CNC Milling for Plugs, Molds,
Stephen M. Hollister
New Wave Systems, Inc.
79 Narragansett Ave.
Jamestown, RI 02835
Tel: (401) 423-1852
Fax: (401) 423-1810
Now that designers are adopting computer
methods for modeling and fairing 3-dimensional hull surfaces, it
seems reasonable to use the computer surface model to mill
full-size male or female plugs, or even produce complete tooling
by CNC machine. The promise is better accuracy, less cost, and
faster turn-around time. This article discusses some of the
things you need to know about the process before you jump in with
both feet. Although the focus of this article will be on using
outside services, the information will still be useful for those
considering whether to buy their own equipment.
Actually, the real question is not about CNC
machines or computers, but whether it is better to do the work
yourself or to contract out the business. The use of the computer
and 5-axis milling machines is only one part of that decision. If
someone can do the job better by hand, then that is the service
you should use. Do not assume that you will automatically get
better results by computer. The following will discuss the
process of CNC milling and the problems associated with obtaining
CNC Milling Process
Before going into the evaluation of benefits,
let's review the basic machining process.
1. Design the boat using some form of CAD hull
or surface design program.
Several programs exist which allow you to
define and fair the 3-dimensional shape of a hull on the
computer. You want a design program which will allow you to
describe the hull as a group of complete surfaces, rather than as
a series of curves. This will allow you to easily transfer the
hull geometry to a CAM milling program without the need to
recreate the shape of the boat.
2. Write a transfer geometry file (DXF, IGES,
etc.) of the hull
Once the surface shape of the hull is complete
in the design program, you need to be able to output the geometry
to a file in a format compatible with the CNC CAM software. If
this cannot be done, then the CNC machining service will need to
recreate the hull shape using their own software, which could
take quite a bit of time.
3. Read the geometry file into the CAM program
Once you create a standard geometry transfer
file, you can put the file on a diskette and send it to the CNC
machining service. You could even send the file immediately by
e-mail. The company machining the part does not need to have the
same hull design program that you have. They will have their own
software which specializes in the machining of surfaces.
4. Adapt or correct the geometry to meet the
needs of the CAM software
Depending on the complexity and details, the
CNC program operator may have to adapt the CNC process to meet
the needs of the part. For example, concave creases and local
cutouts may require special cutting procedures. Smooth or
sculpted surfaces are easier to handle than creased surfaces.
5. Define the cutter tool paths over the
surfaces using the CAM software
There are many ways to have the CNC machine cut
the plug. The skill and experience of the CNC operator can have a
big effect on the outcome and how much finishing work is needed.
6. Break the job into pieces that will fit on
Many hulls are too large to be cut as one
piece. In addition, you may want smaller pieces to be able to
truck the parts to the construction site.
7. Mill the individual pieces
Each piece of foam to be cut has to be oriented
in the machine coordinate system and the CNC program set up to
cut that piece.
8. Drill the connection pin locations or
alignment marks for the milled parts
If a part is to be cut into pieces, the CNC
machine needs to cut or drill alignment holes or marks while the
piece is still fixed in place. This is critical for large parts
cut into several pieces. You want a system which is accurate and
foolproof when the pieces get to the construction site.
9. Prepare the plug for use or use it to create
the final mold
After the plug is cut out of foam (or some foam
variation), there is always a certain amount of finishing
required to make the plug (male or female) usable. The amount of
processing depends on the type of foam used, the type of coatings
used, and the desired end product; a one-off prototype boat or a
master mold for production use.
The promise of CNC milling is accuracy
(including perfect symmetry), speed, and cost savings, each of
which will be discussed in detail.
If the full 3D hull surface is completely
designed on the computer, then a milling machine will reproduce
the shape exactly as it is defined on the computer. The following
problems, however, can arise.
The goal in CNC milling is to be able to cut
the plug automatically without any lengthy final preparation by
hand. The assumption is that the input 3D computer surface shape
is accurate to begin with. This depends on the program used to
define and fair the hull and the skill of the program's operator.
Since there is no automatic way for a hull design program to
guarantee fairness, it is up to the designer to make sure that
what is sent to the milling machine is accurate and smooth.
Surface irregularities which are nearly invisible on a small
computer screen get magnified greatly when the hull plug is
milled full size. In addition, a hull model may look smooth when
rendered in 3D with colors, lights, and reflections, but the
underlying surface may not be accurate enough for construction
purposes. Most photo-realistic rendering software gloss over and
hide many surface irregularities. That may be fine for the
company brochure, but it is not accurate enough for the milling
The traditional approach to hull construction
is to base the shape on a number of frames, where there is a lot
of hand work which can deal with any inaccuracies and unfairness
in the design. To get the best advantage from computer milling,
however, you need to start with a very accurate 3D computer
model. This is a problem with all CNC cutting and construction.
To eliminate expensive cutting and fitting, everything has to be
very accurate every step along the way. Designers need to spend
extra time evaluating the entire fairness of the computer model
beforehand. Do not rely on examining just the standard stations,
waterlines, and buttocks, because the goal is to avoid having to
fair the milled plug.
Now that you have a fair and accurate hull
surface model, you want to transfer it to the milling software
without losing any accuracy or fairness. The only way to do this
is if the hull modeling technique you are using is mathematically
equivalent to one used by the machining software. This means that
the hull model should be some variation or subset of a NURB
(Non-Uniform Rational B-spline), because all of the major surface
milling CAM software (e.g., MasterCAM, SurfCAM, Catia, CADAM) use
NURB surfaces to define the milling paths.
If your hull is defined using some technique
other than a NURB surface, you must make sure that the milling
CAM software can accept your hull definition and match the shape
accurately using NURB surfaces. For example, if your hull design
software does not use NURBs, you still may be able to produce a
detailed surface mesh and have it accepted by the CAM software.
The CAM program must be able to read this mesh file format and it
must be able to interpolate or fit the surface mesh accurately.
Fitting a surface mesh with a NURB surface is not a precise
process. Depending on the density and shape of the mesh, the
resultant NURB surface might not be accurate or fair enough for
milling purposes, or the milled plug might require too much hand
fairing. If the surface mesh fitting process is not accurate
enough, then the CAM software must be able to correct the
problems. This might be impossible, since most CAM programs are
geared toward milling and have little or no control over detailed
A more basic problem is that the CAM software
must be able to read the geometry file produced by your hull
design program. The two main geometry transfer file formats are
DXF (Data Exchange File) and IGES (Initial Graphics Exchange
Specification). The DXF format was defined by Autodesk and IGES
is defined by a national standards committee. The main difference
between the two formats is that DXF does not allow for the
definition of NURB surfaces, but does allow for the definition of
mesh surfaces. IGES, on the other hand, does allow for definition
of NURB surfaces, and is the most common file type used for
transfer of NURB surfaces. You have to be careful, because the
IGES specification (630 pages) defines many types of geometric
entities and it is rare that a CAD or CAM program will handle all
geometry types. This means that you must make sure that the hull
design software that you use can produce the proper entity type
required by the CAM software. The IGES entity type used most for
transfer of NURB geometry is entity type 128: Rational B-Spline
Surface Entity. This is one of those details that can cause a big
problem unless you check it out beforehand.
Once you have tested the transfer of the hull
geometry to the CAM program, you need to determine if there are
going to be any special shape problems related to the detailed
geometry. Are there certain shapes that cannot be done accurately
by the machine? Do these detailed shapes require extra
pre-processing in the CAM software (more time means higher
costs)? It is hard to describe many of these problems ahead of
time. Usually, when the CAM software operators see the geometry,
they will be able to immediately pick out difficulties and
problem areas. Try to find out whether these problems are due to
the CAM software they are using, or if it is a limitation of the
milling machine, or if it is a problem with the transferred hull
geometry. Also, determine if the difficulties affect only the
time of setup and milling, or if they affect the accuracy of the
milling process. The more post-milling hand work that is
required, the less cost effective is the whole process. Provide a
sample geometry file to various milling services to see what kind
of feedback you receive about the model and the final accuracy of
the milled plug. Remember that even though the milling machine
might be very accurate, the input geometry and details it is
cutting might not be as accurate. After the geometry conversion
process is complete, try to obtain some form of output from the
CNC program of the hull geometry for validation purposes. Some
CAM programs can output 3D renderings or tool-path diagrams.
These may not be perfect for validation, but anything is better
than being surprised after the plug has been milled.
The amount of post-milling finishing that is
required depends on the accuracy of the input geometry, the
required hull details, the capability of the CAM software, the
accuracy of the machine, and the type of material being cut. The
difficulty of finishing a plug depends on the accuracy of the cut
and the type of material being used. Most milling services use
some form of foam, which can vary greatly in density and bubble
size. Some materials require less preparation than others and
which type of material you choose might depend on your goal. Are
you going to construct a prototype boat from the plug, or are you
going to use the plug to produce a master mold? Discuss your
goals with several milling services, since each seems to have
their own strong opinions about the subject. There are a number
of tradeoffs depending on what you plan to do with the milled
plug. Keep in mind that more hand finishing means more
inaccuracies in surface shape. This may be a critical concern for
parts such as airfoil keels and rudders.
Depending on the size of the boat and the size
of the milling machine, you may have to mill the plug in pieces.
You may also have to mill the plug in smaller pieces than the
machine is capable of because you need to truck the plug to your
construction site. Errors can occur when fitting plug pieces
together. The typical solution is to have the milling machine
drill alignment holes so that the plug pieces can be pinned
together at the construction site. The accuracy of this process
depends on how tightly the pinned alignment holes hold the pieces
together. Very small alignment problems between the plug pieces
can have a dramatic effect on the finished hull. The slightest
continuity problem between two connected curved surfaces might be
easily visible in the reflected surface of the finished part. In
addition, when multiple plug pieces are pinned together, you may
get progressive or additive errors. It would be best to align
each piece to some accurate external structure or grid.
One of the main advantages of CNC milling is
the promise of a fast turn-around time. Often, the success of a
project may depend on how quickly you can get a product to
market. Whether it is to build a prototype to bring to the show
or to build a master mold for production use, CNC milling
promises speed. Let's review some of the areas that can help or
hinder a fast turn-around.
Although the speed of the CNC milling machine
is main reason for the speed of plug production, there are many
other factors that can contribute. One is the experience of the
CNC company providing the service. The more and varied jobs they
have done, the better they will be able to solve any unusual hull
geometry you may have. In certain cases, the milling time ends up
being only a small portion of the time it takes to do the overall
job, and the "special" problems dominate. Depending on
your goal (one-off or production boat), the experienced tooling
company can foresee problems and suggest optimum choices in
things like the type of foam used and whether to mill a male or
Before milling, the geometry has to be as
perfect as possible, and this can take time. As mentioned before,
if you provide your hull geometry using the same mathematical
type and format as the CAM software, then you are 80 percent
there. The last 20 percent will be needed to take care of special
details such as cutouts, creases, and fillets. If you do not
provide the hull geometry using the same mathematical definition
as the CAM program, then the pre-processing time can go up
dramatically, especially if the geometry translation is not done
accurately. In addition, if you cannot produce the proper DXF or
IGES input file for the CAM software, then the CNC milling
company will have to define the hull geometry from scratch.
As mentioned before, the time it takes to mill
the plug may be just a fraction of the time it takes to do the
whole job. If the goal is to produce just the milled plug out of
foam, then the process can be very quick. If the job is to
produce the master mold or the complete tooling for a production
boat, then the time savings are less dramatic. This whole process
can still provide a lot of savings in terms of time and cost,
especially if your yard does not have the experienced labor to do
the task quickly.
The major benefits of CNC milling are accuracy
and fast turn-around. It is more problematical to expect a great
savings in cost. The following discusses some of the reasons.
Large gantry, 5-axis milling machines are very
expensive and require a huge capital expenditure. Even if you
keep the machine busy all of the time, the company providing the
service still has to charge enough to obtain a reasonable
return-on-investment. In addition to the cost of the machine,
there are the facility costs, the maintenance costs, the
insurance costs, the software costs, the people costs, and the
training costs. For example, CAM software can cost up to $50,000
or more and the operators have to be highly trained. Eventually,
cost will become more of a benefit for this process, but for now,
it remains more difficult to prove.
Your choices are to continue to do things the
way you always have, vs. buying the CNC milling machine for
in-house use, vs. subcontracting the work to one of several
companies who specialize in the task. This is not a new decision.
Even before CNC machines, there were companies who offered
complete tooling services. As you might expect, however, it takes
quite a large volume of work to justify the cost of machinery,
facilities, people, and training for in-house work. Traditional
in-house methods will also become more difficult due to the
increased lack of skilled tooling labor and its slow turn-around
time. It seems that as more and more hulls and parts are designed
by computer, there will be a greater cost benefit to using CNC
milling and tooling services.
Some parts that are difficult to construct by
hand are easy to produce by CNC machine. Do not assume that the
CNC machining costs will be relative to traditional methods. Some
stylized or complicated part shapes that you would normally avoid
due to difficulties of hand construction might be very
inexpensive to construct by CNC machine. This might open up whole
new styling options that you have never considered. The point is
that you might find that for certain projects the cost, accuracy,
and turn-around time are all benefits. The only way to know for
sure is to submit the geometry and obtain quotes from many CNC
machining services. The quotes can vary greatly.
The promise of accuracy, fast turn-around time,
and lower costs can be achieved using CNC milling machines if you
have a good understanding of the process and its advantages and
limitations. Some people expect too much and are disappointed
with the results. You should start with an easy project and
progress to more difficult projects. Don't wait until a
complicated hull has to be built in a short time to learn about
the CNC process.
As companies learn to use this service
appropriately, they will begin to obtain secondary and tertiary
benefits from the results. When more and more parts are CNC
machined accurately, the boat will be built faster and go
together with less rework and hand fitting. Hull modules can be
built outside of the hull and dropped into the hull with no
fitting problems. This lack of hand fitting has a multiplying
effect throughout the boat and can result in dramatic
Sidebar on CNC Companies
20 Toro Road
North York, ONT M3J 2A7
Has 5-axis CNC cutting with up to 10' X 15'
Goetz Marine Technology
48 Ballou Blvd.
Bristol, RI 02809
Contact: David Sprague
Specializes in CNC rudder and keel
Janicki Machine Design
24595 State Route 20
Sedro-Wolley, WA 98284
Contact: John Janicki
Has large 68' X 19' X 8' 5-axis CNC machine.
3880 North Courtenay Pkwy.
Merritt Island, FL 32953
Contact: Jeremy Mollica
One of the original leaders of 5-axis
137 Trout Stream Dr.
Vernon, CT 06066
Contact: Mike Coderre
Machines excellent half-hull and full-hull
models up to 4'.
North End Composites
P.O. Box 548
Rockland, ME 04841
Contact: Dave Janson, Ext 315
Can perform the whole tooling process.
Sea Ray Boats
100 Sea Ray Drive
Merritt Island, FL 32953
Performs in-house 5-axis CNC.
17825 59th Ave. NE
Arlington, WA 98225
Performs in-house 5-axis CNC.