While building the Stellar Hercules clone I mentioned earlier, I ran across two standard techniques that just don't work for me.

The first is the block-sanding of the fins. When I put a set of fins together to sand the edges, no matter how careful I am, I never get them matched exactly, and I often sand a subtle curve into the fin edges. This frustrates me no end, especially given that I try hard to make every single rocket show-quality.

But I've discovered, with laser-cut fins, that I don't need to do that. The point of block-sanding fins is to make them identical. This is useful with hand-cut fins, which are always a bit irregular, and I grit my teeth and struggle through it when I have such fins to work with. Laser-cut fins are already identical.

So I just sand off any bumps on the root edges, and I call that good. Later, I sand down the "flash" as part of the regular finishing work. My end results are acceptable that way, and for some reason I never get those subtle, unwanted curves.

The second thing that doesn't work for me is gluing multi-part fins together before attaching them. This is mainly a problem when each part has a root edge which is meant to attach to the sustainer; when I glue the fins together first, the compound root edge always wants to pull away from the sustainer at one end or the other. My Ultimatum is such a rocket, and I swore like a sailor putting those fins on. In the end, I had to work around a small gap at the upper end of one fin.

So on the Stellar Hercules, I attached the larger, lower part of the fins to the rocket first; after they were stuck on good, I glued the upper part in place. This was so easy to do that I wondered why anyone ever did it any other way.

But the answer is actually the same... hand-cut multi-part fins are liable to be irregular. Gluing them up first with a ruler to verify alignment gives at least some chance that they will go on the rocket okay. For my Ultimatum, I should have sanded the compound root edges, but the problem was, the fins were too large for any sanding block I had. I should have taped a piece of sandpaper to the workbench, I guess.

For laser-cut fins, such as the nice set I got from Semroc for the Hercules, this isn't an issue. They are already perfectly shaped. Yet, with other similar rockets I've built previously, I've blindly done it the standard way, and had to deal with those not-quite-straight root edges.

Gah, that was long. Anyway, I'm just curious if anyone else finds the "standard" methods don't always work for them.

__________________
NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org

I think most of us try to sand stacked fins "in the air". By that I mean we hold the fin stack in one hand and the sanding block in the other. This really doesn't work too well. If you notice in the instructions, the sanding block is a flat surface sitting on bench. That eliminates half of the random motion that gives you curved fin edges.

Today I bought a 9"x12" slab of granite that's allegedly flat to 0.001 inch. I will mostly use it with a sheet of sanding film to put "Scary Sharp" edges on my woodworking tools. The principal is the same.

Put your abrasive surface on the bench. Hold the fin stack in your hands and sand by moving them across the stationary abrasive. Much better results I'll bet.

As to the other practice of gluing the fin parts together before attaching them to the fuselage, well that's the way I've always done it. Just seems easier to get straight fins that way. YMMV.

Bottom line, do what works best for you.

__________________
Kit (aka Cranky Kong)

I'm just a roadie for the banned...

Well don't feel alone. I use to have the same problems block sanding and matching the fins, whether I hand cut them or if they were laser cut. I was doing the same thing with laser cut fins, just sanding the bumps and then coming back later during the final prep before primer and really detailing and cleaning up the edges. Now I just go ahead and sand them all the same way.

What solved my problem was I purchased a good block sander.

http://www.hobbylinc.com/htm/gpm/gpmgpmr6169.htm

I never had any problem after that, but you have to make sure that the block is straight and sand slowly and intentionally and apply equal pressure, almost like controlling the motion and speed. Not just hacking at it back and forth but. being aware of how you are holding the fins and the sanding block and taking your time. Even doing so you still might sand a curve in it. I also find that using 180-220 grit makes quick work of it and get them straight level. You have to make a lot less passes back and forth, which means less chance of screwing up. I'm like you and I'm sure a lot of modelers out there want to build the best possible rocket or kit. It's a passion we all share and love to do. For me it never fails, every kit or rocket I designed and built I always managed to goober it in some way, not real bad, but enough to irk you. Just take your time and enjoy it and you will build much better rockets and kits. There's no rush, at least I tell myself that. At times balsa can be difficult to work with. I use to hate filling and sanding. I tried skinning fins and I also tried using CA to fill and sand, but that was messy and just a waste of the CA. If I had a choice between the two I would skin the fins...but the only time I think you need to do that is if you are sanding an airfoil in your fins. I went back to Fill & Finish, why? For one it doesn't smell and you can get messy and it cleans up nice and not toxic. Second I learn to perfect it more or less. Mix until it's like peanut butter, apply with a brush or finger...and use a putty knife to scrape excess off for a level surface. After it dries lightly sand with 400 and you are done. If there is any grain showing who cares. I figure I will come back fill it where it needs it after I primer it. It usually takes 2-3 coats of primer and filling to get close to perfect. Once I get it to where I like it ...I lay down the first topcoat...

Dan

In his EMRR review of the Semroc Aerobee-Hi kit (the second of the two here: http://www.rocketreviews.com/review...trobee_hi.shtml ), Chan Stevens wrote that the instructions said to glue on the three fins first, then the three strakes (they were conduits on the full-scale vehicle). He found it easier to glue the strakes to the fins *before* gluing them to the body tube. Being laser-cut, the fins and strakes fit together perfectly.

__________________
Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see:
http://www.lulu.com/content/paperba...an-form/8075185
http://www.lulu.com/product/cd/what...of-2%29/6122050
http://www.lulu.com/product/cd/what...of-2%29/6126511
All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com.
NAR #54895 SR

Hah. You give me inadequate credit in the thinking arena. I know how that works; however, even drawing the fins in just one direction against a stationary sanding block, I get curves. And I just can't get those fins perfectly aligned with each other. It's not my thinking, it's my manual skills that give me trouble.


(Boldface is mine.)

Doing it my way, I never get a curve. It's just not worth it for high-quality laser-cut fins.

__________________
NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org

He's better than me. No question about that.

One thing I noticed doing the Hercules fins is that the alignment was better than other similar rockets I've built. This is because the lower parts were made straight with the lines on the body tube, then the extensions were added, also straight. If the composite fin has any warp at all, it can be hard to see, and even if you do see it, it can be hard to control while the glue is drying.

This was the second problem I noted with my Ultimatum. It was an absolute pain in the butt getting the fins flat. I laminated them with 110# paper, and they came out with noticeable warps. So, I put them under a big pile of books for four or five months; that made them visibly straight. But they still warped away from the sustainer. My edge-sanding deficiencies might account for that.

Gluing the fin parts to the sustainer ensures that any deficiency in the cutting or sanding will result in nothing more than a slight fin-to-fin gap that I can FnF my way through. If the gap were ever more than very slight (so far hasn't happened), I'd note it in dry-fitting and do a little judicious sanding. It seems I can sand one fin at a time to fit just fine... but sanding a pile of them all at once eludes me.

__________________
NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org

Y'know, it almost seems you guys are trying to tell me that I should do it the standard way, even though it doesn't work for me.

__________________
NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org

I'm not--I use jigs and fixtures whenever possible to get around my hands' lack of virtuosity. I was always envious of my father (who built houses "on the side" from his career as a Fire Chief)--he could sand a perfectly-airfoiled, mirror-image pair of wings for a boost-glider or a balsa chuck glider using nothing more than a hand-held sanding block. His hands had the same "genius of dexterity" as an ancient Greek marble sculptor, which enabled him to transform a mental image into a perfectly-contoured finished piece, whether it was a set of glider wings or a scratch-built storage shed built without the aid of any drawings.

__________________
Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see:
http://www.lulu.com/content/paperba...an-form/8075185
http://www.lulu.com/product/cd/what...of-2%29/6122050
http://www.lulu.com/product/cd/what...of-2%29/6126511
All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com.
NAR #54895 SR

I thought I was the only one also... I would sand the fins on the bench but if you did not get them exactly perpendicular to the bench, then you have 4 messed up fins.

Also, why do instructions always have you apply sanding sealer after you attach the fins? I do it before removing the fins from the sheet.

Actually I said "Bottom line, do what works for you."

__________________
Kit (aka Cranky Kong)

I'm just a roadie for the banned...