Nuts and bolts

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A nut is a small piece of metal drilled with a hole threaded to match the threads of a bolt, screw, or stud. If the thread of the nut does not fit that of the bolt, the teeth will be jammed or stripped.

Nuts, like bolts, screws, and studs, are made of a variety of metals, such as steel, aluminum alloy, brass, and the like, and may be plated with rust-preventing metals such as topper and cadmium.

Kinds of nuts

Figure (soon) illustrates the various kinds of nuts used on machines and in the sheet-metal and aviation industries.

Square (four-sided) nuts are generally used on bolts, and hexagonal (six-sided) nuts are usually used on screws; but very often they are interchanged.

There are a number of safety devices used in connection with nuts. Figure shows three of these and some of their uses. The check nut is used with plain nuts and set screws as a locking device to prevent the plain nuts from unscrewing and dropping off.

The castle nut, shear nut, and slotted engine nut all require a drilled hole in the bolt or stud. A cotter pin is inserted in this bolt hole to prevent the nut from coming off. The several slots on the nuts permit of proper adjustment of tension with correct alignment of slot and hole.

The lock nut is another type used with other nuts to prevent them from coming loose. The wing nut is used where great tension is not required and, therefore, where the fingers can be used for tightening. The coupling nut is used with a coupling (in electrical work) to join two sections of conduit or to join a conduit to a junction box.

There are several types of self-locking nuts; all are designed to provide a locking arrangement for the nut without the use of other aids.

Emergency substitutes

When unable to get a lock nut or washer, you can temporarily lock a nut to a screw by winding wire around the threads extending beyond the nut. As soon as possible, of course, the screw should be replaced and the correct lock nut or washer put on.


Bolts and screws are solid metal cylinders with heads; they are threaded to receive nuts or to be screwed into tapped (threaded) holes.

A stud bolt is threaded at both ends and therefore has no head. A blank space between the two sets of threads permits gripping the bolt with a pipe wrench. Usually one end is screwed into the main part of a machine and the other end protrudes through a hole in another part to receive a nut.

Bolts with nuts are used for holding together two or more separate pieces. Screws are generally used in tapped holes. However, the terms are interchangeable, for bolts can also be used in tapped holes and screws can be used with nuts.

Classification of bolts and screws

Bolts and screws are classified according to: (1) length, (2) type of head, (3) pitch, (4) diameter, and (5) material.

Length is the distance from the underside of the head to the end of the bolt. Type of head is the shape of the head, which may be hexagonal (six-sided), square (four-sided), round, flat, and so on. Pitch consists of the number of threads per inch or the distance from a point on one thread to a similar point on an adjacent thread. Diameter is the maximum distance across the bolt or the distance across the cylinder before the thread is cut. The material, as with nuts, may be steel, brass, bronze, aluminum alloy, and the like, or rust proofed with a plating of a rust-preventing metal such as copper or cadmium.


The more common forms of screw threads are called National Coarse series, National Fine series, National Extra-fine series, and American National Taper Pipe Thread. The coarse-thread series is used with soft metals and in designs where rapid engagement is wanted.

The fine-thread series is used in general in aircraft and engine construction. It is widely applied to fastenings such as bolts, screws, and turnbuckles. Since this series has more threads per inch than the coarse series, the engagement is not so rapid. Further, since the threads are not cut so deep as in the coarse series, the fine series is stronger.

The extra-fine thread series is used where thin-walled material is to be threaded, where the depth of the teeth must be kept to a minimum, and where a maximum number of threads per inch is wanted.

The pipe thread is used on taper-threaded pipe joints, the taper (3/4" per foot) permitting a tighter fit with continued engagement of the screw thread.

Usually National Fine is expressed with the letters NF, and National Coarse with the letters NC. Screws are also designated according to the number of threads per inch and the major diameter of the screw. Thus, size 1-8—NC means one-inch diameter, eight threads per inch, with National Coarse thread. Screws of a diameter smaller than 1/4" are designated by gage number instead of inch size. Thus 10-24 means 24 threads per inch on a cylinder that gages 10.

Screw threads are also designated by fit, as loose fit, free fit, medium fit, and close fit. NF-3 means National Fine medium fit. Threads are generally right-hand, designated RH. When lefthand, they are marked LH. Pitch can be measured with a steel scale or with a pitch gage.

Kinds of bolts and screws

Figures (soon) illustrates the kinds of bolts the kinds of screws commonly used by the machinist. In addition to the heads shown, machine screws and bolts come with special screwdriver slots, such as the Phillips. In addition to these, the sheet-metal and airplane industries use other types of screws.

The flathead screw requires a countersunk hole. This provides a surface free from protrusions and aids in streamlining.

  • The roundhead, buttonhead, and washerhead screws are used where projections are unimportant. They require no countersinking. The washerhead is especially good in providing a large contact area.
  • The fillister-head screw, coming with or without drilled head, is generally used in light-mechanism assemblies.
  • Machine and cap screws, being used most often without nuts, require threaded holes.
  • Set screws are used for holding wheels, knobs, or collars rigidly in place on shafts. Socket-type set screws are turned by inserting a wrench made of hexagonal hardened steel into the head of the screw.
  • Lag screws are used for attaching metal to wood, as a vise to the work table.
  • Sheet-metal screws, made of case-hardened steel, can make their own threads when screwed into a tapped or punched hole. They do away with the need for tapping the hole or using a nut. One type resembles a wood screw; another resembles a machine screw.
  • The drive screw, also made of hardened steel, is designed for being driven into an untapped hole of a diameter slightly smaller than that of the screw. It is chiefly used for plugging holes and attaching name plates.


These are discs of metal used with screws, studs, bolts, and nuts to protect the surf ace of the metal, to provide a larger clamping area, or to keep nuts or bolts from loosening.

Counterboring and countersinking

Sometimes a hole must be enlarged for part of its depth to receive the head of a screw so that it will be flush with the surface of the metal into which the screw is turned.

If the underneath surface of the screw is at an angle with its shank, as in the flathead type, then the wider part of the hole must have sloping or tapered sides. For this a countersink is used. The three-flute countersink with a round shank is used in a drill. The rose type, with a square insert, is used in a regular brace.

If the head of the screw has parallel sides, as in the fillister type, the sides of the widened part of the hole must be parallel. For this a counterbore is used. The pilot of the counterbore fits the original hole and leads the cutting edges of the counterbore to widen the hole. Counterbores come in various sizes to fit various holes.