The DeskProto approach offers true Concept Modelling
Article containing background information on the use of DeskProto,
published in Prototyping Technology International 1998 (pp 110-114).
In order to be used as a real Concept Modeler an RP system has to
meet a number of requirements. After explaining these requirements a system is
presented that indeed does meet them: DeskProto. This system is both low priced
and fast, in contrast to many other 'Concept Modelers'.
During the past few years Rapid Prototyping has grown to a mature technology.
It is now generally accepted by the design community as a handy designer tool,
and known by the management as a tool to shorten lead times. It is offered by a
large number of service bureaus in a fair competition, and between the RP
hardware manufacturers even a process of shake- out and concentration has
Considering the above, it is in fact very strange that still no generally
accepted clear definition does exist for a process to be called Rapid
Prototyping. Many different definitions are being used, varying from "Any fast
prototype creation process" to just "Stereolithography". For the sake of clarity
I will start this paper with our own definition, which is based on three
A prototyping process may be called Rapid Prototyping in case:
The process is based on the use of 3D CAD data
The prototype is created (almost) automatically
The 'almost' is added as all current processes still involve some manual labour for pre- and/or
The model is ready within a few days
Rapid has to be taken in contrast with manually creating a prototype, which
generally speaking will take several weeks.
This definition does not include the skilled craftsman who can produce a foam
model by hand in, say, ten minutes (literally speaking Rapid Prototyping indeed
!). Important issue is that the type of process involved does not matter:
incremental techniques (LMT = Layered Manufacturing Technique) as well as
decremental (CNC = Numerical Controlled milling) are included.
||Figure 1: Committed Cost and Incurred Cost versus the stages of the
design process (the development time axis).
The current key application of RP systems is to check a new design when it is
(almost) finished: just before the large expenditure of creating the
manufacturing tooling has to be done. Testing a fully functional prototype at
that moment gives the opportunity to locate design errors and correct them when
the costs of changing are still low. Errors that could have remained unobserved
when testing the 3D CAD model only. See the graph of the incurred cost in the
well known figure 1. This preproduction testing is vital: in many cases even a
small series of prototypes are manufactured for testing purposes, using a Rapid
As this preproduction testing is the current key-application of RP, most RP
systems manufacturers until now have concentrated on developing the high-end RP
systems needed. However, in recent years a divergence has been observed between
these high-end machines and a new type of RP machines: the Concept Modelers
[Throup, 1996] and Wohlers
At this moment a number of these Concept Modelers, also called '3D
Printers', have been put on the market by the RP industry (see the next
paragraph). The marketing information for each of these new machines gives a lot
of information about why the particular machine is in fact a Concept Modeler.
Some criteria given include:
- Low cost
(which can be applied either to the machine or to the
- Office friendly
, or 'nontoxic','environmentally safe'.
operation, or 'fully automatic', 'easy to use', '3D Printer', 'no training
, or 'free choice of accuracy'.
An interesting thought is that these criteria (as given by manufacturers of
RP systems) thus do not apply to the high-end RP systems. This indeed is the
case, and a company considering to invest in a high-end RP system should give
this matter serious thought. Especially the skills (training) needed to operate
a high-end RP machine must not be underestimated. The processes involved in
high-end RP are very sensitive, and a slight change in process parameters (for
instance a one degree temperature rise) may result in a worthless prototype.
The most important issue about Concept Modelers is their key application,
which is completely different from the key application for high-end RP as given
above. High-end RP is used at the end of the design stage; Concept Modelling
right at the start of the development process, at the stage of concept design.
Concept design involves a number of technical choices concerning the functioning
of the new product and styling choices concerning its appearance. Concept design
is mainly done by writing, sketching and creating simple CAD models: all of
these being relatively lowcost activities. The cost of changing a concept are
still low, the effects of the change can be very large. This is clearly
illustrated in figure 1 as well, by the graph of the committed cost: most of the
investments are committed during the stage of concept design.
Taking into consideration this great importance of correct concept designs,
it is absolutely needed in this early design stage to support the designer in
choosing the correct alternatives. One of the means to enhance the quality of
these rough designs is Concept Modelling: creating physical models/prototypes
for evaluation purposes. They can be used for instance to aid in choosing
between design alternatives, to communicate with marketing, to test certain
functional behavior. They are used as an interactive designers tool, during the
design process, just like calculating a rendering or a number of section lines
to clarify a difficult 3D detail. The computer peripheral to be used to output
such models is called a Concept Modeler.
True Concept Modelling
Here again, for the sake of clarity we will use our own definition to define
a true Concept Modeler. It is based on four criteria. In fact the same as the
requirements mentioned above, however more sharply defined here. The criteria
will be further explained below. A Rapid Prototyping system may be called a
Concept Modeler in case:
The price of the total system is below USD 10,000
The system can be used inside a design office
without causing any inconveniences such as noise, stench, toxic materials etc.
A model must be ready within one coffee break
The operation must be really push-button
(as easy as
printing this paper by just pressing the Print button in a word processor).
The desktop-requirement is left out, as the size of the machine is in fact
not important (as long as it is office-friendly). As these requirements are very
sharp indeed, and the exact limits (time and cost) mentioned are in fact
arbitrary, some explanation is needed for each requirement.
The maximum price
is set to ten thousand US Dollar. Reason is that for
true concept modelling the machine needs to be located at the designers desk,
where he/she can use it without any barriers, such as having to go to the
prototyping department, asking a prototype builder to start his job, having to
wait for the current 'prototype queue', applying for a budget, etc etc. A
Concept Modeler needs to be readily available, to be instantaneously started by
the designer. To achieve this availability value a Concept Modeler needs to be
idle most of the time (say 90 %). Compare this to the use of a personal inkjet
printer for 2D output.
It may be clear that this availability can only be reached if the investment
needed is sufficiently low. The prices of most 'Concept Modelers' do force a
high utilization rate to justify the investment done. The exact maximum price is
of course arbitrary and dependant of the actual situation, however the order of
magnitude is correct (ideal would be just the price of a high quality color
monitor). Note that the prices per prototype as given by the manufacturers are
valid only in case of a 100 % utilization, so are in fact far too low.
The no inconvenience
requirement needs no further explanation. All
current Concept Modelers do meet the requirement, most current high-end systems
The build time
of maximum 15 minutes is an arbitrary value as well.
The maximum rises from the observation of designers at work. Designing is a
dynamic process, requiring interactive tools to instantly aid the designer in
taking decisions. Interactive does imply very short waiting times: a coffee
break is the maximum to keep the process going without interruptions. This
method of using RP may well be called Interactive Prototyping.
It is obvious that the accuracy of the resulting prototype is inversely
related to its build time. However, for concept modelling the accuracy, surface
quality etc are less important than a short build time. An ideal Concept
Modeler should offer a free choice: either high accuracy at low speed or low
accuracy at high speed (or anywhere in-between).
is needed to get the new tool accepted by the
designers. Their main job is designing, not model-building, and they do not want
to be bothered with any technical details concerning the build process. Just a
black box: push the "3D Print" button on the CAD screen and a prototype will
come out. Obviously this is not yet the case, and to achieve this a cooperation
will be needed between CAD-developers and RP machine manufacturers (perhaps even
a standard interface for a Microsoft Windows 3D printer driver ? ).
Currently a number of 'Concept Modelers' is commercially available. We have
found four machines, all based on a Layered Manufacturing Technique (LMT): the
, the Genisys
, the Modelmaker II
and the Z
(The BPM Personal Modeler is no longer available). However,
when checking the criteria given for Concept Modelers a disappointing
conclusion has to be drawn: despite their obvious qualities none of the machines
fulfills all criteria. BR>- Prices between USD 50,000 and 60,000 are far too
- On any machine the build process will take at least a couple of
hours, which is too slow for true concept modelling.
- Push-button operation
is advertised by all, however achieved by none. Each machine is delivered with a
software package to do some necessary preprocessing.
The DeskProto approach
In this paper is completely different approach is presented: the DeskProto
system, offering true Concept Modelling using CNC milling. DeskProto has solved
both major drawbacks of the current LMT based Concept Modelers: their high
price and their low speed. A complete system is indeed available below USD
10,000 (the most lowcost even for ca USD 3,000); a prototype can be ready within
ten minutes (when choosing a low accuracy and an easy material like foam).
The use of CNC milling for prototype creation is of course well known,
however until recently this technique was not exactly Rapid. Main problem was
the calculation of the toolpaths, for which a skilled CAM software operator was
needed. This process involved the importing of several surface patches and the
creation and checking of toolpaths for every separate surface, which would take
several hours [Wall, 1992]. In the past the CNC approach was also not suitable
for Concept Modelling because of the high investments needed for machine and
software. Both problems have been solved, and now CNC offers Concept Modelling
possibilities that are in fact superior to LMT.
The DeskProto software has solved the above problems, making CNC very
suitable for Rapid Prototyping. Its main characteristics are:
- import of
instead of IGES.
- no CAM-training needed: automatic
- low calculation times (fast
characteristic is the low cost
of the program, which makes it suitable
for Concept Modelling.
||Figure 2: The most lowcost CNC milling machine we ever saw (Modela): full 3D possibilities,
however suitable for light foam only.
||Figure 3: An affordable
desktop CNC milling machine (CPM 2030), suitable for concept modelling.
On the machine side things have changed too: a new generation of lowcost,
desktop CNC milling machines is available right now. As the basic technique for
CNC is more simple than for LMT, the prices of these milling machines are much
lower. Sufficiently low to 'just buy one', for its availability value. See
figures 2 and 3 for example machines.
The combination of software and machine makes an affordable desktop RP
system, very well suited for concept modelling. It can be operated inside a
design office by the designers, in combination with any 3D CAD software. After
transferring the geometry by STL file (trouble free transfer) the user typically
will change a few parameters like the tool and the accuracy to be used (all
parameters have a suitable default value). From that point on calculation of the
CNC toolpaths is done automatically. Finally the toolpath file is sent to the
milling machine and the model is created.
An important advantage of using a CNC system for Concept Modelling is the
fact that no true solids (3D CAD solid models) are needed. This in contrast to
LMT systems, which cannot function with incomplete solids. During the stage of
concept design in most cases simple 3D CAD models are used, consisting for
instance of surfaces only. Some design bureaus even use special simple CAD
software for concept design, not capable of true solid modelling. It is a known
fact to experienced CAD users that the conversion to a true solid (without any
cracks, gaps, orphan surfaces etc) is not easy and may in fact take a couple of
days. The DeskProto software can handle any 3D geometry, from true solid to
surface only, and can thus directly convert the incomplete CAD-model to a
After mentioning the many advantages, it must be said as well that CNC
milling is not the best choice for all prototypes. The LMT based systems are
superior for complicated prototypes that include many small details: a milling
tool cannot create sharp inner corners and cavities. CNC is best suited for
styling block models in foam or tooling board, a very common type of Concept
Models. Three examples of this type of use are presented: the remote control
unit in figure 4, the counter-display in figure 5 and the hand-tool in figure 6.
Complicated, hollow models can be produced as well, however these will require a
more skilled operator (figure 7).
Other DeskProto applications
In the previous paragraph DeskProto has been presented as a new type of RP
system, perfectly suited for Concept Modelling applications. In addition
DeskProto can be used in a number of other application areas as well, which will
be presented using some more example projects.
Styling block models
||Figure 4: Half of the model of a hand tool, just after the CNC milling is
finished. The second half can easily be attached (figure courtesy of Design bureau IDE, Switzerland).
in tooling board can easily be finished (color
painted in high-gloss, with all product graphics and other details added) and be
used as presentation models. The properties of the tooling board make it easier
to finish such a model than one produced by an LMT system. In many design
projects of electrical equipment like a razor or a drill two preproduction
prototypes are created. One fully functional prototype, using LMT created parts
of the housing that include all inner details like ribs and assembly helps. This
prototype will contain all inner parts of the final product and can be tested
for its functioning. However, it will not look the same as the final product.
The second prototype, the styling block model, will be created to test the outer
appearance. See the hand tool in figure 4, where one half of the model is just
milled and ready to be finished.
||Figure 5: A Remote Control unit for interactive television. For this
design both 'the look and the feel' were very important. A surface modelling CAD
system was used to create the complicated freeform surfaces, and software tools
for surface quality evaluation to enhance the quality of the surfaces. In
addition physical models were absolutely needed: by touching and holding these
models the surface quality could be further enhanced. These CNC milled styling
block models were later finished and used to present the definitive design to
the management. Even a first functional prototype has been created by making a
thin-walled 'copy' of the block model in polyester, and assembling all inner
parts in this cover. An LMT based prototype at that time was not possible as no
solid CAD model of the inside had been created yet. Figure courtesy of DSI and ITCOM, the Netherlands.
In products containing freeform surfaces, of which the outer appearance is
very important, CNC milled prototypes are an ideal tool for surface
. Accurately milling a part of the product with a ball-nose cutter
and then applying high gloss paint offers a very good insight in the quality of
the surface (no sudden changes in curvature etc). A good example of this
application is the remote control unit in figure 5.
||Figure 6: A selling unit, meant to display Bodycards (r) on the sales
counter. A Bodycard is a cover in which a few credit cards can be safely stored.
The display unit consists of a vacuum- formed basis with a number of small slots
for the cards, and a cardboard back to support the cards and to show promotional
text. A concept model in foam has been created, used to test both the
functionality (will the cards remain upright, can they be easily taken off, etc)
and the styling. Later a hollow model in tooling board has been milled as a
master to create a small series of prototypes (figure courtesy of Design bureau DSI, the Netherlands).
The illustration shows a first Concept model in foam.
A styling block model in tooling board is also a very good starting point for
a number of small series production
methods (Rapid Tooling). It can be
directly used as a mould for vacuum forming or for hand-layup in polyester. In
fact the model of the hand tool shown in figure 4 was used as a master to create
a series of prototypes. In case the CNC machine being used is heavy enough,
DeskProto is also suited to produce aluminium tools for small series injection
From aluminium tools to steel is a small step: the DeskProto software is
being used by a number of toolmakers
to create tools in steel. This can
be done either by directly milling or by creating electrodes for spark erosion.
The toolmakers typically already do have one or more large CNC milling machines,
and CAM software to create 2D CNC milling paths. They want to use DeskProto as a
3D addition to their current setup, to create the cavities for complicated 3D
||Figure 7: A detailed, thin walled prototype in tooling board (two parts),
created using DeskProto and a light CNC milling machine (figure courtesy of
PD Models, Great Britain).
It has been stated above that the DeskProto approach is most suited for
styling block models. That prototypes of detailed, thin-walled geometry
can be milled as well proves the housing shown in figure 7. The two parts of the
housing each have been milled from two sides, on a light CNC milling machine
(operated by an experienced DeskProto user). The resulting model (tooling board)
has been used as a master to create a silicone mould, for a series of replicas.
DeskProto is even in use to produce small, detailed wax models for jewelry
production (lost wax method).
||Figure 8: Part of DeskProto screendump: left the geometry (created by 3D
scanning an existing marble bust), right the CNC toolpaths for finishing the
front-side (figure courtesy of RSI GmbH, Germany).
An application area that is often mentioned together with Rapid Prototyping
is Reverse Engineering
. Where in 'normal' engineering a CAD model is
created based on ideas only, in Reverse Engineering a CAD model has to be
created based on an existing physical product. The 3D scanners used to measure
the existing product produce a cloud of point data, and it is quite difficult to
convert such data to a valid CAD model consisting of 3D surfaces and/or solids.
In contrast it is very easy to convert the point cloud data to an STL file: the
basic file- type for DeskProto. See the example in figure 8, where a classic
marble bust has been scanned. The replica is created by milling from two sides.
The Divergence of Rapid Prototyping Systems
Rapid News Europe, 1996 No 2, pp 28-32.
Wall, M.B. etc
Evaluating Prototyping Technologies for Product Design
Research in Engineering Design, 1992 No. 3, pp 163-177.
Will 3D Printers Cannibalize RP systems?
Prototyping Technology International, 1997 Issue 1, pp 14-15.