Milling Flanges to Meet Tighter Tolerance Demands for Wind Turbines
CNC Onsite machine prior to mounting onto the flange.
By Dr. Malene Conlong
Headquartered in Vejle, Denmark, CNC Onsite designs and delivers high-precision mobile machining for wind turbines, including offshore foundations. Machines built by CNC Onsite are designed to be flexible using its proprietary “building blocks” approach, which means machinery can be built to match a range of tasks.
CNC Onsite serves the onshore and offshore wind energy sector delivering as standard solutions: machining of large-diameter steel flanges and blade root ends; specialized repair services covering yaw ring, inserts in blade root, rotor lock, generator shaft, bearing housing, and threaded holes; and removal and replacement of worn and broken bolts.
The company has recently delivered its custom-built “Goliath” machine to the wind tower manufacturer Welcon, which is now using the machine to mill 97 bottom tower flanges for Vestas’s 15-MW V236 turbines destined for projects in Germany and the Netherlands. Production began in April, and the first flanges have already exceeded flatness specifications, ensuring a perfect fit with the transition piece—enhancing reliability and design flexibility and significantly reducing costly post-production checks.
The milling process ensures that the tower and transition piece align perfectly, forming a flat, stable connection between the two flanges, a critical mechanical joint held together with bolts. By implementing CNC Onsite’s machining process, industry-leading tolerances are achieved, which eliminates the need for heat straightening, a common post-production step to correct flatness issues, while optimizing the maintenance of bolts during operation.
The precision machine Goliath is custom-built by CNC Onsite, specifically designed to ensure flat flanges for structural strength and fatigue resistance.
Bolted Connections with Long-Term Benefits
On the first flange, CNC Onsite has achieved a global flatness tolerance of 0.21 mm. The required 1 mm for this project is already well below the common industry standard of 2.5 mm, proving the machine’s performance.
Ensuring a tolerance of 1 mm or better is a significant benefit when it comes to ensuring correct bolt tightening. With 160 bolts in a flange, this is a considerable cost-saver by reducing maintenance and downtime due to loose bolts.
Welcon will integrate the flange-facing process as a standard manufacturing step, replacing the traditional heat straightening method, a controlled heating and cooling process used to correct flatness distortions after welding in metal components, such as wind turbine flanges. By employing Goliath on the 97 bottom flanges, the tower manufacturer can ensure a faster, more uniform method.
Goliath mills Vestas flanges with 7.5-m diameter.
Added Value for Vestas
The consistent flatness across all flanges will provide Vestas, the customer of Welcon, with greater flexibility in flange designs and dimensions, as well as a broader choice of bolt sizes.
“The industry demand for fine tolerances is increasing,” said Johnny Hauggaard Skov, vice director of Welcon. “To meet flatness requirements, we need a machine-based process, as heat straightening cannot achieve the necessary precision.”
He added: “We also expect this to improve health and safety by eliminating manual work.”
“If applied throughout the industry on all flanges, from tower to transition piece, I believe this precision milling will offer a range of benefits,” said Peter Sigfred Mortensen, a senior specialist in offshore tower structure in Vestas’s Towers R&D department.
“When all flanges are completely flat, it eliminates the costly measurements we have seen both at sea and prior to shipping. It will also provide design freedom, including the option of smaller and even maintenance-free bolts.”
The first flange achieved 0.21-mm tolerance after precision
milling.
Meeting New Challenges in Large-Diameter Flanges
As wind turbine tower flanges grow in diameter—now reaching 7.5 m for Vestas’s new 15-MW turbines, the demand for even finer tolerances increases.
It is a challenge to achieve the required precision for 4- to 5-m-diameter flanges. Given today’s 7-m (and larger) flanges, the tolerances are even tighter. This combination of larger diameter and stricter tolerances makes it nearly impossible to meet standards with traditional methods.
Designed for efficient operation, the machine is mounted directly onto the flange. Using adjustable legs with hydraulic cylinders or optional electric actuators, the machine is securely positioned, allowing easy switching between different flange diameters.
Powerful electric motors enable fast, high-precision milling. The machine handles flanges from 6.5 to 10 m in diameter in the standard configuration and can be adapted for larger sizes. Fully CNC-controlled, the machine can mill flat, tilted and double-tilted flanges, as well as features such as gasket grooves.
Goliath is the result of CNC Onsite’s more than 12 years of expertise in machining flanges for the wind energy industry. Developed and built based on this extensive experience, the new machine sets novel standards for precision and efficiency.
The machine for Welcon is the third custom-built Goliath machine since its launch in 2022, designed for large-diameter flanges. It is currently in use by offshore customers in Denmark and Spain. It is available for lease or purchase.