Cast aluminum thread suggestion 6

[isilva]

Hi; I am trying to reduce the cost of a baseplate by removing the thread inserts and having the baseplate made of cast aluminum or impact die casting process and make the threads onto the baseplate. I am trying to calculate what is the minimum baseplate thickness which will be strong enough to be used with M3 and 4-40 screws?

Thank you;
Isaac Silva
Mechanical/Manufacturing Engineer

 

[KENAT]

Do you know what your loading is?

Do you have the strength of the material?

I believe there are the calculations for thread pull out loads on this site somewhere, maybe in a FAQ.

You could take a look at

http://www.boltplanet.com

which has an online calculator but blindly using this without understanding the math behind it may be inadvisable.

I'd expect casting threads that small would be challenging to say the least, or are you going to machine the threads into a casting? This may still be problematic in the event of porosity or other faults in the area to be tapped.

Also consider some of the other issues aluminum threads sometimes have such as galling etc. I've had problems with #4-40 threads in machined Aluminum with Stainless fasteners.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

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

 

[hydtools]

Use a plain hole and a thread forming screw.

Ted

 

[CoryPad]

Here is one thread that addresses required thread engagement:

thread725-35222



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[IRstuff]

The inserts aren't necessarily there for strength, alone. Aluminum tends to gall, so anything that might be disassembled and reassembled needs to be carefully considered before removing thread inserts.

TTFN

FAQ731-376

 

[isilva]

Thank you all for the replies and comments. The baseplate has to be really thin, we are talking about 040”, but on the corners I could go up to 080 thick; where I would put holes and machine the threads. The thread engagement for the customer baseplate could be up to 080 not more as if goes more than that, it will damage components and we also put epoxy to prevent that. I did some destructive tests here tapping various types of cast aluminum and regular 6xxx aluminum alloy plates of 040, 060 and 080 thick. Using a torque wrench they all could withstand torques up to 5inlb.As I started to increase the torque until failure, I started to see some of the threads of the 040 plates to fail, then 060 and 080. With the 080 6xxx plates, all the screws failed before the thread, but with the 080 cast aluminum 380 plates, was about 75% of failure of the screws, all Steel screws failed, pan and flat head screws. The torque and SS hex screws striped the plate thread. So I am trying to calculate the minimum thread engagement (plate thickness) capable of withstand these stronger screws, as we are also trying to avoid having to rely on customers, having to specify max torque limits.

 

[GregLocock]

Are you trying to cast the female thread in, or are you happy to tap the thread?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[isilva]

The baseplate will be cast and the thread will be tap, 4-40 2B some models and M3 others

 

[SincoTC]

Isilva

Is the original baseplate with thread inserts a casting or is it sheet metal?

I'm thinking that the 040" thickness that you require is pretty thin for a casting (depending on overall size), It maybe OK with a pressure die-casting, but the tooling for that would be costly and your'e talking about eliminating thread inserts to cut the cost of the existing baseplate.

We cut M2.5 and 4-40 threads in heat treated LM25 Al alloy; the material is unsuitable for thread forming and the threads are too small for thread milling. Because it is so short chipping, the biggest danger is that the dusty swarf gets jammed in the relief angle on the cutting edges as the tap is backed out, so they have to be done carefully by hand, a horrible job and only acceptable because of the relatively low numbers required!

If you have the luxury of numbers them maybe a pressure die casting with plain holes and selt tapping screws is the way to go, but if they need to be dismantled regularly then choose an alloy that is ductile enough to be tread rolled and use regular screws.

Good luck

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2009x32 SP1.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.11.7751

 

[HDS]

Inserts are usually cheaper than taping. Especially when you consider the cost of taps.
Maybe you need a better insert.

 

[MotorVib]

I have tapped in to cast AL on a number of projects and the only problem that I have see was for dissimilar metals. (Steel Bolt, Al Casting) When I have tapped in to castings in the past 2 time the bolt dia work well, but that was in 300 series castings.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[isilva]

The original plate is stamped, machined, tumbled and then pressed w/ PEM inserts. I got quotes from die cast and impact die cast for the same plates and the price drops to 1/3~1/2 of the original plate. But again I am not sure about the thread on the cast aluminum. In theory with the calculation that I made (Thanks Kenat and CoryPad) a thread on an 84mil thick aluminum cast 380 plate would not fail an over torque of a SS screw; therefore that did not happen at the lab. The thread failed and I cannot really have the baseplate at the mounting locations, thicker than that.

 

[SincoTC]

I was going to ask if you had considered laser cutting the profile from .040" in a reasonably ductile alloy, and then flodrill for the fasteners to give the increased thickness for thread length and improved grain structure for a roll formed thread.

see:

http://www.flowdrill.com/english/introduction.htm

However, when you say the original part's first operation is stamping, I wonder if it's not just for the shape and that some strengthening features are pressed in, so that it isn't just a flat plate with tapped holes in, which would rule out this method.



Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2009x32 SP1.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.11.7751