Drill Counterbores and their uses

Counterboring

.differentiate counterboring and spotfacing

. the types of counterbores and their uses

. determine the correct counterbore sizes of different holes

Counterboring

Counterboring is an operation of enlarging a hole to a given depth, to house head of socket heads or cap screws with the help of a counterbore tool.

Counterbore(Tool)

The tool used for counterboring is called a counterbore.Counterbores will have two or more cutting edges.

At the cutting end,a pilot is provided to guide the tool concentric to  the previously drilled hole. The pilot also helps to avoid chattering while counterboring.

Counterbores are available with solid pilots or with interchangeble pilots. The interchangeable pilot provides flexibility of counterboring on different diameter of holes.

Spot facing

Spot facing is a machining operation for producing a flat seat for bolt head, washer or nut at the opening of a drilled hole. The tool is called a spot facer or a spot facing tool. Spot facing is similar to counterboring, except that it is shallower. Tools that are used for counterboring can be used for spot facing as well.

Spot facing is also done by fly cutters by end cutting action. The cutter blade is inserted in the slot of the holder, which can be mounted on the spindle.

Counterbore sizes and specification

Counterbore sizes are standardised for each diameter of screws  as per BIS

There are two main types of counterbores. Types H and type K.

The type H counterbore are used for assemblies with slotted cheese head, slotted pan head and cross recessed pan head screws. The type K counterbores are used in assembllies with hexagonal socket head cap screws.

For fitting different types of washers the counterbore standards are different type H and Type K.

The clearance hole d1 are  of two different grades i.e. medium (m) and fine (f) and are finished to H13 and H 12 diemensions.