Tapped Holes In Aluminum Casting - Distance From Edge 6

[paulcook]

Designing a cast aluminum part (ADC12 alloy), and trying to determine an adequate distance from the center of the tapped hole to the edge of the material.

Bolt is 5/16-18 stainless + loctite.

Depth > 3X bolt diameter.

For distance from the threaded hole center to the edge, we have about 0.77" 0.85":

Is using the diameter of the circumscribing circle of a standard (aluminum) nut not a reasonable approach to finding the minimum?

I am confused on the standard 1.5 - 2D rule of thumb. Is that from the center of the hole, or the edge?

Have also seen 3X thread height as a rule of thumb, but that seems like much less than >1.5D.

 

[hydtools]

Use thread forming taps.

Ted

 

[Jboggs]

@paulcook

Thank you! A new nugget of knowledge for today.

 

[rb1957]

hummm, clearly it (rolling threads in castings) 'cause people do it, but I imagine it needs to be carefully controlled and well understood.

Forming threads utilizes the plasticity of the parent material ... castings are not known for plasticity, and are known for inclusions.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

castings are not known for plasticity, and are known for inclusions - the combination of which leads to low and variable properties .......

 

[paulcook]

Aerospace guys really setting themselves apart from us land based engineers. Must be nice to have unlimited budgets

 

[mfgenggear]

I been sitting on side lines. Watching and trolling.
Aerospace has safety issues, were there could be safety flight article. Safety is number 1.
Commercial products that ate not with safety issues. Becomes reliability to the customer.

Tapping on raw aluminum is just bad practice.
Rolled or cut. Problem usaully is the threads seeze and fail.
My apologies for the typing. Bad eye site, and cheap phone.

 

[TugboatEng]

What does raw aluminum mean?

Still in stainless steel bolts directly into aluminum threads. Are the norm, not the exception in all but the aerospace industry. There are considerations that must be made. Coatings help.

Having a thread that is open on both ends does not help because capillary action will draw water into the threads and cross corrosion.

 

[paulcook]

Does the corrosion cause part failure, or just seizing of the bolts?

There is a large volume of aluminum compared to SS. My understanding is the corrosion (if it happens, despite the loctite) would be negligible in terms of hurting the material integrity.

Galling the threads on install is a contractor problem, so not too concerned about that.

 

[CWB1]

Is using the diameter of the circumscribing circle of a standard (aluminum) nut not a reasonable approach to finding the minimum?

No, and even the 2D rule of thumb commonly fails to provide enough underhead bearing area to prevent yielding softer base materials like AL. This is one of the classic undergrad examples of why good engineers run the analysis rather than relying on bad rules of thumb, bad forum advice, or other nonsense. And FWIW, aluminum threads are common in both automotive and aerospace applications, even in joints intended for disassembly and reuse.

 

[TugboatEng]

Have you considered hot dip galvanized like the rest of the equipment on the pole?

 

[paulcook]

@CWB1
Thanks - I get frustrated when I see things frequently in real life, yet a certain cohort (whose opinions I appreciate) insists that it is ALWAYS out of the question.

I’d argue that testing is often better than analysis, and that most rules of thumb have that status for a reason - circumscribing diameter of a common aluminum nut did seem small though.

@TugboatEng
Company is pretty adamant about stainless. They’re also socket head cap screws. Not sure how easy that would be to source.

Would you say hot dipped galvanized / other hi-tech coatings are superior for corrosion or other factors (aside from the better mechanical properties)?

 

[TugboatEng]

Stainless is such a finicky metal. Most seem to know about the galling troubles. Few seem to understand stress corrosion cracking. Stainless steels are great for food and pharmaceutical processing where they get washed regularly. They're not so hot in near coastal environments where the get damp in the mornings and then any chlorides on the surface get super concentrated as the part dries out. This is especially true in California where it doesn't rain often but we get morning fog. There are nitrogen containing alloys such as Bumax 88 that resist SCC.

There are many good coating systems for fasteners. The newer generation of "painted" fasteners are very good.

https://www.mcmaster.com/91274A316

There are multiple formulations with different advantages.

 

[MintJulep]

The potential for stress corrosion cracking of stainless steel fasteners introduces some undesirable failure modes and effects.

This compounds with the prevalence and ready availability and low cost of "18-8" stainless fasteners. The problem is that "18-8" isn't a grade, there is no controlling standard, and no limiting requirements for elements that degrade performance. "18-8" is great for shiny forks and spoons and pots and pans; use it at your peril in engineering applications. When you understand that you really need Type 301 or 304 or 316 stainless fasteners made from a material that complies with a controlling AISI/ASTM material standard the price goes up dramatically.

The zinc-flake coated steel fasteners favored by TugboatEng are probably a better and cheaper choice in many cases.

 

[mfgenggear]

Tugboat
Good point , uncoated aluminum alloy
Yes even stainless corodes. Coated fasteners and thread inserts.

 

[mfgenggear]

Paul Cook
Once the threads seize the removal will destroy the threads. Once excessive force us applied the threads will fail. In my early years I fixed aluminum tooling cast plate with failed threads.
The other issue with stainless once corrosion starts it hard to remove.

 

[paulcook]

Edit - I'm going to pay someone to evaluate this hah. Thank you guys for your thoughts/knowledge.