A better way for fin fillets?

[Not Quite Nominal]

Tried the IPA drip method a few days ago with BSI 30. When it works, it works great.
I think it needs moderately soft epoxy to work. Put it on a few minutes too late and the top layer becomes soft while the rest is already hardening, creating a smeary top layer. Put it on at the right time and swipe and it self-levels beautifully.

 

[DeltaVee]

My first HPR I used epoxy and created not so pretty external fillets.
After it was well cured I used wood filler and with a caulking tool made nice rounded fillets. I then wrapped sandpaper around a motor and sanded it until it was smooth. I was afraid it would crack but after a few launches including one minor crash they still look great.

Am I wrong for doing this?

Looks pretty good to me... a lot of times with white glue on low power, bubbles in the fillet will spoil them. I've used elmers' carpenter putty on those for a long time.

 

[neil_w]

Looks pretty good to me... a lot of times with white glue on low power, bubbles in the fillet will spoil them. I've used elmers' carpenter putty on those for a long time.

That's where TB Quick and Thick comes in. No bubbles, except in unusual circumstances. I normally put a thin fillet of regular TBII (choose your favoriate) and then add layers of Q&T to build it up.

 

[bjphoenix]

Yesterday I tried my ideas for epoxy putty fillets. I've watched the Apogee videos on youtube and I already had some JB Weld wood repair epoxy putty from a home repair so I decided to try it. My fins are TTW and I had already put some small liquid epoxy fillets on there so I didn't need as much putty. So I mixed a small batch, rolled it into a snake and pressed it into the fin root. I picked a 7/16" deep socket from the toolbox as being the ideal diameter for what I wanted. I tried putting a piece of waxed paper down and pressing the socket against it, but this didn't work very well because I couldn't push hard enough and because the waxed paper sticks to the soft putty very well. The next step was to tilt the socket slightly and push the trailing edge into the putty, then drag it down the length of the root to "tool" the putty. This worked pretty well and produced a very uniform curved surface, but not perfectly smooth because the putty tears just a little bit. After cleaning up a rough spot or two and scraping off the little crumbs from the edges, I dipped a finger in rubbing alcohol and rubbed the top surface of the fillet. These are the best shaped and most uniform fillets I've ever made! They will need a little bit of sanding, then I'm going to put a thin layer of wood filler on top and sand again. It does take a little time to make fillets on both sides of 4 fins, but as soon as you do one fillet you can go to the next because the epoxy putty isn't going to run, flow or droop as you handle the rocket.

If anybody's interested the socket that I used is probably 5/8" outside diameter or a little more, and I'm working on a rocket with 2.6" diameter phenolic tube and fin roots that are nearly 6" long.

 

[Hooked On Rockets]

One thing I have done is to clip the tip of the leading edge where it meets the body tube. This give the adhesive more surface area.

Minimizing stress risers is also another advantage of fillets, the effects of which can be dramatic.

View attachment 376675

I have actually considered this, chopping off the leading edges of the epoxy and then building the leading edge "point" after the fillets set up.
I was concerned about fillet to leading edge integrity...but YES that seems like a good approach to a good fillet point finish....
I think I figured out what bugs me so much about the finish work on rockets.....
See, I used to make a living in the body work and painting in the automotive field...like 30 years ago!!!
But I figured out that when the filler (epoxy filler for fillets) is harder than the "body" (paper body tube and wooden fins)..things are just FLIPPED for me.....

I keep telling myself that....

1. the finish HAS to be perfect....but...

I keep having to REMIND myself that...

2. "if a recovery failure occurs, that...

3. No mater how perfect it looked.....

it fell to the Earth and is now a pile of wood and paper and plastic........

Right now I always lean towards
# 1.....
It's the fun and pride of the build....
(because I don't have a place anywhere near by to fly....)
Working on that....!!!

 

[bjphoenix]

I inspected my epoxy fillets a little closer this past weekend. They have a nice shape and are very uniform end to end. The surface is a little rough, apparently the epoxy doesn't tear smoothly when you tool it. The edged can be a bit rough for the same reason, the stuff doesn't like to feather to a very thin edge. The imperfections are relatively small, maybe 0.010" at most. I think they would be easy to smooth out with FNF, but then that's more sanding. That will be my next step.

I was only trying the JB Weld Wood Filler putty, other putties might be easier to tool.

 

[Chad]

I thought about making fillets from triangle stock.

Hit both ends with a miter saw to make the points, then glue them to the fins. Now you have a fin with fillets on both sides so next you would hit the bottom of the fin and fillets with a drum sander equal to the diameter of the body tube. Now the fillets are rounded to the same radius as your body tube. Finally, glue the fin with attached fillets to the tube using all that nice surface area. Sand and done.

 

[lakeroadster]

I thought about making fillets from triangle stock.

Hit both ends with a miter saw to make the points, then glue them to the fins. Now you have a fin with fillets on both sides so next you would hit the bottom of the fin and fillets with a drum sander equal to the diameter of the body tube. Now the fillets are rounded to the same radius as your body tube. Finally, glue the fin with attached fillets to the tube using all that nice surface area. Sand and done.

I guess it depends on what the builder is looking for as an end result. The triangle stock fillets would be a very strong attachment at the body tube and would be visually uniform, but the fin strength is the same, it would just snap off at the top of the fillets.

 

[Chad]

I guess it depends on what the builder is looking for as an end result. The triangle stock fillets would be a very strong attachment at the body tube and would be visually uniform, but the fin strength is the same, it would just snap off at the top of the fillets.

would that not also be the case with an epoxy based fillet? In order to increase the strength of the fin after it's on the body tube you'd go the route of fiberglassing correct?

I like the triangle stock idea because it takes the "art" out of making fillets and produces the same fillet every time with the same set of steps every time.

 

[DaveW6DPS]

would that not also be the case with an epoxy based fillet?

Yes. Fillets are not to strengthen the fin, they are to strengthen the fin attachment.

 

[neil_w]

I thought about making fillets from triangle stock.

Hit both ends with a miter saw to make the points, then glue them to the fins. Now you have a fin with fillets on both sides so next you would hit the bottom of the fin and fillets with a drum sander equal to the diameter of the body tube. Now the fillets are rounded to the same radius as your body tube. Finally, glue the fin with attached fillets to the tube using all that nice surface area. Sand and done.

This would work fine and look fine, although you'll get angular fillets which would look different from typical rounded glue fillets. In my experience that "sanding on a drum" step is quite challenging unless you have some sort of clever jig to do it on.

I tried this technique on my IRIS-T and at the relatively small LPR scale I was working at, I found it to be way too much work to try to do that sanding. At larger scales where you could work with real tools it might be easier to pull off. If you do try it and have success, please show your sanding jig.

 

[lakeroadster]

would that not also be the case with an epoxy based fillet?

Technically, from an engineering stress analysis standpoint, not at all. Those nice radiused fillets are much better than angular sharp transitions.

And a little flex is a good thing.

And the radiused fillets probably "flow better" also.

That being said, for 95% of model rocket applications, it's 6 of one, and a half dozen of the other.

 

[Not Quite Nominal]

I thought about making fillets from triangle stock.

Hit both ends with a miter saw to make the points, then glue them to the fins. Now you have a fin with fillets on both sides so next you would hit the bottom of the fin and fillets with a drum sander equal to the diameter of the body tube. Now the fillets are rounded to the same radius as your body tube. Finally, glue the fin with attached fillets to the tube using all that nice surface area. Sand and done.

I had the same thought, only instead of using a drum sander, tape/glue a piece of sandpaper to a spare/sacrificial piece of BT and run the fin root up and down it.

 

[neil_w]

I had the same thought, only instead of using a drum sander, tape/glue a piece of sandpaper to a spare/sacrificial piece of BT and run the fin root up and down it.

If you come up with a reliable way to do that, please share. I couldn’t. It is more challenging than it sounds.

Two hints if you want to try it:
1) use something extremely rigid to wrap the sandpaper around. Any flexing will sink you.

2) Start with something smaller diameter than the body tube you’ll be mounting the fin to.

 

[Chad]

If you come up with a reliable way to do that, please share. I couldn’t. It is more challenging than it sounds.

Two hints if you want to try it:
1) use something extremely rigid to wrap the sandpaper around. Any flexing will sink you.

2) Start with something smaller diameter than the body tube you’ll be mounting the fin to.

i sat down and drew some sketches and the more i did the more i realized how hard sanding would be. If the triangle stock is glued on to the fin beforehand then the sanding has to be perfect, otherwise you'll never get the fin on straight. Also, the triangle stock glued on beforehand means no TTW fins.

I was thinking, maybe don't worry about sanding the bottom of the triangle stock and just leave it flat. Attach the fins as usual and then put on the triangle stock, just use a liberal amount of epoxy on the BT side, let it fill the gap, and wipe off the excess. heh i guess at this point you're sort of making a fillet for the fillet but seems like it would still be worth it. I may try this over the weekend, if it works i'll do a thread.

 

[neil_w]

I was thinking, maybe don't worry about sanding the bottom of the triangle stock and just leave it flat. Attach the fins as usual and then put on the triangle stock, just use a liberal amount of epoxy on the BT side, let it fill the gap, and wipe off the excess. heh i guess at this point you're sort of making a fillet for the fillet but seems like it would still be worth it. I may try this over the weekend, if it works i'll do a thread.

Another approach would be to sand the triangle stock to an obtuse angle such that the tip would rest on the body tube, but the part against the fin would be above. Then (somehow) fill in that void with epoxy when attaching it. Slightly harder to do, but would ensure that the visible parts would be all wood, and consistent.


DISCLAIMER: I have no idea how well this would work.

 

[Chad]

Another approach would be to sand the triangle stock to an obtuse angle such that the tip would rest on the body tube, but the part against the fin would be above. Then (somehow) fill in that void with epoxy when attaching it. Slightly harder to do, but would ensure that the visible parts would be all wood, and consistent.
View attachment 378745

DISCLAIMER: I have no idea how well this would work.

yeah i like that even better. if you have a belt sander you could lay it flat and then rotate slightly, taking off the corner with a straight line to the edge.

 

[Steve Shannon]

It’s probably 50 times easier to make the fillet the right profile in the first place than to try and reshape it after the epoxy has cured. It’s not hard.

 

[Chad]

It’s probably 50 times easier to make the fillet the right profile in the first place than to try and reshape it after the epoxy has cured. It’s not hard.

Yeah, I guess to keep feet on the ground, making fillets (or anything in this hobby) just takes practice and patience. I like to think and try different things in a hobby setting because if it doesn't work there's no real consequence. In my work there's money and people's jobs at stake if a decision is wrong. Here it's just a bruise on the old ego and then you get to move on to your next crazy idea.

 

[Steve Shannon]

Yeah, I guess to keep feet on the ground, making fillets (or anything in this hobby) just takes practice and patience. I like to think and try different things in a hobby setting because if it doesn't work there's no real consequence. In my work there's money and people's jobs at stake if a decision is wrong. Here it's just a bruise on the old ego and then you get to move on to your next crazy idea.

It’s important to explore alternate methods. I think almost all of us have tried to clean up fillets by sanding at some point in our early years. I’ve even used hand carving chisels.
When sanding fillets you frequently end up with dished areas in the fin surface as you try to blend the edges of the fillets. Smoothing partially cured fillets is much easier and results in nicer looking fillets.

 

[Banzai88]

It’s probably 50 times easier to make the fillet the right profile in the first place than to try and reshape it after the epoxy has cured. It’s not hard.

That's why I haven't left 4500 since figuring out how to use it. Others like RP or some blend of fillers and Aeropoxy or West.

When sanding fillets you frequently end up with dished areas in the fin surface as you try to blend the edges of the fillets. Smoothing partially cured fillets is much easier and results in nicer looking fillets.

To prevent the dished area on the fin you need to use a sanding mandrel where the mandrel PLUS the thickness of the sandpaper equals the finished diameter that you want.

 

[jnobels]

I may try 3d printed gussets (if they’re not made of glue, are they still fillets?) on my next larger diameter build. Spoils all the fun of playing with epoxy ... presuming one considers that fun....

Semi-hollow (30% fill) cutsom PETG gussets would stiffen/adhere the fins with only minimal adhesive, less weight and no mess - with any cross section you’d like. You could even use a different adhesive in lieu of epoxy and/or (integrated?) mechanical fasteners to tie the inner and outer or opposite fillets together. An openscad template that is customizable for for the root length, sweep angles, tube diameter and fin thickness should be doable. Feathering these so they look seamless when finished could prove to be tricky...

This may be complete blasphemy in this thread, but it might be a “better way” - though certainly not faster.

 

[Rocket man]

Late response.....Look at surfboard fins, smooth entry and exit.....They use unraveled
fiberglass rope (wetted out squeezed out with rubber gloves on, perfect fillet , cover with 6 oz. cloth
cut to workable size.... "rocket on"

 

[Hobie1dog]

Late response.....Look at surfboard fins, smooth entry and exit.....They use unraveled
fiberglass rope (wetted out squeezed out with rubber gloves on, perfect fillet , cover with 6 oz. cloth
cut to workable size.... "rocket on"

to the forum

 

[David Schwantz]

I have the opposite problem. I just got an OOP SR-71 kit and need to many tiny fillets. Think my fingers are too big

 

[Banzai88]

I have the opposite problem. I just got an OOP SR-71 kit and need to many tiny fillets. Think my fingers are too big

https://www.amazon.com/Ailaxq-Decor...-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

 

[David Schwantz]

Thanks Banzai88, I have seen those in the past, should probably order them, but I just scrunched my little finger in there

 

[David Schwantz]

PS, I did just go and order them.

 

[SkyFire]

Fondant tools work great! I rub graphite from a pencil on a piece of paper then rub the fondant tool on the paper over the fillet area. This leaves lines where to apply tape. It helps to dip fondant tool in alcohol before pulling over epoxy.

 

[MarsLander]

alcohol

Denatured? Isopropyl? %? Makers Mark?