Fillet

[jared185]

Hi all
So my sanding and fitment of my fins on my mega Der red max is done and looking really good. I surprised myself a Lil lol. Anyway I'm gluing them in today and and the next step is making a nice fillet were the fins will meet now usually I run into a bit of trouble here making them completely smooth and consistent anyone got any tips on an easy fillet job for someone who is not so great at it. I love this rocket and want it to look perfect

 

[smugglervt]

What is your adhesive of choice?

 

[jared185]

Using epoxy

 

[tim cubbedge]

What kind of epoxy? Rocketpoxy? Proline 4500? Bob Smith? West System? US Composites? Aeropoxy?
They all have diferent techniques for fillets!

 

[jared185]

Oh I was using loctite epoxy general purpose

 

[Banzai88]

There are lots of fillet tutorials, here and on youtube. Some good 'hands on' techniques there.

Once you go RocketPoxy, you'll never sand fillets with anything less than 600 grit ever again (you won't need to reshape them). Once you go 4500, you'll never sand fillets with anything ever again.

For everything else, it's all about timing and lay down. Mix the same amount of A and B component for the same length of time, lay down the fillets for the same amount of time, pull at the same time for each set, and pull tape at the same time for each set.

The 'time' will be different for each operation and each type of epoxy.

For example, I mix 4500 for 5 minutes, let sit for 5 minutes(10 total minutes elapsed), lay a set of fillets for 10 minutes(20 total minutes elapsed), pull fillets(at the 20 minutes mark), let sit for 10 minutes(30 minutes elapsed), and pull tape promptly at the 30 minutes mark. I then level the rocket across the fins and let it set up. EVERY set of fillets. Shapes perfectly each and every time.

Fillet RADIUS should be 4-8% of fin root chord. So for an 8 inch long fin, you would want a radius of .32-.64 inch, which would be a diameter of .64-1.25 inch. Remember that your fillet shaping tube is measured in measured across itself and gives you a diameter, 1/2 of that is proper radius.

Pull fillets with a piece of tube (copper or PVC) soaked in denatured alcohol. Pull once, wipe clean, pull a second time and LEAVE IT ALONE. You have to figure out when your epoxy is between 'melting butter' and 'peanut butter' thick.

Some folks add colloidal silica or fiberglass or carbon dust to thicken/strengthen basic epoxies. I find that to be inconsistent and hard to get the 'right formula'. YMMV

Don't forget to sand the tube some at the base of the fin. Glassine will peel off easily on a hard landing, so sand it off.

 

[mpitfield]

Here is a link to some sanding grips that I have found to be invaluable for working with fillets https://www.leevalley.com/en/wood/page.aspx?p=20183&cat=1,42500

The convex set are the ones I use most, although I also use the 30degree wedge quite a bit for the areas around the leading and trailing edges of the fillet for blending. You can also use dowels with a sandpaper wrap, however the downside of using dowels for me was that I had a lot more hand cramping or fatigue and sanding fillets can be one of the most laborious tasks when it comes to building your rockets...YMMV.

As for the choice of grit, it really depends on a a few factors as others have said the epoxy plays a big role in that as some epoxies are harder to sand than others. Your choice also depends on the materials your fins ad rockets are made with as sanding fillets can easily create concave grooves (the negative of your sanding grip) in your rocket body that run parallel with the fillet and extend beyond your fin, as well as angled where you blend the fillet into the leading and trailing edges of the fin.

Regular Paper tubes can get out of control quickly if you're not paying attention, also they will fuzz up once you sand the gloss off so you just have to pay attention to what you are doing and be patient. FW/FG is much easier to work with and CF even easier, although it can also be a bit tricky if you sand too deep into the fibers.

One way I cope with sanding fillets is by selecting one fin and one side to do per evening, it will take a bit longer, but if you have the time, it can make the process a bit more enjoyable.

 

[Bat-mite]

I don't think you'll like using 5-minute epoxy. It starts to set before you can get the bubbles out, and before you can get it smooth.

With RocketPoxy, you can let it sit for 30 minutes before applying to get a nice, smooth, creamy surface. Use a piece of PVC pipe to round it.

If you want to forgo RocketPoxy, use 30-minute epoxy. You might still get some bubbles, but not as many.

 

[smugglervt]

+1 what Bat-mite said.

 

[bclark989]

Here is a picture tutorial I made. I am using Loctite Marine epoxy off the shelf from the store. Its right next to the 5 minute stuff =)

As others have said RocketPoxy and Proline 4500 are the end game in the world of epoxy for rockets, but this marine stuff has been working great for me. It has a 2 hour working time, high enough viscosity that you don't get runs, a glassy finish similar to Proline, and its cheap and available in small quantities.

If you want this rocket to look perfect, I would suggest practicing on something you care less about first.

Good luck =)

 

[ksaves2]

Duralco 4525IP is the Rolls-Royce but not necessary. Could be helpful for some really extreme stuff. Kurt