Self tapping screws in 304 stainless

[bsr]

Hi

I need to make 12x holes of diameter 4mm in 25x3mm 304 stainless steel strip. It's the central steel support in a bench.

Can I use self drilling screws in this steel? I've read they're only suitable for mild.

Are they ok to use with stainless? The strip seems very soft, as easy to bend as mild steel.

Thanks

 

[just pumps]

Any self tapping screw needs to be harder than the material they screw in to.

 

[RandomGrinch]

Hi,

Do you mean self-drilling, or self-tapping?
...and sorry if I'm being silly, but I don't quite understand what you want to do!
Do you want to use the screws to make the 12 holes instead of drilling them?

There are specialist self drilling bi-metal screws; stainless screws with a hardened carbon-steel tip:
https://www.orbitalfasteners.co.uk/categories/pancake-head-self-drilling-screws-bi-metal
They will drill well into stainless, but are on the pricey side!

 

[Deleted member 174758]

Self drilling are OK for thin (1 to 2 mm) steel, but the thicker the steel gets the more likely that they won't work and just end up burning the tips out or snapping off before they are fully in

 

[bsr]

@RandomGrinch thanks, that's exactly what I want. To fix 3mm stainless steel to 22mm wood. Expensive though! Says they're suitable for up to 3.5mm steel.

@JobAndKnock what about the long ones for drilling through RSJs? Do you not rate them? For example these are rated for 12.5mm steel. I've never used them though.

1. https://www.orbitalfasteners.co.uk/products/55x85-csk-timber-tek-5-40-1250mm-bi-metal-a2

 

[bernardgreen]

RSJs

are seldom made from stainless steel

If the wood is to be pulled tight to the metal by the screw then the screw has to be a clearance fit through the steel.

 

[Deleted member 174758]

The issue is thickness, Self drillers work well enough in thinner metals, but when you start to drill more than say about 3mm (RSJs, stainless steel, etc) you can and will blunt the tips and snap some unless you pre-pilot for them

what about the long ones for drilling through RSJs? Do you not rate them? For example these are rated for 12.5mm steel. I've never used them though.

1. https://www.orbitalfasteners.co.uk/products/55x85-csk-timber-tek-5-40-1250mm-bi-metal-a2

I've had three projects in the last 5 years where I've had to drill multiples of holes in thick wall RSJs and box section beams (5 to 10mm wall thickness) and one project where I had to fix through oak sections into a welded stainless steel framework (about 5mm thick walls). In all instances I've used Hilti self-drilling fasteners (what the firm supply), probably about 20 boxes in total (or 1000 fastenings) over the last 5 years, and the results were the same every time - if I pre-piloted the steel I got a fix first time probably 90+% of the time, whereas if I tried to use the self-driller to form the hole I got a percentage which just burned out the screw (maybe 20+% on rolled mild steel, less on the the stainless) and a percentage which jammed in the hole and twisted, then snapped off like a carrot (more on the stainless than the mild steel). Swapping from a conventional impact driver or drill to a torque sensing impact driver (the Makita DTD171 which has a T-drilling setting to handle this type of screw) made some improvement, but I just found it slower to drill and screw in one shot than it was to pilot drill and then screw in. When I had problems on a couple of occassions I talked to the Hilti technical rep and the answer was the same both times - if drill/driving is burning up the tips, or you are experiencing jamming when thread cutting, swap to piloting the hole first because you'll get less failures. And at Hilti prices that matters. Hence my comments

 

[bsr]

4mm cobalt bit it is! Thanks for your help.