Deep Hole Drilling Capacity Planning: Spindle Needs

A job requiring 50,000 holes at 12 minutes per hole needs 10,000 spindle hours. At 75% utilisation, that is 7 single-spindle machines or 2 four-spindle machines on two shifts. A 20% error in this calculation means a $200,000–$800,000 mistake in capital allocation.

The Capacity Calculation Framework ​

Capacity planning for deep hole drilling follows a six-step process:

Annual Required Parts → Cycle Time per Part → Spindle Hours Required
→ Utilisation Adjustment → Shifts Calculation → Machine Count

Step 1: Determine True Cycle Time ​

Cycle time for deep hole drilling includes much more than cutting time:

Step 2: Calculate Spindle Hours Required ​

Spindle hours = (Annual parts × Cycle time per part) / 60

Example: 50,000 holes × 7.8 min / 60 = 6,500 spindle hours/year

Step 3: Apply Utilisation Rate ​

Realistic utilisation for deep hole drilling operations:

Required machine hours = Spindle hours / Utilisation rate

Example: 6,500 / 0.75 = 8,667 machine hours/year

Step 4: Determine Shifts and Machines ​

Available hours per machine per year:

Machines required = Machine hours / Available hours per machine

Example (2 shifts): 8,667 / 3,840 = 2.3 machines → 3 single-spindle machines

Multi-Spindle Consideration ​

Multi-spindle machines change the calculation fundamentally:

Multi-spindle becomes cost-effective above approximately 4,000–6,000 spindle hours per year on the same part.

Worked Examples ​

Example A: High-Volume Hydraulic Cylinder Bore ​

Spindle hours: 25,000 × 14.5 / 60 = 6,042 hours
Utilisation 78%: 6,042 / 0.78 = 7,746 hours
Available (2 shifts × 240 days): 3,840 hours
Machines: 7,746 / 3,840 = 2.0 → 2 single-spindle OR 1 twin-spindle BTA

Example B: Low-Volume Aerospace Job Shop ​

Spindle hours: 3,000 × 22.3 / 60 = 1,115 hours
Utilisation 60%: 1,115 / 0.60 = 1,858 hours
Available (1 shift): 1,920 hours
Machines: 1,858 / 1,920 = 0.97 → 1 single-spindle gun drilling machine

Growth Capacity Planning ​

When planning capacity, allow for growth:

FAQ ​

How do I estimate cycle time for a new part I have not drilled before? ​

Use gun drill manufacturers' recommended feeds and speeds, calculate cutting time, add 30–50% for peck retraction and idle time. Refine with actual test cuts.

What is a realistic utilisation rate for a first deep hole drilling operation? ​

65–70% for the first year. Do not plan for 80%+ until the process is stable and operators are experienced.

When should I choose multi-spindle over multiple single-spindle machines? ​

When the part volume exceeds approximately 4,000 spindle hours per year and the parts are similar enough that a single machine can run them without excessive changeover.

How does automation affect capacity planning? ​

Robotic loading increases utilisation by 10–15 percentage points by reducing load/unload time and enabling lights-out operation. A machine running 20 hours per day instead of 16 effectively adds 25% capacity without a second machine purchase.

Should I buy one large machine or two smaller ones? ​

Two smaller machines provide redundancy — if one is down for maintenance, the other keeps running. One large machine has lower capital cost per spindle but creates a single point of failure.

Capacity calculations depend on specific part geometry, material, and operating parameters. The examples in this article are illustrative. Perform detailed time studies on actual test cuts before committing to a machine configuration. This article reflects industry practice as of 2026.