Uptower See all →
Yaw Ring Teeth Repair Patented in-nacelle machining. The ring is repaired where it stands - no removal, no crane. Rotor Lock Repair Full rotor lock seat restoration in-nacelle. No component removal required. Bolt Removal & Thread Repair Failed or seized bolts removed and threads recovered without replacing the host component. Yaw Sliding Surface Repair On-site repair of worn yaw sliding surfaces. No yaw ring exchange, no crane. Yaw Gear Mounting Flange Repair Worn yaw gear mounting flanges and damaged threads repaired uptower. Brake Disc Repair Damaged brake disc sections milled out and replaced uptower. Generator Shaft Repair Worn generator shafts machined back to tolerance in place. No generator replacement.
Blade See all →
Blade Root Contact Surface Repair CNC-controlled machining of worn or damaged blade root contact surfaces. Blade Root Bushing Repair Restore damaged blade root inserts onsite, reducing downtime from months to days. Blade Surface Milling Repair 3D milling of damaged blade areas to an exact surface - uptower or on ground.
Support Structures & Flanges See all →
Monopiles & TPs Sub-millimetre flatness on monopiles and transition pieces. Precision machining performed before installation. Tower Flanges Tower section flange flatness restored on-site. No re-fabrication, no transport. Industrial Flange Machining Sub-millimetre flatness restored on site for industrial flanges on large production machinery. Sea Fastening Machining sea fastenings and towers stands to the required tolerances without transporting the structures to a workshop.
General Industry See all →
Machine Plane Restoration On-site machining of machine plane. Flatness restored within specs without disassembling the press. Line Boring Portable line boring for large bores in-situ. Heavy industry, marine, energy. Axle & Shaft Repair Worn or damaged axle and shaft journals restored on-site. No removal, no replacement.
Measurements See all →
3D Laser Tracker Measurement Portable laser tracker measurement of large components. Hole patterns, reference points, machine alignment and flange geometry - up to 0.05 mm. Easy-Laser Measurement Easy-Laser alignment and geometric measurement. Flange flatness, tilt, shaft and machine alignment - verified and documented on site. 3D Scanning Handheld ZEISS 3D laser scanning on site. Reverse engineering, CAD comparison, wear and damage analysis - metrology-grade digital documentation.
Custom Solutions Can't find your repair? We design bespoke solutions for unique geometries - wind or industry.
→
Case study

First Vestas V236 tower flange milled to 0.21 mm global flatness

Welcon built CNC Onsite's Goliath flange milling machine into its production of 97 bottom tower flanges for Vestas' 15 MW V236 turbines. The specification was 1 mm. The first flange came out at 0.21 mm.

Customer
Welcon
Machine
Goliath
Published
Goliath flange milling machine mounted inside a 7.5 m Vestas V236 tower flange at Welcon
Measured result

Global flatness on the first flange

The project specification was 1.0 mm global flatness. After milling, the first flange measured 0.21 mm.

  • 0.21 mm Global flatness, first flange
  • 1.0 mm Project specification
  • 97 Bottom tower flanges
  • 7.5 m Flange diameter

All figures are from the published case and the 22 May 2025 press release. 0.21 mm is the result on the first flange in this project, not a general capability figure.

The situation

Offshore turbines keep growing, and the flanges grow with them. For Vestas' 15 MW V236 turbines the bottom tower flange is more than 7.5 m in diameter. Larger diameter and tighter tolerance is a hard combination: the flatness that traditional methods manage on a 4-5 m flange gets nearly impossible at 7 m and above.

The project specification for global flatness was 1 mm. The common industry standard is 2.5 mm. Welcon, one of Europe's leading tower manufacturers, needed a process that could hit the tighter number consistently and stop relying on heat straightening to correct flatness after welding.

Why heat straightening falls short: it corrects distortion by heating and cooling selected areas so the steel contracts back into shape. It is manual work, and on flanges this size it cannot reach the precision the specification asks for.

What we did

CNC Onsite delivered a custom-built Goliath flange milling machine to Welcon, and production started in April 2025. Goliath is milling 97 bottom tower flanges for V236 turbines destined for offshore projects in Germany and the Netherlands.

The machine is mounted directly on the flange. Adjustable legs with hydraulic cylinders position it, so switching between flange diameters is quick. Fully CNC-controlled, it mills the contact surface between tower and transition piece flat and uniform. The standard configuration covers flanges from 6.5 to 10 m and handles flat, tilted and double-tilted flanges as well as gasket grooves.

Welcon runs the milling as a standard manufacturing step, in place of heat straightening.

Why the flange face matters

The bottom tower flange is the bolted joint between the tower and the transition piece. Each flange in this project carries 160 bolts. A flat contact face is what lets the bolts be tightened correctly and stay tight. Loose bolts mean repair costs and downtime.

Vestas' Peter Sigfred Mortensen, Senior Specialist, Offshore Tower Structure, sees it going further: if all flanges from tower to transition piece were milled flat, he believes it would remove the costly measurements done at sea and before shipping, and open up smaller and even maintenance-free bolts.

Section drawing of a bolted tower flange connection in assembled form
Fig. 01Section of a bolted flange connection in assembled form. Infographic: CNC Onsite.

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, reducing maintenance costs and downtime due to loose bolts, is a considerable cost saver.

Søren Kellenberger, Sales Director and Partner, CNC Onsite

The result

The first flange came out at 0.21 mm global flatness against the 1.0 mm specification. That is the result on the first flange of this project, not a general capability figure.

  • Heat straightening dropped as a post-production step
  • A controlled, repeatable machining process instead of manual correction
  • Better conditions for correct bolt preload around the whole flange connection
  • Less need for costly flatness measurements and corrective work
  • More freedom in future flange and bolt designs

Welcon expects the change to improve health and safety too, since it takes manual work out of the process.

From the project

Goliath at Welcon

Goliath mounted on the machined face of the first flange
Fig. 02The first flange after milling. Global flatness measured at 0.21 mm.
Goliath flange milling machine lifted by crane before mounting on the flange
Fig. 03Goliath being lifted into position before mounting on the flange.
Customer quotes

“The industry demand for fine tolerances is increasing. To meet flatness requirements, we need a machine-based process, as heat straightening cannot achieve the necessary precision. We also expect this to improve health and safety by eliminating manual work.”

Johnny Hauggaard Skov, Vice Director, Welcon

“If applied throughout the industry on all flanges, from tower to transition piece, I believe this precision milling will offer a range of benefits. 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.”

Peter Sigfred Mortensen, Senior Specialist, Offshore Tower Structure, Towers R&D, Vestas
Related solutions

The solutions in this case

Talk to us

Facing a similar challenge?

Describe the component, the tolerance you need and where it sits. We come back within one working day with an assessment of what is possible on site.

Request a quote See all solutions
Ole Andersen
Your direct contact Ole Andersen Technical Sales – Ad Hoc oan@cnconsite.dk +45 31 51 11 48