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The Torque Specifications for Sheet Metal Screws

Tamara Wilhite

Sheet metal screws are used to hold sheet metal in place or attach molding to the sheet metal. Sheet metal screws are also called tek screws and type "A" tapping screws. They can have pointed or tapered tips. Sheet metal screws can have a slotted pan head or Phillips head. Sheet metal screws range in size from a quarter of an inch to several inches long. They range from 6 gauge to 12 gauge in diameter, measured across the screw thread diameter. Sheet metal screws can be torqued with a screwdriver.

Sheet metal on car bodies are commonly attached with sheet metal screws.

Sheet metal screw manufacturers recommend torque. The ideal torque for a sheet metal screw will securely hold the two items together without damaging the screw or warping the sheet metal. The proper torque will also keep the sheet metal screw tightly. Drive sheet metal screws through a hole slightly smaller than the screw diameter. According to "Collision Repair and Refinishing," "when the screw is driven into the punched hole, the metal driven through when punched is pulled back by the metal screw. This makes the bond tighter and stronger than if the hole is drilled."

Factors Affecting Torque

Thicker sheet metal and harder alloys require more torque to drive in a sheet metal screw.

Sheet metal screws require more torque to go through hard material than soft. Thicker sheet metal requires more torque than thinner sheets. Torque requirements decrease as the prepared hole size approaches the diameter of the sheet metal screw diameter. Steeper screw pitches require more torque to insert. About 10 percent more torque is required to remove the screw from wood than is required to insert it.

Determining Maximum Torque

Tightening torque can be any force between the thread stripping torque, when the sheet metal screw's threads are stripped from the bolt, and above the maximum tapping torque. The maximum torque a sheet metal screw can receive when being driven into particle board is 1.36 times the density of the particle board plus 27.98. The resulting value in inch-pounds equals the maximum torque that can be applied to the fastener. For metric units, the maximum torque will be 0.0096 multiplied by the density of the particle board plus 3.16. This will give the maximum torque in Newton-meters.

American Society of Mechanical Engineers (ASME) standard B18.6.4 is the self-tapping sheet metal screw torque testing standard. ASME B18.6.4 is the standard process for determining the maximum torque a sheet metal screw can withstand. The American National Standards Institute (ANSI) has adopted this standard. American Society for Testing and Materials (ASTM) International standard A548 was the specification for tapping or sheet metal screws. ASTM A548 has been withdrawn without a replacement.