What screws to use.

[skye16]

Hi
I want screw a piece of wood to a metal strip. The wood is 20mm thick. The metal strip is about 2 or 3mm thick. I can't get behind the metal strip, to use nuts and bolts. The gap behind the metal strip is about 15mm.
Please could you tell me what sort of screws I should be using? It would be better if they were rustproof.
Thanks.

 

[rsgaz]

For 'rustproof', they need to be stainless, zinc plated (BZP) only last so long outside.

If you drill the right size pilot, any normal self-tapping stainless screws will work.

"Self drilling" screws are available, but not sold in as many head types/lengths/finishes/etc, so might be impossible to find exact ones to suit your dimensions.

If it needs to be very strong also, then drilling and tapping the hole first is the best option IMHO(without going overboard with rivnuts, etc). That way you are then free to use whatever machine screws you like in terms of head style, etc.

 

[foxhole]

Hi
I want screw a piece of wood to a metal strip. The wood is 20mm thick. The metal strip is about 2 or 3mm thick. I can't get behind the metal strip, to use nuts and bolts. The gap behind the metal strip is about 15mm.
Please could you tell me what sort of screws I should be using? It would be better if they were rustproof.
Thanks.

Would be more helpful to tell us what you are trying to achieve.?

 

[endecotp]

https://www.screwfix.com/p/easydrive-countersunk-head-self-tapping-screws-8ga-x-1-100-pack/7496h

 

[skye16]

Thanks for the advice on self tapping screws. I had never heard of them before. And thanks for the Screwfix link.
I only need half a dozen screws. Can you tell me what drill bit size I would need for drilling a hole in the metal, if I get no. 8 screws?
Thanks

 

[rsgaz]

what drill bit size I would need for drilling a hole in the metal

What metal is it?

 

[skye16]

I'm not sure. I think it is just painted steel. Does it make a difference?

 

[rsgaz]

Use a 3.5mm HSS drill then. If it was something softer like aluminium, I would have said 3.0mm.

 

[skye16]

I suppose what I was thinking is it a smaller hole that the screw size, or is it a size 8 drill bit, for the self tapping no. 8 screw?
Thanks.

 

[skye16]

Thanks rsgaz. I think I posted just as you did. I'll see if I can get a small amount of screws and drill bit from ebay.

 

[rsgaz]

is it a smaller hole that the screw size

Yes, a number 8 screw has a nominal size of 4.17mm, but they usually measure about 4.05 to 4.10.

 

[^woody^]

I'm not sure. I think it is just painted steel. Does it make a difference?

You wont need rust-proof screws if drilling into ferrous metal.

 

[Roger928]

You wont need rust-proof screws if drilling into ferrous metal

So a screw driven into an aluminum roof will never rust?

 

[^woody^]

So a screw driven into an aluminum roof will never rust?

Does this hypothetical screw contain iron, and is this hypothetical roof on Mars?

 

[Roger928]

Does this hypothetical screw contain iron, and is this hypothetical roof on Mars?

Roof is on earth. Screw contains metal.
You say it won't rust? I say it will.