I still don't understand about countersinking screws. I have been in lowes and home depot and have never seen packages that say countersinking screws. I still don't know the purpose of countersinking screws. I only know I drill a pilot hole for the screws insert the screws by hand a little bit then finish drilling the screws down with a power drill??? I am a novice not a woodworker, little jobs around house, hanging shelves, hinges, pictures,mirrors, etc. thanks
A countersink is a feature of a hole. The corresponding feature of a screw is a taper under the head. This taper appears on flat head and also oval head screws. So in Lowe's or Home Depot you would see packages of flat head screws most commonly (where countersinking is concerned).
The primary purpose of a countersunk screw is achieving a flush surface; the head of the screw does not protrude above the work. On a deck, say, this is important because screw heads would be a tripping hazard, among other things.
Properly installing a flat head screw involves three operations (although they can be combined with a specialty drill bit):
- Drill the pilot hole, the same or slightly smaller than the root diameter of the screw (the root is the solid shaft bit under the threads, roughly speaking). The pilot hole extends as far as the screw will into the back-most material.
- Drill the clearance hole through the front-most piece only. It should be as big as the full diameter of the screw (measured across the threads, but not the head). The thread should not bite into the clearance hole.
- Form a countersink as deep as the screw head. This can be done with a "countersink bit" which may just be called a "countersink".
The screw is then pushed through the clearance hole, started and screwed into the pilot hole, and cinched up tight such that its head fits into the countersink tapered hole and sits flush with the top of the work.
GRK makes what they call a "self-countersinking" screw. Above the screw threads there are some "cutters" that widen the hole and allow for the screw to be set flush with the surface without having to use a countersink drill bit.
From GRK's web site:
GRK's R4™ self-countersinking screw has a patented underhead with saw-blade like cutting teeth and six self-contained cutting pockets. Together they act similar to a circular saw-blade, transporting the drill dust away from the edge of the screw hole while cutting a perfectly clean hole into even the most brittle materials without cracking any surface treatment.
This design enhances the R4's versatility by allowing the fastener to countersink into even the hardest woods. The head of the screw closes the hole off with precision leaving no damaged fibers around the head.
R4™ screws 2" and longer have a four threaded CEE Thread. This enlarges the screw hole for the non-threaded portion of the fastener, allowing the wood to settle easily. It increases the screw's drawing strength and reduces the friction on the screw shank that lowers the driving torque.