Morning all

Hope this finds you all happy and well. I need to pick the collective brain on self tapping screws.

I want to attach some 1.5mm thick aluminium extrusion through a fibreglass side panel and into one of the 1" square chassis tubes. I intend to use two extrusions per side to attach flared side panels to.

So what sort of size self tapper should I be considering using as I haven't a clue

No.6 x 1/2" - 3.5mm dia
No.8 x 1/2" - 4.1mm dia
No.10 x 1/2" - 4.7mm dia

or the above but 3/4" rather than half?

And lastly if given the diamter of the screw in mm what size relation should the hole be (i.e 0.5mm smaller etc)

Any help greatly appreciated.

Nigel

Dunno about size, but a good rule of thumb for hole diameter is if you have e.g. M6x1 then hole size should be (6-1) = 5mm. (This works for taps but I guess it should also work for self-tappers).

I agree, just used a drill bit which was .5-.8mm less than the screw when I've used them. I find if it's too easy to screw in the first time, they always strip the hole really easily, if it's a bit harder to get it to cut the thread at first, they seem to cope with being being in and out a few times better. I sometimes have to use a couple of screws to 'cut' the thread first.

HTH

Edit: Not sure if this is any use?

[Edited on 7/9/11 by Daddylonglegs]

In this sort of situation we've just taped the chassis rail

You can then use stainess bolts/cap screws to hold it on if there going to be visable and are always easier to remove and get to the correct tightness.

Can't say i'm a fan of self tappers where avoidable.

Cheers for the info. Hadn't considered tapping the chassis rail itself, didn't think the metal was thick enough. Will look into it.

Thanks again

Typical 16 gauge RHS isn't thick enough to drill and tap unless you are using some properly tiny screws e.g. M3. with an M6 you'd barely get 1.5 turns of thread.

A rivnut may work for you, though personally I don't like putting them in the main chassis rails as the hole required is pretty big.

I second tapping the chassis rail.

Thread lock and nice button flanged socket screws

does it for me.

Len.

no6 1/8th drill
no8 5/32 drill
no 10 11/64 drill.

You may get away with coming down by 1/32 on each but thats the sizes we use at work for these self tappers.

rivet?

Self drilling self tapping 'stich' screws - used on sheet metal roofing.

Whatever you do, don't buy Screwfix self tappers.

good quality and black I got from bluebeehive.