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Below is a table summarizing the process, as well as the approximate time that each step took.
| Approx. Time | Notes | |||||
|---|---|---|---|---|---|---|
| Design fins | 10 hours | We used the design from Hermes 1. Next time, we should do more detailed analysis. | ||||
| Total design time | TBD | |||||
| Waterjet G10 | 3 hours | Make sure it doesn't delaminate by leading in to the cut | ||||
| Waterjet phenolic | 3 hours | Make sure to weight down the part because it's long and thin and moves around. | ||||
| Cut G10 tab | 4 hours | It's going to be difficult to get this accurate because sheet stock has a variable thickness. | ||||
| Cut phenolic slot | 4 hours | Use machine oil or it'll get spicy. | ||||
| Phenolic taper | 4 hours | Used 15-degree taper for Hermes 2. Taper may need to change for future designs. | ||||
| Mill down phenolic | 2 hours | May or may not need to do this depending on the total thickness of CF on each side of the fin | ||||
| Assemble fins | Bevel fins | 2 hours | Don't use 5-minute epoxy. | |||
| Bevel fins | 4 hours | Did this by hand- could make a jig to get a better result and save time. | ||||
| Total manufacturing time | 26 hours | The more you do it, the better you get, but allow for set-up and things going wrong. |
Things to improve for next time:
- Make a jig to bevel the fins or figure out some better way of doing it than by hand
- Consider using honeycomb for the fins- would save mass while maintaining strength
Test Fin Can Fins
Below is a picture of the fins used for the test fin can layup. These dimensions were obtained from our OpenRocket sim, but were not the updated shape of the fins. This was acceptable for the test fin can layup, but for the flight candidate layup the shape was different.
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Talk about design of phenolic slot (0.3" deep, 1/16" thick)
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Flight Candidate Fin Manufacturing Process
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Part One: Waterjet G10 Fin Core (~2 ~3 hours)
*Delamination is when the layers of your composite start coming apart. This can happen if the waterjet is cutting too quickly/strongly, and especially if your composite sheet is thin (1/8" is considered pretty thin).
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Part Two: Waterjet Phenolic (~2 ~3 hours)
- Make a DXF drawing of your phenolic leading edge.
- Follow the steps for waterjetting based on the waterjet you're using. Make sure to make extras, like with the G10 fin cores. Use the same settings as for the G10- even though the phenolic is thicker (1/4" inch), this will help you be sure to avoid delamination.
- When waterjetting, make sure to weigh down your part. Since the phenolic pieces are long and thin, there's a higher risk than with the G10 that the cut will be uneven because the part moves around as it's being cut out.
- After waterjetting, file down any burrs.
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Part Four: Cut Phenolic Slot (~4 hours)
- Clamp your phenolic piece horizontally (the 1/16" cutting bit will come in horizontally) on either side of the phenolic. It does not matter which side you select for the slot, as when you flip over the piece, it is symmetrical.
- Zero the cutting piece at the center of the phenolic.
- Before turning the mill on, have some machine oil ready, and set the mill to low speed because the phenolic heats up while it's being cut.
- When you turn the mill on, make sure it's set to "Forward" and cutting in a clockwise direction (to the right). Cut out the 1/16" slot.
- After cutting, use the air hose to remove the phenolic dust and use ethanol* to remove the machine oil.
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Part Five: Cut Phenolic Taper (~4 hours)
- Since the phenolic piece is long (11.66"), you'll need to use two clamps to clamp it vertically.
- Zero the end mill at the middle of the part.
- Use a 15 degree end mill to cut a taper on either side.
Part Six: Mill Down Phenolic (~2 hours)
This step was required because since the phenolic is twice as thick as the G10, some material had to be taken off each side so that it better matched up with the G10. You may wonder why we didn't simply start out with 1/8" thick phenolic: this is because the phenolic has to be slightly thicker than the G10 so that when the carbon fiber plies are added during the fin can layup, the carbon fiber is flush with the phenolic on either side. Therefore, we took 0.020" in off either side to account for the thickness of two plies of carbon fiber. Ideally, the sides would be milled down before we cut the taper, but this worked out fine.
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Part Seven: Assemble Fins (~2 hours)
- Make sure that each G10 fin core fits into the phenolic slots.
- Use West Systems epoxy (do NOT use 5-minute epoxy) to attach the phenolic leading edge to the G10. In reality, this step was done after we bonded the fins to the fin collar because the phenolic leading edge did not fit into our fin can jig, but this step could also be done before the fins are attached to the fin collar if the jig is designed to account for this.
- Wait a bit and wipe away the excess on the sides and oozing out of the slot.
- Let cure for 24 hours.
Part Eight: Bevel Fins (~4 hours)
Lessons Learned:
- When waterjetting 1/8" G10, delamination is a concern because G10 is a composite (made of several layers of fiberglass) and 1/8" is pretty thin. Make sure to use the "Low Pressure" and "Brittle Material" settings on the OMAX waterjet.
- Make extra fins and phenolic leading edges. That way, if anything goes wrong, you won't have to go back and make them all over again.
- When making the tab on the G10, don't mill everything in one go- use several passes on the mill so that you take off a little bit of material each time. This way, the tab will be more uniform/even.
- When cutting the phenolic slot, use oil because the phenolic will heat up from the cutting piece.
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