In manufacturing, a small design detail can make the difference between a smooth assembly and a costly production delay. One of the most overlooked yet critical choices is whether to use a tapped hole or a drilled hole.
A drilled hole is simply an unthreaded opening created by removing material with a drill bit. A tapped hole, on the other hand, is a drilled hole with internal threads cut into it to accept a fastener directly.
Choosing the wrong type can lead to stripped threads, loose assemblies, or rework — all of which cost time and money. This guide breaks down both hole types, compares their characteristics, and gives you practical decision-making criteria.
What is a Drilled Hole
A drilled hole is a cylindrical opening created by removing material from a solid workpiece with a rotating cutting tool, usually a drill bit. It is unthreaded by default and can serve as a clearance path, a pilot for tapping, a guide for alignment pins, or a passage for fluids.
Process of Drilled Holes
- Mark and position: Use layout lines or a center punch to mark the exact location and prevent drill wander.
- Select the drill bit: Choose the type and size based on material, required diameter, and depth.
- Secure the workpiece: Clamp or fixture it to ensure stability and safety.
- Set speed and feed: Adjust spindle speed and feed rate to match material hardness.
- Drill the hole: Feed the rotating drill bit into the material, using steady pressure.
- Apply coolant or lubricant: Reduce heat, extend tool life, and improve surface finish.
- Evacuate chips: For deep holes, use peck drilling or high-pressure coolant.
- Deburr and clean: Remove sharp edges and chips to prepare for assembly or further machining.
What is a Tapped Hole
A tapped hole is a drilled hole that has been cut with internal threads, allowing it to accept a fastener directly without the need for a nut. It combines the precision of a drilled opening with the functional engagement of threads, enabling strong and compact fastening in assemblies.
Process of Tapped Holes
- Drill the pilot hole: Use a drill bit sized according to the thread’s pitch and diameter specifications.
- Secure the workpiece: Clamp or fixture it to prevent movement during tapping.
- Select the tap: Choose the correct thread form (UNC, UNF, ISO metric, or specialty) and size.
- Apply cutting fluid: Reduce friction, prevent galling, and improve thread finish.
- Cut the threads:
- anual tapping: Rotate the tap slowly, reversing periodically to break chips.
- Machine tapping: Use a tapping head or CNC program for consistent depth and alignment.
- Evacuate chips: Especially important for blind holes to avoid tap breakage.
- Inspect the threads: Check pitch, depth, and engagement percentage.
- Deburr and clean: Remove sharp edges and debris to ensure proper fastener fit.
Tapped Hole vs. Drilled Hole: Key Differences
Threads
- Drilled Hole: No internal threads — purely an unthreaded cylindrical opening.
- Tapped Hole: Contains internal threads to engage with a screw or bolt directly.
Purpose
- Drilled Hole: Acts as a clearance, pilot, or guide hole for fasteners, pins, or fluids.
- Tapped Hole: Serves as a fastening point without the need for a nut, holding the fastener in place.
Machining Steps
- Drilled Hole: Single operation — drilling only.
- Tapped Hole: Two operations — drilling a pilot hole, then cutting threads.
Strength Handling
- Drilled Hole: Relies on an external nut or threaded insert to carry the load.
- Tapped Hole: Threads in the material hold the load directly.
Tolerances
- Drilled Hole: Less demanding; minor variations often acceptable.
- Tapped Hole: Must precisely match thread pitch and diameter standards for proper fit.
Choosing Between a Tapped Hole and a Drilled Hole
Space constraints:
If there’s no room for a nut on the opposite side, a tapped hole is often the better option.
Load capacity:
For high-load applications, drilled holes with a nut or threaded insert can provide greater strength than threads cut directly into soft material.
Assembly speed:
Tapped holes reduce part count but can slow machining. Drilled holes with bolts and nuts may allow faster assembly in the field.
Material properties:
Soft materials like aluminum or plastics are more prone to thread stripping — in these cases, inserts or drilled holes may be preferable.
Cost implications:
Drilling is faster and cheaper. Tapping adds a machining step, tool wear, and inspection requirements.
Maintenance:
Tapped holes can be repaired with helicoils or inserts, but repair is more complex than replacing a nut in a drilled-hole assembly.
