JSS 2. TOPIC 2: DRILLS AND DRILLING MACHINES

Drills are cutting tools used in the production of cylindrical holes in the machine shop. They include the following types:
(i).Twist drill: The twist drill has two, three or four lip types. While the two-lip drill is mainly used for drilling holes from origin, the three and four-lip drills are used for finishing or enlarging already-drilled holes. Eg. (to be drawn on the board).

Twist drills

Drilling Machines
 In drilling larger holes and at faster speed, drilling machines are used. Drilling machines make drilling job easier and faster. They include the following:

(i).Sensitive drilling machine:  It is a machine used for lighter jobs, with drill ranging from 1mm to 6mm diameter. It is electrically powered. Eg.

(ii). Bench drilling machine: This drilling machine is used where a hole of up to 15mm diameter is required on a large structure.eg.

(iii).Pillar drilling machine: This drilling machine is used for heavy-duty work, with drills of up to 20mm and above.eg.

(iv).Radial-arm drilling machine: This is a drilling machine used where a large work has to be maneuvered to bring each drilling point in position under the drill. It is usually larger than the first three mentioned. .eg.Bench, Sensitive, Radial and Pillar  drilling machines.

                                          Screw Thread-Forming

A thread is a line forming a groove round the outside of a bolt and screw or the inside of a nut or a hole. It is used to assemble parts of a machines in metal work.
When a thread is made on the outside of a metal, such as on a bolt or screw, it is called an external thread. However, when the thread is made on the inside of the object, it is called an internal thread.

                       Methods/Tools Used for Thread-Forming

(i).Internal Thread: The tool used for forming internal(female) thread is called the tap. There are three types of taps called taper tap, second tap and plug tap. It is necessary to drill a hole equal to the core-diameter of the thread, before taps could be used. Eg.

Tap              

Taps.

The handles for taps are called wrenches. This enables someone to use the manual method to form an internal thread. There is the adjustable wrench and the fixed-size wrench. Eg.

(ii).External thread: The tool used for forming external thread is called the Die. The handle used on the die is called the stock. There are different types of dies called the split die and solid or rectangular die.

                                                   Types of Dies 
(i). Circular split die: This die contains three screws provided for limited adjustment.eg.
(ii). Rectangular split die: This die has two piece screws marked 1 and 2. This die provides for a wide range of adjustment in size.eg.

                                                    Types of Threads
(i). Vee Thread: This is the most common type of thread used for screws and nuts of lighter jobs. Eg.                            

(ii). Whitworth thread: This thread is slightly different from the vee thread. It is also applied on lighter jobs.eg. 
(iii). Acme thread: The acme thread is used by heavy-duty machines and plants. Eg.                                                                                
(iv). Buttress thread: This thread has a little variation from the acme thread. It is also used for heavy-duty jobs. Eg
(v). Iso thread: This thread is used for gears and chains in most machines, especially for loads not too heavy. Eg. 
(vi). Square thread: The square thread is used for wrenches and adjustable devices or in cranes.eg.           

REVIEW QUESTIONS

(i). Explain internal and external threading.
(ii). List and explain four types of threads.
(iii). How is internal thread formed?


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