How do i tell the size of drill bits for self tapping screws?

Question

i'm going to put timber on to 4x4 box metal to hang door, the metal will be standing away from a wall on its own, in concrete.

Answer 1

The drill bit should be slightly smaller than the screws. I think of it as being the size of the s***w without the threads. ALthough, if it is a self tapping s***w, you shouldn't need to drill a hole first.

Answer 2

There are tables that tell you what you need to know.  Here is one of them

http://www.aaronsself-tappingscrews.com/...

Answer 3

Pick a drill bit just slightly larger than the solid shank of the s***w.

You should be able to see the threads clearly when you hold the bit in front of the s***w.

Answer 4

if you hold the drill bit in front of the s***w you should just be able to se the threads on the s***w

Answer 5

The answer is far from being straight forward.  There are many factors involved.  There are thread "tapping" screws and there are thread "forming" screws, the threading method is different but the result is very similar, with the formed thread being stronger due to work hardening.  

The diameter for the s***w hole would be different, based on forming or cutting.  The relative hardness of the base material can also have impact on hole diameter.  The material of the s***w can also impact the outcome.  The risk you run is this, too small you ruin the s***w before it is all the way in the hole, too big and the threads easily strip out and carry no load.

Most self tapping s***w manufacturers reccommend the pre-thread hole diameters.  Unfortunately, it is nearly impossible to find who made the s***w.  My advice is to use a drill slightly larger than the s***w shank (the unthreaded portion of your s***w).  Attempt to s***w the s***w in.  If it stops prematurely, don't force it, remove it and re-drill with a drill that is 1/64th larger and try again.  This should get you a hole that functions well and does not damage your box or your screws.