Turbine Root Machining Prepress

Creep Feed Fir-Tree Root Grinding & Superalloy Slot Profile Prepress Guide

Comprehensive guide to deep full-profile creep feed grinding of gas turbine blade root fir-tree multi-serrations in nickel superalloys, specific grinding energy, continuous diamond roll dressing (CDCR), and tangent-continuous CAM vector layouts.

1. Creep Feed Deep Profile Grinding Mechanics

Turbine blade fir-tree roots transmit massive centrifugal retention forces ($> 50\text{ kN}$ per blade) to the turbine rotor disk. Machining these complex multi-lobe serrations into nickel superalloys (Inconel 718, René 88DT) requires creep feed grinding—taking the full profile depth ($a_e = 5 - 20\text{ mm}$) in a single pass at slow table traverse feed ($v_w = 20 - 100\text{ mm/min}$):

Maximum Undeformed Chip Thickness ($h_{\text{max}}$)

$$h_{\text{max}} = \sqrt{\frac{4 \cdot v_w}{v_s \cdot C \cdot r} \sqrt{\frac{a_e}{d_s}}} \quad \text{and} \quad P_{\text{spindle}} = u_g \cdot (a_e \cdot b \cdot v_w)$$

Where $C$ is active abrasive grain density, $r$ is grain aspect ratio, $v_s$ is wheel speed ($25 - 35\text{ m/s}$), and $u_g$ is specific grinding energy ($60 - 95\text{ J/mm}^3$).

2. Hydrodynamic Coolant Delivery & Thermal Burn Elimination

Because contact length is huge ($l_c = \sqrt{a_e \cdot d_s} \approx 45\text{ mm}$), extreme frictional heat is generated. To prevent metallurgical thermal burn and tensile residual stresses, specialized coherent coolant nozzles deliver cutting fluid at exact synchronous velocity matching the grinding wheel:

Coolant Nozzle Jet Velocity Matching

$$v_{\text{jet}} \approx v_s \quad \text{and} \quad P_{\text{coolant}} = \frac{1}{2} \rho_{\text{fluid}} v_{\text{jet}}^2$$

At $v_s = 30\text{ m/s}$, coherent nozzle pressure must be at least $4.5\text{ bar}$ to pierce the boundary air barrier generated by the rotating wheel.

3. Fir-Tree Multi-Serration Lobe Stress Distribution

Centrifugal blade load must be distributed uniformly across all lobes. Typical fir-tree profiles feature 3 to 5 pairs of symmetric flanks with convex/concave fillet radii ($R = 0.5 - 1.5\text{ mm}$):

Turbine Stage	Lobe Count	Profile Depth ($a_e$)	Flank Contact Angle	Typical Tolerance
High-Pressure Turbine (HPT)	4 Lobes	8 – 14 mm	30° – 45°	± 0.005 mm
Low-Pressure Turbine (LPT)	3 Lobes	6 – 10 mm	35° – 50°	± 0.008 mm
Heavy Industrial Frame Turbine	5 Lobes	15 – 25 mm	28° – 40°	± 0.010 mm

4. 2D Fir-Tree Profile DXF & Continuous Dresser Contour Vectors

In CNC creep feed grinders (Blohm, Mägerle, Danobat), the vitrified CBN or alumina wheel is continuously dressed by a diamond roll running at synchronous speed. Both wheel and workpiece contours require G2/G3 tangent-smooth arc splines without cusps:

// DXF Multi-Radius Fir-Tree Root Slot Profile
0
SECTION
2
ENTITIES
0
LWPOLYLINE
8
FIR_TREE_FLANK_PROFILE
90
6
70
0   // Open contour
10
-6.500   // Top entry land
20
15.000
10
-6.500
20
12.000
42
-0.4142  // Upper convex lobe radius R=1.2mm
10
-8.200
20
9.500
42
0.4142   // Concave fillet stress relief R=0.8mm
10
-5.800
20
7.000
0
ENDSEC
0
EOF

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