Rigid tapping, thread milling and the cases each one wins

Technical FoundationConsiderationDecember 11, 2026 · 1,522 words · 7 min read

**Technical due diligence memo: Rigid tapping vs thread milling — seven-year total cost for DACH/UK job shops**.

Technical due diligence memo: Rigid tapping vs thread milling — seven-year total cost for DACH/UK job shops

Conclusion: For most DACH and UK job shops making M3 to M16 threads in aluminium, mild steel and annealed alloy steel, rigid tapping has the lower seven-year total cost. Thread milling wins when thread diameter is above about M20, when material hardness is above roughly 35 HRC, when thread depth is more than 2.5 times the diameter, or when one tool must cover several diameters with the same pitch. This memo is a due-diligence note. It separates what we can support with typical industry data from what you must confirm in your own shop.

This memo compares two ways to cut internal threads on a CNC machining centre: rigid tapping and thread milling. It does not compare machine brands. The cost model uses typical industry numbers, not promises.

1. How the two processes work

Rigid tapping

Rigid tapping uses a tap, a cutting tool shaped like a screw. The machine spindle rotates and the Z-axis feeds down at exactly one thread pitch per revolution. The spindle encoder tells the control the exact spindle angle, so the tap does not pull itself in or out. One pass cuts the thread. The tap then reverses out of the hole. Rigid tapping is the fastest standard cycle on most 3-axis vertical machining centres.

Technical terms:

• Thread pitch: the distance from one thread crest to the next, in millimetres.
• Spindle encoder: a sensor that reports spindle angle and speed to the CNC.
• Canned cycle: a pre-programmed machine routine, called with one line of code.

Thread milling

Thread milling uses a milling cutter smaller than the hole. The cutter moves in a circle while the Z-axis feeds down at the thread pitch. The circular motion and linear feed together are called helical interpolation. The cutter makes one or more orbits to cut the full thread profile. Thread milling does not require the spindle to reverse.

2. Where each process wins

Use this list as a first filter.

• Rigid tapping usually wins for:
• Thread sizes M3 to M16.
• Thread depth up to 2.5 times diameter.
• Aluminium, free-cutting steel, mild steel, and stainless up to about 300 HB.
• High volume of the same thread size.

• Low tooling budget per hole.

• Thread milling usually wins for:

• Thread sizes above M20.
• Material hardness above roughly 35 HRC.
• Tough or work-hardening alloys such as nickel-based alloys.
• Blind holes that need full thread close to the bottom of the hole.
• One tool covering several thread diameters with the same pitch.
• Expensive workpieces where a broken tap would scrap the part.

These are not absolute rules. Material, coolant, tool coatings and machine condition move the boundary.

3. Seven-year cost model — baseline

We model one typical job: 20,000 internal threads per year, size M10 x 1.5 pitch, in 42CrMo4 alloy steel, annealed, blind hole 20 mm deep. The machine is a 3-axis vertical machining centre. Labour rate for programming is separated from the machine hourly cost.

All figures are typical industry ranges, not official specifications. Confirm every number with your own data, your tool supplier, and the machine catalogue before making a purchase decision.

Assumptions table

Parameter	Unit	Value used	Typical range	Status
Machine hourly cost	€/hour	75	60–90	Assumption, confirm from own accounts
Productive hours per year	hours	2,000	1,500–2,500	Assumption
Threaded holes per year	holes	20,000	—	Baseline workload
Thread size and material	—	M10 x 1.5, 42CrMo4 annealed	—	Baseline
Rigid tap cycle time per hole	seconds	10	6–15	Assumption, do time study
Thread mill cycle time per hole	seconds	35	25–60	Assumption, do time study
Tap tool cost	€	25	15–45	Assumption, confirm with supplier
Thread mill tool cost	€	100	70–160	Assumption, confirm with supplier
Tap tool life	holes	2,000	1,000–5,000	Assumption, confirm with supplier
Thread mill tool life	holes	4,000	2,000–10,000	Assumption, confirm with supplier
Tool change downtime per event	minutes	5	3–10	Assumption
Programming time delta for thread milling	minutes per new part	5	2–15	Assumption, confirm with programmer
Number of new parts per year	parts	20	—	Assumption
Programming labour rate	€/hour	60	50–80	Assumption
Scrap and rework cost per failed hole	€	100	50–200	Assumption, confirm with own quality data
Tap failure rate	% of holes	0.2	0.05–0.5	Assumption, confirm from own history
Thread mill failure rate	% of holes	0.05	0.01–0.2	Assumption, confirm from own history
Evaluation period	years	7	—	Fixed
Discount rate	%	0	—	Undiscounted cash cost

Seven-year total cost table

Cost category	Rigid tapping €/year	Thread milling €/year	Rigid tapping €/7 years	Thread milling €/7 years
Machine time	4,167	14,583	29,169	102,083
Tooling	250	500	1,750	3,500
Tool change downtime	62	31	434	217
Programming labour	0	100	0	700
Scrap and rework	4,000	1,000	28,000	7,000
Total	8,479	16,214	59,353	113,500

In this baseline, rigid tapping costs about €54,100 less over seven years. The main driver is cycle time. Thread milling takes longer per hole. Lower scrap does not offset that time cost at M10 size.

4. Sensitivity: what changes the ranking

Two variables move the ranking quickly: thread mill cycle time and tap failure rate.

First, thread mill cycle time. If all other assumptions stay the same, thread milling becomes cost-neutral against rigid tapping when its cycle time drops to about 16 seconds per hole. Typical M10 thread milling cycles in the table above are 25–60 seconds, so rigid tapping wins at M10. For larger threads, tap cycle times rise and thread milling can become competitive.

Second, tap failure rate. In this baseline, rigid tapping becomes more expensive than thread milling if tap failure rises above about 0.6% of holes. In difficult materials or deep holes, failure can reach that level. Then thread milling wins on total cost.

Other shifts to watch:

• Hardness above 35 HRC: rigid tapping often becomes unreliable. Thread milling is the safer process.
• Thread diameter above M20: tap cost and spindle torque demand rise fast. Thread milling often wins.
• Depth beyond 2.5 x D: chip evacuation becomes a problem for taps. Thread milling copes better.
• Batch size: short runs with many thread sizes favour thread milling because one tool can cover several diameters with the same pitch. Long runs of one size favour rigid tapping.

5. What we do not yet know

This memo cannot give a final answer for your shop until the following points are checked.

• Your actual tap and thread mill failure rates in your materials.
• Your actual scrap and rework cost per part. In DACH, a scrapped 42CrMo4 part often costs more than €100.
• Your real cycle times on the specific machine and CNC control.
• Whether your CAM post-processor supports thread milling as a canned cycle or needs longhand NC code.
• The low-speed torque curve of the spindle. Large rigid taps need high torque at low rpm. This is a machine specification. Check the official catalogue.
• How often you cut threads above M20 or in materials above 35 HRC. Many shops do this more often than they think.
• Coolant type and pressure. Through-spindle coolant changes tool life for both processes.

What to check before you buy

• Confirm the CNC has rigid tapping with spindle encoder feedback.
• Check the maximum tap size the spindle can drive at low speed without stalling. Ask for the torque curve, not just the kW rating.
• Confirm helical interpolation is available and test it on a sample part.
• Ask a tool supplier for tool life and cost data for your thread sizes and materials.
• Run a 100-hole time study for both processes on your machine.
• If you work above 35 HRC or with nickel alloys, plan for thread milling and do not rely on rigid tapping.
• Check whether the machine control supports thread milling canned cycles, which reduce programming time.
• Ask your Gree CNC distributor to show the spindle torque curve and the rigid tapping cycle in the official catalogue.