Holding power...8-32 vs. 10-24 in 6061 (2024)

Need to tap some holes through 1/4" 6061. Pullout strength is a major consideration. Which thread count would have the greater holding strength? I will be using 18-8 stainless flat head machine screws.

I'm guessing 10-24?

I don't have a machinery's handbook here at home but depending on the thickness you might want to go with an NF thread if you can tap it very carefully.

If they're only loaded in tension (i.e. pullout is a much bigger consideration than shear) I would go with more smaller screws with finer threads, provided you have some depth (i.e. >3/16" thread contact). 6061 is in a weird place wrt hardness where it's pretty hard (not like wood where fine screws are worthless) but not like steel where you get some warning before you strip threads out. aluminum tends to be nice and tight, and getting a little tighter, then about right and then you've stripped out all the threads. especially on shallow coarse threaded stuff.

Not being a full-fledged machinist I personally could be wrong about this but sometimes in applications like this I'll use a slightly smaller tap drill than is recommended (by .002 or so with holes this small) and carefully tap the holes with Moly-Dee, then clean them really really well and use medium strength loctite on everything. it seems to work well for me to make sure the tapped holes are as tight as possible and nothing comes loose when dealing with softer and/or thinner materials.

The chance of pullout is way higher if the fit is sloppy or things aren't tightened properly. Also if it's that critical i would find a torque spec so you don't damage the threads by overtigtening which is always really easy with small stuff in aluminum, especially if the threads aren't cut really nicely.

anyway greater thread depth and longer screws is probably the best possible thing you can do if you're worried about pullout.

can you tap a steel plate or put insert nuts on the far side?

in the end it's how critical this really is to how much work you want to put in it. not tapping it and having something on the far side the screws go into (e.g. nuts or tapped backplate) will save you from having to tighten screws into aluminum. I have had bad luck with smaller coarse thread stuff in 6061. I would go with 8-32 and use a few more of them if you can. Or really any size if you can put something steel (rivnut or tapped plate) behind it. If shear loading is also a problem 10-24 or 10-32 is probably better. For whatever reason I have found smaller finer thread stuff always fits tighter and is harder to strip out than some of the small coarse stuff. I'd go with 8-32.

Just a rough guess but in that material at that thickness you will fail the fastener
under tension before the tapped threads will let go. Strongest will be the number ten.

If you are really worried about the tapped hole letting loose then install a helicoil
first. Helicoil is likewise advised if the fastener will be removed and installed repetitively.

Jim

do people ever use helicoils in other than blind holes? he said through holes which is why i recommended a press fit nut.

if helicoils work by all means use them but you'd probably want to go up to a 1/4-20 if you do that.

Helicoils and other types of thread inserts in aluminum through holes are common in aerospace, particularly where assemblies will be disassembled for maintenance.

As for the original question, there are two possible failure modes for "pull-out" under a tensile load. The first is actual pull-out by shearing of the female threads near the thread roots. The second is fracture of the male thread through the area of the root diameter.

In this example with equal engagement depths the 10-24 would be stronger in tension. This is because the circumference of the #10 screw engages more shear area of the 6061 for any equal engagement length for the two sizes. At some length of engagement the shear strength of the 6061 will exceed the screw strength and the screw will fail in tension. The tensile area of a #10 screw is considerably larger that a #8 screw so obviously the #10 screw is stronger for that failure mode as well.

All of this is easily calculated if the material properties are known. "6061" and "18-8" are not adequate to specify properties. However for the question asked where the materials are the same for both sizes a general answer as above is applicable.

DaveE907 said:

Helicoils and other types of thread inserts in aluminum through holes are common in aerospace, particularly where assemblies will be disassembled for maintenance.

As for the original question, there are two possible failure modes for "pull-out" under a tensile load. The first is actual pull-out by shearing of the female threads near the thread roots. The second is fracture of the male thread through the area of the root diameter.

In this example with equal engagement depths the 10-24 would be stronger in tension. This is because the circumference of the #10 screw engages more shear area of the 6061 for any equal engagement length for the two sizes. At some length of engagement the shear strength of the 6061 will exceed the screw strength and the screw will fail in tension. The tensile area of a #10 screw is considerably larger that a #8 screw so obviously the #10 screw is stronger for that failure mode as well.

All of this is easily calculated if the material properties are known. "6061" and "18-8" are not adequate to specify properties. However for the question asked where the materials are the same for both sizes a general answer as above is applicable.

Click to expand...

This is all true, certainly, and of course a larger screw root diameter will be stronger, but I guess I agree with you that there isn't enough info here.

My advice is based on my experience with coarse thread or poorly tapped holes in 6061 (assume T6) being easy to overtorque and almost 100% of the time having damaged threads after the first use, ESPECIALLY on coarse thread holes.

I think anything between 6-32 and 1/4-20 or 1/4-28 might be appropriate, but this would depend greatly on what the actual application is, which we really don't know. If it's a single screw in tension I would probably pick 10-24 or 1/4-20 or 28 because they will probably fail the aluminum (the threads, which is why you really should have something other than aluminum bear the threads) before the screw, but if there is more than one screw i would go with a smaller finer pitch because of the human factors and assembly since i've seen so many 10-32 and 10-24 holes in Al get stripped without much effort. It seems much harder even to strip out a 2-56 by overtightening it than a 10-24 on thin stuff.

Usually you would want to pick such that your fasteners and material would fail around the same time, especially for fatigue considerations. We really don't know enough except to say that he's probably in the right ballpark.

DaveE907 said:

See Also

Beginner's Guide To Common Wood Screw Dimensions (Explained)

All of this is easily calculated if the material properties are known.

Click to expand...

if only it worked that way in real life with shallow hand-tapped holes in aluminum. i would bet 100% variance in pullout in identical holes using identical materials but done in different shops with different tools by different operators.

"pem" nuts

get an on-line catalog
a host of mirical products for the pathetic strength of 6061

Phil

Do NOT use a 6-32 fastener for this application.

I AM willing to bet a neat 100 bucks that given the parameters discussed
(proper tapped hole in 1/4 inch thick 6061 material) that the stainless
fastener will fail under tension long before the threads give up in the
aluminum.

Jim

HuFlungDung said:

Jim,
I'll go one better and recommend do NOT use a 4-40 fastener for this application

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well I quess a 2-56 and an 0-80 are out of the question then

The center 2 rear cylinder studs on Yamaha FZ600s pull out of the case threads often. Well, used to when people rode 'em back in the days of 16" front wheels. The studs were threaded 8x1.25mm, and the hole had about 11mm of thread. The cases were cast aluminum, pretty soft stuff like 356 or something similar.

Once Helicoiled (or ReCoiled) they wouldn't pull out. Ever.

Helicoil or go as big and coarse as possible.

BOSTON said:

well I quess a 2-56 and an 0-80 are out of the question then

Click to expand...

Just use several dozen M1 screws and you should be cool.

Anybody really for sure think coarse? Or fine? 6061 T6, if T6 is what it is, is one of those materials that isn't really soft nor hard. I would go for a larger coarse or a smaller fine.

At the risk of being labelled a heretic, I have doubts about thread-forming taps in aluminum. My thoughts are that the stuff is barely cold formable in a press without inducing cracking. So, just because the cold forming goes on inside of a hole with a forming tap, I don't see that it's necessarily done anything more than mash the material around to the point that it is almost next to breaking out. Thus cold forming a coarse thread causes even more 'mashing', and the excess material extruded to form the thread crests inside the hole basically adds nothing to the thread strength, so all you really have for the strong part of the thread, begins from where the drilled hole diameter began. The portion of the thread that lies in the parent material around the hole should be good and strong as the original metal, but that's about it.

So I'd vote for a coarse thread in aluminum, but cut thread, not formed thread, because the tap drill hole is smaller for a cut thread.

You also need screws that are formed to full diameter, particularly for fine threads in aluminum. If the screws are a few thousandths undersize, then you'd be losing vital engagement of the threads deep into the threads in the hole. For example, I find that allen socket head capscrews are usually right up near the max diameter, but hardware store screws can be a bit on the loose side.

I too was taught fine threads for steel and coarse threads for aluminum and to drill for 80% thread rather than 70%.

Fine threads should never be used in aluminum.