Self-tapping screw are fasteners that create there own thread in metal, and self-tapping screws can cause damage if you dont use a correct amount of torque. If you apply too much torque, a self-tapping screw may strip the metal or the screw head may snap. If a self-tapping screw strip the metal or the screw head snap, you will have to perform a repair instead of an assembly, and a repair is more difficult and more expensiv than an assembly.
The diameter of a self-tapping screw determine how much force the screw can handle before the metal fail. Small self-tapping screw, such as a #4 screw, require very little torque, but a small self-tapping screw also has a low limit for error. If you apply the same amount of torque to a #4 screw that you would apply to a #14 screw, you will strip the hole.
Using Self-Tapping Screws Safely
Many professional aim to use a torque that is eighty percent of the failure limit, and using eighty percent of the failure limit provide a safety margin because material density can vary. The thickness of the metal also change how you must use a self-tapping screw. Thin metal provide very little material for the threads of a self-tapping screw to grab, so thin metal increases the risk that a self-tapping screw will strip the hole.
Thick metal provide more material for the threads, but thick metal also increases the resistance that you feel when you drive the self-tapping screw. If you do not account for the thickness of the metal, you may not drive the self-tapping screw deep enough, and a self-tapping screw that is not driven deep enough will be loose. Stainless steel is a specific material that behave differently than zinc-plated steel.
Stainless steel is harder than zinc-plated steel, so stainless steel require more force to drive. Stainless steel also have a tendency to gall, and galling occur when the threads of a stainless steel self-tapping screw fuse together due to heat and friction. To prevent galling in stainless steel, you must use a lower RPM and you must use a lubricant.
Using a lubricant help to prevent the stainless steel threads from fusing together. The tool you choose to drive the self-tapping screw will affect the outcome of the work. An impact driver use rapid hammering to drive a screw, and an impact driver may strip a thread if you use an impact driver on thin sheet metal.
A standard drill/driver with a calibrated clutch is often safer for thin metal because a calibrated clutch provide tactile feedback. This tactile feedback tell you when the self-tapping screw has reached the correct depth. You can avoid mistake by preparing the work properly and by testing your settings.
One common mistake is using a Phillips head drive when a hex or Torx drive would provide a better grip, and using a drive with a poor grip cause the tool to slip. Another common mistake is failing to drill a pilot hole in thick metal. You can avoid these mistake by testing your tool settings on a scrap piece of the same metal before you begin the actual work.
Testing your settings on a scrap piece of metal allow you to verify that your clutch is set correctly, and verifying the clutch setting prevent you from stripping a hole or fracturing a screw head.
