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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 7:53 pm 
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Hollow wrote:
Hi,
check your wax expiration date... I have faced the same problem in the past and lost one mold.
Best results can be achieved using wax systems. There is no need to wax the mold every cycle. 2-5 parts can be made in one application.
Follow the mold preparation procedure: http://www.finishkare.com/dynamic_image/1248797021.pdf

Casting mold for epoxy fins:
Take a thin sheet of aluminum cut a precise hole and press the fin model (sealed screw threads) into it. This will be your mold separation plane. Fin surface must be perfect. Follow the new mold preparation steps. Apply a mold gelcoat (epoxy based) 0,5 - 1 mm thick. Wait 5 hours at ~30 C. Apply glass fiber and epoxy layers.
Drill two holes to pour the epoxy in and four more for alignment pins in the corners. Separate two sides. Take out the fin model and aluminum sheet. Install aligment pins. Wax two pieces of female mold carefully then clamp them together again. The mold is done.
The third piece of mold could be done to form screw treads but its to complicated.
Try to cast one make screw treads and you will see why they are expensive.

Racing fins are usually CNC milled from a solid sheet of G-10/FR4 (Rista fins). Low quantities = high price.



Dude thank you!

My mold prep was obvious crap. (need sealer and a bit more cleaning)
Also need a gelcoat to improve surface finish of final mold.
2 problems solved (where do I send the beer?)

Curious why the aluminum plate as a separation plain vs building it with clay in 2 parts.
Like this without the RTV
http://www.youtube.com/watch?v=LSH19G_6Yeo?.


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 Post subject: Re: Building fins
PostPosted: Wed Apr 18, 2012 11:33 pm 
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Original poster mentioned aluminium was not option without CNC, I dissagree. For two years now my aluminium fins have been going fine. Sorry I have no pics, but all I did was copy outline of my desired fin onto some aluminium plate, drilled and tapped screw holes then foiled by hand with coarse rasping file, followed by finer file to get accurate foil shape of fin I copied.

First attempt failed as I drilled holes after foiling and went through side of fin. Much easier to drill when you are dealing with square shape. Less complicated than playing around with molds.


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 Post subject: Re: Building fins
PostPosted: Thu Apr 19, 2012 3:39 am 
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40cm fins that have some flex in them will be very hard to make in a mold. All the good stuff over the years has been g10 for windsurf fins and basically the same tech needed in this application. 3 or 4 big perfectly foiled fins is really a big hurdle for a DIY.


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 Post subject: Re: Building fins
PostPosted: Thu Apr 19, 2012 3:35 pm 
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Johnny,
do not use clay for your mold. If you don't like to use aluminum for separation plane you can do similar think as in the video but more precise. Take Aquapour powder mix with water and sink the half of the waxed fin into it. Let it cure. Apply gelcoat on top + some glass reinforcement layers. Wash away the Aquapour and clean carefully. Apply wax on fin and gelcoat side (spray like in video). Pour some gelcoat + some glass reinforcement layers on it.
The same result but you get a flat mold surface around the outline of the fin which is important because of the sharp edges of the fin.
Gelcoat must be a first coat and not the last one. I do not recomend to paint gelcoat on top of a mold surface and sand it to perfection.

Molding a racing fin is a very difficult task but you can controll stiffness and twist properties by laying various directions and types of the fibers in each section of the fin. Advaced hydrofoils are not cnc milled from G10 especially large ones. CNC milling is the cheapest and most practical technology to manufacture racing fins.


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 Post subject: Re: Building fins
PostPosted: Thu Apr 19, 2012 6:56 pm 
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Location: Jupiter, FL
http://www.ebay.com/itm/G10-FR4-Sheet-1-2-x-26-5-x-28-75-/300647406763?pt=LH_DefaultDomain_0&hash=item45fffb5cab#ht_500wt_1180

For a couple hundred bucks, you can get a .5" thick sheet of G10 to play with. The width of the opening on the mini tuttle boxes on my sector are around 11.9mm which is just under half an inch, so this would be the correct sized base material.


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 Post subject: Re: Building fins
PostPosted: Sun Feb 17, 2013 7:15 am 
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Hey guys digging this thread up cause I wanted to thank all for your advice and
share the knowledge I learned during the process. As well as show off the fact that
I ACTUALLY F'N DID IT!
Image
Image
Here's how I did it.

Foil selection:
Measured up a few windsurf fins, decided most were pretty close to a NACA 0010. Also the Naca Equations are easily available online, it's symmetric (ie works both ways) and is reasonably thin. Model airplane guys seem to like this one, and these guys run similar reynolds numbers...besides, it was my first try, this is about whether or not I can, Not trying ot build a supercavitating foil here.

Making a plug:
Got to know my router....how many turns of the adjustment screw = how many millimeters change in depth of cut.

Translated the equation of a Naca foil into incremental steps on the router and # of turns so I could accurately reproduce the foil. This is why my fins don't taper, cuz I built my plug on a router table..I'm undecided if there are significant advantages to a tapered fin. typically it's a structural thing on airplanes. I don't think the loading on a kiteboard of a solid glass fin is really a concern......If it is, I can make it out of carbon......So screw it, I made it straight because straight was easy to make in my garage.....in hindsight, using a hotwire cutter to a fixed point you can make a tapered foil very easily..... will try this later....I've got other things to build like a race board.
http://www.youtube.com/watch?v=jZkTCeoN79MI

I cut the basic outline out of 2 pieces of scrap 1/4" divinycell Then ran if over on a router table to get the foil . Piece (a) goes through left to right, piece b goes through right to left. Adjust router position, adjust cutting depth and repeat....when done I had 2 squared off mirrored shapes each resembling a fat stepped foil. gently sanded off the steps, hand foiled the tip and planed off any extra thickness....giving me two mirrored ½ foils....put them together and I have my desired fin shape.
I layed these up separately with a few layers of fibreglass to give them some structure so I could use them for plugs.. I used 2 layers of 6oz top and bottom.... layed them up on a flat mirror....

Found THE BEST mold release Loctite frekote FMS sealer and frekote release. . Not terribly expensive, and lasts a long time. (not the spray can) WAY faster than waxing, 1 wipe, and your cured epoxy will fall off under gravity and surface finish is PERFECT.. ...found PVA gives a pretty crappy surface finish While wax is WAY too labour intensive as the release if often times difficult.

Once glass was cured I shined em up
Here’s my plugs

Image
Image

A coat of mold release, and a few mold indexing bumps then covered 1/2 a fin with an epoxy tooling surface coat. (ie molding surface) and reinforced with 10 layers of scrap 6 oz glass I had from building boards. After that’s cured. I stuck the plug halves together and built the second half of the mold ontop of the first. Here’s a pic of the finished mold
Image
Here’s a link to the basic Idea on the how to but on something more complicated.
http://www.youtube.com/watch?v=UgKvDw1E60E

Once I had a mold, I made a few templates of decreasing size and cut out lots of glass (olfa rotary cutter is your friend) Wet layed enough glass in each mold half to fill it (ish) (like 30 layers of 6 oz). Keeping this much wet lay-up organized is nearly impossible so once cured, I pulled the part out and anded the flat side until it was flat with the mold surface using a belt sander. Did the same with the other side. Then put the parts back into the mold. And squeegee’d on a thin mixture of milled glass (for strength), resin(for bonding)and cabosil(to prevent sagging) So that the mold was perfectly filled. Put the two halves of the mold together to bond the pieces in the perfect shape and alignment and vac bagged the closed mold to squeeze out any extra resin.
Now I had a 50cm foil.......that's stiff and strong as all hell.

Needed a fin head, this was done according to these instructions.
http://www.kiteracing.com/Pot_a_Fin.html

For mine I used an epoxy and milled glass mixture. Tried resin and carbon hair but it wouldn’t pour and trapped a lot of air leaving a large void in my casting which had to be repaired. (the one fin with the black fin head

Used 4 degrees rake,... cuz 4 degrees produces the optimal lift drag ratio on a flat planning vessel. (Savitsky) So figured this would be a good angle to have my fins going straight to get the best Ligth wind performance.
Used 4 degrees of cant.....cuz I heard that’s what the fast guys run
Used zero toe, cuz toe would counterproductive. on a raceboard.
Went with 42cm side and 40cm rear fin to start I highly suspect my untapered fins will make the board extremely “overfinned” but for 25 bucks worth of materials I can make more and cut em down smaller

It’s a small part, so not a-lot of actual work but a hell of time spent waiting for epoxy to cure
2 layups for the plugs
2 layups for the mold
3 lay ups per fin.
3 epoxy pours for the fin head
So 16 days spent just waiting for stuff to cure.
I’ll be experimenting with how to do the fin mold it in one shot. Bladder molding and resin infusion both look promising to save a few steps.

Obviously if I had a CNC machine I woulda just cut em out of G10, but this method has a lot of technical advantages
1) I can play with hollow cores or foam cores to save weight.
2) I can play with ridiculous high aspect ratios and use carbon uni for strength I don’t believe they make a carbon equivalent of G10.
3) If there is some desired stiffness, I can use lay up regimen and materials rather than screw with geometry and compromise the foil .
4) This is cheaper, I use about 1 yard of glass per fin (8 bucks) and not a whole lot of epoxy
Certainly not the last set I’ll build.

Know there’s a bunch of board builders out there, thinking about a raceboard but balking at the fin cost, I’d say it’s TOTALLY doable just takes time. (an hour a night for 16 nights)


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 Post subject: Re: Building fins
PostPosted: Sun Mar 03, 2013 7:27 pm 
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Johnny Rotten wrote:
...
So 16 days spent just waiting for stuff to cure.
...


... but this method has a lot of technical advantages
1) I can play with hollow cores or foam cores to save weight.
2) I can play with ridiculous high aspect ratios and use carbon uni for strength I don’t believe they make a carbon equivalent of G10.

... )


NICE write-up there Johnny ; & GOOD Job !!

** That 16 days can be cut down considerably by making a second mold
with the male blank and the best 1/2 of the first female mold.

Most ALL the work is in the male blank , and the first mold 1/2 ; the second Full
mold goes FAST !!

** Cores -- you can get small sections of Nomex-Honeycomb for a reasonable
price ; it's one of the Best core materials available, just a Bitch to work with.
It's waterproof and has Gobs-Better compression strength than Foam !

** The COOL part about making a High AR mold is : -- Ya don't need to
use the entire mold for a lower AR fin ; just make a removable plug to go
in the mold that resembles your wanted tip. The Tip can also be changed
this way and placed into the mold anywhere Ya want. Tapered molds need
a costume tip plug for each desired length. I do this when building rotor-blades
for my RC heli.

Bille


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 Post subject: Re: Building fins
PostPosted: Mon Mar 04, 2013 4:45 am 
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Thanks Billie both for your kind words and your advice along the way.


Bille wrote:

** Cores -- you can get small sections of Nomex-Honeycomb for a reasonable
price ; it's one of the Best core materials available, just a Bitch to work with.
It's waterproof and has Gobs-Better compression strength than Foam !



Have used this stuff on some race car chassis, but always use it when there's a constant thickness core for the sandwich structure.....I don't imagine it machines terribly well, especially with garage tools. Any tips? Considered using it for a core for the raceboard where compression strength is the primary failure mode.

As for the fins bending stress is the primary load and will be carried by the skin. So a weak core that's easy to work with is just fine as it won't carry any load.

Bille wrote:

** The COOL part about making a High AR mold is : -- Ya don't need to
use the entire mold for a lower AR fin ; just make a removable plug to go
in the mold that resembles your wanted tip. The Tip can also be changed
this way and placed into the mold anywhere Ya want. Tapered molds need
a costume tip plug for each desired length. I do this when building rotor-blades
for my RC heli.



Since I'm casting the fin head separately from the foil, all I need to do is chop down the foil and I can make em whatever aspect ratio I need. Or as you said use only part of the mold. Casting the head also allows me to cant the fins to whatever angle I feel like without making separate molds and plugs.


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