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Anonymous Poster

Self taping screws in 304 sheet metal

04/26/2007 9:59 AM

Hi,

I have a question about using self tapping screws in 304 sheet metal. I originally tried to screw in a self tapper made of 304 stainless steel into 304 stainless steel sheet metal. Of course the screw quickly threaded. I then attempted to use a 316 self tapper into the 304 sheet metal to my supprise allthough harder than the 304 this also failed. I guess this was most likely due to the work hardening that was occuring during the tapping process. Can anyone suggest a material type or self tapping type to over come these problems?

Thanks in advance

roger

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#1

Re: Self taping screws in 304 sheet metal

04/27/2007 1:32 AM

As I recall, the "300" series of stainess-steel cannot be hardened appreciably. The fastener must be considerably harder than the material being tapped of course. How much? I really can't say, but one would think that a difference of approximately 30 points Rockwell C would be the norm. I am guessing here, and I am sure there is an industry standard for the hardness of self-tapping screws versus the metal (usually mild steel) being tapped.

The "300" series, as well as the other stainless series tends to be quite plastic in nature when worked due to the high content of nickel. It is the preferred series for machined parts, as it can be cut more easily than the other grades. Will it work-harden? Of yes, that is guaranteed. Will it seize against a similar material? That is guaranteed as well. May I suggest obtaining fasteners made from the "400" series of stainless steels that can be hardened with good success?

I think that is where the solution lies. Will the slight differences in chemical composition between the "300" grade and the "400" grade cause a problem? I don't think so, but of course I do not know the application. Surely someone else here can recommend not only a fastener material and hardness, but perhaps a supplier as well.

Good Luck,

Ing. Robert Forbus

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#2

Re: Self taping screws in 304 sheet metal

04/27/2007 5:54 AM

make sure you use quality self tapping screws! the cheap type are mass produced and the cutting edge is not very good. Had much the same problem only to find that the packet of 500 I bought, maybe 50% worked! the ones with the hex head seem to be the best due to the fact that when you use them, the socket holds them better and you canĀ“t stab the screwdriver point through the fleshy part of your hand! (It really hurts as you have probably found out!)

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#3

Re: Self taping screws in 304 sheet metal

04/27/2007 6:11 AM

We do this quite regularly. What we have is a box of passivated zinc self tappers, run a zinc one in to cut the thread, then put in the ST/ST screw. We use a power tool for the zinc screw, but be advised - do the stainless screw by hand. You have been warned! (I know this is not a high tech engineering solution, but in the production process it works well.)

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#4

Re: Self taping screws in 304 sheet metal

04/27/2007 6:56 AM

Look at a US company called TexTron fastening systems they make a extremely good range of self tapping and self drilling screws for applications in plate upto 1/2" Thk. Thread rolling fasteners might be more suitable for you. If anything they are a good reference base to work from.

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Anonymous Poster
#5

Re: Self taping screws in 304 sheet metal

04/27/2007 11:28 AM

300 series will not heat harden but will work/cold harden.

Part of your problem is 300 series are "sticky". When they cut/thread with other metal, they'll fuse together. Threading 304 parts together will damage the thread and result in almost permanent join.

Use a harden screw. If you need to use stainless then look for harden 400 series screw.

If need to use 304 screw, pre-drill the parts first.


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#6

Re: Self taping screws in 304 sheet metal

04/28/2007 8:15 AM

Using a pilot hole is probably your best solution. Also try a drop of quality cutting oil. I found that there are a lot of cheap screws out there, and the length is also important.

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