steel bolt and thread in aluminium

The weakest steel exceeds the strongest aluminum in tensile strength, except for some exotic alloys where they are about the same.

According to Camilo Ramos :

It is a function of how long a thread engagement you have. If the aluminum tapped hole is about the same length as the diameter of the steel bolt, you are probably pretty close. But if you have several time the bolt diameter in length of threaded hole you can perhaps break the bolt in tension before you strip out the threads -- even possibly with grade eight bolts.

Also -- it depends on how the threads were made in the aluminum. Normally tapped threads in aluminum are weaker than those made with a form (roll) tap.

Good Luck, DoN.

It all depends on the type of aluminum alloy and its heat treatment, and the length of thread engagement.

A good rule of thumb is a minimum of 3 bolt diameter of thread engagement in aluminum to develop the strength of the bolt, ie. the bolt will break before the thread will strip.

For example look at a modern die-cast engine block with the cylinder head bolts. Roughly 4 or more dia. of thread engagement and the 10mm bolts are tightened to approx. 40 foot-lb of torque. (numbers from memory).

Wolfgang

I don't think so!

7075 T6 alloy can have a tensile strength of over 80,000 psi. A36 structural steel may have a tensile strength of roughly 65,000 psi.

Wolfgang

Grade 2 is minimum 74 kpsi up to 3/4 inch diameter.

Like I said, about the same.

If I recall correctly you are right if the threaded length in the aluminum is less than 1/2 the bolt diameter. Look at some nuts. The thickness of a nut is less than the bolt diameter, even in Heavy nuts. Also note that nuts do not come in grades similar to bolts. Yes you can get high strength nuts, but generally a plain steel Heavy nut is all you need with a grade 8 bolt.

Dan

Dan

Rule of thumb for thread engagement in steel is 1 bolt diameter.

You are right on nuts; A 1" dia. A325 (grade 5 equiv.) bolt has a nut height of 63/64". The bolt head height is only 39/64".

Garden variety stuff is less.

Wolfgang

Not correct. For the reasons stated by the other posters and also as all of the bolt falures I have seen, not that I've seen enough to be a significant sample, have resulted in the bolt being sheared. The the engaged part of the bolt was left in the hole with the threads undamaged, such that the stub was easily backed out of the hole.

starbolin

Rule of thumb I was taught was that the minimum engagement for full strength was four threads in steel, six in aluminum. This is a little closer to what you will find in nuts.

Jerry

I made sure to tap the holes by at least 3 bolt diameters deep.

My taps all are common cutting taps. Oddly enough, the threads came out somewhat ragged. I used the same tap in ZA-12 and the threads there look nice. Is there something special to tapping aluminium?

Regards,

Camilo Ramos

I guess that depends on what rackets we were in, right?

Mine was heavy engineering and fabrication.

The rules I stated have stood well by me. The info I gave on the A325 bolt head & nut height stem from a STELCO publication (large Canadian steel mill).

The one instance I know of where the rule-of-thumb was mis-interpreted by the shop people ended in the wholesale stripping of 3" & 4" dia. studs. (The reasons for the 4140 studs stripping are complex and I don't wish to elaborate).

"Four threads in steel and 6 in aluminum" is insufficient for standard thread pitches: A 4" dia. x 4TPI stud would strip a threaded hole very quickly with 4 threads of engagement, as would most, if not all UNC and UNF scews.

If you cannot design a bolted joint it is best to stick to PROVEN rules of thumb.

Wolfgang

OK, admittedly, I was dealing with mostly 1/4 in. diameter and smaller. For years, for no good reason other than it seemed reasonable, I used the rule of thumb of one diameter. Then the company hired a duly trained and certified mechanical engineer and he more or less beat into me the four threads rule...

Jerry

O.K. Then you are possibly deep enough so the failure mode would be the bolt snapping in tension rather than the threads stripping out of the aluminum. I would feel more comfortable with four, but from what has appeared here, three is probably sufficient.

[ ... ]

Some aluminum alloys tend to do that -- and that will weaken your threads in the aluminum.

What lubricant are you using when tapping the aluminum? You can even get better results with WD-40 to lube the tap in the aluminum. There are also waxes for the purpose (you rub the tap in the wax before going into the hole). There is a TapMagic fluid for aluminum, as well as a general purpose one.

And what aluminum alloy are you dealing with? The ragged threads suggest that it is one of the softer alloys, and your threads are tearing. For that, the thread forming tap would certainly be a better choice. (Be warned that it will need a different tap drill size (slightly larger, and likely to be a metric size instead of an imperial size for imperial threads, than that for the usual thread cutting tap.)

Best of luck, DoN.

Well, it so happens that a 1/4-20UNC bolt would have an engagement of

1/20" x 4 = .2" by your rule, which, depending on application, may suffice.

For ordinary bolts nut height for 1/4" bolts is .88 x .25 = .22", for which 4 threads is probably close enough in non-critical applications, or if the threads are in a tough and ductile steel.

For a 3/4" UNC or UNF bolt, for example, 4 threads of engagement is insufficient to develop a tensile load capacity anywhere near the strength of the bolt.

It's that .88 factor on which the 1 diameter rule of thumb is based. This rule also covers the requirements for nuts used on A325 and A490 bolts, or grades 5 and 8 respectively. (For completeness let me point out that you should not substitute a grade 5 bolt when an A325 bolt is specified, because there may be subtle differences, but proof strength is the same IIRC).

Wolfgang

It's been a long time, but Ithink the rule of thumb we used to use on satellite hardware was to thread at least 1.5 times the screw diameter.

3 diameters should certianly be very safe.

Doug White

In fact I used very little cutting oil, as the holes are blind and clearing the chips off them is difficult.

And what aluminum alloy are you dealing with? The ragged

Homebrew alloy from a lot of stuff I threw into the crucible. Anyway after pouring the piece I left it alone until next day, on the idea that slow cooling promotes Al hardening (?).

Regards,

Camilo Ramos

In that case, a thread forming tap is the way to go. That would leave *zero* chips. But you will want to use a threading lube on that tap. A first through suggests a wax, given the nature of your alloy. Castrol makes a wax for the purpose in tubes somewhat like what would go in a larger grease gun. Just stick the end of the tap into the wax before making your threading pass.

Of course a spiral tap can pull the chips out better than a straight tap, but I would go for the thread forming tap, myself.

Yep -- unknown alloy, and one which results in tearing when you tap it, so go for the thread forming taps. (Be sure to look up the proper diameter tap drill for that.) Check the maker's page, or there is a table in _Machinery's Handbook_.

Of course -- it appears that you are living in Columbia, and I'm not sure how easy any of these things would be to acquire.

I believe that if you want good casting alloys for aluminum, go with melting down things which were *originally* cast, such as old automotive pistons (which are cast before being machined to final size and shape).

Things like beverage cans are a terrible alloy for any machining. They are designed specifically for deep drawing, instead.

Good Luck, DoN.

One trick I learned many, many moons ago: when tapping deep blind holes, put into the holes a drop or two of appropriate cutting fluid, and then fill the holes with bees wax.

Worked like a charm even on stainless steel. It was fascinating to see the displaced wax carry the swarf out through the flutes.

Who was it that said that simple minds settle for simple pleasures....?

Wolfgang

Well I don't do so much tapping that I can really justify buying roll taps. BTW, time ago I bought a cheapie set of chinese taps at a sale. Their edges are somewhat dull, take a drill size larger than good cutting taps do, need a lot of force to turn and produce less chips. Now that I think it over they might substitute for roll taps in a pinch! :)

I believe that if you want good casting alloys for aluminum, go

This particular piece (for the Gingery mill BTW) is temporary. For the remaining I used either piston heads or ZA-12.

Good Luck,> DoN.