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.

 

[Jboggs]

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...

 

[paulcook]

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.

 

[rb1957]

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.

 

[paulcook]

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?!

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.

 

[rb1957]

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.

 

[paulcook]

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.

Appreciate your thoughts.

 

[TugboatEng]

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.

 

[paulcook]

@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).

 

[MintJulep]

@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.

It may be your bread and butter, but some prefer oatmeal.

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

 

[rb1957]

AMS2175

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

 

[hydtools]

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

 

[paulcook]

found rolled threads to be preferred over cut threads

Formed threads no doubt better than cut

 

[Compositepro]

rolling threads so near an edge would be difficult.

 

[paulcook]

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).

 

[Compositepro]

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.

 

[paulcook]

Whoa - tight tolerance on those threads!

 

[Jboggs]

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

 

[paulcook]

@Jboggs

https://www.harveyperformance.com/in-the-loupe/8-unique-facts-about-thread-forming-taps/

 

[EdStainless]

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

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P.E. Metallurgy, consulting work welcomed