Gallery, Projects and General > The Design Shop
Strength when threading into Aluminium
Lew_Merrick_PE:
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.
picclock:
@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
Davo J:
--- Quote from: BillTodd on April 08, 2013, 10:58:25 AM ---
--- Quote ---Thats 16 threads, you would break the thread rod pulling those threads out of it.
--- End quote ---
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
--- End quote ---
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
BillTodd:
--- 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.
--- End quote ---
Happy to bow to your superior knowledge Lew :)
Lew_Merrick_PE:
--- Quote from: Davo J on April 09, 2013, 04:24:11 AM ---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.
--- End quote ---
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.
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