Fab Train Model (Frame and Composites With CNC)



Introduction: Fab Train Model (Frame and Composites With CNC)

In this Instructable, I explain a part of my final project that I did at the Fab Academy. To see the complete project click on the following link.

As I have said in this part I explain the use of the CNC for my project. I will create the frame of the model and modeling the landscape by milling the polystyrene to create the composites.

Step 1: Frame Design.

Using the Rhinoceros program, I design the frame of the model. In my case it consists of three modules, which are the same. They have in common the modular Maquetren regulations to be able to join other modules and form a large model.

For the frame I will use a 9mm thick plywood.

Step 2: Aspire. Preparation of the File for the CNC.

Aspire VCarve is the program we use to mill with the CNC of the Fab Lab León.

I open a new project in Aspire, where the first thing I do is configure the board that I am going to use and its measurements. For this exercise I am going to use a board 9 mm thick. When we import the vectors we have to center them on the material. It can also happen to us that the vectors are not joined, to join them we use the union tool. For all the work I use a 6mm single flute bur.

Then we choose the pocketing paths, where I will only mill the wood about 5 mm.

The next thing is to make a milling of a channel to place the cables. Also 5 mm.

Then I make the interior cut of the three dogbones and the holes.

Finally the exterior cut of all the pieces.

In the end, we can do a simulation with all the trajectories, and see the final time.

Step 3: Preparation of the CNC and Milling.

Once we have the files ready to send them to the machine, we have to put the milling cutter and prepare the CNC.

The CNC that exists in the Fab Lab León is the following model.

  • Model: Milling Machine CNC TEC-CAM 500.
  • Software: Aspire VCarveWork area: 1300 x 2500 mm
  • Bridge height: 120 mm
  • Maximum thickness of the material: ~ 40 mm (depending on the mill)This milling machine uses a vacuum table.

To assemble the mill, we will take the collette and two keys to tighten it.

We will place the board and turn on the aspiration.

After milling all the pieces, we will clean the surface.

Step 4: Frame Assembly.

Once all the pieces have been cut (I laser engrave the name Fab Train Model on one of them) I begin to assemble all the pieces.

They fit by press-fit, for greater security it would be advisable to use white glue.

I also take the opportunity to place the electronics boxes and the cables that will go under the track.

Step 5: Landscape Modeling Using Blender.

For the landscape, I create a 3D model of the module and begin to texturize and stretch the pieces to create small mounds or valleys.

Step 6: Design of the 3D Molds for the Composite.

From the Blender design I extract three STL files. Of those three files, using Rhinoceros I make a slicer of each part of the module, to get only the volume of the mountains or the valley.

I cut with planes and make a boolean difference to get the parts I need.

I am attaching all the STL files that I get.

Step 7: Preparation for Milling of Polystyrene.

I will explain the process of one, the rest is the same.

I will use Vectric's Aspire software. Before going to the Fab Lab, I test the toolchain at home, and do the simulation to see how long it takes. I use the trial version of the Aspire 10. I try to perform a rough milling and then finish two of them turned 90º.
Using a 6mm mill polystyrene gives me a total time of 3 hours. I'll have to see how it looks in the Fab Lab León Aspire simulation.

The Aspire of the Fab Lab León is an old version and some things compared to the modern one. The first thing I choose is the size of the material (the material plate is 1250 x 600 x 40 mm). I import the STL and orient it, I tell it that the position plane 0 is down completely.

I choose the 6mm cutter for polystyrene to make the rough. I see in the simulation that it mills all the material (in the modern version of the Aspire, it only mills the part). So Nuria recommends that I adjust the material to the piece.

I choose the 6 mm cutter for polystyrene to make the rough. I see in the simulation that it mills all the material (in the modern version of the Aspire, it only mills the part). So Nuria recommends that I adjust the material to the piece.
Also in my case I don't need it to be a perfect surface, because it will be part of the landscape and I will camouflage it with vegetation and herbs. So I'm going to try doing just the rought machining, without using the ball mill to make the finish finish. It only takes 25 minutes. Once the simulation has been carried out and the strategy that I am going to use has been decided, I move on to the CNC milling machine. These are the two mills, 6 mm for polystyrene and the ball (the latter I will not use). I must tighten the mill well to the spindle.

Step 8: Polystyrene Milling.

Once I have milled with the rough technique, I decide to stay with it because the lines are not going to be noticed much and the landscape is not uniform.

Step 9: Composite Process

As Neil explained in class, it's about applying a resin to a fabric or fiber. Fiber alone is good for tension, but bad for compression. The resin is good for compression and bad for tension. So if we combine the two materials we obtain a compound resistant to tension, but also to compression.

The composites process is as shown in the following drawing. We have a 3D mold, which we protect with a protective film (the kitchen one is worth it). Then we will place the fiber or the fabric impregnated in the epoxy resin. Next we put a release film (it will help us to get the excess resin out of the mold). Then a blanket that covers the entire mold and absorbs excess resin. Finally all this is put inside a vacuum storage bag.

Once I am clear on the process, I protect the floor and the work table and prepare all the necessary materials.

I am going to use an Entropy Resins (Super Sap) brand epoxy laminating resin. Clear, general use, laminating epoxy for composites, coating, and adhesive applications. CLR is a SUPER SAP® Formulation.

The one in the Fab Lab León is the CLF (Fast) model. It has the following Technical Data Sheet (TDS).

  • Mix Ratio (by volume): 2: 1 (Part A / Part B)
  • Mix Ratio (by weight): 100: 45 (Part A / Part B)
  • Pot Life (25ºC): 21 min.
  • Tack Free Time (35ºC): 4 hrs.
  • Recommended Full Cure: 7 days 25ºC

On the Safety Data Sheet (SDS). Good ventilation is recommended, wearing safety glasses, rubber gloves and long sleeves. As we are in COVID-19 period, I use the mask. 😷 This type of resin is corrosive and irritating on the skin and on the eyes, so you have to protect yourself.

The first step is to protect the polystyrene mold that we have milled with the film. So we can use it more times.

Using the textile vinyl sheet, I iron the fabrics that I am going to use in this case burlap. Then I cut it to the size of the mold, and cut the corners a little diagonally. In my case for the final project I will not need the edges, I will have to cut them, because they will rest on the model frame. For this test I will use only one layer of burlap.

Now I proceed to mix the epoxy resin. It is recommended to use two pairs of gloves, because the resin is quite sticky. It consists of two parts, Part A and Part B. I am going to add 100 gr of Part A and 45 gr of Part B. The plastic cup is 5 grams. I have 21 minutes to work with the resin.

With the help of a roller and a brush I spread the epoxy resin on the burlap. It is recommended to protect the table with another plastic since we are going to soak the fabric. Then we will place it on top of the mold.

The next thing is to cut a piece of release film (it is a perforated film to help evacuate excess epoxy resin). In the photo you can see the detail of the little holes.

Later we will place a blanket, to absorb the rest of the epoxy resin and that does not stick to the vacuum storage bag.

The final moment comes, insert the mold with all the previous layers into the vacuum storage bag. So that the suction valve is not blocked, we will place a piece of blanket under it (blue blanket). This way we will continue to vacuum the vacuum cleaner. Thanks to the help of my instructors, Nuria and Pablo I can carry out this assignment. 😍

We close the bag tightly and vacuum with the vacuum cleaner until there is no air left. The bag begins to wrinkle and you see how the excess resin begins to transfer the release film and soaks the blanket. Is incredible. The manufacturer says that after 7 hours the composite can be manipulated, in my case I will leave it for 24 hours. Then until 7 days have passed it will not be completely healed.

Step 10: Completion of the Composite

After 24 hours I return to the Fab Lab, and check that the bag remains the same, it has not lost its vacuum. That is a good sign. So I take out the mold, peel off the blanket and notice that there are little dots of resin left over. Then I remove the realease film, and I get the burlap perfectly sealed, all its holes bonded with the resin.

I will have to let it cure for 7 days and cut it to fit the model frame. One of the advantages of using this technique is that I can recover the mold, reuse the realease film, the blanket and the vacuum bag. I just have to make more resin and more burlap. Incredible. 🤗

Using the Dremel I cut and fit the composites to the frame of the model.

Step 11: ​Decoration

Through different shades of grass, bushes, trees, the ballast for the rails and paint the model is ready.

Step 12: Final Project

This is the result of the whole process. If you want to know more information about the project, click on the following link. Enjoy it.

CNC Contest 2020

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CNC Contest 2020

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