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.