Drills – Parts, Types, Sizes, Cutting Angles, Cutting Speed, Boring Tool, Countersinking and Counterboring

In the ITI Turner trade, drilling is an important machining operation used to produce holes in a workpiece. Various types of drills and related tools are used for different purposes. Understanding their parts, types, cutting angles, and speeds is essential for efficient and accurate machining. This chapter also covers boring tools, countersinking, counterboring, and special drills such as letter, number, and core drills.

Introduction to Drills

A drill is a cutting tool used to make round holes in a workpiece. The most common type is the twist drill, which has helical flutes to remove chips.

Parts of a Drill

• Shank: Held in the drill chuck
• Body: Main part of the drill
• Flutes: Helical grooves for chip removal
• Cutting Edge: Removes material
• Point: Tip of the drill
• Lip: Cutting edges at the tip
• Web: Central portion providing strength

Types of Drills

• Twist Drill: Most commonly used
• Flat Drill: Simple design for rough work
• Step Drill: Used for different hole sizes
• Center Drill: Used for making center holes
• Core Drill: Used to enlarge existing holes

Sizes of Drills

Drill sizes are available in different systems:

• Metric sizes (mm)
• Fractional sizes (inch)
• Letter drills (A to Z)
• Number drills (1 to 80)

Letter and Number Drills

Letter drills range from A (largest) to Z (smallest), while number drills range from 1 to 80 (1 is largest, 80 is smallest).

These are used for precise hole sizes in engineering work.

Cutting Angles of Drill

1. Point Angle

The angle at the tip of the drill, usually 118° for general purpose drilling.

2. Lip Clearance Angle

Prevents rubbing and allows cutting action.

3. Helix Angle

Helps in chip removal and cutting efficiency.

4. Chisel Edge Angle

The angle at the center of the drill point.

Cutting Speed for Different Materials

Boring Tool

A boring tool is used to enlarge and finish an already drilled hole. It improves accuracy and surface finish.

Uses

• Enlarging holes
• Improving hole accuracy
• Finishing internal surfaces

Countersinking

Countersinking is the process of enlarging the top portion of a hole to form a conical shape.

Uses

• Providing seating for countersunk screws
• Removing burrs

Counterboring

Counterboring is the process of enlarging the top part of a hole to a cylindrical shape with a flat bottom.

Uses

• Providing seat for bolt heads
• Ensuring proper fitting

Core Drill

A core drill is used to enlarge existing holes. It usually has three or four cutting edges.

Advantages

• Better accuracy than twist drill
• Smoother finish

Advantages of Proper Drill Selection

• Improves efficiency
• Reduces tool wear
• Provides accurate holes

Precautions While Drilling

• Use correct drill size
• Maintain proper speed and feed
• Use cutting fluid
• Ensure proper clamping

Maintenance of Drills

• Sharpen regularly
• Keep clean
• Store properly

Application in Turner Workshop

In the Turner trade, drills are used for making holes, preparing workpieces, and finishing operations. Proper understanding of drill types, angles, and cutting speeds ensures efficient machining.

Boring, countersinking, and counterboring operations improve accuracy and quality.

Conclusion

Drills and related tools are essential in machining operations. Understanding their parts, types, sizes, and cutting parameters is important for achieving accurate results.

Proper selection and use of drilling tools improve productivity and quality.

In conclusion, mastering drilling operations is a key skill in the ITI Turner trade.