On 15/03/11 15:48, Andy Dingley wrote:
On Mar 15, 1:35 pm, "george
wrote:

Still havent seen a convincing reason
for the short thread at the top ofhttp://www.screwfix.com/prods/88716/

Finer pitch than the main thread, so it locks the board against the
screw head, but still allows them to be tightened down onto the joist.


http://en.wikipedia.org/wiki/Screw_t....2C_and_starts
shows me what pitch is.

But Spax Flooring Screws threads top and bottom
look the same pitch at in the picture at:
http://www.screwfix.com/prods/88716/

Thinking again maybe how it works is that
the lower thread drills through the floorboards into the joist
and when the smooth middle bit is in the floorboard
it pulls the board down with friction on the (not totally) smooth centre
of the screw.

Then, when the top bit of thread gets into the floorboard
that fixes the floorboard into the position it is
so it doesnt ever move again due to changing humidity etc.

Its unlikely that the top bit of the thread on the screw
will start exactly where the new groove starts in the floorboard,
so it will carve a slightly larger groove, which may help if the boards
expand due to humidity changing.

Or something like that.

I'll order a box tonight!

[g]