Threading directly into 6061-T6

[MrGearhead]

Rules about threading into 6061-T6.

I've got a design application that due to it's closeness to the design size envelope limits, and the part's size, force threading directly into 6061-T6 for some #4-40 SHCS.

The other choice is using Helicoils, but their larger dia takes the threaded holes to within .030" of breakout on the edge of the part.

What is the lesser of 2 evils? Using Helicoils close to the edge of the part, or using no insert and not exceeding a calculated max torque?

As a subquestion... How does "number of times cycled" change the decision? Will an aluminum thread "wear" much even if if the torque is calculated and kept under some predetermined, non-destructive limits? I've heard this before, but is there anyone that's ever tried to quentify cycles with or without a steel insert?

 

[MikeHalloran]

You can break (in tension) a steel bolt that's threaded into a _plastic_ part, provided you've got ~>3 diameters of thread engagement.

I've done development work on small assemblies that had #2-56 stainless screws threaded a couple diameters into aluminum, and they survived at least dozens of cycles without trouble. The parts were clear anodized, which may have contributed a little to their durability.



Mike Halloran
Pembroke Pines, FL, USA

 

[MrGearhead]

Thanks for response... I'd love to see proof that an aluminum thread will wear even if it's used for bolts that are tightened to their proper limits and not over-torquing

I've got an application that is being force in the direction of a helicoil insert but I'd rather not doe to size and cost on a producton basis...

 

[israelkk]

As a whole this is bad design. The risk is that without the hellicoils many part can be discarded due to thread shear and wear. On the other hand hellicoils that close to the envelope limits. You should seriously check the option to use stronger aluminum alloy such as 2024, 7075, 7050 etc. You have to decide what is more cost effective: the added cost of using stronger aluminum alloy or the extra cost and loose of dignity when a customer will face the results of this bad design. If the threads are not meant to repeatedly opened and closed you may get away without hellicoils but you should at least seriously consider stronger aluminum.

 

[MikeHalloran]

One other, other choice is using smaller screws... but we know nothing of the limits of your problem space.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

If you are wanting to torque the screws to max because of load requirements, increase the number of screws and use reduced torque. Cut the threads, do not form them. For strength I found cut threads are stronger.

I have used hex socket capscrews into 2024-T4 for hydraulic motors for over 30 years of production with no problems.

Ted

 

[CoryPad]

Internal threads in aluminium alloys can wear after only a few insertions/removals. To avoid this, it is best to use an engineered coating on the screw (e.g. multi-layer zinc organic from Magni, Dorken, MCI). Thread quality (dimensions, roughness, debris) is highly variable for both formed threads and cut threads. If done correctly, formed threads should be better (smoother surface, higher strength due to compressive residual stress). It is possible to have an aluminium internal thread without an insert last through just about any usage, you just need to engineer it properly.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BrianPetersen]

Is there any way to use a larger fastener?

I don't like seeing threads finer or smaller than roughly M6x1.0 (or 1/4-20 if you prefer) threaded into aluminum.

 

[MintJulep]

While they may be hard to find, why not use studs and a nut if disassembly/reassembly is an issue.

Install a stud once with Loctite.

 

[Ron]

If the threaded part is thin enough, you might consider nut inserts (Nutserts). They work reasonably well up to about 1/2-inch thickness.

 

[CorBlimeyLimey]

MJ's solution is one I've used before, but with 3M two-part epoxy (green) for vacuum compatibility. #4-40 threaded stainless rod is fairly easily obtained.

A little vacuum grease on the threads helps reduce galling, and extends thread life.

 

[hydtools]

In our tests formed threads performed less well than cut threads. The aluminum whether 6061-T6 or 2024-T4 does not cold form very well. The finish may be good, the thread strength is not. Manufacturing engineering wanted to change the threading process from cutting to forming. Wanted to eliminate chip production in blind tapped holes. That is what prompted us to test before accepting or rejecting the forming process. Thread sizes were 10-32 and 1/4-20. Formed threads failed consistantly at lower pull out loads than did cut threads.

Ted

 

[GregLocock]

Production suspension parts that are made of forged aluminium generally need to be helicoiled when they are used on prototype cars where they are frequently dissassembled.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CoryPad]

hydtools,

Can you tell me what form tap was used?

Thanks.

 

[IRstuff]

Can you anodize the threads? That seems to work well with Newport aluminum optical bench parts that are threaded. But the 1/4-20 bolts used are also anodized, I think.

TTFN

FAQ731-376

 

[hydtools]

CoryPad, it has been several years since testing was done. I think the tap was a tri-lobe shape but I don't recall for sure. The tap drill was larger, too so that we could get the recommended 50-65% thread. It made sense to me at the time that pushing the T-condition aluminum beyond yield to form the thread resulted in a weaker thread. The tempered alloy does not cold form very well. Not like steel which forms threads very well.

Ted

 

[drawoh]

MrGearhead,

I have had a similar design problem to yours. In my case, I wanted the biggest possible threads into the edge of an aluminium[ ]6061-T6 1/4"[ ]plate in an airborne system. The plate was chemical filmed, rather than anodized, so that we could control EMI/RFI. I anticipated that the structure would be assembled and then never taken apart.

I specified 8-32UNC without the helical inserts. I made the holes as deep as possible as per MikeHalloran's suggestion, above. We did use lubricating threadlocker (Loctite[ ]242). The alternative design would be 6-32UNC with thread inserts. I think I picked the lesser of two evils.

Does your part absolutely have to be aluminium? If it is a tiny part, the weight may not be critical, and you will be able to use brass or bronze.

JHG