Drilling Machine Operations
The different operations that can be performed in a drilling machine are:
|1. Drilling||6. Spot facing|
|2. Reaming||7. Tapping|
|3. Boring||8. Lapping|
|4. Counter boring||9. Grinding|
|5. Counter sinking||10. Trepaning|
Drilling is the operation of producing a cylindrical hole by removing metal by the rotating edge of a cutting tool called the drill. The drilling is one of the simplest methods of producing a hole. Before drilling the centre of/the hole is located on the workpiece by drawing two lines at right angles to each other and then a centre punch is used to produce an indentation at the centre. The drill point is pressed at this centre point to produce the required hole. Drilling does not produce an accurate hole in a workpiece and the hole so generated by-drilling becomes rough and the hole is always slightly oversize than the drill used due to the vibration of the spindle and the drill. A 12 mm drill may produce a hole as much as0.125 mm oversize and a 22 mm drill may produce one as much as 0.5 mm oversize.
Reaming is an accurate way of sizing and finishing a hole which has been previously drilled. In order to finish a hole and to bring it to the accurate size, the hole is drilled slightly undersize. The speed of the spindle is made half that of drilling and automatic feed may be employed. The tool used for reaming is known as the reamer which has multiple cutting edges. Reamer cannot originate a hole. It simply follows the path which has been previously drilled and removes a very small amount of metal. For this reason a reamer cannot correct a hole location. The material removed by this process is around 0.375 mm and for accurate work this should not exceed 0.125 mm.
Boring illustrated in Fig.5.28 is performed in a drilling machine for reasons stated below:
- To enlarge a hole by means of an adjustable cutting tool with only one cutting edge. This is necessary where suitable sized drill is not available or where hole diameter is so large that it cannot be ordinarily drilled.
- To finish a hole accurately and to bring it to the required size.
- To machine the internal surface of a 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 an independent path with respect to the hole.
Boring Operation Counter boring Operation
The cutter is held in a boring bar which has a taper shank to fit into the spindle socket. For perfect finishing a hole, the job is drilled slightly, undersize. In precision machines, the accuracy is as high as ± 0.00125 mm. It is a slow process than reaming and requires several passes of the tool.
Counter boring shown in Fig. is the operation of enlarging the end of a 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 called a counter bore. The counter bores are made with straight or tapered shank to fit in the drill spindle. The cutting edges may have straight or spiral teeth. The tool is guided by a pilot which extends beyond the end of the cutting edges. The pilot fits into the small diameter hole having running clearance and maintains the alignment of the tool. These pilots may be interchanged for enlarging different size of holes. Counter boring can give an accuracy of about ± 0.050 mm. The cutting speed for counter boring is 25% less than that of drilling operation.
Countersinking shown in Fig.5.30 is the operation of making a cone-shaped enlargement of the end of a hole to provide a recess for a flat head screw or countersunk rivet fitted into the hole. The tool used for countersinking is called a countersink. Standard countersinks have 60°, 82° or 90° included angle and the cutting edges of the tool are formed at the conical surface. The cutting speed in countersinking is 25% less than that of drilling.
Spot facing shown in Fig. is the operation of smoothing and squaring the surface around a hole for the seat for a nut or the head of a screw. A counter bore or a special spot facing tool may be employed for this purpose.
Tapping shown in Fig. is the operation of cutting internal threads by means of a cutting tool called a tap. A tap may be considered as a bolt with accurate threads cut on it. The threads act as cutting edges which are hardened and ground. 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.
Tap Drill size
The size of the tap being the outside diameter of its threads, it is evident that the drilled hole must be smaller than the tap by twice the depth of the thread. The amount to be subtracted from the tap diameter depends on the shape of the thread, e.g. B.S.W., B.S.F., Indian Standard Thread (IS) etc. Tap drill size may thus be derived from the following formula
D = T- 2d
Where, D is the diameter of tap drill size, T diameter of tap or bolt to be used and d depth of thread.
Lapping is the operation of sizing and finishing a small diameter hole already hardened by removing a very small amount of material by using a lap. There are many kinds of lapping tools. The copper head laps are commonly used. The lap fits in the hole and is moved up and down while it revolves.
Grinding operation may be performed in a drilling machine to finish a hardened hole. The grinding wheel is made to revolve with the spindle and is fed up and down. A suitable grinding wheel may be selected for surface grinding operation. Grinding can also be done to correct out of roundness of the hole. The accuracy in grinding operation is quite high about ±0.0025 mm.
Trepanning shown in Fig. is the operation of producing a hole 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 as the variation in diameter of the tool is limited by the narrow cutting edge. 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. This is one of the efficient methods of producing a hole.