Bolt material / AL die casting

[NewTechie]

Guys,
I need some help in choosing right bolt material.
I have a 1/4-20 tapped hole on an AL die casting. the vendor screws a stainless steel bolt in that hole and ships to us.

We wanted to check the lenghth of the bolt, so we tried to remove the bolt out. The bolt actually stripped the threads.

I am not sure if the vendor is not tapped the holes right or the Bolt material is too hard for the application.

any help is appriciated.
Thanks
NT

 

[KENAT]

From my limited experience stainless fasteners will sometimes seem to gall in aluminum.

http://www.eng-tips.com/viewthread.cfm?qid=217587

I forget how good the porosity etc. of die cast Al parts is but tapping into castings can sometimes be an issue.

How about looking at a helical insert of some kind in to the aluminum part, especially if it's getting given any significant torque/load.

Another option might be some kind of antiseize/anti galling but I'd lean to an insert.

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[Tmoose]

Were the aluminum threads galled/welded/stuck to the bolt, as near the end of this bolt?

http://www.stu-offroad.com/suspension/arm1/bolt-3.jpg

 

[drawoh]

NewTechie,

I agree with KENAT. It is policy in many organizations to systematically use thread inserts in aluminium to prevent thread galling and stripping. Helicoil inserts are Keenserts both work as thread repair kits, so you might as well order something now!

Anodizing your aluminium parts will reduce, although not eliminate, your thread galling problem.

--
JHG

 

[hydtools]

Do you need the ss screw? The ss screw is not too hard, it has a tendency to gall threads. We use carbon steel or alloy steel screws in cast Al parts, sand and die cast.

Ted

 

[Screwman1]

I agree with the general thoughts of the group: you had galling: it is common with AL die cast and SS screws. One way to reduce it is to use some sort of thread lubricant (wax or torque/ tension fluid). The fastener supplier can supply the screws with the dry lube preapplied; it is very common.

 

[NewTechie]

Thanks for the input from you guys. Appricite it.
Inserts? Will they not pop out?
I am thinking a Blind thread 1/4-20 1.0IN deep. are there inserts avilable for this that strong and wont pop out.
I am planning on using 200-250lbs load on it.

Secondly, I suspect that the tapping might be a culprit too. May be the bevndor isnt consistently using all 3 taps (first second & bottoming taps). But, hard to control this.

Lastly, I like the idea of Lubricant on screws.

Thanks again
NT

 

[Screwman1]

On the other hand; just have him buy some SS Taptite screws with wax and run those in. No worries about loosening (as long as you have 2.5 dia, of thread engagment).

 

[racookpe1978]

It is not just stainless steel screws galling in an aluminum casting with female threads.

I just lost two days on a outage because stainless steel pipe threads (3 inch NPT pipe) galled badly in the IR compressor cast aluminum nozzle.

 

[KENAT]

If you choose the correct type of insert and they are installed properly then no inserts shouldn't be particularly prone to 'popping out'.

You don't say what the load on the screw is, you don't say what type of environment the part is subject to etc.

However, inserts have advantages over just using anti-seize in a tapped cast part for strength and potentially durability reasons. They effectively create a 'larger' thread in the cast aluminum part which can be beneficial for strength.

http://www.helicoil.in/pdf/HeliCoil Catalogue.pdf

only appear to go up to .75 deep but that may be more than enough. How much load is going on your bolt that you need 4X major diameter depth?

We used helicoil inserts in cast & forged (as well as machined) aluminum airborne parts routinely at a former aerospace/defense employer.

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[KENAT]

Doh, you do say the load, sorry just not if that's the tensile or sheer load (I'd guess tensile). Anyway the helicoil data sheet has some info.

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[tbuelna]

1/4-20 UNC threads to 1" deep? That's an L/D ratio of 4. For coarse threads like 1/4-20, you should not exceed an L/D of around 1.0 to 1.5. The galling or interference in the threads was likely due to excessive pitch tolerance accumulation with the large amount of axial engagement.