Page 52 - CITS - Fitter Trade Practical - Volume -2

FITTER - CITS
Job sequence

Cut internal thread in through hole

• Use Ex.No 2.1.67 for this excercise.
• Fix the job in bench vice.
• Fix M 10 first tap in tap wrench and cut internal thread in through hole.
• Similarly, fix M 10 second tap and third tap in tap wrench one by one and cut the internal thread to form full
thread.
• Repeat the above process to cut internal thread in other drilled through hole.
Cut internal thread in blind hole
• Remove metal chips if any from the blind hole by turning it upside down and slightly tapping it on a wooden
surface.
• Fix the M 12 first tap in tap wrench.
• Screw a matching nut on the first tap to the required distance for 14 mm to act as a depth stop.
• Cut internal thread in blind hole to the required depth 14 mm.
• Remove the metal chips, if any from the threaded blind hole.
• Similarly, fix M 12 second tap and third tap in tap wrench one by one and cut the thread to form full thread.

• Clean the threaded hole without burrs.
• Repeat the above process to cut internal thread in other drilled blind hole.
• Check the threaded hole using the M10, and M12 matching bolts by screwing.
• Apply thin coat of oil and pressure it for evaluation.
Note: Use cutting fluid while cutting the thread.

Internal threading of through holes using hand taps holes)

Objectives: At the end of this exercise you shall be able to
• determine the tap drill sizes for internal threading
• cut internal threads using hand taps.

Determining the tap drill size
For cutting internal threads, it is necessary to determine the size of the hole (tap drill size). This can be calculated
using the formula or can be chosen from the table of the tap drill sizes.
Procedure
Drill the hole to the required tap drill size.

Note: Do not forget to give the chamfer required for aligning and starting the tap. (Fig 1)
Hold the work firmly and horizontally in the vice. The top surface should be slightly above the level of the vice
jaws. This will help in using a try square without any obstruction while aligning the tap (Fig 2).
Note: Use soft jaws while holding the finished surface on the vice.

Fix the first tap (taper tap) in the wrench.
Note: Too small a wrench will need a greater force to turn tap. Very large and heavy tap wrenches will
not give the feel required to turn the tap slowly as it cuts.
Position the tap in the chamfered hole vertically by ensuring the wrench in a horizontal plane.
Exert steady downward pressure and turn the tap wrench slowly in a clockwise direction to start the thread. Hold
the tap wrench close to the centre. (Fig 3)

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CITS : C G & M - Fitter - Exercise 62