Calculation Involved to Find Out Drill Size (Metric and Inch) in ITI Turner Trade

In the ITI Turner trade, accurate drilling and threading are essential for producing quality components. Before tapping a hole, it is necessary to drill a hole of the correct size. This size is known as the tap drill size. The calculation of drill size ensures proper thread formation and avoids damage to the tap or workpiece. This chapter explains the calculation methods for drill size in both metric and inch systems.

Introduction to Drill Size Calculation

Drill size calculation is the process of determining the correct diameter of a hole before tapping. If the hole is too small, the tap may break. If the hole is too large, the threads will be weak.

Therefore, selecting the correct drill size is very important for successful threading operations.

Concept of Tap Drill Size

Tap drill size is the diameter of the hole that must be drilled before cutting internal threads. It depends on:

• Major diameter of the thread
• Pitch of the thread

Metric Thread Drill Size Calculation

In the metric system, the drill size can be calculated using the following formula:

Drill Size = Major Diameter – Pitch

Example 1

For an M10 × 1.5 thread:

• Major diameter = 10 mm
• Pitch = 1.5 mm

Drill size = 10 – 1.5 = 8.5 mm

Therefore, an 8.5 mm drill is used before tapping M10 × 1.5 threads.

Example 2

For an M8 × 1.25 thread:

• Major diameter = 8 mm
• Pitch = 1.25 mm

Drill size = 8 – 1.25 = 6.75 mm (approx. 6.8 mm)

Inch Thread Drill Size Calculation

In the inch system, threads are specified in Threads Per Inch (TPI). The formula for drill size is:

Drill Size = Major Diameter – (1 / TPI)

Example 1

For a 1/2 inch, 12 TPI thread:

• Major diameter = 0.5 inch
• TPI = 12

Drill size = 0.5 – (1/12) = 0.5 – 0.083 = 0.417 inch

The nearest drill size is selected from standard drill charts.

Example 2

For a 1/4 inch, 20 TPI thread:

• Major diameter = 0.25 inch
• TPI = 20

Drill size = 0.25 – (1/20) = 0.25 – 0.05 = 0.20 inch

Standard Drill Size Charts

In practice, drill size charts are often used instead of calculations. These charts provide standard drill sizes for different thread types.

However, understanding the calculation helps in situations where charts are not available.

Importance of Correct Drill Size

• Ensures proper thread formation
• Prevents tap breakage
• Improves strength of threads
• Ensures smooth assembly of parts

Factors Affecting Drill Size Selection

Several factors influence the selection of drill size:

• Type of material (soft or hard)
• Type of thread
• Depth of hole
• Required accuracy

Practical Considerations

In actual workshop practice:

• Standard drill sizes are selected nearest to calculated values
• Cutting fluid is used to improve threading
• Proper alignment is maintained during drilling

Common Mistakes

• Using incorrect drill size
• Ignoring pitch or TPI
• Not using lubrication
• Poor alignment of drill

Precautions While Drilling for Tapping

• Use correct drill bit
• Maintain proper speed and feed
• Ensure workpiece is properly clamped
• Use cutting fluid

Application in Turner Workshop

In the Turner trade, drill size calculation is used before tapping operations to ensure accurate threading. It helps in producing components with proper fitting and strength.

This knowledge is essential for both manual and machine tapping operations.

Conclusion

Calculation of drill size is an important skill in the ITI Turner trade. Understanding the formulas for metric and inch systems helps in selecting the correct drill size for threading operations.

By applying these calculations and following proper techniques, trainees can achieve accurate and efficient results.

In conclusion, correct drill size calculation ensures better thread quality, tool life, and overall machining performance.