Manufacturing Processes - Drilling

Introduction:

n  Drilling is the process of cutting holes in metals by using a drilling machine

n  Drills are the tools used to cut away fine shavings of material as the drill advances in a rotational motion through the material.

Drill parameter:

A drill bit is a multi-point tool and typically has a pointed end. A twist drill is the most common  type used

n  The twist drill or  drill  bit is made from High Speed Steel, tempered to give maximum hardness throughout the parallel cutting portion.

n  Flutes are incorporated to carry away the chips of metal and the outside surface is relieved to produce a cutting edge along the leading side of each flute

n  Twist drills are available with parallel shanks up to 16mm diameter and with taper shanks up to 100mm diameter and are made from high-speed steel.

n  Standard lengths are known as jobber-series twist drills, short drills are known as stub series, and long drills as long series and extra long series.

n   Different helix angles are available for drilling a range of materials

Twist drills -  Helix  Types 

n  The helix angle of the twist drill is the equivalent of the rake angle on other cutting tools and is established during manufacture.

n  The standard helix angle is 30 degrees, which, together with a point angle of 118°, is suitable for drilling steel and cast iron.

Typical Drilling Processes

ü  Reaming

n  A reamer enters the workpiece axially and enlarges an existing hole to the diameter of the tool.

n   A reamer is a multi-point tool that has many flutes, which may be straight or in a helix.

n  Reaming removes a minimal amount of material and is often performed after drilling to obtain both a more accurate diameter and a smoother internal finish

ü  Countersinking

n  

A countersink tool  enlarges the top portion of an existing hole to a cone-shaped opening.

n   Countersinking is performed after drilling to provide space for the head of a fastener, such as a screw, to sit flush with the workpiece surface.

n   Common included angles for a countersink include 60, 82, 90, 100, 118, and 120 degrees.

ü  

Counterboring

n  A counterbore tool  enlarges the top portion of an existing hole to the diameter of the tool.

n  Counterboring is often performed after drilling to provide space for the head of a fastener, such as a bolt, to sit flush with the workpiece surface.

n  The counterboring tool has a pilot on the end to guide it straight into the existing hole

ü  Spotfacing

n  

Spotfacing provides a seat or flat surface at the entrance and surrounding area of a hole.

n  This flat surface allows the bottom of a  screw or bolt to seat squarely with the material.

n  Spotfacing is commonly done on castings where irregular surfaces are found.

n  Spotfacing may be performed on a drill press with a counterbore of suitable size for the operation.

n  A proper  size pilot must be used whenever this is done on the drill press.

ü  Spotting

n  Where holes in two parts are required to line up with each other, a technique known as ‘spotting’ is carried out. The top part is marked out and drilled.

n  The two parts are then carefully positioned and clamped together.

 

 

ü Deep hole drilling(Gun drilling)

n  This method was originally developed to drill out gun barrels and is used commonly for drilling smaller diameter deep holes. This depth-to-diameter ratio can be even more than 300:1

ü Tapping

n  For cutting internal thread

n  Multi cutting edge tool.

n  Tapping is performed either by hand or by machine.

n  Minor dia of the thread is drilled and then tapping is done.