What Is Thread Machining?
Thread machining is the process of cutting helical grooves (threads) on a cylindrical feature. Threads are used for fastening (e.g., bolts, screws), sealing (e.g., pipe threads), or transmission of motion (e.g., leadscrews).
The three common thread-machining processes:
Tapping: a multi-point tool (tap) cuts internal threads by rotating and feeding into a pre-drilled hole. Tapping is the most common internal threading process.
Thread milling: a rotating tool with thread profiles cuts threads by interpolating around the hole. Thread milling handles internal and external threads, large thread sizes, and hard materials.
Thread turning: a single-point tool cuts external threads on a turned part by feeding along the thread helix. Thread turning is used for shafts, leadscrews, and external threads.
The choice depends on the thread size, material, lot size, and required thread quality. The Yuqing processing of mechanical parts capability covers all three processes.
When Is Tapping the Right Process?
Tapping is the right process when:
The thread is internal and the diameter is small (typically up to M16 or 5/8").
The thread is a standard size (e.g., M3, M4, M5, M6, M8, M10, M12, M16).
The material is machinable (e.g., aluminum, mild steel, brass, cast iron).
The lot size is small to medium.
The thread tolerance is standard (6H or 6g).
Tapping is the fastest internal threading process (typical cycle time 1–5 seconds per hole for small threads). The tap is fed into a pre-drilled hole at the appropriate speed, and the threads are cut in one pass.
For hard materials (e.g., stainless steel, tool steel), tapping can be challenging due to high tool wear and tap breakage. Thread milling is often a better choice for these materials.
When Is Thread Milling the Right Process?
Thread milling is the right process when:
The thread is large (typically M16 or larger).
The thread is in a hard material (e.g., stainless steel, tool steel, hardened steel).
The thread is non-standard or has special requirements (e.g., Acme, Trapezoidal, buttress).
The thread tolerance is tight.
The part is expensive and tap breakage would be costly.
Thread milling uses a rotating tool with thread profiles that interpolates around the hole. The toolpath is a helical interpolation that cuts the thread profile gradually.
Thread milling is slower than tapping for small threads but is more reliable and produces better thread quality. The tool is also less prone to breakage, which is critical for expensive parts.
Yuqing's CNC machine tool processing capability includes thread-milling cycles for large and hard-material threads.
When Is Thread Turning the Right Process?
Thread turning is the right process when:
The thread is external on a turned part.
The thread size matches the turning capability (typically up to 200 mm diameter).
The thread is a standard size or has a defined pitch.
The material is machinable.
Thread turning uses a single-point tool with a thread profile. The tool is fed along the thread helix at a rate that matches the thread pitch. The process is used on lathes and turning centers.
For external threads on milled parts (e.g., a stud on a bracket), thread turning is not applicable. Thread milling is the alternative.
What Are the Achievable Tolerances and Quality?
|
Process |
Tolerance |
Surface Finish |
Tool Wear |
Best For |
|
Tapping |
6H/6g standard |
Good |
Moderate |
Standard threads, machinable materials |
|
Thread milling |
4H–6H, tighter possible |
Very good |
Low |
Large threads, hard materials, expensive parts |
|
Thread turning |
4g–6g, tighter possible |
Very good |
Low |
External threads on turned parts
|
The thread tolerance depends on the thread profile accuracy, the tool wear, and the machine capability. Tapping is typically limited to standard tolerances; thread milling and thread turning can achieve tighter tolerances.
How Is Tapping Performed?
Tapping is performed with a tap, which is a multi-point tool with cutting edges that match the thread profile.
Steps:
Drill a pilot hole: the pilot hole is drilled to the recommended tap drill size for the thread.
Mount the tap: the tap is mounted in a tap holder (rigid or floating).
Tap the hole: the tap is fed into the hole at the appropriate speed and feed.
Reverse the tap: the tap is reversed out of the hole to clear the threads.
For CNC tapping, the machine synchronizes the spindle rotation and the feed to match the thread pitch. This is called rigid tapping; it is faster and more accurate than floating tapping.
For hard materials or deep holes, the tap may need to be retracted periodically to break the chip. This is called peck tapping.
How Is Thread Milling Performed?
Thread milling is performed with a thread mill, which is a rotating tool with thread profiles that interpolates around the hole.
Steps:
Drill or bore a pilot hole: the pilot hole is drilled to the minor diameter of the thread.
Mount the thread mill: the thread mill is mounted in the spindle.
Mill the thread: the thread mill interpolates around the hole, cutting the thread profile gradually.
Measure: the thread is measured with a thread gauge or optical comparator.
Thread milling can be performed as a single-point tool (one profile cutting the thread gradually) or as a multi-point tool (multiple profiles cutting the thread in one revolution). Multi-point thread mills are faster but more expensive.
How Is Thread Turning Performed?
Thread turning is performed with a thread-turning tool, which is a single-point tool with a thread profile.
Steps:
Turn the shaft: the shaft is turned to the major diameter of the thread.
Mount the thread-turning tool: the tool is mounted in the turning tool holder.
Turn the thread: the tool is fed along the thread helix at the appropriate feed.
Measure: the thread is measured with a thread gauge or optical comparator.
Thread turning is performed on a lathe or turning center. The machine synchronizes the spindle rotation and the tool feed to match the thread pitch.
What Are the Common Defects in Thread Machining?
Common defects in thread machining:
|
Defect |
Cause |
Prevention |
|
Stripped threads |
Oversized tap drill, worn tap, wrong cutting parameters |
Use correct tap drill, replace tap, adjust parameters |
|
Tapered threads |
Tool wear, deflection, machine error |
Replace tool, reduce deflection, verify machine |
|
Poor thread profile |
Worn tool, wrong cutting parameters, wrong tool |
Use sharp tool, adjust parameters, use correct tool |
|
Tap breakage |
Too deep a hole, hard material, worn tap |
Peck tapping, use coated tap, replace tap |
|
Burrs at thread exit |
Insufficient chamfer on the hole, dull tool |
Chamfer the hole, use sharp tool |
|
Galling on stainless steel |
Work hardening, insufficient coolant |
Use coated tool, increase coolant, lower speed
|
Each defect is preventable with discipline. For high-value parts, the first-article inspection is critical.
How Is Thread Quality Verified?
Thread quality is verified by:
Thread gauge: a go/no-go gauge checks the thread diameter.
Thread ring gauge: for external threads, checks the diameter.
Optical comparator: checks the thread profile, pitch, and angle.
CMM with thread probe: measures the thread profile and diameter.
Functional test: the part is assembled with a mating thread to verify the fit.
For most applications, a thread gauge (go/no-go) is sufficient. For critical threads, optical comparator or CMM measurement is required.
How Are Thread-Machining Operations Quoted?
Thread-machining operations are quoted based on:
Cycle time: the time to perform the operation per part.
Tooling cost: the cost of the tap, thread mill, or thread-turning tool.
Setup cost: the cost of setting up the operation.
Inspection cost: the cost of measuring the threads.
For a custom part, thread-machining cost is typically 3–10% of the total machining cost. The cost varies with the thread size, the material, and the lot size.
How Does Thread Machining Affect Part Cost?
Thread machining affects part cost in several ways:
Process selection: tapping is the cheapest for standard threads; thread milling is more expensive but more flexible; thread turning is used for external threads on turned parts.
Tooling cost: taps are inexpensive (5–50 each); thread mills are more expensive (50–500 each); thread-turning tools are similar to taps.
Cycle time: tapping is the fastest; thread milling is slower; thread turning depends on the thread length.
Scrap rate: tap breakage in hard materials can scrap expensive parts. Thread milling reduces this risk.
For a custom part, the engineering response is to specify only the thread standard, size, and tolerance that the function requires.
What Is the Future of Thread Machining?
Trends in thread machining:
Thread milling for small threads: improved thread mills are making thread milling competitive with tapping for small threads.
Hybrid tapping-milling: combination tools that drill, tap, and mill in one operation.
Coated taps and thread mills: advanced coatings (TiN, TiCN, AlTiN) extend tool life in hard materials.
In-process gauging: thread gauges integrated into the machine for real-time measurement.
Additive manufacturing: near-net-shape threads from additive manufacturing, requiring less material removal.
For a custom machine shop, the trend toward thread milling for smaller threads and the use of coated tools in hard materials is making thread machining more reliable and more economical.
Conclusion
Thread machining is the process of cutting internal or external threads on a custom part. The three common processes — tapping, thread milling, and thread turning — each have a different range of capability, cost, and applicability. Tapping is the fastest and most economical for standard threads; thread milling is more flexible and produces better quality; thread turning is for external threads on turned parts. The choice depends on the thread size, material, lot size, and required thread quality. Yuqing's CNC machine tool processing and processing of mechanical parts capabilities cover all three processes.
Frequently Asked Questions
What is the difference between tapping and thread milling?
Tapping uses a multi-point tool with cutting edges that match the thread profile; thread milling uses a rotating tool that interpolates around the hole. Tapping is faster for small standard threads; thread milling is more flexible and reliable for large threads and hard materials.
When should I use thread milling instead of tapping?
Use thread milling for large threads (M16+), hard materials (stainless, tool steel, hardened steel), expensive parts, or when the thread is non-standard.
What thread sizes can be tapped?
Standard taps cover M1 to M36 or larger, in standard pitches. For non-standard pitches or thread profiles, thread milling is required.
What thread sizes can be thread milled?
Thread mills cover M3 to M100 or larger. Thread mills are limited by the cutting tool length and the machine envelope.
How is thread quality verified?
Thread quality is verified with thread gauges (go/no-go), optical comparators, CMMs with thread probes, or functional tests.
What is the typical cycle time for tapping?
Tapping cycle time is 1–10 seconds per hole, depending on the thread size, material, and depth. Small threads in aluminum can be tapped in 1–2 seconds.
What is the typical cycle time for thread milling?
Thread milling cycle time is 10–60 seconds per hole, depending on the thread size, material, and pitch. Thread milling is slower than tapping but produces better quality.
What is the cost of a tap?
Standard taps cost 5–50; non-standard taps cost 50–200. The cost depends on the thread size, material, and coating.
What is the cost of a thread mill?
Standard thread mills cost 50–300; non-standard thread mills cost 200–500. The cost depends on the thread size, profile, and coating.
Can tapping and thread milling be combined?
Yes, combination drill-tap tools are available for tapping, and combination drill-thread-mill tools are available for thread milling. These tools reduce cycle time by eliminating a tool change.
What is the typical tolerance for tapped threads?
Tapped threads typically achieve 6H tolerance. Tighter tolerances (4H, 5H) require thread milling or thread turning.
What is the typical tolerance for thread-milled threads?
Thread-milled threads typically achieve 4H–6H tolerance. Tighter tolerances are possible with careful tool selection and machine calibration.
