Overview

The objective of this fin can is to attempt the first potential flight candidate fin can for Hermes III. This fin can implements the lessons learned from the first fin can. It includes steel leading edges, a different laminate stack up, and the correct fin geometry. 

This fin can is the second in the series of 3 fin cans for Hermes III (Rusty, Dusty, and Crusty.)** This is the story of Crusty.

**Originally, Dusty was supposed to be the 2nd completed fin can, but a series of unfortunate root bonding accidents resulted in Crusty being completed before Dusty. Therefore, despite the fact that the overwrap for Dusty was done a week before the overwrap for Crusty, Crusty is now referred to as the 2nd fin can.

Tube Preparation

1. Kit the necessary materials for the overwrap layup. The sizes listed in the table below are given such that there are 3 layers of carbon fiber that run 22" long on a ~19" circumference tube. 22" is chosen so that there is one inch of exposed left on both sides of the 24" phenolic tube, which will be used for the vacuum tape. The breather, peel ply, and release film are sized to wrap around the tube with several inches spare for vacuum tape. 

   1. Kitting set up: 

      Overwrap Kit Material	Sizes
      High temp vacuum bag (pink, no holes)**	30" x 24"
      Breather cloth (white, fuzzy)	22" x 24"
      High temp peel ply (blue nylon or white polyester)	22" x 24"
      High temp release film (red with holes)	22" x 24"
      Carbon fiber (3 layers, (0/90))	22" x 60"
      Wax paper	24" x 60"

       

      1. Two tables in Gelb lab, with fresh paper laid on top. 

      2. Make sure to wear gloves when cutting everything. 

      3. Be weary of stray pieces of painters tape from the wax paper touching the carbon fiber because this could cause fraying. 

      4. Use the sharp shears for the breather cloth. Everything else can be cut using the pizza cutter. 
      5. For the carbon fiber and wax paper: Kit the wax paper first by laying two 12"x60" sheets next to each other. Tape the two together either directly side by side with no overlap or with two inches of overlap. Use scotch 77 to spray a light layer of spray glue on the wax paper. Make sure to rub out any bubbles of spray glue material that globbed up on the wax paper (any glue globs on the wax paper is an area where there won't be epoxy). Roll out the carbon fiber on a freshly cleaned table such that there's at least 60" of length. Place the wax paper on the carbon fiber, glue side down. Gently flatten the wax paper over the carbon fiber with your gloved hands. Use the pizza cutter to trim the carbon fiber to size. Weigh the carbon fiber and wax paper combination. 

2. Sand the tube with low grit sandpaper to roughen it up.

3. Do the layup. 
   1. Grab a pole and wrap it with wax paper. This pole will slide through the tube and hold it up during layup. 
   2. Secure the pole ends and the tube with two tables, and add weights or something to make sure someone can't accidentally knock the pole off. 
   3. Cover the area beneath the tube with lots of paper/cardboard/etc. Just make sure that any epoxy drips are caught so that the floor isn't covered in epoxy.
   4. Mix Systems 3000 epoxy. Make sure to record the amount used. 
   5. While holding the tube in place, gradually unroll the carbon fiber onto the tube, while pouring epoxy over it. Use squeegees to spread the epoxy. Be careful not to use too much epoxy because this will reduce the overall strength of the part. 
   6. Continue unrolling the carbon fiber from the wax paper until finished. Be careful to keep the same amount of tension on the carbon fiber throughout its length to prevent warping. 
      1. If you notice warps or areas where it doesn't look like the fiber is going in the 0/90 degree direction, pause for a moment and try to massage the fibers back into place. 
   7. The epoxy has a two hour pot life, so hopefully you've finished the layup in 90 minutes and now have time to bag.
4. Vacuum bag
   1. Wrap the part in peel ply
   2. Wrap the part in release film 
   3. Wrap the part in breather. Consider using two layers of breather because our layups are pretty wet. 
   4. Wrap the part in the vacuum bag. Be sure to add vacuum tape to the ends of the tube.
      
      1. Slide the vacuum connector into the part. 
   5. Pull vacuum and listen for leaks. You're aiming for -26 mm Hg. 
5. Let it cure at room temperature for 24-36 hours. 
6. Bring into the oven to cure for an additional 12 hours. 

Root Bond and Root Fillet 

Required Materials:

• CF overwrapped tube
• Systems 3000 epoxy (resin and hardener)
• Acrylic fillet radius tool 
• Gloves
• Respirator
• Carbosil
• Popsicle sticks
• Epoxy boats or paper cup (for mixing epoxy)
• Painters tape
• Ruler
• Acetone and shop towels (to clean up epoxy on acrylic tools)
• Vacuum 
• Root bond jig
• Sandpaper (220 grit)

Root Bond Procedure

1. Measure the outer diameter of the overwrapped fin can. Use this measurement to size the diameter of the hole for 4 root bond jig plates. 

   Root Bond Jig Dimensions	Size
   Diameter of collar	6.27"
   Width of slot	0.131"
   Length of slot
   Diameter of rod

2. Measure the thicknesses of the fins. Use this measurement to size the slots for 3 of the 4 plates. The bottom one should not have slots in order for the fins to rest on it.
3. Use the threaded rods to evenly stack the plates on the root bond jig, and use a level to make sure that each plate is aligned properly. 
4. Prepare the fins
   1. Sand both large faces of the fin and the root face until you can no longer see the shiny, non-stick top coat. Use low grit (like 60 or 80) and finish with higher grit (like 220). 
5. Apply Systems 3000 epoxy on the root chord of the fin preform (Without the leading edge attached), and slide the fin preform into the slot of the root bond jig to attach. 
6. Tape popsicle sticks each slot where the fin ends so that the fin can be held in place.
7. Leave to room temperature cure for 24-36 hours before attaching leading edge and starting fillets.
   
8. Attach the leading edges
   1. Move the plates of the root bond jig such that you have enough room to attach leading edges, but you still have a plate securing the bottom of the fin and a plate securing the tube at the top. Having these plates here is useful in preventing someone from knocking the fin off. 
   2. Clean the inside of the leading edges. I used a paper towel soaked in IPA and a slotted screwdriver to stuff as much of the paper towel into the leading edge slot as possible, and then I ran the paper towel through the slot with the slotted screwdriver. This method removed a lot of gunk that using compressed air didn't. 
      1. You could probably use a cotton swab, but I couldn't find those in Gelb. 
   3. Clean the entire surface of the fin and the carbon fiber wrapped tube with a microfiber cloth and some IPA. Make sure there isn't gunk/oil on the part.
      1. Microfiber is extremely important because using a shop towel or any other paper towel will shred. 
   4. Do a dry fit with the leading edge. It should be a comfy, snug fit, but not one where you have to use a ton of force. You might have to see-saw a little. 
      1. If it doesn't fit (in my case, it never did), then sand the hell out of the fin while it's sitting in the root bond jig. Be extremely careful while sanding because it is truly heartbreaking when a fin breaks after the root bond. 
         1. Use 40 grit or something low. Removing material as fast as possible is the goal. 
         2. After sanding, make sure to thoroughly clean the leading edge surface of the fin preform to remove any dust. Microfiber cloth and some IPA. Multiple times. 
   5. Mix some Systems 3000 epoxy (100 parts resin to 18 parts hardener)
      1. Don't mix up the caps. 
      2. Clean the containers after you pour. It takes 2 seconds with a paper towel. 
   6. Using a small popsicle stick or a needle, cover the inside of the leading edge slot with Systems 3000. Make sure to also cover the surface that goes up against the fin can. 
   7. Insert the leading edge onto the fin, and make sure to have some painters tape handy to tape the leading edge to the fin to prevent it from sliding downwards. 
      1.  Careful that the tape doesn't end up somewhere the epoxy is because then you've cured painters tape to the leading edge. 
   8. Leave it for 24-36 hours.

Root Fillet Procedure

1. Add in carbosil in small amounts and mix until a peanut butter consistency is achieved. 
2. Place the epoxy cup in the vacuum chamber, and wait 2 minutes. The epoxy should rise and then fall after the air bubbles have been pulled out. 
3. Mark the radius of the fillet on the fin and on the fin collar with a sharpie, and then cover in tape. In this case, Rusty had a one-inch fillet. 
   
4. Apply the epoxy in the fillet area, being careful not to go outside the tape. 
   
5. Use the acrylic fillet radius tool to remove excess epoxy. Be sure to swipe perpendicular to the root to ensure that the same amount of epoxy is being removed each time. 
   
6. Let it cure at room temperature for 24-36 hours. 
7. Place in the oven in the vertical position to oven cure for 12 hours. 
8. Sand the fillets.

11/13:

Need to CF kit:

1 more 0/90 20x22

4 more 0/90 20x26

4 more 0/90 20x26

Layup:

Epoxy: 1312.9 grams

Cutting Carbon Fiber on the Epilogue in Makerworks

• speed 10%
• 100% power
• 50 Hz frequency
• 600 ppi

Exporting from Solidworks:

• right click on the face you wish to export and click "Export to DXF / DWG"
• Specify the proper name and file location, and set the file type to dxf
• The Left Panel should say "DFX / DWG Output"
• Ensure "export" is set to "faces / loops / edges"
• Ensure "export options" is set to "Single File"
• Click the green check mark at the top of the panel
• The file will be exported!

Printing a DXF File:

• Open the dxf in your favorite vector graphics editor (e.g. Adobe Illustrator, inkscape, Corel draw)
• In Illustrator
  • Set your document to the size of your piece
  • Place your piece in the top right corner of the laser cutter
  • In illustrator, select file > open and navigate to your dxf
  • In the window that appears, make sure "original" size is selected
  • Hit okay
  • It may complain about not having a text file - just hit okay
  • Move your drawing to the top right corner of the file
  • Delete extraneous lines (solidworks will include a "only for educational use" decal)
  • Select the drawing and set the line width to "hairline" or 0.001" if that isn't an option