Unit 6: Drilling, Threading, Tapping & Reaming

Internal thread

A groove cut in the inside diameter of a workpiece with a tap.

External thread

A groove cut out on the outside diameter of a workpiece with a thread-cutting die.

Thread

A spiral groove made on a round external or internal diameter.

Thread-cutting die

A tool used for cutting external threads.

Tap

A tool used for cutting internal threads.

Twist drill

Has spiral grooves in its sides; used for general purpose hand-drilling.

Counterboring

Increases the diameter of a hole to a certain depth in order to allow a screw head or nut to be positioned flush with or below the workpiece surface.

Thread depth

The depth of a single thread measured from the crest to the root.

Lead

The distance a thread will advance in one revolution.

Pitch

The distance from a point on one thread to the same point on the adjacent thread.

Pitch diameter

The imaginary diameter measured where the thickness of the thread and space within the groove are equal; responsible for the fit between two mating threads.

Major diameter

The largest diameter of the thread.

Threads per inch (TPI)

The number of threads contained within one inch of the workpiece length.

Crest

The peak of one thread; creates the points form which major diameter is measured.

Countersinking

The process of cutting a tapered opening in a hole so that a flat-head screw can be installed flush with the workpiece surface.

Reaming

The process of finishing a hole to a precise size with a smooth finished surface.

Spotfacing

The process of machining a flat spot on a rough surface surrounding a hole opening so that bolts, nuts and washers will be properly seated when tightened.

Drilling

The process of using a cylindrical rotating cutting tool that is sharpened on its end to create a hole.

Class of fit

The relationship between two mating parts.

Minor diameter

The smallest diameter of a thread; measured from the base of one thread to the base of the thread on the opposite side.

Flutes

The spiral grooves in the sides of a twist drill.

Unified Thread Standard

The system of standards for inch series threads.

ISO Metric

The system of standards for metric series threads.

Root

The valley between threads; creates the points from which the minor diameter is measured.

Spot drill

Used when more precision is required for drilling.