Center Drilling-Everything You need to know

Center drilling is a preliminary drilling operation used to create a small, precise starter hole at the center of a workpiece. This guides larger drill bits, ensures accuracy, and prevents the drill from wandering during subsequent drilling.

Center Drilling

1. Definition

Center drilling is a machining operation used to produce a short, accurate conical hole at the center of a workpiece. It is mainly performed to:

Accurately locate the center before drilling
Provide a seat for lathe centers to support long workpieces

It is usually the first operation before drilling, turning between centers, or deep-hole drilling.

2. Purpose of Center Drilling

Prevents drill wandering
Ensures positional accuracy
Provides support for turning between centers
Improves tool life and surface finish
Helps in alignment of holes

3. Machine Tools Used

Drill press
Lathe machine
CNC machining center
Milling machine

4. Center Drill (Tool Used)

A center drill is a short, rigid cutting tool with two distinct sections.

Parts of a Center Drill

Pilot (Small Diameter Tip)
- Starts the hole accurately
Countersink (Conical Portion)
- Forms a chamfer or center seat
Shank
- Held in the chuck

Typical Angles

60° → Standard lathe center
90° / 120° → Chamfering and spotting
Combined drill & countersink tool

5. Types of Center Drills

Type	Description	Use
Type A	Pilot + 60° cone	General purpose
Type B	Pilot + 60° cone + safety chamfer	Prevents damage
Type C	Larger cone angle	Heavy-duty
Type R	Rounded countersink	Stress reduction

6. Process Steps

Workpiece Setup
- Clamp in vice or chuck
Tool Alignment
- Center drill aligned with spindle axis
Speed Selection
- Lower RPM than twist drill
Cutting Operation
- Pilot enters material
- Conical section forms the center hole
Depth Control
- Drilled just enough for:
  - Drill location OR
  - Lathe center support
Tool Retraction
- Withdraw carefully to avoid breakage

7. Cutting Parameters

Speed: Low to medium (rigid but brittle tool)
Feed: Light feed to prevent snapping the pilot
Coolant: Optional for shallow holes, recommended for steel

8. Importance in Drilling Operations

Twist drills are long and flexible
Without center drilling:
- Drill may walk or shift
- Hole position error occurs
Center drilling ensures true starting point

9. Accuracy and Surface Finish

Very high positional accuracy
Provides smooth conical seat
Improves accuracy of subsequent drilling or turning

10. Common Defects and Causes

Defect	Cause	Prevention
Broken center drill	Excess feed	Light feed
Oversized center hole	Too much depth	Proper depth control
Chatter marks	High speed	Reduce RPM
Off-center hole	Poor alignment	Proper setup