CNC Milling Machine Produces Precisely
Shaped, Perfectly
Symmetrical Composite Rudders for America's Cup Boats
By Ben Sprague
Marketing Director
Gary Crosby
Shop Foreman
Goetz Marine Technology
Bristol, Rhode Island
America's Cup boats are getting faster all the time, and part of that
is due to Goetz Marine Technology's (GMT) use of CNC to produce the
precisely shaped, perfectly symmetrical composite rudders many of these
boats use. GMT rudders can be found on several boats competing in the 1995
America's Cup race as well as on the 1992 winner, America3, and contender
Stars & Stripes. Symmetry and accuracy are critical in the production
of a rudder because this part has been optimized by the boat designer to
provide maximum speed and lift. With manual production methods, GMT was
able to get within four or five mm of designers' specifications, but by
using a CNC machine from Techno-Isel, New Hyde Park, NY, rudders are
accurate to within 1/2 mm. The CNC machine, driven by geometry from
AeroHydro's KeelCAM software, is used to cut the two halves of a part's
high-density foam core. In addition to helping GMT produce rudders that
exactly match customers' drawings, the machine has reduced the time needed
to shape a rudder from 16 hours to about 10.
Goetz Marine Technology, in Bristol, RI, is recognized throughout the
boating world as a leader in the design and construction of advanced,
high-quality composite parts such as keels, rudders, masts, and booms. The
company is run by David Schwartz who jointly owns GMT with founder Eric
Goetz. In addition to his role at GMT, Goetz is president of Eric Goetz
Custom Sailboats, the company that built the boats for all three of the
defense syndicates--America3 Foundation, Team Dennis Conner, and PACT
95--competing in this year's America's Cup. Goetz started GMT in 1984 to
provide other boat builders, as well as his own company, with the
composite parts he was using with such success on the boats he was
building. Goetz's knowledge of advanced composites such as carbon fiber
formed the foundation for GMT, which in 1989 was the first company to
offer a carbon fiber mast for cruising boats. GMT started out by supplying
parts primarily for racing boats, but the business now breaks down to
about 70% parts for cruising boats and 30% parts for racing boats.
Carbon fiber has become a popular material for boat parts because it is
strong yet lightweight. According to Ben Sprague, director of marketing, a
rudder made completely of carbon fiber weighs about 60% less than one made
with a stainless steel post and fiberglass blade. Strength and lightness
are especially important in racing situations, and the America's Cup boats
in particular incorporate carbon fiber into many of their parts. GMT
primarily makes custom parts, which means that rather than pouring
material into a mold to form an object--the way parts are mass
produced--they cut a high-strength core to the desired shape and then
laminate it with carbon fiber. The advantage of this approach is that by
laying carbon fiber over a high-density foam that holds its shape well,
the part comes out stronger than one produced in a mold.
Typically, when customers come to GMT with a request for a part, they
provide drawings. The customer has probably done extensive computer
modeling and tank testing to come up with the shape of that particular
part. This is especially true of the foil shapes of parts like a keel or
rudder. It is important that GMT match the customer's drawings exactly
because the designer has optimized the part's shape to give the boat as
much speed as possible. Using the conventional approach, however, this was
difficult to do.
As an example, consider the production of a rudder, the critical component that both steers the boat and generates lift, which translates to angle to windward. A rudder consists of two parts: the post that attaches to the boat and permits the turning back and forth, and the blade which is the foil shape that does the turning. In the manual production process, the blade's foam core was manually carved, one half at a time. Using a circular saw, kerfs or slots were cut into the foam blank every 12 to 18 inches. Templates representing the blade half were inserted into the slots. The foam was then carved with a power plane down to the tops of the templates. This required a skilled operator because it was easy to dip below the templates in the area in between them.
After one side was carved, carbon fiber was placed on that side and allowed to set. The next day, the piece was flipped over so the other half could be carved. It too was covered with carbon fiber and once it hardened, the two halves were joined. A pocket for the post was cut into the blade with a hand saw and the post was bonded into place with glue. The drawback to this method was that because the foam core was cut by hand, accuracy was only possible to about four or five mm. This approach was also somewhat slow, taking about 16 hours to produce a part.
About six years ago, GMT purchased Techno-Isel's CNC milling machine with the Mac100 controller to improve accuracy and reduce production time. This system was chosen because it was the most affordable and highly accurate. The machine has a resolution of 0.0005 inches which is about 100 times the precision that can be achieved by hand. Other points in its favor were its ease of use and compatibility with all the leading CAM packages.
Now after GMT receives instructions from a yacht designer, the rudder production process proceeds this way. First, Gary Crosby, who is in charge of rudder production, re-creates the blade in a software program called KeelCAM. This program was created specifically for modeling foil-shaped objects by John Letcher of AeroHydro Corp. in Maine. To model a rudder blade in KeelCAM, the designer enters points along the leading and trailing edges of the blade in a grid format. Then he builds a table of numbers as an ASCII file from which the software generates the plan form and the foil offsets for the foil shapes that run up and down the blade. Usually, the designer will enter half a dozen foil shapes up and down the blade, and let the computer smooth them out to produce the finished shape.
The smoothed blade produced by KeelCAM becomes the cutting file for the Techno machine. The data describing the plan form of rudder blade is transferred into machine's Mac100 controller. The Techno is a three-axis machine with a z axis travel of 11 inches. It has a work envelope of 40 inches by 40 inches. However, most rudders are longer than that so the machine is programmed to carve 40 inches at a time.
The cutting bit is a 1-3/4-inch ball mill on a 2-1/2 hp router. The carving stroke is in the fore and aft direction. The step-over is about 15 mm, which leaves little scallops that are smoothed out by hand. After one half is carved, the foam is laminated. As in the manual process, the rudder builder shapes a plywood cradle that sits on top of that half of the blade. The purpose of the cradle is to make sure the piece sits flat on the machine's table when it is turned over. The other side is then carved and laminated and from there the process proceeds as it did in the past.
Most rudders can be carved in three 40-inch cutting sessions. Each session requires between 120 and 130 passes and takes about 20 minutes. Usually, the carving for one side of the blade is done in about four hours which includes set-up time. Programming time depends on quality of information provided by the yacht designer. Generally it takes two to three hours for programming and getting the cutting files ready. Therefore, the automated method typically takes about 10 or 11 hours to produce the finished blade.
Compared to the 16 hours required by the manual approach, this is a beneficial time savings but the real benefit of the automated method is that it creates very accurate rudder blades. The Techno machine also allows exact duplicates to be made for spares or testing. The Techno machine has four ground and hardened steel shafts and eight recirculating bearings in each axis. This shaft and bearing system produces very smooth play-free motion which produces high-quality cuts. Also, the machine has anti-backlash ball screws for play-free motion that make it possible to produce circles that are accurate to the 0.0005 inch machine resolution. For a rudder blade that is two meters tall and 500-600 mm fore to aft, GMT is now delivering blades with 1/2 mm accuracy. Compared to the four to five mm accuracy they were getting, this is a substantial improvement.
The Techno machine is sometimes used in the production of keels, as well. However, its primary use is for producing rudders to the precise specifications required for racing boats. In fact, GMT has an impressive list of boats outfitted with its rudders, including several in this year's America's Cup race, although due to the highly competitive nature of the event it is not possible to specify which boats at this time. To boat designers, GMT's highly accurate production process and knowledge of advanced composites is the combination they trust for critical composite parts, making them the preferred supplier.
For more information contact: Techno,
Inc., 2101 Jericho Turnpike, New Hyde Park, NY 11040.
Phone: 516-328-3970 Fax: 516-358-2576 E-mail: TECHNO
CNC ROUTER SYSTEMS