Through drilling is a drilling operation in which the drill bit passes completely through the workpiece, producing a hole that opens on both sides. It is commonly used when full penetration is required for fastening or assembly purposes.

Through Drilling:

Through drilling is a machining operation in which a rotating cutting tool (drill bit) produces a hole that passes completely through the workpiece, entering from one side and exiting the opposite side. It is one of the most common drilling operations in manufacturing, fabrication, and maintenance.

1. Definition and Purpose

Definition: A drilling process where the hole fully penetrates the material thickness.
Purpose:
- Fastening (bolts, screws, rivets)
- Assembly alignment
- Fluid or air passage
- Weight reduction
- Preparation for tapping, reaming, or countersinking

2. Machines Used

Through drilling can be performed on:

Drill Press (Bench / Pillar / Radial)
CNC Drilling or Machining Centers
Lathe (workpiece rotates, drill is stationary)
Hand Drills (low-precision applications)

3. Cutting Tools (Drill Bits)

Common drill types:

Twist Drill (most widely used)
Step Drill
Gun Drill (for deep through holes)
Carbide Drill (high-speed, hard materials)

Drill Geometry:

Point angle: Typically 118° (general use) or 135° (hard materials)
Helix angle: Helps chip evacuation
Flutes: Carry chips out of the hole

4. Process Steps

Marking / Centering
- Center punch or spotting drill used to prevent drill wandering
Clamping the Workpiece
- Fixed firmly using vice, fixture, or clamps
Tool Selection
- Proper drill size and material
Setting Parameters
- Spindle speed (RPM)
- Feed rate
- Coolant selection
Drilling Operation
- Drill enters the workpiece
- Material is cut and chips are removed via flutes
Breakthrough
- Drill exits the opposite side
- Requires reduced feed to avoid burrs and tool damage
Deburring
- Burrs removed from exit side

5. Cutting Parameters

a) Spindle Speed (RPM)

Depends on:

Drill diameter
Material type
Formula:

$\text{RPM} = \frac{1000 \times V}{\pi \times D}$ RPM=π×D1000×V

Where:

$V$ V = cutting speed (m/min)
$D$ D = drill diameter (mm)

b) Feed Rate

Measured in mm/rev
Too high → tool breakage
Too low → poor cutting, heat buildup

6. Coolant and Lubrication

Purpose:
- Reduces heat
- Improves surface finish
- Extends tool life
Types:
- Water-based emulsions
- Cutting oil
- Dry drilling (some aluminum or cast iron)

7. Chip Formation and Removal

Chips must flow freely out of the hole
For deep through holes:
- Peck drilling is used (intermittent drilling to clear chips)
Poor chip removal leads to:
- Tool breakage
- Overheating
- Poor accuracy

8. Accuracy and Surface Finish

Through drilling typically gives moderate accuracy
Tolerances: ±0.1 mm (standard drilling)
For high precision:
- Reaming after drilling
- CNC machines preferred

9. Common Defects and Problems

Defect	Cause	Prevention
Burrs at exit	High feed at breakthrough	Reduce feed near exit
Drill breakage	Excess feed / poor chip removal	Correct parameters
Oversized hole	Worn tool / vibration	Replace tool, improve clamping
Poor surface finish	Dull drill / no coolant	Sharpen tool, use coolant