The builders of my 15-yr-old home used this screw to affix my subfloor to the metal joists. I'm putting in new floors and tightening some of the plywood that has loosened over the years. I'm also adding screws to a few sections where they only used nails. I want to use what they used but none of the hardware / construction suppliers here on Oahu has ever seen this screw or can explain why it's smooth on the bottom third. Any idea what this is, why it is designed this way?
Looks like a self drilling wood to metal screw. Here's a picture from McMaster-Carr's website:
They call them "Stainless Steel Drilling Screw, Wood to Metal," described as
Screws with cutting wings drill a slightly larger hole in the wood to allow for expansion. The wings break off when they contact metal, so the screw can tap the remainder of the hole.
The reason for the long unthreaded drilling tip, I assume, is to allow the drill tip to cut through an underlying metal layer without lifting the wood surface away from the metal layer. The drilling tip takes a bit of time to cut metal, so a fully threaded fastener would lift the wood all the way up to the screw head while the drilling tip worked to penetrate the lower layer of metal. Your fastener has a long unthreaded part to avoid this situation, whereas this more modern fastener has "wings" to overbore the wood and thereby accomplish the same goal with less material.
