Tap and die

A tap is a cutting tool used to create internal threads inside a pre-drilled hole. A die is its counterpart — a cutting tool used to create external threads on a rod or bolt shank. Together, taps and dies let you cut matching threads on any material, repair damaged threads, or create custom threaded fasteners.

How a tap works

A tap looks like a bolt with cutting flutes along its length. It is turned into a drilled hole with a tap wrench, cutting grooves into the hole’s inner wall as it advances. The flutes clear away chips (metal shavings) and allow cutting fluid to reach the cutting edges. A typical threading operation uses three taps in sequence: a taper tap (starts the thread), a plug tap (cuts most of the thread), and a bottoming tap (finishes threads to the bottom of a blind hole).

How a die works

A die is a circular disc with cutting teeth arranged in a ring. It is threaded onto the end of a rod and turned with a die stock, cutting external threads as it advances. The split in the die allows slight adjustment to control thread depth.

When you need them

• Repairing damaged threads: If a bolt has been cross-threaded or a tapped hole is stripped, running a tap or die through it can restore usable threads.
• Custom fasteners: When a standard bolt is too long, you can cut it to length and chase new threads with a die.
• Prototyping: In workshops and machine shops, taps and dies let you create threaded holes and studs without sending parts out for machining.

Cutting fluid

Taps and dies always benefit from cutting fluid — oil or tapping compound — which reduces friction, clears chips, and extends tool life. Cutting dry risks breaking the tap inside the hole, which is extremely difficult to remove.