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Sheet Metal Screws
Sheet metal screws date back to 1913 and are used with metallic and nonmetallic materials too thin to tap and in situations where rear access is somehow obstructed, which prevents installation of a nut. They are reusable, one-piece, high strength fasteners that can be removed to allow for maintenance and repair. Considered thread forming screws, they "tap" their own internal mating threads by pushing material outward and into their threads to create a tight fit that resists loosening. Unlike thread cutting screws, sheet metal screws do not produce chips that can cause electrical shorts, contaminate lubricants, or otherwise inhibit mechanical operation. Used to secure covers and panels and by a variety of trades, especially HVAC contractors to fasten sheet metal—hence the name sheet metal screw—you'll also see them frequently used with wood, particularly resin impregnated plywood. Sheet metal screws are also known as
tapping screws, self tapping screws, thread forming screws, thread forming tapping screws.
Having an expanded thread pitch like wood screws, sheet metal screws differ because they are fully threaded and not tapered. Available in different types, a letter or number designation identifies both their threads and point. The original Type A screws have "coarse spaced threads" while the newer Type AB have "spaced threads," which is a finer pitch (the threads are more closely spaced). Although Type A is considered obsolete and Type AB is said to be more versatile, Type A is still available today so unless you specify, either one may be supplied. Both types have a gimlet point that requires a preformed hole. "A gimlet point is a threaded cone point usually having a point angle of 45 to 50 degrees." Needle point sheet metal screws have a sharp point that can pierce sheet metal and start threading without a pilot hole. Threading of all types is standard right-hand, so turn the screw clockwise to install and tighten.
Sheet metal screws commonly range in size from #2 to #14 in diameter; some head styles are also available in 5/16" and 3/8" sizes. Typical lengths, specified in fractional inches, span from 3/16" to 3 1/2". For flat and oval heads, length is measured from the top of the head to the tip of the thread; all other sheet metal screw head styles (hex, pan, round and truss) are measured from under the head to the screw's tip. They should be long enough to ensure that fully formed threads are in contact with the entire thickness of the joined materials.
Drive styles include slotted, cross recessed (Phillips), combination (both slotted and Phillips), hex (slotted and unslotted) and square recessed. Due to the tendency of cam out (the driver bit slips or twists out of the drive recess), slotted screws are often not used with power drivers although slotted pan heads have superior driving characteristics compared to slotted round heads. Consequently, use Phillips, hex and square recessed drive sheet metal screws with power drivers (square drive is more resistant to cam out than Phillips). Higher drive torque can be applied to hex heads because they are externally wrenched, which also means that cam out does not apply.
Common head styles include flat (countersunk), hex and hex washer, oval (countersunk), pan, round and truss; some hex heads are slotted. The head angle (conical bearing surface) of flat and oval heads is 82°. Hex head self sealing sheet metal screws have a bonded sealing washer for leak-proof fastening of metal roof and wall panels to wood and are ideal for other applications where a weather-tight joint is desired.
Estimates suggest that 99% or more of the tapping screws used are case hardened carbon steel; stainless steel is also available.
Zinc plating, which offers moderate corrosion resistance, is the most common finish for steel sheet metal screws. Stainless steel, though, offers better corrosion resistance and is usually recommended if the screws will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations.
Undersized holes require higher drive torque and risk screw twist-off while oversized holes can result in thread stripping or screw pull-out. Refer to our reference charts—Tapping Screws Type A Hole Sizes and Tapping Screws Type AB & B Hole Sizes—for details relating to pierced and drilled/punched hole sizes in sheet steel, sheet aluminum and castings based on screw size and material thickness.
Use an 82° countersink to create a countersunk recess (a beveled opening) for flat and oval heads mounted in wood. The body diameter of the countersink should be equal to or greater than the head diameter to ensure that a uniform bearing surface is created.
Use flat washers to increase the bearing surface of the screw's head, which can be useful when working with soft materials.
Flat head sheet metal screws, especially Phillips drive, are commonly supplied with packaged barrel bolts, corner braces, hasps, hinges, mending plates, utility pulls and other hardware items. Withdrawal resistance of sheet metal screws is said to be about 10% better than wood screws of the same diameter and threaded length. In denser woods, such as oak, sheet metal screw withdrawal resistance increases to about 16%, and decreases to about 5% in lighter woods, like redwood.
Sheet metal screws are generally intended for use with thin sheets of malleable material with preformed holes. Consider using thread cutting screws, thread rolling screws or self drilling screws for other applications where tapping screws are needed.
Refer to American Society of Mechanical Engineers Standard ASME B18.6.4, Thread Forming and Thread Cutting Tapping Screws and Metallic Drive Screws, and Society of Automotive Engineers Standard SAE J933, Mechanical and Quality Requirements for Tapping Screws, for specifications relating to sheet metal screws.
Sheet Metal Screws |
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|---|---|
| Flat Head | |
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 Phillips  Slotted |
| Hex Head | |
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 Needle Point  Slotted  Unslotted |
| Oval Head | |
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 Phillips  Slotted |
| Pan Head | |
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 Combination  Phillips  Slotted  Square |
| Round Head | |
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 Phillips  Slotted |
| Self Sealing | |
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| Truss Head | |
|
 Phillips  Slotted |
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