Despite the recent economic slowdown, few metalworking markets today are growing nearly as fast as the wind power industry. Wind farms presently represent 1.3% of world electrical generating capacity, a figure that is growing at 29% per year. Even at the 1.3% level, wind turbines produce enough electricity to power 36 million households and, in turn, to reduce greenhouse gas emissions by 220,000 tons/year.
New investment in wind power is projected to create 10 million new jobs in 2010 and exceed $40 billion by 2017. So, before very long, we will have far surpassed 1.3%.
Another reality is that while we may be in a sellers’ market today, it will not be that way forever. Wind power is here to stay, and sure to grow, so it is sure to attract a crowd to the supply chain. Survival will hinge on the ability to continuously machine big parts more efficiently and more profitably, amid a growing competition.
To understand the manufacturing issues in wind power sub-components production, one must first examine a typical windmill station framework and what it teaches about the workpieces a manufacturer will be seeing then taking a closer look at the machining operations involved and current best practices to tackle these operations. The following is advice from Iscar engineers, based on their experience from working with more than 50 companies now in the wind power supply chain.
More Than Meets the Eye
Do not be fooled by what you see from ground level. That tiny windmill atop that pole is actually a complex piece of rotating machinery weighing several tons. You see the slow-moving rotor blades, but not the 750rpm to 3,600rpm generator they drive. Also hidden inside the nacelle is a helicopter-like blade feathering mechanism, the yaw linkage that seeks the wind, the step-up transmission with big bearings, low rpm ring gears, servomotors, and rotary actuators.
Then there is the tower. Along its length are segments with thick flanges of hardened metal on both ends, each with its own bolt circle in diameters up to 6m. From the machining standpoint, the chief operations in wind-turbine manufacture share these three key characteristics:
- Long cycle, often running unattended, putting a premium on process security and long, predictable edge life on the cutting tools;
- Very large, often non-rectilinear and asymmetric, workpieces, which call for a complete machining approach – grip the part once and well, and then do it all; and
- High added-value workpieces that you cannot afford to scrap.
Now here is a look at a typical part: the hub. A face milling cutter can be in a cut for hours, continuously, on a two-ton casting that carries a price tag in the tens of thousands of dollars before making the first cut. Not only must the finished hub be accurate dimensionally, but also balanced dynamically. There are a few opportunities for indexing and re-working, but there is zero tolerance for tool wrecks or scrap.
Rough Milling Big Parts
One of the principal operations – arising in the transmission and on the main shaft and support base as well as that hub – is rough face and shoulder milling. The best solution here is a tangential mill which delivers high removal rates, together with dependably long edge life. The reason: tangential tools present the insert’s strongest cross section to the main force vector of the cut, enabling surprisingly fast hogging with great edge security. In combination with inserts given a proprietary surface treatment and a helical curve to the cutting edge, tangential milling has doubled both the removal rate and edge life on a variety of big wind power parts. Insert geometry and surface treatment complement each other – strengthening the cutting system while reduce cutting forces.
In several shops, larger-area face milling is done smoothly with the Iscar 16Mill cutter. Available in large diameters, it uses 16-edged inserts and a 45° lead angle for smooth entry of each pass. Typically, dozens of inserts populate their big pitch circle, evenly distributing cutting forces while reducing chip loads on each tooth. This cutter has proven a big time saver for face milling of the support base and planetary carrier.
Iscar engineers already work with companies in the wind power supply chain and can offer sound tooling selection advice.
Wind turbine hubs weigh two tons, costing in the tens of thousands of dollars.
For smaller-scale surface rough milling, as in the blade adapter feathering mechanism and the reach-in milling needed for journals in the gearbox, a face mill is producing excellent results and extremely high feedrates. This cutter uses double-sided rectangular-shaped inserts, together with a proprietary treatment and helical cutting edge.
The resulting cutting action is more like scissors shearing paper than an axe chopping wood – you cut fast while preserving the edge.
Speeding up Work
Production of the hub and main bearing housing involves a lot of rough OD and ID machining on large, asymmetrical workpieces, which cannot readily be rotated. In such cases, several manufacturers report exceptional gains by rough orbital milling with a tangential indexable mill. The metal simply comes off faster. The high accuracy of the hub ID bores are obtained by semi-machining rough boring heads using a balanceable dual cut mechanism.
However, the cut micron finishing accuracy can only be achieved with dynamic balanceable fly cutter fine boring heads – despite its unproportional size and weight. Tooling costs are much lower as well, since the tool need not match the hole in diameter.
Conventional turning certainly still has its place here, especially on shafts, bearings, and other rotating parts. For the very roughest turning, Iscar engineers often recommend the Sumoturn tooling with Sumo Tec-treated ISO square, round, or long-edge rectangular inserts. The treatment smoothes out the insert coating at the micro level, thereby eliminating stress raisers while reducing cutting friction and heat. Additionally, the
Heliturn TG turning tool, featuring robust tangential clamping mechanism inserts, with the free-cutting helical cutting edge, has proven well for deep roughing cuts on the main shaft.
For finish turning hardened ring gears and large bearings, the recommended tool is the Isoturn lathe tool with either CBN or ceramic inserts. The inserts provide the requisite hardness with a strong, free-cutting geometry.
Bolt circles abound in wind power components such as, yaw and blade adjustment systems and tower flanges. For deeper bolt holes running on tight machines, twist drilling has become the choice for bigger hole size applications. The twisted coolant ports follow the contour of the flutes to create a stronger tool body, while improving coolant delivery and chip evacuation. This means less re-cutting, cooler running, and less friction – all leading to longer drill life.
For smaller holes, or deeper ones, and hole production on less rigid equipment, Iscar sees the Sumocham replaceable tip drill speeding up the operation and adding process security. Its alloy steel shank is more forgiving of machine instability. The design also saves time for tool replacement. Tip changing takes just seconds, done right in the spindle, for perfect repeatability and exceptional productivity rate.
For threading those bolt holes, an indexable thread mill is preferred, size permitting. In addition, larger holes can be threaded by orbital milling with the right tool. Expensive carbide is used only where needed, chip clearance is better, and again the steel shank makes the operation more secure.
Where a solid threading mill is required, choose a tool which features fewer and stronger flutes, plus a released neck between the cutting zone and main shank. This provides more room for more trouble-free chip evacuation.
A Better Method
Not surprisingly, teeth on gears and some bearing races pose special problems – lots of gear teeth to cut in the softened state and very precise ODs and IDs to finish in the hardened state. For the heavy slotting, one specific gasher gear milling cutter has made a dramatic difference, reducing cycle times on big ring gears by 50% or more. Unlike any other slotter, the gasher gear milling cutter brings the benefits of tangential insert orientation to gear-tooth production. Then, by their geometry and special coating treatment, the inserts themselves reduce cutting forces, friction, and impact to protect the edge while still cutting faster. The gasher has become the standard solution for gear slotting at dozens of wind power component suppliers.
Avoiding the Risk
Wind power component production often requires a non-standard tool. Rather than ordering a special tool from a specialty provider, look instead for a single source full-line tool manufacturer. Especially when you are trying to keep up with a fast-growing market, you need certainty in your tooling supply. In fact, there is a fair chance that he is already making something very similar for someone else – so you save money and reduce the risk.
If you are already playing catch-up in the wind power component manufacturing business, or thinking about jumping in, remember this: The quickest, most cost-effective route to faster production is through retooling that builds on the knowledge base already developed specifically in this industry. It is no exaggeration to say that smart use of the latest tooling could cut machining costs and cycle times – literally by half. Iscar engineers have seen it time and time again, in every corner of the wind power industry. Equally important, smart retooling could shave weeks off the delivery times of components that are presently causing the greatest bottlenecks.
If you want to keep up with this growing field today, take a fresh look at new tooling periodically. Tooling itself, and best-practice expertise, are evolving more rapidly than you may realize. Look at your bottleneck operations and ask an experienced full-line tooling supplier for his ideas. You will get better answers sooner, making you even more competitive and profitable tomorrow than you are today – and a sustainably key player in the Green Energy movement.
Iscar Metals Inc