Author Topic: Strength when threading into Aluminium  (Read 4950 times)

Offline picclock

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Strength when threading into Aluminium
« on: April 08, 2013, 05:31:43 AM »
I am using some 6mm Steel allthread into an aluminium block. The minimal allthread strength is 1000 lbs yield, about 6x my working load. However I am a bit unsure as to the depth of thread needed to transfer the load without fear of stripping the thread. I know that conventional wisdom is 4 threads, but I have always taken this to be with similar materials, and I normally allow double that if the design will tolerate it.

So the question - How is the number of threads required calculated when threading into different materials ?

For this particular case I have a workaround but the information would be useful to know.

Many Thanks

picclock
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Offline Davo J

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Re: Strength when threading into Aluminium
« Reply #1 on: April 08, 2013, 08:51:44 AM »
I have no idea of a chart etc, but I would be going 6 at least. You can always drill and tap it bigger and use helicoils into it. They come in all sorts like stainless, brass etc and you locktite them in, so you then have a good strong thread.

Dave

Offline BillTodd

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Bill

Offline picclock

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Re: Strength when threading into Aluminium
« Reply #3 on: April 08, 2013, 10:34:37 AM »
@ DaveJ
The current design allows up to 16mm for threading, and I can also mill out a recess for a steel flanged nut at the cost of 6mm depth. However, although I feel fairly safe with my 16mm (16 threads in M6), it would be nice to have a more informed answer as to how many threads are really necessary.

@ BillTodd
I looked up the link you posted and whilst it has a lot of information the mixed metals threading issue doesn't seem to be covered, if I've somehow missed it a pointer would be welcome !

Best Regards

picclock
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Offline Davo J

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Re: Strength when threading into Aluminium
« Reply #4 on: April 08, 2013, 10:37:18 AM »
Thats 16 threads, you would break the thread rod pulling those threads out of it.

Dave

Offline BillTodd

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Re: Strength when threading into Aluminium
« Reply #5 on: April 08, 2013, 10:58:25 AM »
Quote
Thats 16 threads, you would break the thread rod pulling those threads out of it.
Dave's absolutely right;

Your 'allthread' is likely to be the weakest link. The  thread rolling process creates micro fractures as the material stretches
and deforms (longer the thread more chance of failure).

If you want a strong stud use a rod with a cut thread at each end
Bill

Offline picclock

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Re: Strength when threading into Aluminium
« Reply #6 on: April 08, 2013, 11:13:02 AM »
@ DaveJ, BillTodd

Thats why I said I would feel fairly safe with 16 threads. But really the question is about how many threads are actually needed. With similar materials the 6 thread +2 option in the weblink is pretty much a nobrainer, and lots of information exists like that. But when you combine two metals with such differing characteristics and yield strengths the answer is is far less clear cut.

In a moment of (dubious) :Doh: thought I wondered if the ratio of yield strengths might give the correct answer. Typically the yield ratio for naff steel/Aluminium is around 1:3 so the implication is that 18 threads would be the point at which they would start to let go, assuming 6 threads for steel/steel. 

Many thanks for your thoughts

Best Regards

picclock
Engaged in the art of turning large pieces of useful material into ever smaller pieces of (s)crap. (Ferndown, Dorset)

Offline andyf

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Re: Strength when threading into Aluminium
« Reply #7 on: April 08, 2013, 11:13:21 AM »
You only need worry about the weaker of the two - aluminium, in this case. I once read a rule of thumb somewhere to the effect that the thread engagement in Al should be one and a half times the major diameter of the thread. I can't cite any authority for that, and suppose that it depends on whether it's pure Al or a stronger alloy.

In your M6 appilication, it would mean 9mm, and thus 9 threads.

Andy
Sale, Cheshire
I've cut the end off it twice, but it's still too short

Offline David Jupp

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Re: Strength when threading into Aluminium
« Reply #8 on: April 08, 2013, 12:03:39 PM »
The  thread rolling process creates micro fractures as the material stretches
and deforms (longer the thread more chance of failure).

If you want a strong stud use a rod with a cut thread at each end

I have to disagree on that - good quality rolled threads are considerably stronger than cut threads (especially if fatigue is an issue).  That doesn't necessarily go for "cheap 'n' cheerful" hardware store threaded rod.

High Tensile bolts are thread rolled as much for strength as for cost reasons.

Offline vtsteam

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Re: Strength when threading into Aluminium
« Reply #9 on: April 08, 2013, 12:33:13 PM »
The 6 threads rule seems a little odd, since it would seem to equate 6 threads for a 28 TPI screw with 6 threads for a 14 TPI screw -- the depth of engagement would be double for the coarser screw and the thread lands twice as wide, assuming they were applied to the same diameter screw.


I love it when a Plan B comes together!
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Offline Lew_Merrick_PE

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Re: Strength when threading into Aluminium
« Reply #10 on: April 08, 2013, 12:51:14 PM »
Picclock,

The strength in tension of male screwthreads in a female tapped hole is governed by: (1) the shear strength of the material; (2) the total tolerance & allowance at the pitch diameter; and (3) the total tolerance & allowance at the minor diameter.  I have tabulated such factors for Unified National threads (#0 through 1-1/2 inch).  I have not performed this task for metric threads as there are still five separate and irreconcilable "standards" for allowances & tolerances across the American ISO, French ISO, British Standard, DIN, and JIC realms.

Now, having said that, an M6(X1) thread is roughly equivalent to a 1/4-28UNF thread.  Assuming a (hand tapped) Class 1 thread equivalent, then a 1/4-28UNF thread has .4474 inē/in of engagement as a nut (i.e. in the aluminum).  If you know the type of aluminum being used, I can look up the shear strength allowance.  Your pull-out strength would be depth of thread * (.4474) * shear strength of material with units being inches and pounds.

Bill Todd -- Properly performed thread rolling increases thread strength by nearly 35% when compared with thread cutting.  This is an area where I have done a lot of work for NASA and the USAF over the years.  The tearing you refer to only occurs when either an improper sizing before rolling/forming is used or insufficient lubrication is used in the rolling/ forming operation.

Offline picclock

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Re: Strength when threading into Aluminium
« Reply #11 on: April 09, 2013, 04:04:57 AM »
@Lew_Merrick_PE

Thank you for your very illuminating and informative reply.

The aluminium claims to be 6082, although I have no way of verifying this. My experience of the metal types written on the end of stock is that this is a very hit and miss affair, very annoying when you order a grade for good machinability and you end up with something far from that. Generally, if I make something I will always assume the materials are of the lowest grade though they will likely be better.    According to google the shear is around 30,000psi, so 0.7inches of thread will give an astonishing 9,400 lbs. Working it backwards only 1.9mm would be required to equal the 1000 lb studding estimate.

I did the metric calculation of metal engagement with an M6 thread and the 0.4474 figure was pretty spot on (I made it 0.4867 - 393mmx0.8mm - allowing 80% thread engagement).

Many thanks for your time and your willingness to share this knowledge.

Best Regards

picclock
 
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Offline Davo J

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Re: Strength when threading into Aluminium
« Reply #12 on: April 09, 2013, 04:24:11 AM »
Quote
Thats 16 threads, you would break the thread rod pulling those threads out of it.
Dave's absolutely right;

Your 'allthread' is likely to be the weakest link. The  thread rolling process creates micro fractures as the material stretches
and deforms (longer the thread more chance of failure).

If you want a strong stud use a rod with a cut thread at each end

If where talking about the same thread rod, I know the hardware stuff and similar over here can be snapped with a bend or 2, and the threads strip so easy, absolutely crap.
We just tried the other night to put a bearing out of my sons bike using a bit and it jambed the nut because the threads let go.

Dave

Offline BillTodd

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Re: Strength when threading into Aluminium
« Reply #13 on: April 09, 2013, 07:20:16 AM »
Quote
Bill Todd -- Properly performed thread rolling increases thread strength by nearly 35% when compared with thread cutting.  This is an area where I have done a lot of work for NASA and the USAF over the years.  The tearing you refer to only occurs when either an improper sizing before rolling/forming is used or insufficient lubrication is used in the rolling/ forming operation.

Happy to bow to your superior knowledge Lew :) 



Bill

Offline Lew_Merrick_PE

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Re: Strength when threading into Aluminium
« Reply #14 on: April 09, 2013, 12:51:12 PM »
If where talking about the same thread rod, I know the hardware stuff and similar over here can be snapped with a bend or 2, and the threads strip so easy, absolutely crap.

We just tried the other night to put a bearing out of my sons bike using a bit and it jambed the nut because the threads let go.

Let us start with one agreement, crap is crap.  My local big box hardware store sells uncertified, ungraded HRS bar for $6.50/lb and CRS bar for $12.00/lb.  These are materials I buy from reputable steel yards with mill certifications for alloy & condition for $0.80/lb and $1.45/lb respectively.  The same local big box hardware store sells uncertified socket head cap screws (supposedly alloy steel) for upwards of $0.70 each.  I purchase such screws with manufacturer's material and testing certificates for $7/100 to as much as $15/100 in common small (i.e. <.500 inch) sizes.

I honestly cannot remember the last time I bought retail hardware store threaded rod.  I normally purchase such stock from my local (certified) screw supplier with manufacturer's material and testing certificates in 6 foot lengths.  I use tiewire tags to ID such stock to the appropriate certificate because about 40% of the things I build end up going for military, aerospace, or medical usages.  That is the world I live & work in and, if you want to play in that sandbox, that is the way things are done.

Please understand that I am not denigrating anybody for dealing with mystery metal.  I have done my fair share of that over the years.  However, most reputable steel yards and (certified) screw suppliers will sell to the general public for a small premium (and, in recent years, a Saturday surcharge for entry into their drops area) -- at least here in the U.S. and Canada.  Even with such premiums, the price is a lot less than retail hardware stores charge for such things.  Now, mind you, I have relations with most of my suppliers that date to the 1960's, so I fall in the old friends category in such dealings.  (I also boost good suppliers to my design & development customers at every opportunity -- and they know it!)

As my final observation on this subject, if you want a stud to project accurately, you need at least 1.5X the major diameter in bearing to hope for an accurate projection.