Deep Hole Drilling for Additive Manufacturing and Hybrid Processes

Additive manufacturing is changing how we think about deep hole drilling. Instead of drilling a solid bar to create a hole, additive processes can build the part with the hole already partially formed. But the as-printed surfaces are not suitable for most engineering applications, so post-process drilling is still needed.

I have worked with hybrid manufacturing processes that combine additive deposition with subtractive machining in the same machine. The deep hole drilling step comes after the additive build, and it presents unique challenges compared to drilling from wrought material. The approach to selecting a machine for the application becomes more complex when the machine must serve both additive and subtractive functions.

Additive Near-Net-Shape Bores

The most promising application I have seen for additive in deep hole drilling is near-net-shape bores. The additive process builds the part with a rough bore that is slightly undersized. The deep hole drilling step then finishes the bore to the final size and surface finish.

This approach reduces the amount of material that needs to be removed by drilling. For a 50mm bore through 500mm of material, the additive process might leave a 45mm rough bore. The drilling step removes only 2.5mm per side instead of 25mm from solid.

The material savings are significant:

A 90% reduction in material removal translates directly to faster cycle times and lower tooling costs. The trade-off is that the additive build adds time and cost on the front end.

Challenges with As-Printed Surfaces

The as-printed surface of an additively manufactured bore is rough and inconsistent. I have measured surface roughness of Ra 6-20um on laser powder bed fusion bores. The surface also has partially fused particles that can break off during drilling and create defects.

The main challenges I have encountered:

Variable stock allowance. The additive process does not produce perfectly uniform bore diameters. I have seen variation of 0.5-1mm in the as-printed bore diameter. The drilling tool must handle this variable stock without overloading.

Surface irregularities. Partially fused powder particles on the bore surface act as abrasive inclusions. They wear the cutting edge faster than wrought material of the same composition.

Internal voids. Additive parts can have internal porosity or lack-of-fusion defects. When the drill encounters a void, the cutting forces change suddenly, and the drill can grab or chatter.

For drilling additively manufactured Inconel 718:

I run at 20-30 m/min for the first pass through additive material, then increase to standard parameters for the finishing pass. The conservative first pass prevents tool overload from the variable stock and surface irregularities.

Tool Selection for Hybrid Work

Tool selection is critical for hybrid additive-subtractive work. Standard carbide gun drills work well for wrought material but can chip on the abrasive as-printed surface.

I prefer gun drills with:

• Higher cobalt content (10-12%) for toughness
• AlTiN or AlCrN coating for heat resistance
• Sharper cutting edge geometry (lower edge hone)
• Multiple guide pad configurations for stability

The sharper cutting edge is important. A standard gun drill has a 0.02-0.05mm edge hone for durability. For additive material, I use a 0.01mm hone or less. The sharper edge cuts through the rough surface with less cutting force.

I also find that inserts for BTA drilling need to be changed more frequently on additive work. I change inserts every 2 meters of drilling on additive Inconel versus every 4 meters on wrought material.

Coolant Considerations for Additive Materials

Additive materials often have internal porosity that affects coolant flow. If the part has interconnected porosity in the bore wall, coolant can leak out of the bore into the part interior.

I have seen this happen with laser powder bed fusion parts that were not fully dense. The coolant pressure was 1000 psi at the pump, but the pressure at the cutting edge was only 400 psi because of leakage through the porous wall.

The solution is to seal the bore surface before drilling. I have used:

• Surface impregnation with low-viscosity resin
• Thin-wall sleeve insertion for pressure retention
• Increased pump capacity to compensate for leakage

For production work, the resin impregnation is the most reliable method. It penetrates the surface porosity and seals the leak paths without affecting the material properties.

Hybrid Machine Considerations

Some of the hybrid machines I have worked with combine additive deposition with machining in the same setup. The part is built on the machine, then the drilling head moves into position and drills the bore without the part being moved.

This is ideal for large, expensive components where setup time is significant. The part stays on the machine between the additive and subtractive steps, so there is no alignment error between the two processes.

The challenge is that the machine must serve two different functions. The additive head requires a different spindle interface and control system than the drilling head. I have seen hybrid machines that switch between heads automatically, but the changeover takes 15-30 minutes.

Key Takeaways

• Near-net-shape additive bores reduce material removal by 90% compared to drilling from solid
• As-printed bore surfaces have Ra 6-20um roughness and require conservative first-pass parameters at 20-30 m/min
• Gun drills for additive work need sharper edge geometry (0.01mm hone) and AlTiN coating
• Internal porosity in additive parts can cause coolant pressure loss; resin impregnation is the most reliable seal method
• Hybrid machines eliminate re-alignment error between additive build and drilling, saving significant setup time