Threading Titanium Guide
Threading titanium sounds straightforward — it's just cutting a helical groove. The challenge is that titanium actively fights this process: it galls, it clogs chips, and it'll break a tap in a blind hole if the process isn't controlled.
Key data: thread milling preferred over tapping for most applications. When tapping: 30–50 SFM, TiCN coating, sulfurized tapping oil. Galling is the primary failure mode.
If you are designing your first threaded titanium part, here is the essential context before diving into parameters. A thread is a helical ridge cut into or onto a cylindrical surface — it is what allows a bolt to screw into a hole. There are three ways to create internal threads (threads inside a hole): tapping, where a hardened multi-flute tool is driven axially into a pre-drilled hole to cut the helical groove; thread milling, where a smaller tool traces a helical path using the CNC machine's 3-axis interpolation; and thread rolling (form tapping), where the tool cold-forms the thread shape without cutting chips.
Two terms you will see repeatedly: thread class and thread engagement. Thread class (e.g., 2B, 3B for inch threads; 6H for metric) specifies the allowable tolerance range on the thread dimensions — a higher number means tighter fit and less clearance. Thread engagement is the percentage of the theoretical full-depth thread that is actually cut — 75% engagement means the thread flanks extend to 75% of the full triangular profile. Higher engagement means more holding force but also higher tapping torque and galling risk.
In titanium specifically, threads fail differently than in aluminum or steel. The same parameters that work fine in those materials will cause tap breakage, galled thread flanks, or oversized pitch diameters in titanium. There are three root causes: titanium's tendency to gall (cold-weld to the tool under pressure), poor chip evacuation in deep holes, and springback that affects the finished thread diameter. Each failure mode has a straightforward countermeasure — but you need to know what to specify on the drawing and what to communicate to the shop.
Key Takeaway
Default to thread milling for titanium. Reserve tapping for small diameters or high-volume production with proven parameters. Always specify thread class and lubrication requirements on assembly drawings.
Why Titanium Threading Is Different
Titanium presents three unique challenges when threading that differentiate it from aluminum and steel: galling propensity, tap breakage in blind holes, and spring-back that affects thread pitch diameter. Understanding these challenges drives the method selection and parameter recommendations below.
Galling (Cold Welding)
Titanium's chemical reactivity causes the thread flanks to cold-weld to the tap or mating thread under heat and pressure. Galling can destroy the tap, seize the fastener, or strip the thread — all invisible until the joint is loaded. Prevention: correct SFM, TiCN coating, sulfurized oil.
Tap Breakage Risk
Titanium's high strength and poor chip evacuation cause chip packing in blind holes, which torques the tap beyond its failure point. Standard HSS taps break at higher tapping torques. Mitigation: use spiral-flute taps (chip evacuation), thread milling (no breakage mode), or peck tapping cycles.
Springback
Titanium's intermediate elastic modulus causes thread flanks to spring back after tapping, potentially resulting in a tighter-than-intended pitch diameter. Use 2B class fit for Ti-Ti mating thread pairs; verify with go/no-go gauges. A 3B class may be over-tight after springback.
Tapping Titanium: Parameters and Tooling
| Parameter | Ti-6Al-4V | CP Grade 2 | Notes |
|---|---|---|---|
| Cutting speed (HSS-E) | 30–40 SFM (9–12 m/min) | 40–50 SFM (12–15 m/min) | Below 30 SFM increases BUE and galling risk |
| Cutting speed (TiCN/TiAlN coated carbide) | 40–60 SFM (12–18 m/min) | 50–70 SFM (15–21 m/min) | TiCN preferred; TiN-coated reacts with Ti at temperature |
| Tap type (through holes) | Spiral point (gun) tap | Spiral point | Pushes chips forward; reduces chip packing |
| Tap type (blind holes) | Spiral flute (helical) tap | Spiral flute | Pulls chips up and out; critical for blind holes in Ti |
| Thread class | 2B recommended | 2B | 3B may be over-tight after springback; use 2B as default |
| Tapping fluid | Sulfurized or chlorinated cutting oil | Same | Apply generously to tap before entry; water-based alone insufficient |
| Peck tapping | Recommended for holes ≥ 2× diameter depth | Same | 1/3 to 1/2 diameter peck increments; retract fully to clear chips |
| Thread engagement | 65–75% (1.5–2× diameter length) | 65–75% | 65% engagement provides adequate shear strength in Ti; reduces torque 15% |
Thread Milling Titanium (Preferred Method)
Advantages over Tapping
- • No breakage mode — thread mills are single-flute interpolation tools; failure is dimensional, not a locked insert
- • Adjustable pitch diameter — major/minor diameter corrected by tool path radius; no new tooling required
- • One tool, multiple sizes — same-pitch thread mill cuts different diameters; reduces tool inventory
- • Better chip control — helical interpolation path continuously clears chips from the bore
- • Deeper thread engagement — easier to mill full depth in blind holes vs. peck tapping
Thread Milling Parameters
Preventing Thread Galling in Titanium
Galling occurs when titanium-to-titanium (or titanium-to-steel) thread surfaces cold-weld under combined heat and pressure during assembly or disassembly. It is distinct from machining galling — this section covers both machining and assembly galling.
Machining Galling Prevention
- Use TiCN-coated tap (not TiN — TiN reacts with Ti at elevated temperature)
- Maintain 30–50 SFM — exceeding 60 SFM activates galling mechanism
- Apply sulfurized or chlorinated tapping compound, not standard oil
- Use 2-flute spiral-flute taps to minimize dwell time at each cutting edge
- Prefer thread milling (no tap-to-wall contact on approach and exit)
Assembly Galling Prevention
- Apply anti-galling compound: Molykote P-37, Loctite 771, or Jet-Lube SS-30 to threads before assembly
- Specify class 2B fit for Ti-Ti thread pairs (not 3B — too tight)
- Install titanium fasteners at low RPM with torque control (do not air-ratchet)
- Use fluoropolymer-coated titanium fasteners (Xylan, Nylok) for frequent reassembly
- Never torque titanium into a titanium tapped hole without lubricant
Thread Callout Specifications for Titanium
| Thread Type | Drawing Callout | Tap Drill | Notes |
|---|---|---|---|
| UNF #10-32 | 10-32 UNF-2B THRU (or min depth) | #21 (0.159 in.) standard; #19 for 65% | Fine thread for small precision fasteners |
| UNC 1/4-20 | 0.250-20 UNC-2B ×0.500 MIN FULL THD | #7 (0.201 in.) standard | Common structural fastener in titanium brackets |
| UNF 1/4-28 | 0.250-28 UNF-2B ×0.500 MIN FULL THD | #3 (0.213 in.) standard | Fine thread for vibration-prone or thin-wall applications |
| Metric M6×1.0 | M6×1.0 6H ×12 MIN FULL THD | 5.0 mm standard | Per ISO 965 class 6H; 2B UNF equivalent |
| Metric M8×1.25 | M8×1.25 6H ×16 MIN FULL THD | 6.8 mm standard | Common in high-performance metric fastener applications |
| Helicoil insert | #10-32 UNC-2B HELICOIL PER MS21209-F3D10 OR EQUAL | Helicoil tap drill specific | Use Helicoil for repeated assembly Ti threaded holes; recommend for ≥ 5 insertion cycles |
Frequently Asked Questions
What is the difference between tapping and thread milling?
What is "galling" and why does it happen in titanium threads?
What is the best way to thread titanium?
What cutting speed should I use when tapping titanium?
How do I prevent galling when threading titanium?
What tap drill size should I use for UNC threads in titanium?
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