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

[les]

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?

I have found that I still mess up, but have become a "master" on how to fix the different problems....

 

[Woody's Workshop]

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.

Very good idea. It's called a shooting board in woodworking and generally used with a plane.
I had never thought of using sandpaper like this, but it should work equally as well as with a plane!

 

[samb]

Using the vertical stack sand method to level laser cut tabs on balsa fins is a sledgehammer approach. And almost always guaranteed to end with a bow shaped root edge. This is what I do. Use an emory board from the Walmart cosmetic section. Carefully sand the tabs left from the laser cutter. Done. If you want to cut the fins from balsa sheet then go ahead and stack ‘um and use the horizontal sanding approach others have mentioned.

 

[Sooner Boomer]

I'm working on a set of Baby Berthas right now. Just sanded the first set of fins. Two things come to mind. 1) I carefully align the stack of fins and run a few (2 or 3) T-pins through them to hold them together. I then sand the ganged stack. The alignment is based off of the root edge. All the fins have to be parallel and aligned on this egde. The other edges aren't as critical (usually). 2) let the tool do the work. You may be using too fine of a grit sandpaper, or pressing down too hard. This causes you to rock the fins as you sand them. I'm using 120 to sand the general shape, then 220 to round over the edges (except root). Depending on how hard the materiel is, you could go down to 80 or even 60 grit for plywood.

Gang sanding especially helps when you're making your own design and cutting the fins from a sheet of balsa (rather than laser or die cut). These fins are less likely to be uniform, and need a bit of extra care (also - cut large and sand to shape).

 

[tab28682]

My technique is to lightly sand each root separately.

As others have mentioned, the reason that the gang sanded root edges become curved is that ganged fins are rocking very slightly as you change direction on the sandpaper. If you have to gang sand, do it in one direction only, very carefully.

After my root edges are looking good, I do stack the fins to see how the non-root edges match up and do a little careful, unidirectional gang sanding, if needed. Usually just enough to get most of the laser burn off the edges, assuming the fins are laser cut.

Don't waste time sanding with worn out or clogged up or too fine of a grit sandpaper. Pick the right grit to get the job done.

 

[Angel_of_the_Skies]

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.

I am building my second Zypher Jr, this time with the intention of building with the best finish I can achieve. I accidentally lopsided the edge on one of my fins. Tried to fix with 220 grit. Would correct itself no matter how slow and careful I went

I thought maybe 400 would be more gentle on the edges, this allowing a more ‘stable’ see saw motion. This was my solution. Turned my messed up fins to almost near flat edges (at least to the human eye). 400 grit FTW

 

[Angel_of_the_Skies]

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?

Just got back to the hobby as a 20 yr old, last building a rocket at 10. On my second MPR build, I’m trying my best to give it the best finish I can, although that may not be the case.

But hey, I’m going to agree with you, we are suppose to see improvement right ? (I really hope so)

 

[SolarYellow]

I'm down with the stack. Use a square (I have a couple of 30-60-90 drafting squares and use them a lot) to set the stack up square and then clamp them together. I find binder clips from my office work well without denting the wood. If the wood does get dented by the clip, the CWF or whatever you use to fill the grain will fill that, too. Then set the sandpaper on a flat surface and only pull the edge of the fins across it. You want the vertical pressure to be toward the trailing side of the fins on the paper, so it offsets the tendency to rock forward in the direction of travel. Typically, for the root, this means holding the flat faces of the stack between thumb and middle finger, with index finger pressure on the leading edges toward the forward tip. Pull the stack toward the trailing edge, feeling the forces and keeping everything in balance to avoid rocking moment as much as possible. Only pull it, don't reverse the direction and the forces.



Works for me so far.

 

[Joekeyo]

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

It's small (thus the low price), but it works. The sandpaper is better than I get at Ace Hardware.

 

[jahall4]

Very good idea. It's called a shooting board in woodworking and generally used with a plane.
I had never thought of using sandpaper like this, but it should work equally as well as with a plane!

I still use the one I modified in college to build a bridge for statics class, originally used for model rockets and planes I built as a kid.

 

[Black Bart]

I use DA sanding disks for auto refinishing shop. They range from 36 gt to 1200 gt. Stick them on a plywood, this will let you use both hands to hold the fin.