I've never managed to get the hole size right for self-tapping screws (nasty, 'orrible things). Now I have a situation where I need to use a few made of stainless steel, so I want to get the hole size right first time.

Obviously the hole must be smaller than the outside diameter (der!). If I drill a hole a fraction larger than the core diameter then the effort of inserting the screw is very high and I risk stripping the thread, or even snapping the screw. So the proper diameter is somewhere between the two.

Is there some formula somewhere that tells me the correct hole size, or have I got to go out in the garage and muck about for hours trying to work it out for myself?

David

Try a test hole in some alluminium first, starting small and going up a size if needed - shouldn't take more than a couple of test holes to get it spot on

The screw maker will have recommended a specific size dependant upon material etc...

If you know who made them (i.e. they're in a box not just lying around the garage) do a search on the web.

No 6 screw 1/8 drill
No 8 screw 5/32 drill
No 10 screw 11/64 drill

These are the sizes we use for up to 6mm thick steel.
If i were you I'd drop a size for ally i.e. use 1/8 drill for a No 8 ans 5/32 for a no 10. As said by CalvinX they will force through all anyway.

i pretapped all my holes along the transmission tunnel with a mild steel 8g self tapper first, then once the threads were cut, just ran the stainless ones through the cut threads.

i think the holes were probably still a bit undersize at iirc 3.5mm, as it did eat the first few threads of the steel screws, but using 4mm holes, i found it would pull the threads out if nipped up too tight.

hth
tom

the way I've done it is drill an undersize hole, screw in the screw whcih normally doesn't go in too far, use a file or dremel (before it broke:mad to open up the hole, try the screw again, and repeat until it goes all the way in.

a bit time consuming but not that bad for a couple of screws that I had to do up to now.

Just to confuse the issue, I'm screwing into 1.2mm ali over 1.6mm mild steel...

'omega' - I'll play with those sizes, and see how they work out (I'll have to convert to metric as I've got hardly any imperial drill bits )

Thanks to all,
David

Stainless tappers will not go into steel mate the threads will flatten on the screws. As said in a previous post you'll need to pre tap the holes with a normal hardened self tapper. It's a bummer cause you'd think stainless was harder, but the threads will be torn to bits if you don't pre thread.
For drill sizes look here
Drill sizes http://www.simetric.co.uk/siselftap.htm

[Edited on 31/3/06 by omega 24 v6]

That's a good link, that is!

I'm starting to think about setting a small rivnut instead of using self-tappers - I don't like the things anyway. I had forgotten about SS screws into mild steel; I had experienced that problem before. It's just that the current task - locating a boot panel, with no structural requirements whatsoever - was an obvious case for a self-tapper.

Alternatively, I have some plastic trim fasteners that I might use - they'll be plenty strong enough, although the head might be too big.

David

The hole in your aluminium can be any size smaller than the screw head as it's obviously only acting as a cover for something. I usually use a drill just less than the tapper thread diameter and nothing usually falls off. You can get inserts for holes for your screws to fit into made of plastic from some trim suppliers or you could use expanding clips like the ones for door panels.

Dont know if they are still available but I have used a small nylon insert that pushed into an 8mm square hole which stopped them turning . Think they may have been used on Mini wings to fit the headlights.