On a mid-range vertical machining centre (VMC), the right coolant strategy for titanium and Inconel is high-pressure through-tool coolant at 70–150 bar.
On a mid-range vertical machining centre (VMC), the right coolant strategy for titanium and Inconel is high-pressure through-tool coolant at 70–150 bar, not standard flood coolant. This single decision typically reduces cycle time by 20–40% and tool cost per part by 30–50% compared with flood coolant, which is the core of the business case you will present to the board.
These are typical industry ranges, not machine-specific guarantees. As a procurement engineer, you are not buying coolant. You are buying a machining process that must hit a cost-per-part target. Titanium alloys such as Ti-6Al-4V and nickel alloys such as Inconel 718 have low thermal conductivity. Heat stays in the cutting zone. Standard flood coolant cannot reach the cutting edge fast enough. The result is short tool life, low cutting speeds, and scrap risk.
Thermal conductivity is the physical reason this decision matters. Titanium conducts heat at roughly 7 W/m·K, Inconel 718 at about 11 W/m·K. Carbon steel conducts at about 45 W/m·K, and aluminium at about 200 W/m·K. In titanium and Inconel, the heat from cutting goes into the tool and the workpiece, not into the chip. That heat shortens tool life and changes part dimensions.
A flood coolant system delivers coolant at 0.5–2 bar over the workpiece surface. It cools the outside, not the cutting edge. In titanium and Inconel, the chip sticks to the tool edge and forms a built-up edge (material welded to the cutting edge). Flood coolant cannot break that bond fast enough. High-pressure through-tool coolant, also called through-spindle coolant (TSC), delivers coolant through the spindle, through the tool, and directly to the cutting zone at 70–150 bar. This breaks the chip, forces it away, and removes heat where it is generated.
For the board, the coolant system is a machine capability, not a consumable. A high-pressure system may add 5–10% to the machine purchase price. On titanium and Inconel work, it can reduce total cost per part by far more than that through cycle time and tooling savings.
Start with a real feature from your part. For this example, use a roughing operation in Ti-6Al-4V on a mid-range VMC.
Material removal rate Q = ap × ae × vf = 12 × 1.2 × 382 = 5501 mm³/min = 5.5 cm³/min.
For Inconel 718, start lower. Cutting speed Vc of 35 m/min is a typical planning figure. That gives a spindle speed of 929 rpm and a feed rate of 223 mm/min with the same tool and feed per tooth. Material removal rate is then about 3.2 cm³/min.
The mechanical cutting power at the tool is small. Specific cutting energy (the energy needed to remove one cubic millimetre of material) for titanium is typically 2–4 J/mm³. Use 3 J/mm³ as a planning figure. For Ti, Q = 5501/60 = 91.7 mm³/s. Cutting power = 91.7 × 3 = 275 W. For Inconel, use 4 J/mm³ and Q = 53.5 mm³/s, giving about 214 W. The problem is not bulk heat removal. It is that this heat is concentrated in a few square millimetres at the cutting edge. Coolant must reach that point, break the chip, and carry it away before it is recut. That requires pressure, not just flow.
Use a water-miscible semi-synthetic or synthetic coolant. Do not use neat oil for high-pressure titanium or Inconel work. Neat oil has poor heat transfer and creates mist at high pressure, which is a fire and health risk.
For titanium, avoid coolants with active chlorine or active sulphur. Chlorine can cause stress-corrosion cracking on titanium parts. For Inconel, select a coolant with extreme-pressure (EP) additives that are approved for nickel alloys. Check with the coolant supplier and get written compatibility for your exact alloy.
Concentration is measured with a refractometer, an optical instrument that shows coolant concentration as a percentage. For titanium, maintain 8–12%. For Inconel, maintain 8–10%. Too low a concentration gives poor lubrication and invites corrosion. Too high causes sticky residue, foaming, and skin problems.
The table below shows typical coolant delivery options for a mid-range VMC. All values are typical industry ranges. Exact values for a specific Gree CNC machine must be confirmed against the official Gree CNC catalogue.
| Delivery type | Pressure (bar) | Flow (L/min) | Use for Ti/Inconel | Limitation |
|---|---|---|---|---|
| Flood coolant | 0.5–2 | 10–30 | Not suitable as primary strategy; only light finishing or non-critical features | Cannot penetrate vapour barrier at cutting edge |
| Low-pressure through-spindle | 20–30 | 15–25 | Shallow features up to 2×D, light roughing | Chip evacuation limited in deep pockets |
| High-pressure through-spindle | 70–150 | 20–50 | Pockets and holes up to 5×D, roughing and finishing Ti/Inconel | This is the target for production work |
| Ultra-high-pressure | >150 | 20–60 | Deep holes >5×D, heavy roughing | Often needs special seals and tool holders; may exceed mid-range VMC spindle rating |
On many mid-range VMCs, through-spindle coolant is an option, not a standard feature. The maximum pressure rating depends on the spindle design and rotary union. Check the official Gree CNC catalogue for the specific model’s through-coolant pressure rating. Also confirm whether an external high-pressure unit is required. Do not assume a 70 bar pump will deliver 70 bar at the tool. Pressure drop through tool holders and small coolant channels can be 10–30%.
Pump. Select a multistage centrifugal or positive-displacement pump rated for continuous duty at the required pressure and flow. For a system delivering 40 L/min at 100 bar, the hydraulic power is (flow × pressure) / 600 = (40 × 100) / 600 = 6.7 kW. Add motor efficiency, so electrical demand is roughly 8–9 kW. Use this number in your energy cost calculation.
Tank. The coolant tank should hold 3–5 times the pump flow per minute. This allows air release and fine chip settling. At 40 L/min, a 150–200 L tank is typical. Check the official catalogue for the tank capacity on the model you are evaluating.
Filtration. Use a two-stage filter. The first stage, 50–100 micron, catches coarse chips. The second stage, 10–25 micron absolute, protects the high-pressure pump and tool nozzles. Titanium and Inconel produce fine, abrasive swarf. Poor filtration clogs nozzles and wears pump seals quickly.
Temperature control. Keep coolant at 20–30°C. Titanium and Inconel machining raises coolant temperature. Above 35°C, thermal growth on a mid-range VMC can shift dimensions by 10–20 µm. A chiller of 3–5 kW cooling capacity is often sufficient for the heat loads shown above, but size it against total spindle power and duty cycle.
Use dynamic milling, also called trochoidal milling. This is a tool path strategy with a small radial engagement, typically 5–15% of tool diameter, and high axial depth, 1–2×D. The tool stays in the cut for a short time and the high-pressure coolant flushes chips from the narrow slot.
Never use full-width slotting in titanium or Inconel on a mid-range VMC as a primary roughing strategy. Full slotting generates excessive heat, causes built-up edge, and can break tools. For finishing, reduce radial engagement to 0.2–0.5 mm and keep high-pressure coolant on to prevent recutting of fine swarf.
Set coolant pressure at the maximum the tool holder and tool are rated for, typically 70–100 bar for solid carbide tools with through-coolant holes. Check the tool manufacturer’s rating before applying full pressure.
The table below shows a comparison for a hypothetical part in Ti-6Al-4V. All numbers are planning assumptions based on typical industry ranges. They are not a guarantee for any specific machine or part. Run a test cut on your own part before presenting the business case.
| Item | Flood coolant | High-pressure TSC | Notes |
|---|---|---|---|
| Cycle time per part (min) | 12 | 8 | 33% reduction typical for a Ti-6Al-4V pocket; not guaranteed |
| Tool cost per part (EUR) | 8.00 | 4.50 | 44% reduction from longer tool life; typical range 30–50% |
| Shop rate (EUR/hour) | 90 | 90 | Labour and overhead; DACH/UK typical range 80–120 |
| Labour cost per part (EUR) | 18.00 | 12.00 | Cycle time × shop rate / 60 |
| Total variable cost per part (EUR) | 26.00 | 16.50 | Excludes coolant consumables and extra electricity |
| Annual volume (parts) | 5,000 | 5,000 | |
| Annual variable cost (EUR) | 130,000 | 82,500 | Saving: 47,500 |
| High-pressure system additional cost (EUR) | 0 | 12,000–18,000 | Typical through-spindle coolant option plus pump/filtration on a mid-range VMC; confirm against official Gree catalogue |
| Payback (months) | n/a | 3.2–4.6 | Based on saving of 47,500 and capex of 12,000–18,000; not a guarantee |
This example assumes the part geometry allows a 33% cycle reduction. Deep pockets, very small tools, or poor machine rigidity will reduce the benefit. Always test on your actual part.
High-pressure coolant cannot fix every problem. Be honest with the board about these limits.
In these cases, choose a larger, heavier machine or a specialist process. Do not force a mid-range VMC into an application it will fail at. The board will remember the scrap cost and downtime, not the lower capital cost.
Use these points to structure your investment case:
Run the buyer self-check & book a machine selection call — contact the Gree CNC team.