Primitive Engineering

The past few days have been quite the roller coaster…

So we got the part out…

Easy way to release the vacuum bag - add a little pressure from a shop vac

And it was terrible! It couldn’t hold it’s own weight and just folded over…. Just too weak. The solution? T ribs.

Why? Moment of Inertia. We can’t change the material properties - but we can change the material dimensions. This is why plate steel can hold some load while plate aluminum won’t hold crap (instead, Aluminum is shaped into a channel or box).

This is where we apply engineering - on the table. We had a few different geometries to work with, the one with a wide base and high Moment of inertia (MoI) wins. For those interested, MoI increases cubically with respect to height changes and linearly with changes in base width. That means, an increase in height is more significant than an increase in width.

That’s how we came up with the T rib idea…

So foam ribs were spay glued on the inside of the part. Laid up some glass over them and put it back in the bag.

So here’s the other problem….

Those hazy areas are resin rich regions… Because the vacuum wasn’t applied fast enough, the layer of resin cured before the vacuum could pull it through the cloth :/

It’s dead weight and looks like crap :/

We brought it to the senior design lab for an ASME presentation we had yesterday (3/22). That picture shows the extent of the resin rich zones.

I spoke with our adviser - had him walk to the lab to take a look. He’s generally pretty stoic with his emotions, but he seemed very happy with our mold quality (which doubles as a transportation device) as it has a very very smooth finish. He agreed that the part isn’t optimal. I told him we have enough material and I personally want to have the part redone. He agreed and we now have both the time and material to do so.

By the way - the color in that picture is all wrong - we didn’t remove the mold release so there’s a green tint

Did you know you can sew fiberglass?

You can! So, we can’t do T ribs as it’s a manufacturing problem (we can’t spray glue on wet resin and the ribs want to spring to straight). So we’re changing to one piece “L” ribs. We’re making them by cutting foam with rectangular cross sections and then using a router make the section. It’s an “L” with a rounded inner corner to help the glass get over it….

So, back to sewing… These new ribs are too big for our 3″ fiberglass tape… We have to double up to get proper coverage. Laying up two lengths of tape over these curves is a PITA. So to make the job easier, I’ve sewn double wide sections of fiberglass tape (no budget or time to order wider tape right now). I’m glad I ordered the extra roll of glass tape

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Finally!
So yesterday, that paste epoxy was applied to cover up some goofs… Now all that had to be sanded off…. Check this out…

The tool was then sanded again up to 800 grit.Then, we draped…

It looks a lot better in person (my camera didn’t pick up the fine detail) Here I am cutting! To help find a cutting line, remove a few strands (just tug o them) and you’ll get a perfect line

So, we got all the lay up stuff organized, tested the vacuum pump, and started the lay up (oh, we waxed and sprayed PVA again). I want emphasis, tested - that is, I turned it on, held my finger over the input, pulled full vacuum, then turned it off….

So we started lay up. First, a gel coat like layer… Then some thin resin - then the carbon was draped, coremat was placed, ribs were placed, glass tape was placed, Kevlar was draped. Placed the peel ply, draped the breather and then got it in the bag, sealed the bag and added a vacuum port…

We used a shop vacuum to pull the bulk of the bag… Then, switched over to the vacuum pump… Switched it on - NOTHING. Switched it off - and a flash of light came up under the switch. First, I thought it seized like the last time, so I began tear down while the rest of the team worked on adapting a fridge compressor to fit.

I took off the vac manifold - and the vanes still spin…. Tried turning it on, nothing… Took off the electrical service panel - the insulation was cracked and rotted away… The pump was shorting out :/ By that time, a hacked solution came together, so I abandoned the pump….

So, no one was working on the bag - getting wrinkles out, so I attempted to get some of that out…. Then the vacuum came full on.

You can see some of the rib structure…So, now, time for a beer. Celebratory beer followed by flipping the whole thing, throwing a space heater in and bringing the temperature way up to get the process started

Woo!

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Report was submitted today! Woo! That’s done! But the work is still on

Today we got a largish list of stuff done

• Cleaned Shop
• Built Support Frame #2
• Sanded First Plug (up to 800 grit)
• Made a jig and cut foam ribs
• Sanded said ribs
• developed hatch cut lines
• developed rib layout
• developed coremat placement
• cut coremat
• cut glass tape to cover foam ribs
• cut Kevlar (for extra abraision resistance)
• cut peel ply
• cut breather
• made vacuum bag
• fixed tool holes and dry spot with homemade epoxy putty paste

So here’s our tools

And here’s everyone (minus me) making ribs. They’re 1/8″ foam strips (1″ wide) with a hand sanded chamfer so the vacuum bag pulls a fiberglass cover over the edges without air gaps.

Here’s the ribs

Laying out ribs

Coremat is really strong… but a lot heavier than foam when impregnated with resin. So only thin strips are used where needed for super support. That door is hinged at one point - so that point needs some beefy support to hold that load.

Peel Ply and breather laid out

Be organized… All of those pieces are labeled as is the actual tool. Even the fiberglass backing has a matching label. This will speed us up while laying everything up so we can get under vacuum faster.

Peel ply (back) - breather and Kevlar (middle) - custom vacuum bag (front)

Tomorrow (sunday)

• sand tools (from epoxy)
• wax
• dry lay up
• cut carbon
• spray PVA
• lay up first part!

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After several meetings and model revisions (to save on material) - this came out of the FDM machine… Rapid prototyped in 9 hours - the material is ABS and the process was fused deposition : thumbup :

So we spoke wit another professor who controlled the measurements lab - which has a water tunnel…. Basically, he said go do it - do it now, just figure it out and get it done… My kind of attitude!

This shows the stagnation point at the front of the model…

Flow over the model - sticky all the way…

Iso view… Some turbulence due to surface finish (ridges that haven’t been fully sanded down)

Smoke screen - basically, pump the dye tank pressure up and open the valve all the way….

More sticky flow

So we also did some sample testing… These were tensile tests - ripped apart until destroyed…

I’ll tell you the forces later : thumbup :

And… At the end of the day, my friend on the swirl burner design project was doing some cold flow tests with compressed air and a fog machine… Here’s some CFD (compressor fluid dynamics) with smoke…

That’s all for today - we’ve also been working on our design report… All engineering type stuff, not so much with the actual build… But the extra cure time for the tools is a good thing…

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