Week 14 - Composites

Week 14, one of the extra fun weeks again. This week, making composites. Composites manufacturing is quite a dirty process and has some time limited issues like resin hardening time. For this week I desided to try to make a wing for a vertical wind turbine. I had time to do one wing but I am sure I will get back to this project when I have more time.

Here is a picture from a www-page talking about vertical wind mills. I used this a a starting idea, and the wing profile is not that complicated.

First I thought that designing a wing profile might be a bit too much for one week project in all, but after looking around I found that I can install an add-on for Fusion 360 that can generate NACA airfoils. So I desided to go with that.

Airfoil Generator and Fusion 360

Searching the internet I found an Airfoil Generator add-on to Fusion 360 CAD-software that I have been using. It was pretty easy to find by just googling "wing profile fusion 360". The link got me into Autodes app store. To go to store is easy, but when downloading, you need to sign in. As I have Fusion 360 account, I did not need to create an account. I logged in and downloaded the package.


By following the instructions of downloading the app, I installed it to my computer.


Not the add-on is installed the add-on can be found from Sketch pull down menu at the bottom.


As explained on the add-on www-page, Airfoil Generator will sketch NACA 4 and 5 series airfoils with few spesific design options. NACA stands for National Advisory Committee ofr Aeronautics and is know today as NASA (National Aeronautics and Space Administration). NACA/NASA has done a lot of on wing profiles and research is available for public. Here is a link to NACA airfoil wikipedia page.

NACA 4 or 5 profiles means that there is 4 or 5 digits that are used to define the profile of the air foil. Here is a short article about it from Stanford www-pages.

Here is a nice www-page from Airfoiltools.com to understand and test different 4-digit profiles. I will not go into defining airfoil geometry as it is not part of the weekly assingment.

I also googled if there would be information of what NACA profile would be good for vertical axis wind turbines. By googling "vertical wind turbine blade design naca" got a lot of answers. I shortly browsed through some of them and found out that NACA 0018 was quite commonly used profile.


After finding a good profile for my project I used the Airfoiltools www-page to see how it would look like. It is quite thick and symmetric profile. At this point I think and hope that it will be easy to make as well.


CAD-Design

Now I was ready for CAD-designing the profile for the project. I opened my Fusion 360 and used the new Airfoil generator add-on. From Sketch pull down menu all the way to the bottom, there is Airfoil. This will open defenition window where desired foil can be defined. NACA profile is the 0018 that I found to be good for this. Profile points defines how many points it takes to draw the foil. I used quite low value of 50 as I am sure it will be enought. Half cosine spacing lets you have more points at the leading edge of the wing. Leading edge is the most critical part of the wing desing for aerodynamics, so it is good to have some more points there to make leading edge better defined. Finite Thickness TE (TE = Trailing edge) makes the TE non-zero value. I dont know if this really makes any difference but I desided to use it.


Resulting profile can be seen here.


Now I wanted to scale the profile to right dimensions. At this point I found out that it is quite hard to scale the sketch. Especially when the sketch has 50 points it is defined by. So I had to do a extruce before scaling. Extrude in found directly from the top bar or under Create -> Extrude.


Now scaling could be done on an object. I used 10x scaling to make the profile be 10 cm chord line i.e. from leading edge to trailing edge.


Now that the scaling is done I want to have the wing profile longer. I used a tool from Modify menu -> Press pull. This command lets me select a surface for example and change the dimension of the part in perpendicular dirention from that sufrace.


Here is the ready wing. It is about 10cm x 40cm x 2cm in dimensions.


Next issue was how to mill this kind of profile for composite process. At the end it has to be laminated from all sides. And even how to make the milling. I desided to try adding pieces of material to the ends of the wing. Those could be used to attach the material to the milling machine and maybe could be used to aling the part when milling the other side. I used Sketch and extrude commands to do this.


At this point I realized that it might be easier just to mill to halves of the profile in separate runs and clue them together. So I deleted the parts at the end of the wind strcuture and used Modify -> Split face tool to cut the profile half. This will remove the aligning problem with milling.



Making milling routes

Here are the settings for rough milling parameters in Fusion 360.



For finishing cut there are only minor changes for the values.


Removing the part from the stock i.e. contour routing is explained in week 7 documentation.

To be able to draw some parts from the laminate material, I generated a 1:1 drawings from Fusion 360. Mainly I needed the wing profile for wing tips but as training I attached the wing top profile as well. In Fusion 360 the processa was surprisingly easy. From File -> New Drawing -> From Design. Select the part or parts to be placed on the drawing. Select scale, units and document size and click OK. This will create a new Drawing.


In drawing orientation and placement as well as scale in determined. More parts or different orientations can be added. Moving and orienting can be done as well.




Finally the drawing can be exported in multible formats from output manu.

Making mold for the composite

For making the composite structure we have materials from FabLab stock. For making the mold for laminating we use EPS sheets that are 2x50mm thick making it 100 mm in total. Our longest mill bit has 80 mm reach so we are able to use most of the materials thickness. This longest mill bit is flat end. We also have some shorter ball ended ones.


From foil 3D-model I milled the mold or in this case a core for the wing. Using of our large scale milling machine as well as how milling files are done in Fusion 360 are explained in week 7. After finishing the milling files from Fusion CAM-prosessor I used the milling machine and made 2 pieces of the foil profile.

They had to be cut separately, and on the first one, I realized that I had made too thick stock piece for milling. This meant that first two layers of milling cut through air before the milling reached the actual stock. I corrected this to the next run and managed to make the cut with two roughing rounds instead of four.

I used two sided tape to attach the stock piece to the machine table. I worked fine except that getting the parts out at the end was quite hard. I even managed to rip part of the other foil half and broke the trailing edge.


Now I have two halves of the wing profile for the core of the wing. I cleaned the edges a bit with a knive and clued them together and then it was time to start the composite laminating.

Composite laminating

For laminate we use a two component resin and hardener (parts A and B). They come in two different containers that have pump action mechanism. To use the components one pump action of both will conclude correct mixable amouths of the parts. Components must be mixed well when combining.

There is a exotermic reaction when mixing the components. It container should start to melt be prepaired for correct action to cool it down with water!

Resins are hi-bio content but still hazardous to skin, eyes or breathing. They are also hazardous to environment so waste must be handled correctly. When working with the resin, use protective eye glasses, cloves and protection for clothes.

Here are links to Techical Data Sheet and Safety data sheet for the resin.




For the fiber we use burlap fabric.


After making the mold, cutting the fiber and mixing resin the process needs the next components: cover blastic film, some punctured full of holes some as is. Soft breather material and a vacuum bag. These are used to vacuum compress the laminates tightly when curing.


The process goes:
1. Ready cut fiber must be soaked to resin using a stick or a brush.
2. Cover mold with thin plastic for protection.
3. Soak fiber with resin.
4. Put soaked fiber in place over the mold.
5. Repeat for as many layers as you intend to use.
6. When all layers in place, cover with punctured plastic.
7. Cover punctured plastic with breather material.
8. Cover breather with intact plastic.
9. Put everything in to vacuum bag and create the vacuum.
10. Let it dry for long enough duration. 24 hour in this case.

Making the composite

Now I was ready to star the composite laminating. I cut all the parts ready and set them up, so everything would be available when I mix the resin and hardener. Most work took the wing side profiles, first I tried to cut them by hand, but the fabric was falling apart when worked with. Then I desided to cut the pieces out with the laser cutter. For that I made the cut .pdf file from Fusion 360 drawing file using photoshop and Inkskape software. The processes for this are explained on week 3.




Rest is applying resin to the fabric and attaching it to the core material. Applying the resing to the fabric is best done using hand (with cloves). When I had spread the resin evenly to the fabric, I rolled the fabric to the core material and backed everything in the vacuum following the process flow explained above. The vacuum bag was not a success as it was leaking so much that the vacuum stayed minutes before leaking into atmosphere.


When attaching the fabric to the core, I found out that it was quite hard to get the fabric tight enought. It might have been better to use multible small slices of fabric attached on top of each other. This option just makes it hard to handle the edges of the fabric.


After 18 hours in the bag, I opened it and found out that the result was good. there was some extra fabric on the leading edge but otherwise very nice result.



Files used with this weeks assignment

The first .NC milling file
Optimised .NC milling file
Fusion 360 .f3d foil file
Fusion 360 .f3d half foil file
Drawing exported from Fusion with wing profiles in .pdf
Full foil .stl file
Half foil .stl file