How to Correctly Use Sheet Metal Screws
Sheet-metal screws were originally designed for metal, like for use in a B-Vent Pipe, but can be used for a number of other applications too. They can be used with plastics, rubber, or plywood. Regardless of the material, the driving point needs to be larger than the depth of the material being driven into. Without a long enough drive point, the threads will contact the materials before the hole is adequately bored, and the screw will snap.
Step 1 – Select Appropriate Sizing
Choosing the correct sheet-metal screws for the project is important, as the screw will be the main binder affixing materials together. Screws are defined by a series of three numbers, and each are an important identifier when you are considering the effectiveness of the screw for a given project.
The size of a screw is characterized by a proportional number from 0-24, which defines the head and shaft diameter, not the length. A size-8 screw is twice the shank diameter of a size-4 screw. To obtain the actual diameter, multiply the screw size by 13 and then add .060 inches.
Screws have a pitch that equates to the number of threads possible within 1 inch of screw length. By multiplying the length (with respect to 1 inch) by the thread count, the number of threads for the screw can be derived, indicating the adhesion capacity.
The last number indicates the physical length of the screw. At least half of the length of the screw must be embedded into the material being affixed together.
The numbers are always ordered by size, thread count, and then length. In this way, the standard screw is defined as 8-32 x 1 inch. The size-8 screw has 32 threads per inch and is 1-inch long.
Step 2 – Choosethe Correct Material
Sheet-metal screws come in a few materials to best suit the environment where the project will be housed. Standard screws are the most economical but can discolor with rust or corrosion when exposed to moisture. Zinc-plated galvanized screws and stainless-steel sheet-metal screws are more expensive but stand up to the elements without deteriorating.
Step 3 – Determine the Appropriate Head Style
The head style of the screw allows for the materials being affixed to be protected. Pan heads are raised above the surface. Oval heads provide a finished appearance. Flat-head screws are best countersunk. Hex washers come with a neoprene washer designed for use with water. With all screw heads, over-tightening or over-driving the screws can strip the heads or mar the materials.
Step 3 – Know the Difference Between Pilot Holes and Self-Tapping Screws
Sheet-metal screws can be either self-tapping or standard. Self-tapping screws come with a notched tip that acts as a drill bit to drive the screw and keep thin metal from denting. If these screws are being used, the screw can merely be fitted to the drill bit and driven. Standard screws will need to have a pilot hole drilled in order to prevent splitting within the materials being fixed together.
Your pilot-hole drill-bit size will depend on the size of your sheet-metal screw. A size-4 screw should have a pilot-hole drill size of 3/32 inches; size 6, 7/64 inches; size 8, 1/8 inches; size 10, 9/64 inches; size 12, 5/32 inches; size 14-1/4 inches, 3/16 inches; and 5/16 inches, 1/4 inches.
Step 4 – Drill Correctly
Always use protective eyewear to prevent metal shavings from penetrating the eye. Drill slowly and cautiously when driving sheet-metal screws to prevent shavings and be sure the screw can adequately grab the materials. Before driving another screw, clear the drill bit of metal shavings.