Self tapping sheet metal screws

Hi All,

Not really a model engineering question but... what are peoples opinion on self tapping screws for sheet metal - specifically the screw material?

My experience is that SS screws are pretty much useless - make the hole too small and they will bind and rip the head or shank off, make the hole correct and it just strips the entire thread, make the hole slightly oversize and it blunts the thread and leaves the screw loose it the hole. Whereas I don't experience this as often with BZP screws.

I would like to know if other people's experiences are the same and, if so, if anyone could offer a technical answer to why. (-10 points to the first person who offers the "Chinese Chocolate" trope  

Thanks

Can you not use spire nuts or some other type of cage nuts.?

regards Martin

Have you looked at self drilling screws for sheet metal, vast array of types and sizes.

Just an example here.... **LINK**

BZP screws when hardened have a strength of 1200 N/mm^2 minimum. Stainless, if A2 or A4 type has a strength of 800N/mm^2 at best, and possibly down to the 450 N/mm^2 mark depending on the amount of work hardening and temp at which it was work hardened (some are done at high temp to soften the steel, which reduces the amount of work hardening possible). Thus they are considerably weaker and more likely to strip the thread.

The other problem is the wretched material has a habit of micro welding (galling) under pressure which produces a very effective weld just where & when you don't want it. Having welded itself to the substrate, something else breaks. And frequently, under sod's law, so does the strong weld that caused the 'something else' to break.

So, yes I have experienced it & always try to use BZP. Then slather with paint/goo to stop rusting if outside.

Regards,

Richard.

The Metric/European method of grading nuts and bolts strength is rather confusing, the American system of grading 1-8 with 8 being the heaviest duty use (industrial/farming machinery typically use these), is much simpler and easier to understand. They also have little dashes around the head of the bolt to indicate which grade to the trained eye. 

An easy way to get a decent strength screw though is just to look for the high tensile self colour screws. 

Michael W

Edited By Michael-w on 14/02/2018 15:52:33

I find if I need st/steel screws in the finished item I use a normal self tapper first, well, untill there's a good start then put the st/steel screws in to finish......if that helps ??

clogs

Same trick with brass screws in wood - cut the thread with a steel screw first.

It always helps to lubricate self tapping screws I always use RDT tapping grease and if putting brass screws it to wood a bit of wax.

David

Hi Everyone,

Just looking at that supplier of self tapping screws,and I see that they also sell hand files. Has any other Member bought any of these files and what are they like,quality wise. I need to replace most of my files,something to do with "Old Age",like myself.

Larry, ARCEurotrade sell files that are really good quality for the price, I am in the same boat with most of my files now being well past their use by date.

Dave W

Excuse me, but what are BZP screws? Is it a type, a make or a material and are they self tapping? Just curious. ...

They used self drilling screws into RSJ’s at a build where I used to work but they were using dedicated drivers for the job, not plain battery drills.

BZP is Bright Zinc Plated. Just basic, cheap fasteners.

Thanks for the explanation, Nicholas.

For self tapping screws in sheet metal that need to be removed and replaced often, aircraft inspection panels, spring steel nuts called Tinnerman Nuts are used, these come in a number of types, riveted on, clipped on or loose. My memory of self tapping screws is the number of Philips screw drivers I wore out until I found a brand the retained it's point. The screws we used back in the 60s were cadnium plated steel. Stainless(NO), it work hardens and breaks, they are not as strong as plain steel.

Ian S C