Though drilling is the primary operation performed in a drilling machine, a number of similar operations are also performed on holes using different tools.The different operations that can be performed in a drilling machine are:
Drilling is the operation of producing a cylindrical hole of required diameter and depth by removing metal by the rotating edge of a cutting tool called drill. Drilling is one of the simplest methods of producing a hole. Drilling does not produce an accurate hole in a workpiece. The internal surface of the hole generated by drilling becomes rough and the hole is always slightly oversize due to vibration of the spindle and the drill. A hole made by a drill of size 12mm will measure approximately upto 12.125mm and by a drill of size 22mm will measure upto 22.5mm.
The size of hole made by drilling may not be accurate and the internal surface will not be smooth. Reaming is an accurate way of sizing and finishing a hole which has been previously drilled by a multi point cutting tool known as reamer.The surface obtained by reaming will be smoother and the size accurate. The speed of the spindle is made half that of drilling. Reaming removes very small amount of metal ( approx 0.375 mm). In order to finish a hole and bring it to the accurate size, the hole is drilled slightly undersize.
Boring is the operation enlarging the diameter of the previously made hole. It is done for the following reasons.
- To enlarge a hole by means of an adjustable cutting This is done when a suitable sized drill is not available or the hole diameter is so large that is cannot be ordinarily drilled.
- To finish a hole accurately and bring it to the required size.
- To machine the internal surface of the hole already produced in casting.
- To correct out of roundness of the hole.
- To correct the location of the hole as the boring tool follows independent path with respect to the hole.
Boring tool is a tool with only one cutting edge. The tool is held in a boring bar which has a taper shank to fit into the spindle or a socket. For perfectly finishing a hole, the job is drilled undersize slightly. Boring operation in some precise drilling machine is performed to enlarge the holes to an accuracy of 0.00125mm. The spindle speed during boring should be adjusted to be lesser than that of reaming.
Counterboring is the operation of enlarging the end of the hole cylindrically. The enlarged hole forms a square shoulder with the original hole. This is necessary in some cases to accommodate the heads of bolts, studs and pins. The tool used for counter boring is known as counter bore.
The counterbores are made with cutting edges which may be straight or spiral. The cutting speed for counterboring is atleast 25% lesser than that of drilling.
Countersinking is the operation of making a cone shaped enlargement at the end of the hole. The included angle of the conical surface may be in the range of 60° to 90°. It is used to provide recess for a flat headed screw or a counter sunk rivet fitted into the hole. The tool used for counter sinking is known as a countersink. It has multiple cutting edges on its conical surface. The cutting speed for countersinking is 25% lesser than that of drilling.
Spot facing is the operation of smoothing and squaring the surface around a hole. It is done to provide proper seating for a nut or the head of a screw. A counterbore or a special spot facing tool may be employed for this purpose.
Tapping is the operation of cutting internal threads by means of a cutting tool called ‘tap’. Tapping in a drilling machine may be performed by hand or by power. When the tap is screwed into the hole, it removes metal and cuts internal threads which will fit into external threads of the same size.
Trepanning is the operation of producing a hole in sheet metal by removing metal along the circumference of a hollow cutting tool. Trepanning operation is performed for producing large holes. Fewer chips are removed and much of the material is saved while the hole is produced. The tool may be operated at higher speeds. The speed depends upon the diameter of the hole to be made. The tool resembles a hollow tube having cutting edges at one end and a solid shank at the other to fit into the drill spindle.