How Self-Drilling Screws Work
As their name implies, self-drilling screws operate on the same principles as drill bits and other cutting tools. For any cutting tool, performance is governed by cutting speed, feed rate, depth of cut and the work material itself. Therefore, installation performance of self-drilling screws can be linked to the basic cutting tool parameters. Suggested optimal parameter values are listed by nominal screw size in the table below.
Point Geometry is the designed shape of the screw’s drill point.
RPM is the speed at which the driver motor runs while the screw is installed. This is often adjustable using a variable pull trigger or different driver motor.
Applied Force is a measure of the user applied force as the screw is installed. More force is not necessarily better.
Work Material Hardness can be viewed as a material’s resistance to drilling or cutting. In most instances, the harder the work material, the more difficult it is to cut.
Optimal Cutting Conditions by Screw Size
* Suggested combined maximum values. Individual values may be increased if other, associated variables are decreased proportionally. Stated speeds may require a variable-speed screwdriver motor and a partial trigger-pull.
When selecting a self-drilling screw, consider the material thicknesses and types of materials to be joined. Following are some key design features to look for when selecting suitable fasteners.
Drill Flutes allow drilled material to exit the hole. Completely embedded flutes can no longer remove these chips, which contain approximately 80% of the heat created by the drilling process. A buildup of this material can cause the point to over-heat and fail.
Point Length determines the material thickness which the screw can reliably penetrate. The unthreaded portion of the point, (pilot section) must be able to completely drill through the material before the threads engage. If the threads engage before drilling is complete, the fastener can bind and break.
Point Wings are used on some screws that fasten thicker materials, such as wood, to metal. The wings enlarge the hole in the fastened material, allowing the threads to pass through without contacting the fastened material. This added clearance prevents separation of the fastened material from the base metal (knownas “jacking”). The wings will break away on contact with the metalbefore the threads engage in the metal.
Drill-Point Material is generally plain carbon steel which is less stable at high temperatures than equivalent high-speed steel (HSS) drill-bits. To reduce wear on the drill point, fasten using a drill motor rather than an impact driver or hammer drill.
High Temperature Stability affects how quickly the drill point fails due to the heat generated by the drilling operation. Refer to the troubleshooting guide at the end of this section for some visual examples.
Drilling Temperature is directly proportional to motor RPM, applied force, and work material hardness. As each value increases, so does the heat generated by the drilling operation.
Reducing Applied Force can increase durability and allow the drill point to penetrate thicker materials (i.e., remove more material before failing due to heat buildup).
Reducing Motor RPM can improve performance in harder materials by allowing the user to push harder during the drilling process and extending the life of the drill point.
Basic Self-Drilling Screw Anatomy
Work Material Thickness by Screw
- Total thickness of all steel, including any spacing between layers.
Cold-Formed Steel Thicknesses for Framing Applications
- For reference only.
- One "mil" is 1 /1,000 (0.001) of an inch. Mil thickness measures the uncoated base material.
Self-Drilling Screw Troubleshooting Guide
|Failure Mode||Likely Cause(s)||Suggested Action|
|Split at point (web)||Excessive force (feed) applied while drilling||Reduce application force|
|Outer corners worn or melted||Drill RPM (cutting speed) too high||Use slower motor or partial trigger pull|
|Cutting edges chipping or breaking||Excessive force (feed) applied while drilling||Reduce application force|
|Point melted or diameter significantly reduced||
|Screw spins without drilling a hole||