Machining Titanium and Nickel Superalloys: 6.7 W/m·K, Segmented Chips and the Tool-Life Arithmetic Behind Every Ti-6Al-4V and Inconel 718 Quote

A 12 mm carbide end mill that runs 300 m/min in aluminium will destroy itself in four minutes in Ti-6Al-4V at the same speed, and the reason is a thermal conductivity of 6.7 W/m·K against 167. This guide works the physics and the arithmetic that follow from it: heat partition and edge temperature, adiabatic shear banding at 10³–10⁶ s⁻¹, Taylor exponents of 0.25 and 0.18 that turn a 20% speed increase into a 52% tool-life loss, the 2.8 mm deflection and 504 Hz natural frequency of a 2 mm titanium wall, cost-optimal speed with ₹900 insert edges, specific cutting energy of 2.2 J/mm³ against 0.7 for aluminium, buy-to-fly ratios from 40:1 to 1.2:1, and the DFM limits that separate a manufacturable bracket from an unquotable one.

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Machining Titanium and Nickel Superalloys

A shop that machines aluminium and mild steel takes its first titanium job: a bracket in Ti-6Al-4V, 120 × 80 × 45 mm, from a 2.8 kg billet. The programmer applies the aluminium recipe — 12 mm four-flute carbide, 250 m/min, 0.10 mm/tooth, 6 mm radial engagement, flood coolant — and the tool is gone before the first pocket is finished. Not worn: gone, with the corners rounded off and a weld of titanium stuck to the flute. The second tool goes at 120 m/min and survives twenty minutes. The third, at 50 m/min with a 1.2 mm radial step, runs all afternoon.

Nothing about the machine changed. What changed was that the process was moved into the region where the physics allows it to live. Titanium and nickel superalloys are not "hard" in the sense that hardened D2 is hard — Ti-6Al-4V is 33–36 HRC and cuts like a soft metal right up to the moment it welds itself to the tool. They are difficult because of four coupled properties: heat that will not leave the cut, a chemical affinity for every common tool material, an elastic modulus about half that of steel, and a chip formation mechanism that localises shear into narrow bands at strain rates of 10^3 to 10^6\ \text{s}^{-1}.

This guide works through those four properties, the tooling and parameters that follow from them, and the cost arithmetic that decides whether a part should be machined from billet at all. Every number is either a handbook value, a cited study, or an arithmetic step shown in full so it can be checked.

1. The two alloys, metallurgically

Ti-6Al-4V (Grade 5, UNS R56400)

Titanium at room temperature is hexagonal close-packed alpha. Above the beta transus — 995 ± 15 °C for Ti-6Al-4V — it becomes body-centred cubic beta. Aluminium is an alpha stabiliser, vanadium a beta stabiliser, and the 6/4 ratio is the compromise that gives useful strength at 400 °C with enough ductility to fabricate.

Two heat-treatment conditions dominate:

The properties that matter to a machinist are the thermal ones. Thermal conductivity is 6.7 W/(m·K) at room temperature, rising to roughly 20 W/(m·K) at 600 °C. Specific heat is 526 J/(kg·K). Density 4.43 g/cm³. Elastic modulus 114 GPa — 43% lower than steel.

Inconel 718 (UNS N07718)

A nickel-chromium-iron alloy with 5.0–5.5% niobium, 2.8–3.3% molybdenum and 0.65–1.15% titanium. Its strength comes from two precipitate families: gamma-prime \text{Ni}_3(\text{Al},\text{Ti}) and, dominantly, gamma-double-prime \text{Ni}_3\text{Nb} — body-centred tetragonal discs, about 10–20 nm across and 2–3 nm thick, coherent with the matrix and responsible for most of the 718's yield strength.

Gamma-double-prime is metastable. Above about 650 °C it transforms to the orthorhombic delta phase, which is incoherent and does not harden. That transformation, not melting, sets the service ceiling: 718 is a 650 °C alloy, with short excursions to 700 °C. The standard age is two-step — 720 °C for 8 h, furnace cool at 55 °C/h to 620 °C, hold 8 h, air cool — which is why heat treat is a 20-hour operation and why the material is normally supplied already aged (AMS 5662 bar, AMS 5663 forgings).

Aged 718 runs 36–44 HRC, UTS 1240–1375 MPa, yield 1030–1100 MPa. Thermal conductivity is 11.4 W/(m·K) — 70% higher than titanium and still a fifth of steel's. Elastic modulus 200 GPa. Density 8.19 g/cm³.

The comparison that sets the machine settings

Property · 1045 steel · 304 stainless · Ti-6Al-4V · Inconel 718 (aged)

Density (g/cm³) · 7.85 · 8.00 · 4.43 · 8.19

Elastic modulus (GPa) · 205 · 193 · 114 · 200

Thermal conductivity (W/m·K) · 52 · 16 · 6.7 · 11.4

Specific heat (J/kg·K) · 490 · 500 · 526 · 435

Thermal diffusivity \alpha = k/\rho c_p (m²/s) · 1.35\times10^{-5} · 4.0\times10^{-6} · 2.87\times10^{-6} · 3.20\times10^{-6}

UTS (MPa) · 585 · 505 · 895–1000 · 1240–1375

Hardness (HRC) · 15–20 · 20 (annealed) · 33–36 · 36–44

Relative machinability (B1112 = 100) · 55–65 · 45 · 22–25 · 12–15

Specific cutting force k_c (N/mm²) · 1800–2200 · 2200–2800 · 1800–2400 · 2800–3500

Read the diffusivity row. Titanium spreads heat through itself 4.7 times more slowly than 1045 steel. In a cut, the heat generated in the primary shear zone has nowhere to go: it cannot conduct into the workpiece fast enough and it cannot all leave with the chip, because the chip is moving slowly (low cutting speed) and is in contact with the rake face over a short distance. It accumulates in the tool edge.

2. Where the heat goes

The heat generated per unit time in a turning operation is the cutting force times the cutting speed:

and the flux per unit area of the shear plane is roughly

where b is the width of cut and h the uncut chip thickness. For a 2 mm depth of cut, 0.2 mm/rev feed, F_c = 1400\ \text{N} at v_c = 1\ \text{m/s}, the flux is 1400 \times 1/(0.002 \times 0.0002) = 3.5\times10^{9}\ \text{W/m}^2 — a few gigawatts per square metre concentrated into a contact zone a few hundred micrometres long.

In steel, 75–80% of that heat leaves with the chip, because the chip is thick, fast-moving and in contact with the tool for a short time. In titanium the partition inverts: the chip is thin, the contact length is short, the speed is low, and the material conducts poorly, so a large fraction — published estimates put it around 80% for Ti-6Al-4V — ends up in the tool and the machined surface. Edge temperature at 60 m/min in titanium can exceed 900 °C while the bulk workpiece is still at 40 °C.

Three consequences follow directly.

Diffusion wear. At 800–1000 °C, cobalt from the carbide binder diffuses into the titanium chip and titanium diffuses into the tool. The tool loses its binder and the carbide skeleton collapses. This is why coated carbides help less in titanium than in steel — the coating delays the onset but the substrate still sees the temperature.

Built-up edge and adhesion. Titanium seizes to the rake face. Once a stable titanium layer forms, the effective rake geometry changes, chip flow becomes unstable, and fragments of the built-up edge break away taking tool material with them (attrition wear). The classic signature is a tool that looks fine on the flank but has a ragged crater and a welded lump near the nose.

Chemical reaction with the environment. Above roughly 500 °C titanium reacts with oxygen, nitrogen and hydrogen. In air this produces alpha case, a hard, oxygen-enriched, brittle surface layer 0.05–0.2 mm deep that must be removed by chemical milling or by taking a finishing pass after heat treat, because it is a fatigue crack initiation site. In the shop it means three rules: no chlorinated coolants (stress corrosion and toxicity), clean segregation of titanium swarf from other metals, and a Class D extinguisher within reach — fine titanium turning can ignite, and water is the wrong response.

Inconel 718 adds a fourth mechanism. The aged microstructure contains hard gamma-double-prime and delta precipitates that abrade the flank, while the matrix work-hardens rapidly under the cut. All four wear mechanisms — abrasion, adhesion, diffusion and oxidation — operate at once. Notch wear at the depth-of-cut line is the characteristic 718 failure: the tool survives in the middle of the engagement and fails at the entry and exit where the chip thickness changes discontinuously and the surface has already been work-hardened by the previous pass.

The visible signature of titanium cutting is the chip. Instead of a continuous ribbon, it comes off as a saw-tooth with a pitch that scales with feed, and each tooth is a localised shear failure rather than smooth plastic flow. Hold one next to an aluminium chip and the difference in mechanism is obvious.

3. Chip formation: adiabatic shear banding

In most metals, cutting is continuous plastic flow. In titanium it is not. At strain rates between 10^3 and 10^6\ \text{s}^{-1}, the heat generated in the primary shear zone cannot conduct away within the deformation time. The temperature rises locally, the flow stress falls, deformation concentrates into a band 5–20 µm wide, and the band shears catastrophically before the cycle repeats. The result is a segmented chip with a saw-tooth profile and a pitch that scales with feed.

The mechanics are captured by the adiabatic shear criterion: instability begins when thermal softening outpaces strain hardening,

The practical implication is counter-intuitive. A light feed produces a thin chip, low shear-plane temperature, and a continuous chip that rubs rather than cuts. Rubbing work-hardens the surface, raises the next pass's cutting force, and accelerates notch wear. Titanium wants a decisive feed and a light radial engagement — not a light feed. Programmers who "sneak up on it" with 0.03 mm/tooth and a full-width pass produce exactly the conditions that destroy tools.

The same phenomenon drives force oscillation. Segmented chips produce cutting forces that cycle at the segmentation frequency (often 1–10 kHz), which excites tool and workpiece modes and shows up as chatter or as a poor surface finish with a pitch equal to the segment spacing.

The Taylor equation and what a speed increase really costs

Tool life is governed by the empirical Taylor relation:

where V_c is cutting speed in m/min, T is tool life in minutes, C is a constant for the tool–workpiece pair, and n is the exponent. The exponent is the whole story:

Tool material · Workpiece · n · Implication

HSS · Steel · 0.10–0.15 · Very speed-sensitive

Carbide · 1045 steel · 0.20–0.25 · Standard

Carbide · Ti-6Al-4V · 0.20–0.30 · Similar to steel — the limit is thermal, not wear rate

Carbide · Inconel 718 · 0.15–0.20 · Worse than steel

Ceramic (whisker) · Inconel 718 · 0.30–0.40 · Tolerates speed better, but short absolute life

CBN · Hardened steel · 0.40–0.60 · Speed-insensitive by comparison

Take a carbide tool cutting Ti-6Al-4V at 60 m/min with a 30-minute life. Then C = 60 \times 30^{0.25} = 140.4. Run the same tool at 72 m/min — a 20% increase, the kind of change a programmer makes to shave cycle time — and

A 20% speed increase costs 52% of the tool life. At 84 m/min it is 7.8 minutes. In 718, where n \approx 0.18, the same 20% increase cuts life by 64%. This is why titanium and superalloy cycle times do not respond to the lever that works in aluminium, and why the correct productivity strategy is not "spindle faster" but "keep the tool in the cut with a smaller radial step and a larger axial depth."

4. Rigidity: the 114 GPa problem

Titanium's elastic modulus is 114 GPa against 205 for steel. A tool pushing on a titanium wall deflects it 1.8 times as far for the same force, and the wall springs back after the tooth passes, so the flank rubs on the way out. That is both a dimensional problem and a wear problem.

For a cantilever wall of height L, width w and thickness t,

Take a 40 mm tall, 20 mm wide, 2 mm thick Ti-6Al-4V wall, with a 200 N radial cutting force:

Two point eight millimetres of deflection from a cut that would move a 6 mm steel wall by a few hundredths. The same wall's first natural frequency is

with k = 3 \times 114{,}000 \times 13.33 / 40^3 = 71{,}200\ \text{N/m} and m = 40 \times 20 \times 2\ \text{mm}^3 \times 4.43\ \text{g/cm}^3 = 7.09\ \text{g}:

A 504 Hz wall will chatter when the tooth-passing frequency or one of its harmonics lands on it. With a four-flute cutter, that is 7,560 rpm, 15,120 rpm, and so on — right in the working range of a 12,000–24,000 rpm spindle. The stability limit for orthogonal cutting is approximately

where K_s is the specific cutting force per unit width and G the tool–workpiece transfer function. The practical reading: stability depth is inversely proportional to the real part of the compliance, and wall compliance scales as t^{-3}. Halving wall thickness makes the wall eight times more chatter-prone.

The process response, in order of effectiveness:

  1. Reduce radial engagement, not feed. Trochoidal or dynamic milling at 4–10% of cutter diameter keeps the chip thin geometrically while the feed per tooth stays high enough to cut rather than rub.
  2. Use the axial depth. A 1 D axial depth at 6% radial engagement removes material at a stable force level and distributes wear along the flute instead of concentrating it at the tip.
  3. Shorten the tool. Deflection scales with L^3; a tool hanging 40 mm instead of 60 mm is 3.4× stiffer.
  4. Damp. A tuned mass damper or a high-damping boring bar can raise the stable depth two to five times, at a cost that is trivial against the cycle time it buys.
  5. Support the part. Fill pockets with a low-melt alloy or leave sacrificial webs; support the wall from behind when the geometry allows.

5. Tooling: substrate, coating, geometry

Carbide

Fine-grain tungsten carbide with 6–10% cobalt. For titanium, the trend is toward tough, cobalt-rich, fine-grain substrates with a sharp, positive edge — not the hard, wear-resistant grades used for steel. A sharp edge (edge radius 5–15 µm) matters because titanium's low modulus means a honed edge rubs before it penetrates; a 30 µm hone that works well in 4140 will work-harden Ti-6Al-4V and initiate notch wear.

Coatings

Coating · Hardness (HV) · Max use temp · Titanium · Inconel 718

Uncoated carbide · 1500–1800 · — · Common choice; avoids coating spall · Not recommended

TiN · 2300 · 600 °C · Marginal; spalls under adhesion · Obsolete for this duty

TiAlN · 2800–3300 · 800 °C · Usable, but coating adhesion to Ti is poor · Baseline PVD coating

AlTiN · 3200–3500 · 900 °C · Fair; edge sharpness suffers · Standard for milling

AlCrN · 3000–3400 · 1100 °C · Better oxidation resistance · Good for roughing

TiSiN · 3500–4000 · 900 °C · Limited benefit · High-speed finishing

The counter-intuitive point: in titanium, a coating can be a liability. Titanium adheres to the coating, the built-up layer spalls, and it takes coating with it. Many shops get the best life from uncoated, sharp, fine-grain carbide in titanium and reserve AlTiN/AlCrN for 718. Where coatings are used in titanium, they should be thin (1–3 µm) and applied to a polished substrate.

Ceramics, CBN and diamond

Geometry

6. Parameters that work

The tables below are starting points, not recipes. Tool suppliers publish values for their specific grades, and the correct number depends on rigidity, coolant delivery and the age of the machine. Start at the low end, cut one part, inspect the tool, and move in 10% steps.

Milling Ti-6Al-4V

Operation · Tool · v_c (m/min) · f_z (mm) · Radial a_e · Axial a_p · Coolant

Rough, dynamic · 12 mm carbide, 4-flute, AlTiN · 40–60 · 0.05–0.10 · 6–10% D · 1.0–2.0 D · Flood or HPC 70 bar

Rough, conventional · 16 mm carbide, 3-flute · 30–45 · 0.08–0.12 · 30–50% D · 0.3–0.5 D · Flood

Finish walls · 10 mm carbide, 5-flute · 50–70 · 0.04–0.07 · 3–5% D · 0.5–1.0 D · Flood, high pressure

Face milling · 50 mm insert cutter · 45–60 · 0.10–0.15 · 60–75% D · 1–2 mm · Flood

Slotting · 8 mm carbide, 3-flute · 25–40 · 0.04–0.06 · 100% · 0.3–0.5 D · Through-spindle

Turning and drilling

Operation · Material · Tool · v_c (m/min) · Feed · Depth

Rough turn · Ti-6Al-4V · CNMG carbide, positive · 30–45 · 0.15–0.25 mm/rev · 1.5–2.5 mm

Finish turn · Ti-6Al-4V · VNMG, 0.4 mm nose · 45–60 · 0.08–0.12 mm/rev · 0.3–0.6 mm

Rough turn · Inconel 718 · CNMG, AlTiN · 25–35 · 0.12–0.20 mm/rev · 1.0–2.0 mm

Finish turn · Inconel 718 · VNMG, ceramic or CBN · 40–60 (carbide) / 200+ (ceramic) · 0.05–0.10 mm/rev · 0.2–0.4 mm

Drill · Ti-6Al-4V · Carbide, internal coolant · 18–25 · 0.06–0.12 mm/rev · Peck 1 D

Drill · Inconel 718 · Carbide, internal coolant · 12–18 · 0.05–0.10 mm/rev · Peck 0.5–1 D

Tap · Ti-6Al-4V · HSS-E spiral flute · 3–6 · — · —

The arithmetic behind the numbers

Cutting speed and spindle speed are related by

A 12 mm cutter at 50 m/min runs at n = 1000 \times 50/(\pi \times 12) = 1326 rpm. At f_z = 0.06 mm and four flutes, the feed rate is

At a_e = 1.2 mm and a_p = 12 mm, the material removal rate is

The spindle power at the cut follows from the specific cutting force:

Seventeen-hundredths of a kilowatt. The same cutter in 6061 aluminium at 300 m/min, 0.12 mm/tooth, 6 mm radial and 12 mm axial removes Q = 275{,}000\ \text{mm}^3/\text{min} — 60 times the volume — and draws about 3.7 kW at k_c = 800 N/mm². The titanium cut is not power-limited; it is thermally limited. That is the single most useful sentence in this article for a programmer: you cannot buy your way out of a titanium cycle time with a bigger spindle.

Specific cutting energy, u \approx k_c/1000, makes the same point in energy terms: 2.2 J/mm³ for Ti-6Al-4V against 0.7 J/mm³ for aluminium and 3.2 J/mm³ for Inconel 718. Removing a cubic centimetre of titanium costs three times the energy of aluminium and eight times the tool wear.

Chip thinning

At small radial engagement, the actual chip thickness is less than the feed per tooth. For side milling with a_e \le D/2,

At a_e/D = 0.06 the factor is 0.245, so a programmed f_z of 0.06 mm produces an average chip 0.015 mm thick — below the edge radius, which is rubbing, not cutting. This is why dynamic milling programs use higher feed per tooth than conventional ones: f_z 0.12–0.18 mm at 6% engagement gives the same chip thickness as 0.05 mm at full width, but with a fraction of the force and heat.

Drilling torque

For a twist drill,

A 10 mm carbide drill in Ti-6Al-4V at f = 0.10 mm/rev and k_c = 2200 N/mm² gives M = 2200 \times 0.10 \times 100/8000 = 2.75 N·m. Modest — but the thrust is the problem, and a 10 mm drill at 0.10 mm/rev in titanium will work-harden the bottom of the hole if it dwells. Peck, keep the feed engaged, and never let the drill rub.

7. Coolant: the highest-leverage variable

If heat is the constraint, coolant delivery is the lever. Conventional flood coolant at 3–10 bar never reaches the tool–chip interface in titanium; the pressure is consumed by the chip curl and the vapour blanket. High-pressure coolant (HPC) at 70–200 bar, delivered through the tool or through a directed nozzle, breaks the vapour layer and reaches the edge.

The published gains are large. In turning Ti-6Al-4V with uncoated carbide, raising coolant pressure from conventional flood to 11 MPa improved tool life by 68–123%; at 20.3 MPa the improvement reached 111–150% depending on grade. The same study found coolant pressure above the critical value reduced life for some grades because the jet swept the tool surface too quickly for heat transfer — an optimum exists, typically 7–15 MPa for turning and 7–20 MPa for milling.

What to specify:

MQL works for finishing passes and small-diameter tools where flood cannot reach; it does not carry enough heat away for heavy roughing in 718. Cryogenic LN₂ has produced dramatic laboratory results — one study recorded 91 minutes of tool life at 110 m/min in Ti-6Al-4V, with a 10% speed increase from 100 m/min raising life 83% — but the economics depend on nitrogen cost, fixturing and the shop's willingness to plumb a cryogenic line to the spindle. For most job shops, HPC is the better first investment.

8. Surface integrity is the deliverable

A titanium part that is dimensionally correct and metallurgically damaged will pass inspection and fail in service. Three layers matter:

  1. Alpha case. Oxygen diffusion above 600 °C produces a brittle, hard layer. Depth follows a diffusion relation x \approx \sqrt{Dt} and reaches 0.05–0.2 mm after typical heat treatment in air. It must be removed — by chemical milling (HF/HNO₃), by machining after heat treat, or by processing in vacuum/inert atmosphere.
  2. White layer and microcracks. Severe rubbing or a dull edge can produce an untempered, over-tempered or plastically deformed layer 5–50 µm deep, with microcracks that act as fatigue initiation sites. The remedy is a sharp edge, a decisive feed, and a finishing pass that cuts below the damaged layer.
  3. Residual stress. Machining induces compressive stress at the surface and balancing tensile stress below. In thin walls, removing material releases the imbalance and the part moves. The sequence matters more than the parameters: rough, stress-relieve, semi-finish, finish, with the final pass removing 0.3–0.5 mm.

Fatigue life in titanium is notoriously sensitive to surface finish. A rule of thumb for machined Ti-6Al-4V: reducing R_a from 1.6 µm to 0.4 µm can improve fatigue strength 10–20%, and shot peening adds another 15–25% by converting the residual stress at the surface to compression. For flight-critical parts, the surface integrity specification is on the drawing, and the process must be qualified to it.

Inspection follows from the same logic:

9. Buy-to-fly, distortion and the case for near-net shape

Aerospace titanium parts are famous for buy-to-fly ratio:

Conventional practice for complex brackets runs 10:1 to 20:1, and parts have been documented at 40:1. At 30/kg for Ti-6Al-4V bar, a 1 kg finished part from a 15 kg billet carries 450 of material, of which $420 becomes swarf worth perhaps 10–20% of that as scrap. Add the machine time to remove 14 kg of titanium at 4.6 cm³/min and the reason for near-net-shape processes is obvious.

Additive manufacturing changes the ratio to roughly 1.2–1.5:1 for LPBF and 1.5–3:1 for DED, at the cost of powder (Ti-6Al-4V aerospace powder at 180–350/kg, Inconel 718 at 83–100/kg), residual stress, and a post-build machining operation on every critical interface. The hybrid route — print near-net, then CNC the mating surfaces, bores and threads — is now standard for brackets, impellers and housings. A representative case: a 280 g topology-optimised titanium bracket from a 2.8 kg billet needed 14 hours of 5-axis machining; printed near-net in 6 hours and finished in 3.5 hours, it used 340 g of material and shipped in 7 days instead of 18.

The same economics apply to 718, with an extra constraint: additively manufactured 718 needs a hot isostatic press (HIP) and a full heat treat to close porosity and develop gamma-double-prime, and the as-built surface roughness in internal channels may require chemical or abrasive flow finishing.

Distortion is the other reason to think about sequence. Removing 90% of a billet releases residual stress that was locked in at the mill. The part bows. The conventional answer is a stress-relief anneal between roughing and finishing (for titanium, 540–650 °C for 1–2 hours; for 718, a full re-age is usually impractical, so the sequence is designed around the stress state), plus a semi-finish allowance of 0.5–1.0 mm.

10. The cost model

Cost per part for a machined feature is the sum of machine time, tooling and material:

where C_o is the machine and labour rate per minute, t_m the cycle time, T the tool life, C_t the cost per tool edge, t_r the tool change time, and C_m the material cost per kg.

Minimising with respect to speed gives the cost-optimal cutting speed:

Work it with real numbers. C = 140, n = 0.25, machine rate ₹1,800/h = ₹30/min, insert edge ₹900, tool change 2 min:

Forty-five metres per minute — below the 60 m/min that maximises tool life per edge, because at ₹900 per edge the tool is expensive relative to the machine. Change the insert to ₹250 and the optimum moves to 59 m/min. This is why two shops with the same machine quote the same titanium part 40% apart: one is optimising for tool cost, the other for cycle time, and neither has written down which.

A worked part cost for a 1.2 kg Ti-6Al-4V bracket, 4.5 kg input:

The same geometry in 6061 aluminium might be ₹6,500. The 7× multiplier is not a markup; it is the physics, and any quote that does not reflect it is either misunderstanding the alloy or planning to lose money.

For Inconel 718, add 1.8–2.5× the cycle time, 2–4× the tool consumption and a narrower supplier base. Expect ₹80,000–150,000 for a comparable 2 kg 718 housing.

11. DFM rules for designers

The cheapest titanium part is the one designed for the process. These are the limits that matter, expressed as rules a designer can apply before the model reaches the shop:

Feature · Ti-6Al-4V · Inconel 718 · Why

Minimum wall · 0.8–1.0 mm · 1.5 mm · Deflection scales as t^{-3}; below this, chatter and taper

Pocket depth : width · 3:1 · 2.5:1 · Tool length-to-diameter; 4:1 needs a special approach

Internal corner radius · ≥ 0.5 × tool Ø, min 3 mm · ≥ 3 mm · A sharp corner needs a smaller tool, which is less rigid

Floor thickness · ≥ 1.0 mm · ≥ 1.5 mm · Springback under axial force

Hole depth : Ø · 4:1 (peck) · 3:1 · Chip evacuation and work hardening

General tolerance · ±0.05 mm · ±0.05 mm · Thermal growth and deflection

Critical tolerance · ±0.010 mm · ±0.010 mm · Requires finishing passes and temperature control

Surface finish · R_a 0.8–1.6 µm · R_a 1.6–3.2 µm · Tool wear and BUE

Threads · Cut or formed, ≥ M3 · Thread mill, ≥ M4 · Taps break in 718

Five design moves that reduce cost more than any parameter change:

  1. Replace deep narrow pockets with open profiles. A 4:1 pocket forces a long, slender tool; opening one side lets a 12 mm cutter do the work of a 6 mm one at four times the removal rate.
  2. Use standard tool radii. A 2.5 mm corner forces a 5 mm cutter; a 3 mm corner allows a 6 mm cutter with twice the stiffness.
  3. Put critical tolerances on one datum face. Every additional tight feature is another setup, and each setup in titanium is a chance to scrap a ₹12,000 billet.
  4. Design the near-net shape. If the part starts as a forging or a print, the machining allowance should be 1.5–3 mm, not 10.
  5. Specify the finish you need, not the finish you can imagine. R_a 0.4 µm on a non-functional surface triples finishing time for no benefit.

12. What to put on the purchase order

A titanium or 718 PO that says "material: Ti-6Al-4V, per drawing" is an invitation to dispute. The specification should carry:

A supplier who cannot answer these questions in writing is not a titanium supplier, whatever the quote says.

The numbers to take away

If you are sourcing a titanium or superalloy part, the CNC machining guide covers the general process envelope, the feeds and speeds guide derives the parameters from cutting mechanics, and the spindle and tool-holding guide covers the rigidity side of the same problem. For near-net-shape alternatives, the metal additive manufacturing guide covers LPBF, DED and the post-processing chain; the should-cost model shows how to build the quote from the same arithmetic used above; the non-destructive testing guide covers the inspection methods named in section 8; and the fatigue and fracture mechanics guide explains why the surface layer matters more than the tolerance.

The bracket that opened this article had a 250 m/min program, four broken tools and a delivery date. The one that shipped had a 50 m/min program, a 1.2 mm radial step, 90 bar of coolant and a tool-change interval the programmer could predict to within ten minutes. Same machine, same alloy, same drawing — the only difference was that the second process was designed around the four properties that make the alloy worth its price.

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