CNC Threading: Cut, Tap or Mill — and the Tolerance Each Process Gives
Single-point cutting gives the tightest, most controllable thread tolerance; tapping is the cheapest for small holes in soft materials; thread milling is the safest for large diameters, hard materials and blind holes. The tolerance class you actually need — 6H, 2B, or tighter — should decide the process, not the other way around.
Threads are the most inspected feature on many CNC parts, yet they are also the feature most often specified without understanding how the manufacturing process limits what a shop can deliver. A tapped hole and a thread-milled hole both end up with the same nominal size, but they carry different risks of bellmouth, taper, lead error, and pitch-diameter drift. This guide explains the three main CNC threading processes, the tolerance each one realistically holds, and how to spec threads so you get fits that work without paying for classes you never check.
What Thread Tolerance Classes Actually Mean
Before comparing processes, get the tolerance language straight. Metric threads use ISO classes where the number is the grade and the letter is the position — 6H is the standard internal (nut) tolerance, 6g the standard external (bolt) tolerance. Unified threads use 1A/2A/3A external and 1B/2B/3B internal, where 2A/2B is the standard commercial fit and 3A/3B is precision. The class controls pitch diameter mostly, plus minor and major diameter allowances.
| Class pairing | Typical fit | Where it is used |
|---|---|---|
| 6H / 6g (metric), 2B / 2A (unified) | Standard clearance fit | General mechanical parts, most production |
| 6H / 4g6g | Tighter external | Precision screws, gages |
| 5H / 4h, 3B / 3A | Precision | Instrument threads, aerospace fittings |
| 7H / 8g | Loose fit | Coated threads, high-temperature service |
The pitch diameter tolerance for a standard M6×1 internal thread in class 6H is roughly 0.125 mm total — not a tight number in absolute terms, but one that interacts badly with tool wear, tap drift, and deflection if the process is uncontrolled. When you see a drawing calling 3B or 5H on a deep blind hole, recognize that you are asking for a level the process may not be able to hold without special tooling and inspection.
Single-Point Cutting: The Tolerance Reference
Single-point threading on a CNC lathe cuts each thread groove with a shaped tool, moving along the axis in coordination with spindle rotation. Because the tool path is generated by the machine axes rather than by a pre-formed tool, the operator controls pitch, depth, and profile directly, and can compensate for wear. That makes single-point cutting the reference process for accuracy, and the standard for large diameters and unusual pitches that no tap or thread mill covers.
| Process | Typical pitch diameter control | Size range | Best for |
|---|---|---|---|
| Single-point cut (lathe) | Tightest, correctable on the fly | Any, especially large | Odd pitches, tight classes, external threads |
| Tapping (cut tap) | Good, class 6H/2B typical | Up to ~M24 by hand; larger by machine | Small internal holes, high volume, soft/mid materials |
| Thread milling | Good, class 6H/2B typical, controllable | M2 up to very large | Hard materials, blind holes, big diameters, one tool many sizes |
| Thread rolling (external) | Excellent surface, 3A possible | Small–medium | High-volume external threads on screw-machine parts |
The catch with single-point cutting is cost per thread and cycle time. It is a slower operation than tapping, and on multi-start or deep threads it needs multiple passes with accurate machine synchronization. When the drawing needs the best possible thread, this is the process — our CNC turning cells run single-point threading as a standard operation and verify the result with thread gages on the first article.
Tapping: Cheap, Fast, and Bounded by the Tap
Tapping forms an internal thread by driving a fluted tap down an existing hole. It is the lowest-cost method for small holes — from about M1.6 up to roughly M20 — and it is fast, which is why production runs of threaded holes in housings, brackets, and covers are almost always tapped. Cut taps remove material; form taps displace it, which can improve the thread's grain structure but requires a differently sized pre-drill hole and more torque.
Tapping's limits are tolerance and tool-life drift. A worn or misaligned tap produces bellmouthed entries and oversize pitch diameters, which is why tapped threads are usually guaranteed to class 6H/2B rather than to precision classes. Rigid tapping on a CNC machine — where the spindle feeds in exact synchronization with the Z axis — removes most alignment risk, and a quality tap held in a precision holder keeps the entry clean. If your drawing calls a 3B internal thread in a production part, expect the shop to ask whether tapping is acceptable or whether thread milling is required to hold it.
| Threading decision | Choose tapping | Choose thread milling |
|---|---|---|
| Hole size | Under ~M20 | Any size, excels large |
| Material | Aluminum, steel, plastics | Hard alloys, titanium, stainless |
| Hole type | Through holes ideal | Blind holes with full thread to bottom |
| Volume | High, short cycle | Low–medium, one tool for many sizes |
| Tolerance class needed | 6H/2B standard | 6H/2B and tighter, adjustable |
Thread Milling: The Flexible Precision Option
Thread milling cuts the thread with a rotating tool that interpolates a helical path into the hole. It is not a tap — the tool enters at the hole diameter, moves in a circle while advancing one pitch per revolution, and exits. Because the thread form is generated by machine motion, one thread mill cuts any diameter of the same pitch, and the effective pitch diameter can be adjusted by changing the programmed path radius. That makes thread milling the standard answer for large holes, hard materials like titanium and heat-treated steel, and blind holes where a tap would leave an incomplete thread or risk breakage.
Thread milling is also forgiving of tool failure: a broken thread mill does not get stuck in the part the way a broken tap does. The downsides are cycle time — it is slower than tapping per hole — and the need for a machine with helical interpolation. On CNC milling work, thread milling is the default for anything above about M12 in production, and for any precision thread in a blind hole where chip evacuation and full-form depth matter.
Specifying Threads So the Quote Is Honest
The fastest way to overpay for threads is to call a precision class without a functional reason, or to leave the class off entirely and let the shop assume the tightest one. Write the full callout on the drawing: size, pitch, class, and whether internal or external — for example M10×1.5-6H internal, or 1/4-20 UNC-2B. If the thread must accept a specific gage, name the gage class. If it is a structural thread that only needs a free fit, say so; the shop will then pick the process that costs least while meeting the callout.
Also flag coating and plating on the drawing, because they change final size. A thread that gets zinc or nickel plated needs pre-plate allowance — often cut to a slightly looser class so the plated part still fits its gage. Shops that know the finish in advance build the allowance into the threading pass; shops that find out after machining scrap the batch.
Tolerances You Can Rely On
For production reality: tapped threads in aluminum and steel hold 6H/2B reliably when the tool is fresh and the spindle is rigid; thread milling holds the same classes with better consistency on hard materials and adds the ability to dial in pitch diameter on the first article; single-point cutting holds the tightest classes and any pitch. Precision threads — 3B, 5H, and gage-critical forms — go to thread milling or single-point cutting with inspection on a dedicated thread gage or CMM, as covered in our hole tolerance by process guide and the wider CNC tolerance reference.
Send a drawing with clear thread callouts and the required gage class, and a competent shop will match the process to the class — cut, tap, or mill — rather than the other way around. That is exactly how we handle threading on CNC turned parts, milled parts, and the mixed geometry that runs through our precision components line: process chosen for the tolerance, verified on the first article, and quoted honestly on the risk.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Frequently Asked Questions
Q: Which CNC threading process holds the tightest tolerance?
A: Single-point cutting holds the tightest and most controllable pitch diameter because the tool path is generated and corrected by machine axes. Thread milling is the best internal-thread option for tight classes in hard or large holes; tapping is typically limited to standard 6H/2B classes.
Q: When should I thread mill instead of tap?
A: Thread mill when the hole is large (roughly above M12), the material is hard like titanium or heat-treated steel, the hole is blind and needs full thread to the bottom, or you need to adjust pitch diameter without changing tools. Tap when holes are small, volume is high, and 6H/2B is sufficient.
Q: What does 6H mean on a metric thread drawing?
A: 6H is the ISO tolerance class for an internal thread — grade 6 with an H (zero allowance) position — and is the standard commercial internal fit. It pairs with 6g external threads for a normal clearance fit. Tighter classes like 5H or 4H are precision fits that cost more to produce and inspect.
Q: Why do tapped blind holes have incomplete threads at the bottom?
A: A tap needs clearance below the thread to exit its chamfer, leaving the last few threads incomplete unless the hole is deep enough. Thread milling avoids this because the tool enters and exits the hole along its own path, producing full-form threads close to the bottom.
Q: How does plating affect thread tolerance?
A: Plating adds thickness to the surface, shrinking internal threads and growing external ones. Pre-plate threads are usually cut to a looser class so the coated part still fits its gage. Always state plating on the drawing so the threading pass includes the allowance.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
