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# Tapped Holes In Aluminum Casting - Distance From Edge6

## Tapped Holes In Aluminum Casting - Distance From Edge

(OP)
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.
Replies continue below

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

First: I hope you plan to use some type of steel thread inserts. Bare tapped holes in aluminum are generally a bad idea, especially if they will see much repeated use.

Second: Yes, that 1.5-2D rule of thumb is based on the center of the hole.

BUT!!! -- If you use thread inserts, the "D" in that equation becomes the OD of the insert rather than the base thread size.

Bottom line - you haven't provided enough information to get a good answer. Working environment? Repeated use? Critical nature? Etc...

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)

#### Quote (Jboggs)

I hope you plan to use some type of steel thread inserts. Bare tapped holes in aluminum are generally a bad idea, especially if they will see much repeated use.

No, the bare aluminum will be tapped. It will not see repeated use. Maybe adjusted once or twice within a month of initial install, but generally just installed once.

We do spec high-strength loctite, which hopefully will prevent oxidation.

It is a critical connection, but we have > 4X FOS at this bolt dim.

Working environment is outdoors. It clamps equipment to a pole (pole out of scope for our design). 4X 5/16-18 18-8 Stainless Fasteners - 70,000 lbs Tensile Strength (per supplier).

Load is at most 55lbs, with center of gravity cantilevered about 12 inches from the clamps face.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

tapped castings ... why not ?? oh, but wait, "hi-strength" loctite ... that'll make it ok !!??

This is IMHO a very bad idea, forget edge distance (which is measured to the center of the hole, 2D eD preferred).
And you're not even using a Heli-coil ??

The fastener strength is irrelevant. The threads will shear long before the bolt fails.

There have got to be better ways, particularly as you described the situation as "critical".

Is it a Class I casting ??

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)

#### Quote (rb1957)

And you're not even using a Heli-coil

Other engineers here nixed the helicoils... I would have preferred that.

The purpose of the loctite is to lubricate the thread, avoid/reduce oxidation, prevent loosening from vibration.

There is nearly an inch of thread engagement (between 3/4 and 1"). You really think those threads would shear out under the described load?!

#### Quote (rb1957)

Is it a Class I casting ??

Can you elaborate?

I should note, we loaded samples to 8X load (granted, indoors, controlled environment) and saw no issues after 48 hours.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

troubling you don't know about classes of castings. A Class I casting is for critical structures (as you've described yours) where there is almost a one for one testing, coupons to verify the process, many process controls and such.

Yes, the casting thread will fail long before the fastener. It has little to do with the length of thread engagement, and particularly in castings the first thread will fail (and not yield) and the house of cards will come tumbling down. If you doubt this, run a test ... simple enough.

Wow, people prefer to tapp a casting over a helicoil. Sure, helicoils are not perfect, but the better than "loctite". Loctite IMHO does little to redistribute load, but will help corrosion and dissimilar metals.

Is a barrel nut out of the question ? Is a plane nut on the other side of the flange out of question ?

At this stage I might prefer 3D printed fitting (over a cast fitting).

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)

#### Quote (rb1957)

troubling you don't know about classes of castings

We use NADCA standards for casting specifications. There is no overarching "class". Different parameters (e.g. surface finish, linear tolerance, etc.) each have their own specification, from class 1 - 4.

What classification system are you referring to? Whose standard?

In testing, over-torque of bolts lead to bolt heads failing at about 2X our specified torque values - no failure at/on the threads.

As mentioned, the Loctite is predominantly a corrosion / vibration mitigation measure.

Unfortunately, no. A helicoil could work, but, cost-wise, tapping was chosen.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

The automotive industry uses steel bolts directly in aluminum threads. They use bolts with a lubricating coating. Dacromet is common in automotive. Xylan is common for industrial applications. If your joint is closed and you don't use lock washers you won't need to worry about corrosion of the threads and won't need Loctite.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)
@TugboatEng

Great to know on coatings - thanks.

Joint is not closed. The bolts span a gap (0.125" to 0.875") between the mating parts.

Hopefully Loctite is good enough.

Aluminum part is powder coated (though threads are plugged during painting).

It won't see any type of submersion. Rain, and perhaps limited salt spray (in a beachfront application - not in close proximity to ocean water).

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

@rb1957
Without a citation for the defining standard "class 1 casting" means nothing.

Not to the OP. Not to me. Probably not to many others.

Please enlighten us all and tell us the standard that we should look up.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

AMS2175

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

We used A356 cast aluminum and found rolled threads to be preferred over cut threads. We used helicoil inserts where assembly/disassembly may be required. The application was hydraulic powered hand tools subject to up to 2000psi. Fasteners were hex socket capscrews.

On edit: thread forming rather than rolling. Got the benefit of some cold-working in the threads.

Ted

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)

#### Quote (hydtools)

Formed threads no doubt better than cut

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

rolling threads so near an edge would be difficult.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)
Yes, I do worry about it deforming the part, especially on exit as the wall thickness goes down to zero.

Also need to ensure thread goes fully through the part in case the wrong screw length is used (so you can still clamp effectively without bottoming out).

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

One experience I had once was an oil filled case where the cover was screwed on to threaded holes in the edges of the case. These were blind holes. When the first screw was tightened there was a pop and the case wall split because of the hydraulic pressure generated by the oil in the blind hole.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)
Whoa - tight tolerance on those threads!

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

Rolled threads on a tapped hole? Please enlighten me. How does one do that?

(OP)

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

All that we used were forming taps.
Nice smooth threads a more strength.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Ted

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

@paulcook

Thank you! A new nugget of knowledge for today.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)
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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

#### Quote:

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

(OP)
@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)?

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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.

### RE: Tapped Holes In Aluminum Casting - Distance From Edge

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

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