Type I coating process, 0.0003"-0.0007" (~0.008 mm to ~0.018 mm) thick.

According to MIL-A-63576A para. 4.6.2.1 the coating must endure 1000 hours in a 5 percent salt spray test. I do not see how such a thin coating will meet such requirement.

MIL-A-63576A para. 3.5 recommends 0.002" nominal thickness or even 0.002"+/-0.0002" (~0.046 mm to 0.056 mm). To my best experience this is the minimum thickness necessary to meet the 1000 hours in a 5 percent salt spray test for the full range of aluminum alloys. Thinner coating will not meet this requirement .

It may help the thread dimension issue but opens another issue of corrosion protection. not to mention that it will not meet MIL-A-635676A requirements.

 

[tbuelna]

israelkk-

MIL-A-63576 states that unless otherwise specified the anodize coating thickness will be a nominal .002". But if your read further, you'll note that the anodize coating should conform to MIL-A-8625 type III which lists a min. coating thickness of .0005"+/-.0002". All anodize coatings have permeability issues unless given a post anodize seal. The PTFE resin impregnation used with MIL-A-63576 is very effective at sealing even a thin hard anodize coating.

There is no hard requirement in MIL-A-63576 for the 1000hr salt spray acceptance test. Section 6.3 notes that any use of the term "capable of" in the spec means that the supplier need not perform the test unless specifically directed to by the customer. Section 3.6.1 Corrosion Resistance, uses the term "capable of" in the very first sentence.

Lastly, in my post the first thing I said was that I thought it would be better to use a cres material for both fittings rather than anodized aluminum.

Interesting discussion.
Regards,
Terry

 

[Linghunt]

I don't have any issues with the parts in the picture. That's a 7/16-20 thread for size aspect. A small touch of anti seize can also be used. The parts are only temporary assembled together, say 3 - 8 hours. and same time frame for being in salt water.

The new part will be smaller going to a 9mm-1 thread. If I go for Stainless done. no issues. If I switch to Aluminum for a tad bit less weight was the thought.

The PTFE is interesting, have to look into it. Lots of good information in this thread.

 

[Linghunt]

One of my mechanical engineering buddies referred me to these guys. He had used them before with great results. He also thought having a hard coat on the male part would work. Making some sample parts and do some testing would be next step.

http://www.tiodize.com/aluminium_anodizing.html

 

[tbuelna]

Linghunt-

Couple questions. What is the purpose of the o-ring on the two upper fittings shown in the photo? What is the dark colored round rod or tube the fittings are attached to made of, and how are the fittings attached (bonded, swaged, threaded)? Or is the strut made from one piece of material?

 

[Linghunt]

tbuelna,

Great question. The Black rod is an anodized black aluminum tube (6061 - T6), the Stainless part has a Male thread on both sides and is held in place with loctite into the tube. Multiple length of 2 foot poles are assembled for making portable long poles for shooting fish. Some go as long as 12 foot with 2 foot tips on top of that.

Here is a customer with my Pole next to him.

http://www.iusarecords.com/ViewRecord.aspx?id=754

The long spud has 3 purposes, does proper alignment so diver can't get a cross thread condition and also to take the threads out of the equation for strength. I figure my weak spot is on the large radius on the end of the spud for a side load. The diameter of the spud is just a couple thou under the tube ID. 3rd is alignment for straight pole connections. CNC-ing all the parts makes seem to easy to do.

O-ring: Two purposes; Its a seal for salt water (don't wanted added mass inside pole, and toss off the CG, speed, and balance. I didn't want divers to be dealing with Teflon tape for a seal.

Second purpose is a semi-thread locker. I sized the gland (contour inside tube and on the male part so the O-ring will turn into a semi square cross section. Now the diver can focus on sharks and diving vs continuing to check that his pole is tight. The net positive is he can change length sand various setup quickly based on hunting conditions.

Best field test I have had so far was a 7 foot white tip shark. The shark was in process of stealing a large hog fish off the pole and ended up in a battle with my customer. Everyone was waiting to see the pole snap but once the fish was eaten, the pole "snapped back straight" and just had some teeth marks on the aluminum pole. The entire 18 inch tip and part of the pole was shot through the fish. I recall he had the pole set up in a 10 foot configuration.

Here is a picture of a Pole setup that a young lady uses in the Bahamas, It is an 8 - 1/2 foot long pole. Since she was only 100 lbs wet, I added an additional section of ring grips for her (her wing span and arm/shoulder strength). She can hold her breath like 2- 3 minutes. Un-real.... Ben in link above is same type, but he's a strong kid.

This probably explains my name LINGHUNT as well. I hunt for ling-cod on the northern CA coastline. As Paul Harvey says,"that's the rest of the story".

 

[CoryPad]

Beautiful products. Tiodize is a reputable source for anodizing. I think any of your material options (stainless or aluminium, with or without anodizing) would benefit from a solid lubricant like PTFE or Mo₂S. Graphite is not good for galvanic corrosion with aluminium.