Danish mobile machining specialist CNC Onsite has delivered its custom-built “Goliath” milling machine to Welcon, a leading wind tower manufacturer. The machine is now in use at Welcon’s facility, where it is machining 97 bottom tower flanges for Vestas’ 15 MW V236 turbines. These turbines are set for offshore wind projects in Germany and the Netherlands.
Production started in April, and the first milled flanges have already exceeded required flatness specifications. This ensures a precise fit with the transition piece, improving both reliability and design flexibility. The improved flatness also reduces the need for post-production checks, which are often costly and time-consuming.
Boosting Precision and Efficiency
The flange milling process ensures a perfect alignment between the tower and transition piece. This connection is critical, as it is secured with bolts and must remain stable over time. By using CNC Onsite’s precision machining, Welcon achieves tighter tolerances. This eliminates the need for heat straightening, a traditional method used to correct flatness after welding. It also reduces bolt maintenance during turbine operation.
Goliath, the machine used for this process, was developed specifically to improve flange flatness, which supports structural strength and resistance to fatigue.
Long-Term Benefits for Bolted Connections
“We reached a global flatness tolerance of 0.21 mm on the first flange,” said Soren Kellenberger, Sales Director and Partner at CNC Onsite. “For this project, the tolerance requirement is just 1 mm — already much stricter than the industry standard of 2.5 mm.”
Kellenberger noted that tighter tolerances also improve bolt tightening. With 160 bolts in each flange, better flatness can significantly reduce maintenance costs and prevent downtime caused by loose bolts.
Welcon has now made the flange facing process a standard step in its manufacturing. This replaces the older heat straightening method, allowing for faster and more uniform results across all 97 flanges.
Improved Value for Vestas
With consistently flat flanges, Vestas gains more flexibility in how it designs flanges and selects bolt sizes. This supports greater efficiency during turbine assembly and operation.
“The demand for higher precision is growing in the wind industry,” said Johnny Hauggaard Skov, Vice Director at Welcon. “Heat straightening can’t meet today’s flatness requirements, but with Goliath, we can. The process also improves worker safety by reducing manual labor.”
Peter Sigfred Mortensen, Senior Specialist for Offshore Tower Structures at Vestas, added: “If all flanges across the tower and transition piece are milled flat, we can avoid expensive pre-shipping and offshore measurements. It could also allow for smaller or even maintenance-free bolts, opening up new design possibilities.”
Solving New Challenges in Large-Diameter Flanges
As turbine designs grow, so do the challenges. Flanges now reach up to 7.5 meters in diameter, especially for Vestas’ new 15 MW turbines. Larger flanges require even tighter tolerances, making traditional methods less effective.
“It’s already difficult to achieve precision on 4–5-meter flanges,” said Kellenberger. “With flanges over seven meters, it’s nearly impossible without advanced machining like Goliath.”
A Machine Built for Speed and Flexibility
Goliath is mounted directly onto each flange using adjustable legs and hydraulic or electric supports. This allows fast setup and makes it easy to switch between different flange sizes. The machine uses powerful electric motors for fast and accurate milling.
In its standard form, Goliath can handle flanges from 6.5 to 10 meters in diameter. It can also be adapted for even larger sizes. Fully CNC-controlled, the machine can mill flat, tilted, and double-tilted surfaces and can cut features like gasket grooves.
Developed from over 12 years of experience in the wind sector, Goliath sets a new standard in flange machining. The machine at Welcon is the third custom-built unit of its kind since the product line launched in 2022. Goliath is available for lease or purchase and is currently used by offshore wind customers in Denmark and Spain.
About CNC Onsite
Based in Vejle, Denmark, CNC Onsite delivers high-precision mobile machining for wind turbines and offshore foundations. Its flexible “building blocks” design approach allows machines to be customized for a wide range of tasks.
CNC Onsite’s core services include flange and blade root machining, yaw ring repair, rotor lock servicing, and generator shaft work. The company also offers solutions for damaged or broken bolts, ensuring safe and efficient turbine operation both onshore and offshore.
