In machining, using the wrong term on a drawing can trigger the wrong setup on the shop floor — leading to wasted time, scrapped parts, or expensive rework.
One of the most common sources of confusion? The difference between a tapped hole and a threaded hole.
Every tapped hole is a threaded hole, but not every threaded hole is tapped.
This guide explains what each term means, how they differ, the main ways to create them, and how to choose the right method for your application.
What is Threaded Hole
A threaded hole is any hole with internal threads that can engage with a matching fastener. The term describes the result, not the process.
Threaded holes can be made by:
- Tapping (cutting or forming)
- Thread milling on a CNC machine
- Thread rolling/forming
- Installing threaded inserts
- Molding threads directly into plastic parts
If the final hole has an internal thread, it’s a threaded hole — no matter how you made it.
What is Tapped Hole
A tapped hole is a threaded hole created using a tap — a tool designed to cut or form threads inside a pre-drilled pilot hole.
- Cutting taps remove material to form the thread profile.
- Forming taps displace material to shape threads without producing chips, creating stronger threads in ductile metals.
Key Differences Between Tapped and Threaded Holes
A tapped hole is just one type of threaded hole, but differences in tooling, material compatibility, and performance matter in production.
1. Definition Scope
- Tapped Hole: Made with a tap, either cutting or forming.
- Threaded Hole: Any internal thread, regardless of method.
2. Manufacturing Process
- Tapped: Requires a size-specific tap and a precisely drilled pilot hole.
- Threaded: Can be tapped, milled, rolled, inserted, or molded.
3. Tool Breakage Risk
- Tapping: Cutting taps, especially in small sizes, are brittle and prone to snapping. Removing a broken tap can damage the workpiece.
- Thread Milling: Much lower risk — the tool is smaller than the hole and easier to extract if it fails.
4. Blind Hole Depth
- Tapping: Bottoming taps can reach deeper than other tap types, but still leave an unthreaded space at the bottom.
- Thread Milling: Can produce perfect threads to full depth.
- Forming Taps: Chipless process ideal for blind holes.
5. Material Limitations
- Cutting taps: Work with almost any machinable material.
- Forming taps: Only for ductile metals like aluminum, brass, and mild steel.
- Thread Milling: Excellent for hard alloys such as titanium and Inconel.
- Threaded Inserts: Best for plastics, composites, or worn hole repairs.
6. Flexibility
- Tapped: Each diameter/pitch requires its own tap.
- Thread Milling: One tool can cut multiple diameters (same pitch) and custom profiles.
7. Cost and Speed
- Tapping: Fastest per hole in high-volume runs.
- Thread Milling: Slower per hole, but more cost-effective for low-volume or mixed-size work.
Choosing Between a Tapped Hole and Other Threading Methods
1. Material Considerations
- Ductile Metals (Aluminum, Brass, Copper, Mild Steel)
These materials deform well without cracking, making them ideal for forming taps (roll tapping). - Hard or Heat-Resistant Alloys (Titanium, Inconel, Hardened Steel)
These materials generate high cutting forces and heat during tapping, increasing the risk of tap breakage. Thread milling is preferred because the smaller cutter and intermittent engagement reduce stress, extend tool life, and improve thread accuracy. - Brittle Materials (Cast Iron, Certain Composites)
These don’t deform well, eliminating forming taps as an option. Cutting taps work, but care must be taken to avoid chipping. Thread milling is a safe alternative if equipment allows. - Soft Materials (Plastics, Wood, Composites)
Cut or formed threads in these materials wear quickly. Threaded inserts are the standard solution, providing durable threads that can handle repeated assembly and disassembly.
2. Production Volume and Part Mix
- High-Volume, Standardized Production
If you’re running millions of identical parts, tapping or forming will almost always be the fastest and cheapest per hole. - Low-Volume, High-Mix Production
In prototyping, job-shop work, or short production runs where thread sizes vary, thread milling can be far more efficient overall.
3. Geometry and Access
- Blind Holes
Blind holes are harder to tap because chips must be evacuated without packing at the bottom. Forming taps solve this by eliminating chips altogether. Thread milling also excels here because it can cut full threads to the bottom without chip issues. - Through Holes
Easier to tap, since chips can exit out the other side. Spiral point taps are the fastest choice for these applications. - Large Diameter Threads
For very large holes (e.g., >M20), taps become expensive and require high torque. Thread milling is often the only practical solution.
4. Risk Tolerance and Cost of Scrap
- High-Value, Late-Stage Parts
Breaking a tap in a nearly finished aerospace component could mean scrapping thousands of dollars of work. Thread milling reduces this risk — a broken mill is easier to remove without damaging the hole. - Low-Value, Early-Stage Parts
Inexpensive parts made early in the machining process can justify the higher scrap risk of tapping in exchange for faster cycle times.
5. Thread Form Requirements
- Custom or Non-Standard Threads
Proprietary forms, multi-start threads, or unusual profiles are often best made with thread milling, which can be programmed for almost any geometry. - Standard Threads
Metric or Unified standard threads can be cut, formed, or milled — choice comes down to speed, material, and volume.
6. Environmental and Operational Factors
Some less obvious but important considerations:
- Lubrication and Coolant Availability
Forming taps require generous lubrication to prevent galling. If coolant access is poor, thread milling may be safer. - Operator Skill Level
Manual tapping is straightforward but prone to misalignment in untrained hands. Thread milling requires more programming knowledge but is easier to automate with consistent results.
Conclusion
A tapped hole is one way to create a threaded hole — but not the only way.
The right choice depends on your material, part geometry, production volume, and tolerance for scrap risk.
Get the terminology right, and you get better tooling choices, fewer mistakes, and stronger, more reliable threads.
