Combination Drill – Appropriate Selection of Size from Chart in ITI Turner Trade

In the ITI Turner trade, accurate center drilling is an essential step before performing turning operations. A combination drill, also known as a center drill, is used to produce a center hole that guides further machining operations. Proper selection of the correct size of combination drill from standard charts is important to ensure accuracy and avoid damage to the workpiece. This chapter explains the construction, types, and selection of combination drills in detail.

Introduction to Combination Drill

A combination drill is a special type of drill used to produce a center hole in a workpiece. It combines two tools in one: a drill and a countersink. The drill portion makes the hole, and the countersink provides a chamfer for proper seating of centers.

It is widely used in lathe operations where the workpiece is supported between centers.

Construction of Combination Drill

A combination drill consists of the following parts:

• Pilot Drill: Small diameter portion for initial drilling
• Countersink Portion: Larger diameter for chamfering
• Body: Main part of the tool
• Shank: Held in the drill chuck

The tool is made from high-speed steel (HSS) for strength and durability.

Types of Combination Drills

Combination drills are available in different types based on shape and application:

• Type A (without protection)
• Type B (with protective countersink)
• Type R (radius type)

Standard Sizes of Combination Drill

Combination drills are available in standard sizes, usually numbered from 0 to 6. Each size corresponds to a specific pilot diameter and countersink diameter.

• No. 0 – Small size for light work
• No. 1 – Small components
• No. 2 – General purpose
• No. 3 – Medium work
• No. 4 – Larger workpieces
• No. 5 & 6 – Heavy-duty work

Combination Drill Size Chart

The selection of combination drill size is based on standard charts. These charts specify:

• Pilot diameter
• Countersink diameter
• Recommended application

Example:

• No. 1 → Pilot diameter approx. 1.5 mm
• No. 2 → Pilot diameter approx. 2.5 mm
• No. 3 → Pilot diameter approx. 3.15 mm

Selection of Combination Drill Size

Proper selection of combination drill size depends on:

• Diameter of the workpiece
• Type of operation
• Size of lathe center
• Strength required at the center

Guidelines for Selection

• Small workpieces → Use smaller size drills (No. 0–2)
• Medium workpieces → Use medium size drills (No. 2–4)
• Large workpieces → Use larger size drills (No. 4–6)

Importance of Correct Selection

• Ensures proper seating of lathe center
• Prevents damage to workpiece
• Improves machining accuracy
• Increases tool life

Uses of Combination Drill

• Making center holes
• Preparing workpieces for turning
• Ensuring alignment in machining

Advantages

• Combines drilling and countersinking in one tool
• Provides accurate center holes
• Reduces machining time

Limitations

• Not suitable for deep drilling
• Requires proper handling to avoid breakage

Precautions While Using Combination Drill

• Select correct size from chart
• Maintain proper speed and feed
• Use cutting fluid
• Ensure proper alignment

Maintenance of Combination Drill

• Keep tool clean and sharp
• Store properly
• Avoid excessive force

Application in Turner Workshop

In the Turner trade, combination drills are used before turning operations to create accurate center holes. This ensures proper support of the workpiece and improves machining quality.

Correct selection of drill size from charts is essential for achieving precision and safety.

Conclusion

The combination drill is an important tool in the ITI Turner trade. Understanding its construction, types, and size selection helps in producing accurate center holes.

By selecting the correct size from standard charts and following proper techniques, trainees can improve efficiency and quality of work.

In conclusion, proper use of combination drills plays a key role in successful machining operations.