Designing Mechanical Parts - The Whoosh Machine
The Whoosh Machine was a farewell gift made by Dick Tiekink for a colleague. As well as a lot of inside jokes the Whoosh Machine has several mechanical parts. This tutorial is a practical guide showing you how the Whoosh was designed and how several issues common to mechanical items that are 3D printed are solved.
These issues that are dealt with are accuracy, clearance & the design of the parts themselves. We hope that this will aid people in making more complex 3D printed items and eventually 'machines.' We also hope that some of the technical limitations of the 3D printing process are explored and made clear to the Shapeways community member. This is not for everyone, we are trying to give you deep insight into designing for 3D printing based on our own experience. We recommend that you read the Designing Mechanical Parts for 3D Printing Part I before reading this one.
- Clearance is very important when designing mechanical parts for 3D printing.
- Parts have to be cleaned also. You must allocate space for this.
- Powder is trapped in enclosed spaces. If you do not design so that it can be removed your part will not work.
- Two surfaces(printed in the same orientation) that rub against each other cause more friction than one would expect due to the layerwise production in 3D printing("hooking").
- If you design some kind of mechanical device such as a coil spring please keep in mind that since it is produced in an integrated fashion(ie part of the housing) there is no(or not much) pre-load in the spring. It will be weaker than a traditional spring made out of two parts and then compressed in the housing.
- Paint changes the mechanical properties of your model.
- You could manually correct your model with a file but this is tough going.
The Whoosh was built in Pro Engineer a CAD tool and uploaded to Shapeways. The material used was White, Strong & Flexible. This material is also called Selective Laser Sintering or SLS material. The numbers in this tutorial are only valid for the White, Strong & Flexible material. Different 3D printing materials and processes have very different material properties, limitations and strengths. The Whoosh was printed out on an EOS p100 Formiga 3D printer using the PA2200 material. For more background on material properties you can look at this tutorial.
When making objects for moving parts clearance is very important. There needs to be space between a dial and the base of this dial for example. This space is necessary so that the part does not fuse during production and so that the part can be cleaned while the support material is to be removed.
Theoretically the minimum clearance in White, Strong & Flexible(SLS) is 0.5mm but we would recommend using 0.9mm to make sure your part does not fuse. You can also 'design around' clearance and fusing issues. In the original construction see 1) and 2)("no") the minimum required clearance was 0.2 mm. This was clearly too little. After making the object with 0.5 mm clearance, the knobs still would not turn. After changing to the "Yes" construction it works.
2) Space between the flange of a knob and the housing.
The relative position of the moving parts with regards to the production of the object is also important. If it is tilted, as in the figure under "No", a larger gap is necessary to avoid interlocking due to the layerwise production process.
The way to increase the space for “cleaning” but keeping the clearance low is to add small bulbs.
One of the problems with small bulbs is the question what shape to make them, with regards to the way the object is built up. The right shape of those bulbs could be a half sphere or a cylinder. Because objects are built up in layers a spherical shape when made in the direction of the layers might pose some problems. The definition would be less and the ammount of material used in the top parts of the sphere would be less also. This would mean that a sphere would be more fragile in terms of wear and tear than a cylinder.
The Whoosh had two nobs, one had a spring. Both nobs were designed to have a 'blocking function' so they could not be turned past a certain point.
The cross section of the spring was 4 by 2 mm. The length of the spring was 40mm. This made for a strong spring.
Rotating drum in a housing
The inside of the Whoosh has a rotating drum inside of a housing. This drum is similar to a washing machine drum. Indeed the Whoosh machine is a play on words in Dutch of washing machine. The image illustrates how it is constructed.
Push knob with a spring
Another moving part in the Whoosh is a push knob with a spring in it. The coil spring inside the button has a diametre of 16mm. It is comprised of 4 coils. The spring has a pitch of 6mm. A spring with these dimensions turns out to be a rather weak spring. This construction also has too much friction between the coil and the housing so the button does not work as hoped.
Another important finding is that because 3D printing produces objects out of layers when you have two adjacent surfaces(printed in the same orientation) there will be increased friction due to the layers "hooking" in to one another. With better clearance and positioning this can be avoided. This would be a very important consideration when one would like to design things such as pistons.
Integrated ON button
For this on button cleaning and the ability to remove the support material is a chief consideration. The part can be made out of one piece and can also be a part in a more complicated assembly but space must be made somewhere to remove support material.
When working with White, Strong & Flexible(SLS) parts and the process you have an advantage as opposed to other 3D printing processes. Because the part is made from a fine powder the support structures(the remaining powder in the case of SLS) remain behind easily. But, there stil has to be a way to access any cavity so that the powder can be removed. If this is not the case then any mechanical part will not work.
In this case the button dit work but since it does not have a "pre-load" of the spring itself since it is constructed as one integrated part the button is less firm and more wobbly than one would like.
The open and porous structure of White, Strong & Flexible means that it absorbs paint well. The object actually sucks in paint. You can read more on this in the painting tutorial. This means that special care has to be taken when applying paint to symetrical objects. A difference in the ammount of spraypaint or other paint applied to one side will be easily visible in a symetrical object.
This is why at Shapeways we have opted for dye because this produces more reproducible results. You can see how we dye objects in the Dyeing tutorial.
Another important consideration is that paint adds weight and surface when it is applied. This will mean that paint has an influence over the performance of your moving part. Worst case a part with minimal clearance will get stuck together because too much paint was applied.
Oops..last minute modifications
If you should already have your part and need to modify it manually because it has a mistake somewhere you could do this with a file or sanding paper. Selective Laser Sintered parts are tough however and it is a difficult material to modify by hand. Sanding will also to a certain extent change the structure of the top layer of your part. This will be especially visible if the part is then painted.
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