Chapter 7: Machine Shop Estimation – Drilling Operations

7.1 Introduction to Drilling Operations

Drilling is a fundamental machining process used to create round holes in a workpiece. It is one of the most widely used processes in manufacturing and plays a critical role in operations like fitting, assembly, and fabrication. Estimating the time, cost, and resources for drilling operations is essential for efficient planning and production control in a machine shop.

7.2 Fundamentals of Drilling

Drilling is the process of cutting a hole into a solid material using a rotating cutting tool called a drill bit. The drilling machine or drill press provides the rotational motion and axial feed required to perform the operation.

7.2.1 Common Drilling Machines

• Bench Drilling Machine

• Pillar Type Drilling Machine

• Radial Drilling Machine

• Gang Drilling Machine

• Multiple Spindle Drilling Machine

• CNC Drilling Machine

7.2.2 Types of Drill Bits

• Twist drill (most common)

• Spade drill

• Step drill

• Center drill

• Countersink and counterbore drills

7.3 Parameters Affecting Drilling Estimation

To accurately estimate the drilling time and cost, the following factors must be considered:

1. Material of Workpiece

2. Drill Bit Material and Diameter

3. Hole Depth

4. Cutting Speed (V)

5. Feed Rate (f)

6. Machine Capability

7. Tool Setup and Change Time

8. Coolant and Lubrication Requirements

7.4 Cutting Speed and Feed Rate in Drilling

7.4.1 Cutting Speed (V)

The speed at which the cutting edge of the drill bit moves across the surface of the workpiece.

Where:

• = Cutting speed (m/min)

• = Diameter of the drill (mm)

• = Spindle speed (rev/min)

7.4.2 Feed Rate (f)

The distance the drill advances during one revolution.

7.5 Machining Time Calculation for Drilling

7.5.1 Formula

Where:

• = Time for drilling

• = Length of hole (including approach and clearance, typically 1.3 × depth)

• = Feed per revolution

• = Spindle speed

7.5.2 Example

Problem:
Estimate the time to drill a hole of 20 mm diameter and 50 mm depth in mild steel. Assume cutting speed = 30 m/min, feed = 0.25 mm/rev.

Solution:
Given:

• D = 20 mm

• V = 30 m/min

• f = 0.25 mm/rev

• L = 1.3 × 50 = 65 mm

First, calculate spindle speed :

Now, calculate machining time :

Answer: 33 seconds

7.6 Cost Estimation in Drilling

To estimate cost, the following components are considered:

1. Material Cost – Raw material for the part

2. Labor Cost – Based on time and wage rate

3. Machine Cost – Hourly rate of the machine

4. Tooling Cost – Drill bit usage and wear

5. Overhead Costs – Utilities, administration, maintenance

7.6.1 Total Drilling Cost Formula

7.7 Tool Wear and Life

Tool life in drilling depends on:

• Drill bit material (HSS, carbide, etc.)

• Workpiece hardness

• Speed and feed settings

• Coolant/lubrication

Using Taylor’s tool life equation can help in determining optimum cutting conditions.

7.8 Safety Considerations

• Always secure the workpiece properly.

• Use appropriate cutting fluid.

• Wear protective eyewear and gloves.

• Do not attempt to clear chips with bare hands.

• Maintain proper speed and feed to avoid tool breakage.

7.9 Practical Applications of Drilling Estimation

• Job shop and batch production planning

• CNC programming and cycle time calculations

• Costing and quotation preparation

• Tool selection and procurement

• Production scheduling

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7.10 Summary

Drilling is an essential machining operation that requires precise estimation of time and cost for effective workshop management. By understanding the key variables—cutting speed, feed, hole depth, tool type, and machine capacity—engineers can plan accurately and optimize production efficiency.

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7.11 Exercises

1. A 16 mm diameter hole is to be drilled to a depth of 40 mm in aluminum. Calculate the drilling time. Take cutting speed = 60 m/min, feed = 0.3 mm/rev.

2. A hole of 12 mm diameter is drilled into mild steel with a depth of 30 mm. Determine the spindle speed and machining time if cutting speed is 25 m/min and feed is 0.2 mm/rev.

3. Estimate the total cost for drilling a 10 mm hole 25 mm deep in brass, if machine rate = ₹300/hour, labor rate = ₹100/hour, and tool cost = ₹5 per hole.

4. Describe the impact of tool wear on drilling cost and accuracy.

5. Prepare a comparative table showing typical cutting speeds and feeds for different materials (steel, aluminum, brass, cast iron).