Is it OK to screw a steel screw into aluminum? 3

[tmech77]

If we disregard galvanic corrosion, is it OK to screw a #8-32 a low strength steel screw into 3/16" thick aluminum sheet? I seem to remember that it is not generally recommended because the aluminum like 5052-H32 is not as hard as a steel screw and that is why we use helicoils. But what about if you use 6061-T6 aluminum? It appears to have a Rockwell B hardness of about 60 and a tensile strength of about 47,000 psi. This may be harder than a low cost low strength hardware store zinc plated screw, right? So would the steel screw work ok in 6061-T6 aluminum or is there some other consideration that I have not thought about?

 

[3DDave]

Without a set of conditions to determine 'work ok' it's not possible to be certain. A helicoil or a keensert increases the shear area and provides a harder surface that will better resist multiple insertion/removal of the screw. If that's not required then there's no intrinsic problem.

 

[tbuelna]

As 3DDave notes, one purpose of installing an insert in threaded holes in aluminum is to balance the axial strength between a steel bolt/screw and the mating threaded hole in the aluminum. Another common reason for using a thread insert is to provide a locking feature.

Whether installing a zinc plated low cost low strength #8-32 hardware store steel screw directly into a tapped hole in 3/16" aluminum plate is "OK", that's between you and your customer.

 

[IRstuff]

Isn't there some risk of galling with aluminum contact with steel?

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //

http://www.engineering.com/AskForum/aff/32.aspx

faq731-376 forum1529

 

[CoryPad]

For IRstuff - yes, although proper design/specification and manufacturing allows direct connection between steel screws and threaded aluminum holes. Every automobile has high strength steel screws (UTS ≥ 800 MPa) mated with tapped holes in aluminium castings, extrusions, forgings, and sheet. A good coating goes a long way, hence the widespread use of Magni 565, Geomet 321, etc.

 

[BrianPetersen]

I've lost count of the number of motorcycle engines that I've had apart. Every single one of them has cast aluminum cases, cylinder blocks, and heads, and are held together with steel screws threading into the aluminum.

Minimum typical thread size for a screw threading into aluminum in that application is M6x1.0 and with more thread engagement than you are proposing, though.

I use a torque wrench on every single fastener. Gorillas will do damage.

 

[BUGGAR]

Check the Aluminum Association Aluminum De$ign Manual

 

[IRstuff]

CoryPad,

Thanks. That probably explains why I munged one of my carburetor threads a long time ago. It galled so bad that the screw took all of the thread with it.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //

http://www.engineering.com/AskForum/aff/32.aspx

faq731-376 forum1529

 

[drawoh]

tmech77,

I systematically call up helical thread inserts for airborne stuff. On occasion, I have had to make the choice of thread inserts or larger screws, for example, edge of 1/4"[ ]plate and 6-32UNC and helicoils versus 8-32UNC screws. I went for 8-32UNC. I now know that aircraft approval people don't like screws into edges of plates, helicoiled or otherwise.

If the thing is going to be taken apart a lot, I recommend thread inserts. If it does not fly, and you are in control of the assembly and the screw torque, and it does not come apart, you don't need the inserts.

Have you considered press-in nuts like PEM? These may be cheaper than tapping the holes. The floating ones cost more, but they let you get away with sloppier tolerances. The float typically accounts for sheet metal bending tolerances.

--
JHG

 

[BUGGAR]

Forget the Aluminum Design Manual, it only covers self-tapping screws which have their own thread form.

Coincidentally I'm building something with 3/16" 6061 T6 and #8-32 flat head socket screws. I'm using nuts on the screws, but I've tried threading them into the aluminum. As mentioned by drawoh, the threads in the aluminum quickly become loose if you reassemble several times.

 

[berkshire]

Tmech 77,
One thing you have not mentioned is the usage of this screw. Is this a one time insertion , or does this screw have to be removed and replaced frequently.
The Helicoil increases the effective diameter of the steel portion in the aluminum and gives you a more gall resistant surface on the screw threads.
If a rolled thread tap is used to tap a 3/16" thick Aluminum plate, the work hardening of the threads will reduce the tendency to gall a little, versus a cut thread.
When all else fails do a test.
B.E.

You are judged not by what you know, but by what you can do.

 

[BUGGAR]

For my test, I didn't tap the holes, I just force-screwed the screw into the smallest hole I could drill without breaking the screw when forcing it in. It still came loose when re inserting it a couple of times. Nuts. That's what I ended up using.

 

[tmech77]

Thanks to all of you for sharing your suggestions. We have decided to countersink the back side of the aluminum plate and use flat head screws with nuts. I should have considered this option days ago. Thanks again for all your time and help. Have a great day.

 

[MikeHalloran]

Take a look at PEM Studs.
Hole prep is a little easier than countersinking, you just press 'em in, and you don't have to worry about them working loose.



Mike Halloran
Pembroke Pines, FL, USA

 

[tmech77]

Thanks, good idea.

 

[hydtools]

Steel screws into threaded aluminum 30+ years producing powered hand tools.

Ted

 

[tbuelna]

tmech77- I like the approach you decided to use. A separate screw and nut is always a good choice if your design allows for it. No need to tap holes. Better compatibility between the fastener materials. And much easier to fix any threads that get damaged.

 

[rorybagleys]

Screws with nuts. That's what we do when bolting together aluminum frames for fabric covered buildings. I've never seen it result in a problem.

 

[KENAT]

Sorry to resurrect an old thread but...

We put SST fasteners into threaded holes in aluminum all the time in 'non structural' joints in fairly benign environments (think office or lab) with few issues.

On sheet metal we routinely use pem type fasteners.

All comes down to more detailed application information as to whether it's right for you.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​