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Desk Proto / Delft Spline
DeskProto Mask Screenshot
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Delpft Spline / Desk Proto
The Three Methods for Shaping Materials
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Scopulus
The Berlin Model Being Machined
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Scopulus
The Finisherd Berlin City Model
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Cardiff Metropolitan University
CNC Machined Parts in Tooling, by Student Matthew Cain
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Cardiff Metropolitan University
Styling Block Model of a Vortex Handheld Vacuum Cleaner
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Ferm
Block Model in Tooling Board and Final Model
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Delpft Spline / Desk Proto
CNC Toolpaths in DeskProto for One Half of a Styling Block Model
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Klaveness
Milling Insoles in Flexible Foam
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Delpft Spline / Desk Proto
CNC Toolpaths in DeskProto for Stainless Steel Facade Panel
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Roland DG
Dental Milling in Zirconium
Any reader of TCT magazine will have noticed that the mainstream media (at last) have discovered 3D printing. All talk shows have shown a 3D printer with an enthusiastic evangelist user, major newspapers have published page-size stories, even some retail stores are now offering 3D printers to the general public.
For us specialists this is of course good news, after many years of first having to explain what 3D printing is about. In addition all this press coverage will help to make engineering more interesting and ‘cool’ to people who didn’t yet know: any engineer will surely welcome that.
In recent years we have also seen that the number of 3D printer manufacturers is rapidly growing: almost every month a new supplier enters the market. Cause for this sudden growth is the expiration of a patent by Stratasys on their FDM system, allowing others to launch lowcost printing systems. Both commercial and open source. At the same time prices for these lowcost systems are falling, due to both the larger series and the steeper competition. Altogether many causes for the intense media coverage just mentioned.
Adding material or Removing material
Because of all attention for 3D printing it may seem that adding material (stacking many thin layers) is the only available method for “Digital Manufacturing”: so for transforming a virtual (CAD) model to a real model or product. And that is of course not true: removing material (CNC machining) is a good alternative, for many applications even far more efficient. This paper will show some of these applications. (See Slide 1 in Slideshow)
CNC machining is the exact opposite of 3D printing:
- 3D printer starts with an empty space, and keeps adding small chunks of material (voxels, or 3D pixels) until the desired part is there.
- CNC milling machine starts with a solid block of material, and keeps removing small chunks (chips) until the desired part is there.
The abbreviation CNC stands for Computer Numerical Control, as a contrast to conventional milling machines where the cutter’s movements can be controlled by turning a hand-wheel. The complexity of the process depends on the software that is used: for CNC machining as well as for 3D printing. Where traditional CAM software (the software to calculate the CNC toolpaths) was meant to be used by skilled CNC specialists, nowadays also a new type of CAM software is available, meant to be used by users without CNC knowhow and experience.
This software makes operating a CNC milling machine just as simple as operating a 3D printer. All that is needed to prepare a new job is following a wizard, the rest of the process is taken care of by the software. In order to be complete a third method of shaping material needs to be mentioned as well: deforming. Deformation is applied in manufacturing technologies like casting (including injection molding), forging and bending (for instance bending sheet metal). For mass production these technologies offer unrivaled low prices, however for a one-time conversion from CAD model to tangible model they cannot be easily applied. Of course some exceptions can be found, where deforming is most efficient also for small numbers (like a programmable CNC bending machine), still generally speaking for small numbers the choice is between adding and removing material.
The old name Rapid Prototyping made it easy to distinguish between these two methods: Additive RP and Subtractive RP. These names are no longer correct, as nowadays digital manufacturing is applied forSelection of the optimum process much more than just prototyping. Custom manufacturing, so making a product that perfectly fits one unique client, is possible, and enthusiastic users even predict mass production of custom products (mass customisation).
Panoramic Predictions
Future visions round 3D printing predict big changes: “Personal fabrication is the next revolution that will impact our lives” (says Neil Gershenfeld of the Massachusetts Institute of Technology, the ’father’ of all FabLabs).
In this vision, digital manufacturing will cause a transition from current mass production (one design for all clients) to custom production (each client get a unique design). Production then can be done on a small scale: by small and medium size companies or even in-house by the end-user. This transition will absolutely follow - for some products, and will offer important advantages (personally I would for instance be very interested in an affordable custom made car seat).
However, for most products such transition will not come, as the low prices offered by mass production just cannot be beaten. 3D printing single products makes these so much more expensive than injection molding in large numbers that custom manufacturing will be used only when needed. For instance needed for a good fit (example: custom insoles in shoes) or needed for a personal touch (example: custom iPhone covers). A useful analogy can be found in 2D printing on paper: ‘printing on demand’ is perfectly suited for small batches and for personalised prints. However, for the large series that are needed for books, magazines and newspapers offset printing remains cheapest by far.
About the idea of a personal 3D printer in every house my reservations are even greater. Here the analogy is the bread-baking machines that are available: perfect for having fresh bread every morning. However: it is much more efficient to have a specialist bake the bread and just buy it at the local bakery. So these baking machines, despite the advantages they offer, are not commonly used. Custom manufacturing by selecting from a number of available options is something different, as that can be achieved at normal prices.
Key factor here is a tight control of the processes for assembly and logistics; the actual production of the parts is standard mass production.
Selection of the optimum process
Back from these future vision to the current day: being a designer or engineer you have a 3D CAD model and you need to make (or have made) a model, product or mold. The question is when it is best to 3D print and when it is best to CNC machine. Key factors for this selection are of course the advantages and disadvantages of either method. An advantage for the one method most times also being a disadvantage for the other. In this section for each method a list will be presented of its main advantages.
The actual choice however is more than just comparing the scores on advantages: in many cases one of the factors will tip the balance. Some examples will make it clear why for very similar cases the best choice still may be different.
Advantages of 3D printing:
- Easy to use: few required preparations
- Price independent of part complexity
- No limit on part complexity
- Price per part independent of batch size
- Easy to switch to a next part (flexibility).
Some of these advantages need to be elaborated on. The preparations that are needed to start a job: 3D printing does require some (so it’s more than simply pushing the button 3D print), like finding the optimal orientation of the part and adding a support structure where needed. And after printing this support structure again needs to be removed. Still this is an advantage for 3D printing, as for CNC machining more preparations are needed: decide which side(s) need to be machined, which cutter to use, prepare a block of material in the correct dimensions, decide how to fixture this block on the machine’s working table.
The advantages of few preparations and high flexibility are closely connected, though with an important exception: when all parts to be manufactured are similar the same preparations can be used for all parts. Then the flexibility advantage no longer applies. Examples of such similar parts are: rings (jewelry), insoles (orthotics), crowns (dental). Each part is different, however from a manufacturing point of view they are all equal. The process then can use standard material blocks, a standard fixture and standard settings for CAM. Allowing any level of complexity is a clear advantage of 3D printing.
Well-known examples are manufactured by Freedom of Creation ( www.freedomofcreation.com ), a company that exploits this advantage of 3D printing to create products that otherwise would have been impossible. The part price for 3D printing will not rise with the part’s complexity. This is a clear contrast with CNC machining, where more details do cause a higher price, as more toolpaths will be needed. And the smaller these details, the smaller the cutter that is needed, and thus the higher the machining time.
Price and batch size being independent (making 10 identical products will cost ten times as much as making one product) is an advantage for small series, however when large series are needed this turns to be a disadvantage.
Advantages of CNC machining:
- Free choice of material
- Free choice of resolution
- High surface quality
- High accuracy
- Price independent of size and volume
- Low cost of ownership (both for machine and supplies)
Where for 3D printers the choice of materials is limited to a few materials only (or even one), CNC milling machines can handle a wide range of materials. Not every machine can mill up to steel or stone, still any machine can handle many materials. Most important factor for the resolution of the part is the layer thickness (for 3D printers) resp. the toolpath distance (for CNC machines). The layer thickness cannot be varied (or only within close boundaries), the toolpath distance can be freely chosen. The same machine can produce both ‘quick and dirty’ parts and ‘slow but perfect’ parts. The surface quality (smoothness) for CNC machining is much better than for 3D printing. Even when the CNC toolpath distance is much larger than the layer thickness of the printer: the cutter is much wider than this distance and will smoothen the transition between the toolpaths.
For CNC machining the production time is related to the surface area to be machined (and to the volume to be removed, though that can be done quickly in a Roughing operation). For 3D printers the production time is related to the volume that needs to be solidified. Though current building software applies smart algorithms to replace a solid volume by a shell with a 3D raster inside, still for large models machining will be much quicker than 3D printing. CNC machining as said can be done in almost any material. This does not only apply to the type of material, it does to the supplier as well. Which makes it impossible for the system manufacturer to offer the machine at low price and have all revenues from the very expensive patented material cartridges,
Special selection cases
In many cases the selection can indeed be done based on the advantages and disadvantages mentioned above. An example from the field of model making (so Rapid Prototyping): a model is needed of an electric hand drill. When making a styling block model (only for the outside appearance and for ergonomic tests) it will be best to CNC machine a solid model in tooling board: quick, low cost, and a perfect surface quality. When making a functional prototype (one that is capable to actually drill a hole) a model of the thin-walled casing will be needed, including all inside ribs, screw holes and other details. For such part 3D printing is best (small volume to be printed, high volume to be machined, needing thin cutters). In many other cases one of the advantages will be the deciding factor, making other factors irrelevant. This will be illustrated by the examples mentioned below.
- The functional prototype just mentioned (electric hand drill), in case for durability testing (overheating) the prototype needs to be in exactly the same material as the final product.
- The products by Freedom of Creation, already mentioned above, that just cannot be manufactured by CNC machining.
- A series of unique facade panels in stainless steel (ca 1 x 1 meter) for the computing center of the Dutch tax administration in Apeldoorn: savage-looking masks designed by Dutch artiest Rob Birza. Here the size of the part was decisive, making CNC milling by far the most costeffective. Material was irrelevant, as the part was only used to cast a concrete mold. Production technology for the panels was hydroforming. (See Slide 9 in Slideshow)
- For small products 3D printing is the obvious choice as not much material needs to be solidified. Still CNC machining can be the best process in case of special requirements for the material: crowns and bridges (dental) will be machined from zirconium, as after sintering (and finishing) that material can be used in the client’s mouth. (See Slide 10 in Slideshow)
Jewelers make a personal decision about which process to use: the deciding factor will not be the same for every jeweler. The required parts are jewelry wax models, to be converted to gold via investment casting. Some jewelers prefer CNC machining for it’s superb surface quality, or because of the lower investment for an in-house system, others use 3D printing for the complexity that can be achieved. Some jewelers subcontract to an outside service bureau for the ease of use, others do not want to go outside for reasons of confidentiality and/or speed. Finally it will be clear that in many case the deciding factor simply is the fact that a certain system is in-house available and the alternatives are not.
Example projects
A perfect example of custom manufacturing is the production of insoles, to solve problems with feet and/or posture. A podiatrist or pedorthist can solve such problems by designing a custom insole for the client. This is a sole to be used inside a normal shoe, with for each client different locations with extra support and pressure relief. Such insoles are made of a flexible foam, in many cases using a different density for the front and the back.
Specialised software is used to quickly design the insole, and the product is created using CNC machining. 3D printing cannot produce flexible foams, and apart from that it would be more expensive. The milling process can be completely automated as from a machinist point of view all soles are created equal. Calculating toolpaths is done by pushing one button (using the default settings). The machinist needs to place one or two foam blocks in the machine, push the button to start the vacuum pump for fixturing these blocks, and then start the machine. Ready in 10 to 20 minutes per pair. See the illustration by footwear specialist Klaveness in Portugal. Production can be done either small scale (a lowcost machine for each podiatrist) or at a central location by a service provider using a fast machine. (See Slide 7 in Slideshow)
An electric driver for the Dutch brand Ferm (www.ferm.com), by Brandes en Meurs industrial design in Bunnik, NL (www.brandesenmeurs.nl). The driver belongs to a complete series of power tools that they have designed for Ferm. Brandes en Meurs use Creo Elements (Pro/E) for design, presentation renderings and computer simulations. Tangible models (Rapid Prototyping) are used at several stages of the design process: quick and dirty foam models at the start, ergonomic models later, and finally perfect presentation models of the resulting designs. These models will help the designers, and will also facilitate communications with the client and with the manufacturer in China. Brandes en Meurs have a light CNC milling machine in their in-house modelshop, which makes it easy for them to machine a model when needed. (See Slides 6 & 7 in Slideshow)
Completely different in size is the beautiful city model of Berlin, created by Scopulus in Braunschweig, DE (www.scopulus.de). The model has been machined in wood, with metal inlays for all open water. Dimensions of the model are 3.5 x 1.2 meter, which is scale 1:5000. Such model can be achieved using 3D printing, however machining was chosen as that was cheaper, and as the result would look much better. The model was not only large in size, the file-size for the CAD data (both terrain and buildings) was huge as well. It has been shown on various exhibitions in Germany. (See Slides 2 & 3 in Slideshow)
In education the creation of tangible models results in an important lesson: the student will realise that his/her design in real life looks rather different from what he/she thought based on computer generated renderings. This makes the use of models important, made either by 3D printing or by CNC machining. An special advantage of CNC machining for education is that it forces the student to think about manufacturing issues. Designs with undercuts cannot be easily machined, and not easily produced either. As said before: most products will require a mass production technology like injection molding, where undercuts come with higher cost. In addition most schools will be happy with the low cost of ownership for CNC machining (materials to be machined). (See Slides 4 & 5 in Slideshow)
