Fillet Shaping

[Mad Rocketeer]

I just did the fillets on my new Graduator. The butt joints and fillets between fin tab and MMT and the MMT to CR joints and fillets are in J-B Weld. The internal fin tab to BT fillets and the CR to BT joints and fillets are 30 minute Bob Smith epoxy with some milled glass fiber added. The external fin to BT fillets are 30 minute Bob Smith epoxy with a little milled glass fiber and a good dose of microballoons mixed in. (I had originally intended to do that in layers but forgot.) I think I remembered to wet out the joint on two sets of external fillets with straight 30 minute epoxy and forgot on the third. Ditto on the internal fin tab to BT filets, except I think I remembered once and forgot twice. [Forgetfulness comes with old age? ]

I did the external fillets with a 1/4" radius of curvature, since that puts it near the low end of the 4%-8% of fin root chord (which, in this case, is 6") that Bill Stine recomended in his Handbook of Model Rocketry. I first measured it for 3/8", but that looked huge, so I went with 1/4", which still looks honkin' big (even proportionally) from my LPR perspective.

When smoothing the external fillets, I used 91% isopropal (rubbing) alcohol in a pump spray bottle to squirt the back of a plastic fast food spoon and the fillet surface, then I smoothed the fillet with that. It was kind of grabby though, not smoothing out as nicely as I'd like. Still, it smoothed out fine as it began to cure.

The fillets are flatter than I'd like though. As I understand it, a perfect fillet should make a circular path from the fin to the tangent line of the BT at the point where the fillet and BT meet. Mine make a flatter line from side to side, which also makes the epoxy thicker and heavier than I want.

My idea was to take a 1/4" dowel, wrap it in 330 grit sandpaper, lay that along the fillet, and sand until the sandpaper just touches the BT and the fin. Is that a good idea?

Then I could just clean the rocket with a tack cloth and begin to paint. The sanding will take away any amine blush that would otherwise have needed wiping away, but I'd probably wipe it down with the same 91% alcohol before painting, just to be safe.

Comments? Improvements? Experiences to share?

Thanks!


[EDIT] I should have said a 1/2" dowel, which would have a 1/4" radius.

 

[cls]

the sandpaper on dowel trick works great.

if you take too much off and it really bothers you, build up again with dabs of glue on that spot. in other words, you can't really screw it up, so go for it.

 

[DynaSoar]

Originally posted by Mad Rocketeer
The fillets are flatter than I'd like though. As I understand it, a perfect fillet should make a circular path from the fin to the tangent line of the BT at the point where the fillet and BT meet. Mine make a flatter line from side to side, which also makes the epoxy thicker and heavier than I want.

My idea was to take a 1/4" dowel, wrap it in 330 grit sandpaper, lay that along the fillet, and sand until the sandpaper just touches the BT and the fin. Is that a good idea?

Then I could just clean the rocket with a tack cloth and begin to paint. The sanding will take away any amine blush that would otherwise have needed wiping away, but I'd probably wipe it down with the same 91% alcohol before painting, just to be safe.

Comments? Improvements? Experiences to share?

Thanks!

I've found a rat tail file to be a lot easier to work with than a dowel and sandpaper. I recently got some larger ones for HPR work. I've used the same set of jewlers files for fillets and other stuff for 30 years.

 

[r1dermon]

put masking tape at the points where you want your filet to stop. then run a popsicle stick down the fillet while its still wet. purrrrrfect fillet. whipe the excess with denatured alcohol and your rocket is clean and purdy.

 

[Mad Rocketeer]

Originally posted by r1dermon
put masking tape at the points where you want your filet to stop. then run a popsicle stick down the fillet while its still wet. purrrrrfect fillet. wipe the excess with denatured alcohol and your rocket is clean and purdy.

That's what I did, except I used a plastic spoon, and all the excess came up as I peeled away the masking tape. Maybe I needed still more balloons to make it stiffer/thicker so it would hold a curved shape better?

I actually used a small rat tail file to file away a small drop that landed where it shouldn't have, despite the masking tape. It filed amazingly easily.

I'm thinking I should apply masking tape again before I do this filing job, just to protect the body tube and the fins at the very edge of the fillet when I get the file down to the point where it makes contact.

I like the rat tail idea in that there's no need to hold the sandpaper on the stick and a file will be heavier, stiffer, and easier to use than a 1/4" dowel will be. I'd probably still need to finish with sandpaper to get as smooth as I'd like, wouldn't I? Maybe not. Hmmm. I need to go out tomorrow and look for a 1/4" diameter rat tail. Better to get one with a handle or without? I think some come with a removable / reusable handle.

 

[DynaSoar]

Originally posted by Mad Rocketeer
That's what I did, except I used a plastic spoon, and all the excess came up as I peeled away the masking tape. Maybe I needed still more balloons to make it stiffer/thicker so it would hold a curved shape better?

I actually used a small rat tail file to file away a small drop that landed where it shouldn't have, despite the masking tape. It filed amazingly easily.

I'm thinking I should apply masking tape again before I do this filing job, just to protect the body tube and the fins at the very edge of the fillet when I get the file down to the point where it makes contact.

I like the rat tail idea in that there's no need to hold the sandpaper on the stick and a file will be heavier, stiffer, and easier to use than a 1/4" dowel will be. I'd probably still need to finish with sandpaper to get as smooth as I'd like, wouldn't I? Maybe not. Hmmm. I need to go out tomorrow and look for a 1/4" diameter rat tail. Better to get one with a handle or without? I think some come with a removable / reusable handle.

They usually come without. You can buy one handle and use it on any number of files.

I've tried finishing using a drywall corner sanding sponge. It worked OK but it was very slow. I found it much easier to file down more than "perfect" and add another thin layer of fillet. The epoxy putty from Apogee (I'm pretty sure it's JB Weld based) is excellent for fillet cosmetics.

 

[OKTurbo]

"The epoxy putty from Apogee (I'm pretty sure it's JB Weld based) is excellent for fillet cosmetics."

No JB weld...it has metal in it to make it heat resistant and very hard.

Here's the stuff that Apogee sells.
https://www.avesstudio.com/Products/Fixit/fixit.html

 

[teflonrocketry1]

Here is where I first purchased and bookmarked information about Fixit:

https://www.avesstudio.com/FIXIT_Flyer__red__2003.pdf

I also bought some Apoxie Sculpt from Aves Studio:
https://www.avesstudio.com/Products/Apoxie_Sculpt/apoxie_sculpt.html

I hope to use it for molding nose cones an other small parts.

Bruce S. Levison, NAR #69055

 

[Mad Rocketeer]

I should have said a 1/2" dowel. It's the radius that's 1/4". I picked one up this evening.

Yep. A swappable handle that can be moved from file to file was what I was thinking of.

I looked for big rat tails at Lowes this evening, but I didn't see one over 3/8" diameter, and they were all pretty coarse. If I see a selection of fine ones that go to 1/2", I may grab 'em.

I can see the similarity of J-B Weld and the FixIt clay. Sounds like they both have high heat resistance and cure very hard and strong. I'll bet they're both heavy too, but to put a really smooth and tough skin on a fillet sounds good to me. Though the layer of clay over the top would be wide, it could be applied thinly. It would still add up to a lot more than I put on my Graduator's MMT though.

Sounds like FixIt can be handled without worrying about gloves? How about J-B Weld? Both claim non toxicity.

Would a thin layer of straight epoxy, painted over the epoxy-balloon mix (two fillets at a time), before it fully cures but after it's stiff, work the same? I.e. would it cling and level well enough to provide a mirror smooth surface? There would be the added benefit of having a full chemical bond between the layers, rather than just a physical one. I know you couldn't do all the fillets at once like you could with the Fixit though, and the epoxy might just run and puddle downhill anyway.

For this rocket, I plan to just sand the fillet to the right curve, wipe it down with alcohol, clean it carefully with a tack cloth, and begin priming.

 

[daveyfire]

When filleting, I use Aeropoxy with filler as the material and the plastic spoon trick. One additional thing I do to get a perfect mirror fillet is to come back after the epoxy has come fairly close to the "leather" stage and use a gloved finger dipped in alcohol to smooth it out. Takes off the high spots and smooths it out, and you avoid filler/sanding later.

HTH!

 

[Mad Rocketeer]

Originally posted by daveyfire
When filleting, I use Aeropoxy with filler as the material and the plastic spoon trick. One additional thing I do to get a perfect mirror fillet is to come back after the epoxy has come fairly close to the "leather" stage and use a gloved finger dipped in alcohol to smooth it out. Takes off the high spots and smooths it out, and you avoid filler/sanding later.

HTH!

Will the epoxy still move enough to correct the curve at that point? When I did mine, the curve wasn't circular like I'd wanted. It was flatter side-to-side. It would be great to be able to come back, once it had stiffened enough to not sag any, and mold it back to a perfect curve while it is still shapable but too stiff to lose the curve and too cohesive and unsticky to pull the surface apart. I have a few low spots in the surface, but it's too high to begin with, so when I file or sand it down I should be OK. If not, I'll adjust, as suggested above.

I've seen denatured alcohol mentioned as something to put on the gloved finger, and I've seen rubbing alcohol mentioned for cleanup. The rubbing (isopropal) alcohol I get at the grocery store is 91% alcohol (wiith the remaining 9% presumably being water). I can get denatured alcohol (which ought to be 100%) at Lowes. That will be ethanol with just enough methanol mixed in to make it unusable in any beverage. Is one better than the other at these concentrations? Will one be "slicker" on the epoxy surface? My plastic spoon grabbed somewhat with the isoproponol.

Are vinyl gloves as good as latex gloves for this stuff? My "fits all" latex gloves are so tight I often just take my chances with the epoxy in order to avoid taking 3 or 4 minutes per hand to get them on and the same to get them back off.

 

[wwattles]

I recall seeing a report from someone who used a piece of PVC piping to get the epoxy/balloon mixture to the right shape while it was still wet. Perhaps coating your dowel with a vinyl tape or something to prevent it from sticking to the fillet would work as well. The pics I saw of the results were incredible - extremely smooth fillets that were perfectly round, and even blended into the BT on the ends of the fin (guy dragged the pipe around the end of the fin).

WW

 

[Mad Rocketeer]

What a great idea! You could probaby wrap the pipe in teflon plumber's tape to make it extra slick, but even if the PVC bonded, at first, a sharp rap would probably break it loose. Epoxy doesn't grab PVC all that well. The rap might crack a ballooned fillet in the process though.

 

[Stymye]

probably thinking of the skypirate site

https://www.skypirate.net/rocketry/con_fil_01.htm

 

[Mad Rocketeer]

Beautiful fillets!

I added more microbaloons to the mix when doing the launch lug fillets. They're narrower, so that might make them easier, but the extra stiffness of the mix let me do both fillets at the same time, and the fillets retained their curve, just as I'd formed them, instead of flattening out some, as the fin fillets did.

 

[Mad Rocketeer]

My fin fillets are proving to be pretty difficult and slow to sand with 220 grit sandpaper attached to a 1/2" dowel with Super77. I can file small spills relatively easily with a needle file, and it sands pretty well with an emery board. I'm thinking I'm using too fine a grit maybe. I'll try something in the 80-150 grit range tomorrow if I get the chance.

Meanwhile, I bought a 3/8" diameter rat tail file (coarse, single cut). I'll see if that will get me in the ball park faster. I haven't found a 1/2" one yet in stores.

By the way, be careful in buying / ordering these. I looked on the Grainger web site and found a file like the one I bought today, except it stated that it was 1/2". It wasn't. I went by one of their stores and had the guy get one for me. Putting a tape measure to it, it was clearly 3/8" in diameter. Catalog and computer called it 1/2" though, and the catalog numbers matched. Misleading. Maybe the wrong file got put into the package though, so I can't point any definite fingers.

Has anyone done research on different grits and their effects on sanding epoxy with various thickeners and other additives?

Next time, I think I'll keep adding the microballoons until I get all the way to the "peanut butter" stage.

 

[Stymye]

are you forming a radius when you apply the epoxy? needing a coarse rat-tail file seems extreme.. normally it shouldn't take more than a little sanding to blend the edges and surface nicely.

 

[Mad Rocketeer]

Yep. I masked off the right width for a 1/4" radius, mixed Bob Smith 30 minute epoxy with milled glass fiber and microballoons until it seemed thick enough to my totally inexperienced eyes, dipped it into the wetted out (most of the time) fillet area with my stiring stick, poking it down into the corner, and with a spray bottle of 91% isopropal (rubbing) alcohol in one hand and a plastic fast food spoon in the other hand, did the best I could to shape a nice curve. It looked pretty good and pretty close, so I pulled up the masking tape almost immediately (to reduce the tape dam) and left it alone for 2-8 hours before rotating the tube for the next pair of fillets. The fillets are too thick though and are too flat. I.e., instead of a nice circular curve from fin to tube, I have something that's thicker than that and flatter at the bottom of the "U", if you get what I mean.

I want the extra epoxy out of there both for the look and to reduce the weight. It'll help the aerodynamics some too.

220 grit sandpaper worked, but way too slowly. I worked with 150 grit tonight, and that's better. The file (coarse, single cut) works better still, but of course I have to be careful since it's too narrow. I use the file to grind away some epoxy, then work with the sandpaper to get more of a proper curve to the fillet, then take one of the new style 3M sanding sponges (gray, thin), overlap the fillet evenly on both sides, and sand in the curve with that to smooth it further and to reduce the dam-like transitions from the edge of the fillet to fin and tube.

I'm getting there, but it's slow go. Fortunately, I'm very patient with this one, taking it as a slow, fun, learning project, and trying new techniques as I go along. I don't have any reloadable casings (nor have I even seen one "live and in person"), and I don't have any single use motors large enough to fly this puppy on anyway, so why rush?

Once I get the fillets ground down and the rocket tack clothed really well, I'll weigh it to see what motors I can reasonably use with it. I'll then make up some zip-loc bags with rocks or fishing weights in them to poke into my 29mm-24mm adapter tube and insert as dummy "motors" (matching the weights of the corresponding motors) so I can determine the center of gravity for each and see whether I'm going to need any nose weight for stability. If I do, I'll have to factor that in, tape some weights to the nose to simulate the nose weight, and do it all over again until I have what I need. Once construction is "final", I'll do a final check and tweak the nose weight one more time as needed.

As for now, I think I want some eye drops and a shower. That epoxy stuff is itchy when you sand it. Cough. Cough. Hmmm. The coughing I added, but I really have been sanding it unprotected, which I understand to be a significant no-no.

Next time, I'll mix the epoxy all the way to the peanut butter stage, rather than the mayonnaise level, and I may use waxed PVC pipe as described on the sky pirate link you sent. I'll also mask it with blue masking tape to within 1/8" of the fillet edge, instead of all the way to the edge, and then put Scotch transparent tape (de-stickied somewhat by sticking it to my fingers a few times first) over that and up to the real fillet edge. That should reduce that step at the fillet edge and make sanding that much easier next time. I'll be careful peeling it up and not wait too long.