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Metallic Drive Screws (Type U)
Metallic drive screws have a round head, no drive recess, external threads and a blunt pilot point. Designated Type U, they are self tapping screws designed to create their own mating threads and permanently fasten metal and plastic objects, which makes them tamper resistant. Like other tapping screws, they require a properly sized predrilled or punched hole and rear access isn't needed because they are a "blind" fastener (a nut is not required). Being thread forming instead of thread cutting, chips aren't created that could contaminate lubricants, cause electrical shorts or similarly inhibit equipment operation. Install using a hammer or press into position. They are also known as:
hammer drive screws, round head drive screws, U drive screws.
The threads of metallic drive screws are widely spaced with a helix (spiral) angle between 45° and 60° (in contrast, the helix angle of a 12-24 machine screw is a little over 4°). The number of start threads increases with an increase in screw size and begins at 6 for #00 and increases to 12 thread starts for 3/8" diameter screws. They are fully threaded although threads under the head and at the tip may be incomplete due to manufacturing limitations.
Commonly available sizes (diameters), expressed as a whole number for smaller sizes and as a fraction for larger sizes, range from #00 (0.060") to #14 (0.242"). Typical lengths, specified in fractional inches, span from 1/8" to 1". Length is measured from under the head to the tip including the pilot point. Pilot length varies with the length of the metallic drive screw.
Only round heads are available. Standard materials are steel and stainless steel. Steel is zinc plated.
Table 1 lists nominal size, as a whole number for smaller sizes and as a fraction for larger sizes, and maximum outside diameter in decimal inches; number of thread starts; maximum head diameter and height, in inches; and recommended hole size as drill size number and hole diameter in inches.
Refer to American Society of Mechanical Engineers Standard ASME B18.6.3 Machine Screws, Tapping Screws, and Metallic Drive Screws (Inch Series), for specifications relating to metallic drive screws.
Metallic Drive Screw (Type U) Dimensions and Recommended Hole Sizes
|Nominal Size||Number of
|Max||Max||Drill Size No.||Hole Dia.|
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