Root Edges of Fins Keep Getting a Curved Shape to Them: What am I Doing Wrong?

[brockrwood]

This is the second time I have encountered this problem.

While building the Custom Rockets "Sport" kit, I needed to sand the root edges of the fins to get rid of the little balsa tab that holds each fin into the balsa blank. The fins are laser cut except for tiny little tabs that hold them into the sheet of 3/32" balsa.

So, what I do is I grab all of the fins and stack them together the same way.

Then I place piece of 220 grit sandpaper flat on my cutting mat. I try to hold it as flat and tight as I can.

Then, holding the group of fins together, I sand them on the 220 grit sandpaper. I use a straight back and forth motion, running parallel to the root edge.

When I examine each fin, the root edge of each fin has a slight curve in it! The top of the root edge curves away from the rest of the root edge just a tad. Same for the bottom of the root edge.

This keeps the fin from lying nice and flat on the body tube when I glue it on.

What am I doing wrong?

I tried used a circular motion, and even a side to side motion, and that seems to "flatten out" the root edge somewhat. I started to take quite a bit of wood off with all of this sanding.

The desk and the cutting mat seem pretty flat to me.

Any suggestions?

 

[Buzzard]

Been There. No matter how "flat and tight" that you feel you are holding them flat, you are likely getting some rocking motion in there. Done that.
I suggest getting a sanding block or T-sanding tool and lightly use it on the root edge to remove the small tab. Do each one individually.

Chas

 

[Buzzard]

eRockets Contoured Sander 2" x 5" with 220 Grit Sandpaper ERO 9128

One idea. Some hobby shops have similar tools.

 

[brockrwood]

I suggest getting a sanding block or T-sanding tool and lightly use it on the root edge to remove the small tab. do each one individually.

OK, I will try using the sanding block to just "knock the tab off". The laser cut root edge of the fin is pretty darned flat as is. All I am doing is making it LESS flat, darn it!

 

[brockrwood]

OK, I will try using the sanding block to just "knock the tab off". The laser cut root edge of the fin is pretty darned flat as is. All I am doing is making it LESS flat, darn it!

@Buzzard

Maybe I will just learn to "embrace the curve". As I get older, slight imperfections are no longer "defects". They are "character".

 

[brockrwood]

eRockets Contoured Sander 2" x 5" with 220 Grit Sandpaper ERO 9128

One idea. Some hobby shops have similar tools.

Wow, that is a good price for a sanding T. Looks like Randy will get another order from me.

 

[Buzzard]

You could always use a sharp knife blade and notch the offending tab out. The glue on the root edge will hide it, especially if you use glue fillets.
Or give Randy the business. Haven't been to his store, but my first Air Force assignment was at Wright-Patterson AFB in the ealy '70s when they had a SAC Bomb Wing, so I know Fairborn. Maybe the next time home.

Chas

 

[Ronz Rocketz]

I use emery boards on the fins. I usually paper the fins then run a bead of thin CA along the non-root edges. The emery sticks make a nice smooth finish when rounding or beveling the edges.

https://www.amazon.com/gp/product/B07MV5772J/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[Bill S]

I find that if I am not super careful, I get the fin root ends rounded more than the center. I remedied that by carefully moving the fins sideways instead of back and forth from front to back. Don't push hard and take it slowly, but it works.

 

[brockrwood]

You could always use a sharp knife blade and notch the offending tab out. The glue on the root edge will hide it, especially if you use glue fillets.
Or give Randy the business. Haven't been to his store, but my first Air Force assignment was at Wright-Patterson AFB in the ealy '70s when they had a SAC Bomb Wing, so I know Fairborn. Maybe the next time home.

Chas

I was thinking. Maybe it is the tab that is causing this. As I push the stacked fins up and down on the sandpaper, maybe the fins are "see-sawing" back and forth on the tabs and making the fins come out curved on the root edge?

Definitely give Randy some business. He is keeping many of those old Semroc model rocket kits available for us old BARs who want to build them. He is also a big supporter of the hobby. I like to spread the love around. Sometimes I order from Randy, sometimes I order from Apogee (Tim Van Milligan), sometimes I order from other vendors who support the hobby and "pay it forward".

 

[brockrwood]

I find that if I am not super careful, I get the fin root ends rounded more than the center. I remedied that by carefully moving the fins sideways instead of back and forth from front to back. Don't push hard and take it slowly, but it works.

I do tend to push hard and go at it a bit aggressively. That's how I bend body tubes trying to get couplers inserted. I need to learn to use a lighter touch.

 

[David Schwantz]

Really, really simple fix. Pic shows how I sand the edges. Use the vertical edge of table to guide your sanding block, and the flat surface to lay your part down on.

 

[brockrwood]

Really, really simple fix. Pic shows how I sand the edges. Use the vertical edge of table to guide your sanding block, and the flat surface to lay your part down on.

Ah! I like it!

 

[brockrwood]

Really, really simple fix. Pic shows how I sand the edges. Use the vertical edge of table to guide your sanding block, and the flat surface to lay your part down on.

A drawer, perhaps, would work in this situation, for those of us who don't have a table shaped like this?

 

[Steve Shannon]

Don’t push back and forth. That rocks the stock.

 

[brockrwood]

Don’t push back and forth. That rocks the stock.

As I re-read these posts, I see how many of mine, at least, can be mis-interpreted to be thinly veiled, prurient comments. I must be very, very careful. I have a reputation to protect...

 

[David Schwantz]

Hi Brookrwood. anything with a 90 edge would work. You could use a 2x4, just something to guide the sanding block.

 

[SharkWhisperer]

I find that if I am not super careful, I get the fin root ends rounded more than the center. I remedied that by carefully moving the fins sideways instead of back and forth from front to back. Don't push hard and take it slowly, but it works.

That's the trick. No unconscious see-sawing action. But don't accidentally put an airfoil on your root edges of the outer pieces

 

[neil_w]

1) Light touch
2) 400 grit paper
3) hard, flat sanding surface
4) Periodically rotate the stack of fins in your hand, so you'll somewhat "undo" the lopsided sanding you apply in one direction. This is imperfect but definitely helps, in many sanding situations.

 

[brockrwood]

4) Periodically rotate the stack of fins in your hand, so you'll somewhat "undo" the lopsided sanding you apply in one direction.

Ah! Doh! I will try that with the next set of fins I need to sand!

 

[brockrwood]

1) Light touch
2) 400 grit paper
3) hard, flat sanding surface
4) Periodically rotate the stack of fins in your hand, so you'll somewhat "undo" the lopsided sanding you apply in one direction. This is imperfect but definitely helps, in many sanding situations.

Man, getting good at this model rocket building skill means I have to build a lot of rockets and mess up on a lot of rockets. But, with each new rocket I build, the skill should improve a little, no?

 

[Cape Byron]

Man, getting good at this model rocket building skill means I have to build a lot of rockets and mess up on a lot of rockets. But, with each new rocket I build, the skill should improve a little, no?

Oh no! You have to build a lot of rockets, and that's something we all hate, right?

My stuff started looking a lot better when I slowed down a lot. Look at some of @James Duffy 's builds. Some say he is a God, but others see how much time he takes over lots and lots of tiny steps. Inspirational and educational.

 

[Buzzard]

I know James Duffy. He is a mere mortal like us, but who is only god-like.

Forgive James, for I have sinned...

Chas

 

[Cape Byron]

I know James Duffy. He is a mere mortal like us, but who is only god-like.

Forgive James, for I have sinned...

Chas

You know James Duffy? I'd really like to meet the man. He changed my appreciation of greebles.

 

[Ronz Rocketz]

Man, getting good at this model rocket building skill means I have to build a lot of rockets and mess up on a lot of rockets. But, with each new rocket I build, the skill should improve a little, no?

C6-3 and forget it…

Buy the builders kit and start building a bunch of 3FNC or download the plans from JimZ and YORP. If they’re not perfect, use a bigger motor with a streamer and enjoy.

 

[brockrwood]

Oh no! You have to build a lot of rockets, and that's something we all hate, right?

My stuff started looking a lot better when I slowed down a lot. Look at some of @James Duffy 's builds. Some say he is a God, but others see how much time he takes over lots and lots of tiny steps. Inspirational and educational.

I must see these @James Duffy models. Their reputation precedes them.

 

[Cape Byron]

This as good a place as any to start...

https://www.rocketryforum.com/threads/revisiting-the-estes-maxi-brute-v-2.140236/

 

[Ronz Rocketz]

@James Duffy said to buy two so he can get them out of his garage.

 

[Cape Byron]

Well, the double pack is a good deal...

 

[milehigh]

Hi, Brock,
I would like to add another technique to that shown in reply #12.
If I want a very precise 90 degree edge on a fin, I will use the sanding block vertically on the edge of the bench, and the fin is placed on a sheet of clean plate glass. This allows me to move the the fin stock smoothly against the sanding block. This allows a larger measure of control on the sanding operation