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Learning how to build Lotus Seven replicas...together!
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PostPosted: July 9, 2020, 8:22 pm 
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For my BEC build, I decided to try to build my own seat. I wanted a nice fitting seat that would fit me and the small car.

Overview/steps:

1. Build a wood/foam seat that fit me and inside the cockpit.

2. Scan that seat/pattern into CAD to resurface, smooth make symmetric add other features.

3. CNC foam blocks to make a final foam buck to laminate the actual seat.

4. Make the final CF seat lamination.

5. Final details.

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PostPosted: July 9, 2020, 8:28 pm 
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Step 1.

I build a wood frame from 4 or so sheets of plywood. I have a nice square spot at the bottom of the cockpit that it locks into. I then added foam blocks to fill in to fit me better. A wire wheel on the angle grinder works great for shaping it as well as making it look like it's snowing in the summer time.

I also used some of the cheap expanding foam and sat in it (with a plastic sheet over it) to try to form it better to me.


Attachments:
seat_pattern1.jpg
seat_pattern1.jpg [ 193.64 KiB | Viewed 1254 times ]
seat_pattern3.jpg
seat_pattern3.jpg [ 177.02 KiB | Viewed 1254 times ]

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Last edited by Johnsinski on July 10, 2020, 3:30 pm, edited 1 time in total.
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PostPosted: July 9, 2020, 8:48 pm 
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Step 2.

Scan that pattern into CAD. I used an old Microsoft Kinect as the hardware scanner and ReconstructMe for the scanning software. Lots of youtube examples how to do that.

I've also scanned objects using a line laser set-up with free software too. David3D was the freeware at the time. Lots of youtube examples how to do laser line scanning.

Now you have a very rough point cloud (x y z points/triangles) or a mesh. You could probably import this directly into your CAD system, but I prefer to clean it up first. Some CAD systems can get a bit sluggish with mesh data. I used the open source Meshlab to clean it up and export it as a .obj file, one of the files I can import into in CAD. Lots of Meshlab tutorials can be found on youtube.

Then I import that "simplified" mesh into CAD, Proe in my case. Then I like to try to build a set of datum planes that make sense and split it symmetrically. Then you need to develop a plan for the shape you want. I like to draw a bunch more planes that go through important cross sections. Then I can draw a simple curve that matches that section that will be used to reconstruct a nice clean surface.

I added 6 "hardpoints" where I plan to mount the seat with bolts. I added belt openings and a lip around the whole thing.

Now I make up an assembly of "blocks" that will make the final mold. This is done using assembly mode and is all setup so if I make a change to the seat, the blocks and NC stuff will update.


Attachments:
seat_scan_curves.jpg
seat_scan_curves.jpg [ 35.45 KiB | Viewed 1250 times ]
seat_scan_mesh.jpg
seat_scan_mesh.jpg [ 53.26 KiB | Viewed 1250 times ]
seat_scan_surfs2.jpg
seat_scan_surfs2.jpg [ 24.54 KiB | Viewed 1250 times ]
seat_foam_block_07.jpg
seat_foam_block_07.jpg [ 28.02 KiB | Viewed 1250 times ]
seat_foam_buck_cad_1.jpg
seat_foam_buck_cad_1.jpg [ 37.61 KiB | Viewed 1250 times ]

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Last edited by Johnsinski on July 10, 2020, 3:33 pm, edited 1 time in total.
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PostPosted: July 9, 2020, 9:08 pm 
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Step 3.

A 4" thick block was about the max I could do on my CNC router. I glued 2 2" pink foam blocks together to make a 4" block. I used a couple modified spade drill bits to do the whole thing. The first roughing I used a 1" spade drill that I had ground the spot point off. For finishing I used a 1/2" spade I had custom ground into a BEM (spherical cut). As I recall I used about .050" step over finish, I would go a bit smaller next time to make hand finishing easier.

Once each block was off the CNC, I lined each one up with another and glued them together. I used a construction glue which turned out to be rather difficult to sand/smooth out compared to the foam.

I skipped trying to CNC many of the slight undercuts and finished them by hand. That was kind of tricky, I ended up using of one of those "cheese grater thingies" to help shape it.

I used some foam safe "smoothing" epoxy (Smooth-on epsilon pro?) to seal the foam to prevent melting, etc. from the next smoothing products, bondo and primer. I used from 60 grit to 240 to get it smooth to my liking.


Attachments:
seat_foam_buck_cnc1.jpg
seat_foam_buck_cnc1.jpg [ 334.13 KiB | Viewed 1249 times ]
seat_asm2.jpg
seat_asm2.jpg [ 177.17 KiB | Viewed 1249 times ]
seat_foam_buck_finish1.jpg
seat_foam_buck_finish1.jpg [ 361.07 KiB | Viewed 1249 times ]
seat_foam_buck_finish3.jpg
seat_foam_buck_finish3.jpg [ 160.68 KiB | Viewed 1249 times ]

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PostPosted: July 10, 2020, 4:28 am 
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Superb effort and brilliant technical write up, thanks for sharing your success! The shadow of the airline on the plug had me wondering what was going on for a second. What lamination scheme are you going with?


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PostPosted: July 11, 2020, 1:27 am 
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That shadow caught me too, until I figured it out.

Step 4. (The Nightmare)

I use many coats of Johnson's Paste Wax, and a few coats of PVA. I must have sprayed the PVA too thick cause it ran. So I wiped that off and sprayed lighter coats. I let that dry for a few hours.

I had previously ordered a few yards of 2nd quality 5.7oz CF, some 9.4oz fiberglass and 1/2 gallon of thin epoxy and slow cure hardener from Composite Envisions.

I cut 2 large pieces of CF to fit the majority of the mold and overlap in the butt area. I mixed up some epoxy for the first layer. I brushed on a good coat over half the mold and started draping the CF over it. I used "chip" brushes for all the work. It took some working to get it to conform to all the little details. I thought it went down well, but there were some dry areas after demolding.

I missed getting the overlay in the middle so I tossed another small CF patch over than.

I also had some really thick CF fabric laying around and used it to bulk up the middle of the seat.

Then I started bulking up around the "hard points". I used small "U" shapes to thicken up around those features. I also used small patches to go completely over them. Those areas are probably close to 1/8" thick.

Unfortunately I don't have any pictures of this process. I was super focused on getting as many layers on as I could while the epoxy was setting. I probably put 5 or 6 additional layers of fiberglass fabric over it. It was kind of frantic for about 6 hours! I didn't have a major plan to the fiberglass, I just kept building it up.

Oh, I beefed up the outer lip too with strips of fabric along the way.

A day later...

It seemed pretty cured so I tried to start demolding it. It didn't go well. I was kind of hoping to save the foam buck, but it became clear that it wasn't going to come off easily.

I could peel the edges up, but not very far. After the fact somebody suggested "melting" the PVA with water, which might have worked.

I took the electric chainsaw to the majority of the foam and then some claw hammer work. It wasn't pretty.

I do have a few pics of the demolding somewhere.

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PostPosted: July 11, 2020, 2:27 pm 
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Thank you very much for the write up. I used to make custom seats for my kayaks. The difference in comfort was monumental. Whereas with stock seats I'd start to lose feeling in my lower body after 15 minutes, with a good custom seat, the only limit would be my bladder. One key that people need to understand is that with a good fit, padding is unnecessary. If the load of your weight and impact is spread out evenly over your lower body and legs, you're good.

Here are instructions that would lend themselves well to car seats as well. Making an insert for an aluminum seat would be perfect.

First make a 4-sided cardboard form that delineates the outer limits of the seat. The bottom is open to the boat, and the cardboard is taped to the hull. Then you line it with 4 mill plastic.

Next you put on a pair of disposable painters coveralls and mix up a big batch of expanding foam and pour it into the mold. Then sit in it in a paddling position and things get interesting :D

The expanding foam grows around your lower body like something from a science fiction story. It is undergoing a chemical reaction so it's also warm but not hot. 10 minutes later it's encased your lowers and has hardened up. At this point, you look desperately for something to cut your way out of the foam. Fortunately, you're a smart guy and have a knife handy. Cut your way out and try and avoid your peripheral artery.

At this point you'll have a perfect impression of your lowers - hopefully including lumbar and under thighs. You break out a rasp and a sureform and start whittling away till it looks fairly attractive, then apply epoxy and fiberglass. Note: Do not use polyester resin, as it dissolves the foam. Use the good stuff.


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PostPosted: July 12, 2020, 1:18 pm 
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Thanks for the writeup, Johnsinski. Cool 3D router--did you make it, or is it a commercial product. In either case, what did it set you back? And what software did you use for the CAD part of the operation...and for the CAM as well.

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PostPosted: July 13, 2020, 12:19 am 
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Kayaks freak me out! But that does seem like a place you want a good fit.

Jack,
I've got a thread on that too:

viewtopic.php?f=15&t=20525

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PostPosted: July 13, 2020, 6:09 pm 
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Here's a pic of of early demolding, once I got the foam chipped down thin enough I could get my fingers in an snap another piece off.


Attachments:
seat_part_removal1.jpg
seat_part_removal1.jpg [ 238.84 KiB | Viewed 1069 times ]

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PostPosted: July 20, 2020, 5:04 am 
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Step 2 and 3. (supplemental)

The scanning through making NC code could actually be done with open source software. The only money you'd have to spend would be on an old Microsoft Kinect (or make a laser line scanner).

a. I used Reconstructme (free software) for the scanning software with the Microsoft Kinect.

b. Meshlab (free) to smooth/simplify the design a bit (all triangles at this point)

c. Blender (free). I have very limited use of Blender, but I guess it is quite powerful. It's kind of like a CAD/graphics/animation system, but I believe it only works with triangles.

d. BlenderCAM is a free "addon" to Blender, which I've looked at briefly. It appears to have some basic 2D and 3D machining options.

e. FreeCAD is looking interesting. It looks like it is trying to be a Proe/Solidworks kind of thing. I was actually looking into it and it appears to have some powerful maths behind it. Possibly from Euclid3D.

Nice thing is that you can play with all these and learn them for free.

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PostPosted: July 20, 2020, 5:01 pm 
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Fusion 360 is free for homeuse/small start ups, it's CNC mode is pretty useful.


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PostPosted: July 22, 2020, 2:53 am 
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I did find one picture after the first layer of carbon. The gap in the butt was not intended. Just think, those words have never been put together before.


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seat_cf_first_layer.jpg
seat_cf_first_layer.jpg [ 278.54 KiB | Viewed 950 times ]

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