COUNTERSINKING

COUNTERSINKING]

What is countersinking?
Countersinking is an operation of beveling the end .of a drilled hole. The tool used is called a countersink.Countersinking is carried out for the following purposes,

❖▪To provide a recess for the head of a countersink screw, so that it is flush with the surface after fixing

❖ To deburr a hole after drilling
❖ For accommodating countersink rivet heads
❖ To chamfer the ends of holes for thread cutting and other machining processes

Angles for countersinking

Countersinks are available in different angles for different uses.
1. 75°  countersink riveting
2. 80°  countersink self-tapping screws
3. 90° countersink head screws and deburring
4. 120° chamfering ends of holes to be threaded or other machine processes.

Countersinks
Countersinks of different types are available.
The commonly used countersinks have multiple cutting edges and are available in taper shank and straight shank. (Fig 2)

For countersinking small diameter holes special countersinks with two or one flute are available. This will reduce the vibration while cutting.

COUNTERSINKS WITH PILOT  (Figure):

For precision countersinking, needed for machine tool assembling and after machining process, countersinks with pilots are used.
They are particularly useful for heavy duty work. A pilot is provided atthe end for guiding the countersink concentric to the hole. Countersinks with pilots are available with interchangeable and solid pilots.
The countersink holes according to Indian Standard IS 3406 (Part 1) 1986 are of four types: Type A, Type B, Type C and Type E.
Type A is suitable for slotted countersink head screws, cross-recessed and slotted raised countersink head screws. These screws are available in two grades i.e. medium and fine.
Type 'B' countersink holes are suitable for countersink head screws with hexagon socket.