Why Does Deep Hole Machining Cost More?

In the field of custom CNC machining and non-standard parts manufacturing, customers often ask a question: why does deep hole machining cost significantly more even though it is still “just drilling”?

In fact, deep hole machining (typically defined as a length-to-diameter ratio L/D ≥ 5) is not simply drilling deeper. It is a precision machining process that places much higher demands on equipment, tooling, process control, and engineering experience.

Below is a breakdown of the key cost factors.

1. Physical Limit Challenge: Chip Removal and Cooling

In standard drilling, chips can be naturally evacuated through the helical flutes. However, in deep hole machining (e.g., L/D > 10), the cutting zone is deep inside the part.

Chip evacuation becomes the #1 challenge:
Chips easily accumulate inside the hole. Once chip evacuation is blocked, chips will scratch or compress the hole wall, causing surface roughness failure. In severe cases, chips can jam the tool, leading to tool breakage and scrapped parts.

Cooling cannot reach the cutting edge easily:
Conventional external coolant spraying cannot reach the cutting zone. High-pressure internal coolant systems are required to deliver coolant directly through internal channels in the tool. This requires specialized machines and significantly increases tool cost.

2. Precision Control Challenge: Deflection and Vibration

The deeper the hole, the lower the rigidity of the tool—similar to a thin long rod that easily bends under force.

Tool deflection (deviation) risk:
In deep holes, the tool is affected by material inconsistency and asymmetric cutting forces, which can cause deviation. Maintaining straightness and concentricity over hundreds of millimeters requires high-precision machines and advanced guiding techniques.

Vibration and surface finish:
Deep hole machining is prone to high-frequency vibration (chatter), which severely affects internal surface quality. To achieve Ra 0.8 or better, cutting feed rates must be reduced and multiple processes are often required (drilling → reaming → boring → honing/roller burnishing). This increases machining time several times compared to standard drilling.

3. Expensive Specialized “Tools”

Deep hole machining cannot be done with standard twist drills.

Special tooling required:
We must use expensive gun drills, BTA deep hole drilling tools, or custom long-reach solid carbide internal-coolant drills. These tools typically cost 5–10 times more than standard drills.

Frequent maintenance and regrinding:
Tool wear must be monitored closely. If wear is not detected in time, it may cause hole damage or tool breakage.

4. Higher Risk and Scrap Rate

Deep hole machining is considered a high-risk process:

• Tool breakage may directly scrap the part
• Chip evacuation issues may cause internal surface damage
• Internal quality cannot be visually inspected in real time

Therefore, manufacturers must include additional costs in pricing, such as:

• Risk cost
• Trial machining cost
• Quality control and inspection cost

At Shenzhen PANS Technology, we specialize in high-precision CNC machining and custom non-standard parts manufacturing. We have strong capability in complex deep-hole structures.

If you have drawings or sample requirements, feel free to send us an RFQ. We can help evaluate the process and optimize the cost solution.