James Wilkinson Sword wrote:

On Tue, 23 May 2017 12:07:04 +0100, Roger Hayter wrote:

James Wilkinson Sword wrote:

On Mon, 22 May 2017 21:12:33 +0100, newshound
wrote:
snip

No, in fact it doesn't. As long as it is hammered fairly accurately, it
is in pure compression. It only has to be stronger than the wood, which
is why you can make nails out of copper.

The torsional stresses on a screw head can be high, especially with an
impact driver. This is why harder materials are used.

Funny how nails bend easily, yet screws never shear with rotational torque.

Have you really never sheared a screw trying to remove it? It happens
to me all the time. I've occasionally even sheared a screw tightening
it, when too lazy to take it out and enlarge the pilot hole. You
probably need a well-fitting screwdriver and a lever to do it, though.

I've always found the head burrs long before the screw shears.

Then you're not careful enough. With a clean head and undamaged
pozidrive screwdriver under adequate pressure with a spanner applied you
can easily shear a 3.5mm screw.

--

Roger Hayter