How Do Tests and Inspections Ensure Quality of Deep Hole Drilled Parts?

Validating Internal Surface Integrity

Tests and inspections are critical for deep hole drilled parts because the process inherently risks creating internal defects that are not visible to the naked eye. The primary focus is on the internal bore surface quality. Techniques like boroscopy provide direct visual inspection to identify micro-cracks, tears, re-cast layers, or debris left from the deep hole drilling process. Identifying and quantifying these flaws is the first step in ensuring that the cooling channels or hydraulic passages will not serve as initiation points for fatigue cracks under the extreme thermal and mechanical stresses seen in aerospace turbines.

Ensuring Dimensional Accuracy and Straightness

Beyond surface quality, the geometric precision of the hole itself is paramount. Airflow and cooling efficiency in a turbine blade depend on the diameter, straightness, and circularity of the cooling channel. Specialized internal gauging and air probe tools are used to verify the hole's diameter and consistency along its entire length. For critical applications, this is complemented by non-contact laser scanning to measure straightness and detect any unwanted deviation or "walking" of the drill bit that could compromise the component's function and balance.

Detecting Subsurface Defects with Advanced NDT

Some of the most dangerous flaws are subsurface. Eddy current testing is exceptionally effective for this, as it can detect minute surface and near-surface cracks and variations in material conductivity around the drilled hole. For parts that have undergone processes like Hot Isostatic Pressing (HIP) prior to drilling, these inspections confirm that the drilling operation has not introduced new defects into the densified material, ensuring the structural benefits of HIP are not negated.

Verifying Cleanliness and Preventing FOD

A crucial yet often overlooked inspection is for cleanliness. Any residual metallic swarf, abrasive media from honing, or other contaminants inside a deep hole can become Foreign Object Debris (FOD), causing catastrophic failure in a rotating engine component. Validation processes include rigorous washing, followed by boroscopic inspection and high-precision air flow tests to ensure passages are completely unobstructed and meet specified flow parameters before final assembly or the application of coatings like Thermal Barrier Coating (TBC).