Machine Screws

Machine screws are headed fasteners with external machine screw threads. They are commonly used with internally threaded parts and machine screw nuts. Available in coarse (UNC) and fine (UNF) threads, right-hand threads are standard (turn clockwise to tighten).

Ranging in size (diameter) from #0 (very small) to #12 and 1/4" to 1/2", nine head styles (two countersunk), four drive styles and three materials are common (see below).

The standard head angle (conical bearing surface) of flat and oval head machine screws is 82°; flat heads are also available in 82° undercut and 100° versions. Use a matching (82° or 100°) countersink to create a countersunk recess (a beveled opening) for flat and oval heads. 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.

Machine screws are measured like other screws: For flat heads, length is measured from the top of the head to the tip of the thread; measure oval heads from the edge where the dome top meets the sloping bearing surface to the threaded tip; all other machine screw head styles (Binding, Fillister, Hex and Hex Washer, Pan, Round and Truss) are measured from under the head to the screw's tip. Length is specified in fractional inches.

Use machine screw flat washers to increase the bearing surface of the screw's head or nut, which can be useful when working with soft materials and oversized or irregular shaped holes.

Slotted flat head and slotted round head machine screws are also called stove bolts when supplied with hex nuts.

**Table 1. Machine Screws.** Types and styles of machine screws commonly available.
Machine Screws
Binding Head
Slotted, Brass Slotted, Steel, Zinc Plated	Slotted
Fillister Head
Phillips, Steel, Zinc Plated Slotted, Brass Slotted, Steel, Zinc Plated	Phillips Slotted
Flat Head (countersunk)
Phillips, Brass Phillips, Stainless Steel 304 (18-8) Phillips, Steel, Zinc Plated Slotted, Brass Slotted, Stainless Steel 304 (18-8) Slotted, Steel, Zinc Plated	Phillips Slotted
Hex Head & Hex Washer Head
Unslotted, Steel, Zinc Plated Slotted, Steel, Zinc Plated Unslotted, Stainless Steel 304 (18-8)	Unslotted & Slotted
Oval Head (countersunk)
Phillips, Stainless Steel 304 (18-8) Phillips, Steel, Zinc Plated Slotted, Stainless Steel 304 (18-8) Slotted, Steel, Zinc Plated	Phillips Slotted
Pan Head
Phillips, Stainless Steel 304 (18-8) Phillips, Steel, Zinc Plated Slotted, Nylon Slotted, Stainless Steel 304 (18-8) Slotted, Steel, Zinc Plated	Phillips Slotted
Round Head
Combination Drive, Steel, Zinc Plated Phillips, Stainless Steel 304 (18-8) Phillips, Steel, Zinc Plated Slotted, Brass Slotted, Nylon Slotted, Stainless Steel 304 (18-8) Slotted, Steel, Zinc Plated	Combination Phillips Slotted
Truss Head
Combination Drive, Steel, Zinc Plated Phillips, Stainless Steel 304 (18-8) Phillips, Steel, Zinc Plated Slotted, Stainless Steel 304 (18-8) Slotted, Steel, Zinc Plated	Combination Phillips Slotted
Miscellaneous
Wall Plate Screws	Slotted

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