cutting out the fins

[flyinghigh]

Hi, I we are doing a rocket for 4-H and we have to cut out the fins. What should we use to cut those out so the edges are smooth?

[Pem Tech]

Hi, I we are doing a rocket for 4-H and we have to cut out the fins. What should we use to cut those out so the edges are smooth?

Depends..
If it is balsa or basswood, use a new hobby knife blade and a metal straight edge. Make several light passes to cut through the fin material rather trying to cut through in one swail foop.

If it is plywood, a jigsaw or band saw are recommended.

[McKailas Dad]

I am going to assume that this rocket is a store-bought LPR kit, with balsa fins? I use an x-acto knife with a small (#11 blade) with a metal straight edge. A light sanding with 400 grit paper should make it smooth after cutting.

I am kind of peticular; when I'm sanding balsa fins, I check to see if the edges are square by standing the fin on its edge. If it falls to the right, I sand the left...It's probably more work than it needs to be, but I'm funny like that. I'm sure others will chime in and tell me I'm nuts for that... :o Good luck, and welcome to the Forum!

[o1d_dude]

I am going to assume that this rocket is a store-bought LPR kit, with balsa fins? I use an x-acto knife with a small (#11 blade) with a metal straight edge. A light sanding with 400 grit paper should make it smooth after cutting.

If cutting fin patterns out of a sheet of my own balsa, I use a Zona razor saw and a thick metal straight edge. This goes very quickly and requires only a little sanding to true up the edges.

I can't recommend highly enough a set of sanding blocks with various grades of paper for model construction. Great Planes makes very nice aluminum sanding blocks that are about six inches long. That's the stuff.

Zona Razor Saws (http://www.zonatool.net/razor-saws.html)

Great Planes Hand Sanding Block (http://www.greatplanes.com/accys/gpmr6169.html)

I don't recommend using anything that's made of plastic to guide the saw or modeling knife. The blade will cut into the plastic and skip with bad/painful results. The local big box home improvement stores all carry thick aluminum straight edge stock you can use. L-shaped aluminum stock works pretty well and you can keep your fingers out of harm's way by using the upright side of the stock as your cutting edge.

Very Expensive Safety Cutting Ruler that demonstrates how to use a piece of T-bar or L-bar aluminum molding (http://www.cutting-mats.net/safety-ruler-cutting-straightedge-2720.html)

[Pippen]

Hi flyinghigh, welcome to TRF! I was going to start with my first round of 4-H kids today but one had the flu. Found my lost sunglasses in the build box though...:rolleyes:

For balsa I have them use an exacto knife with a sharp blade, but I like this wide quilting rule with a craft mat beneath as it seems to make it easier for them. With younger beginners I usually help hold the ruler over the wider section of the balsa until they get the hang of it. The clear ruler seems to help them see better if the ruler has slipped off the cutting line.

http://www.amazon.com/Olipfa-45-Degree-Angle-Ruler/dp/B000YZARXQ/ref=sr_1_2?ie=UTF8&s=home-garden&qid=1276556177&sr=8-2

Some kids will get pretty smooth cuts that only need a little sanding, while others will need some work.

I've found it's a good idea to have them sand on a piece of scrap balsa before they go at it on their fins so they can see how much damage can be done quickly if they're not careful.

[MarkII]

I always place the metal straightedge on the "fin side" of the line. That way, if the cut should ever "wander" out of line, it will go away from the fin and not into it. I can always trim away excess material from around the fin, but it's pretty hard to add material back in if I cut away too much.

I always use an X-Acto knife with a brand new blade. I have tried to use jeweler's saws, coping saws, and razor saws, but I could never get precise enough cuts. (I don't have any power saws.) I recently cut out a complete set of fins with a razor saw for the first time, (In all previous attempts, I gave up part of the way through the first fin.) I was cutting 1/8" aircraft plywood, which was a bit too much even for a heavy duty utility knife. (I didn't want to try using a hatchet or a machete. :rolleyes: ) I used the saw to cut out a rough outline of each fin, and then I trimmed them to the precise outline with, you guessed it, a No. 2 X-Acto knife. I haven't had to cut out a fin yet that was large enough and simple enough in outline to be able to use a razor saw alone for all of the cutting.

For balsa fins, using an X-Acto knife with a metal straightedge is the best method, short of a laser cutter.

[Trident]

flyinghigh,

I'd agree with all here, and especially if your group is first-time or relatively inexperienced rocketeers. If you were doing a larger model, or material other than balsa, AND have access to power tools, the method below might open up some options.

For basswood and plywood, like making a set of balsa fins, I trace the fin patterns onto the material. I roughly cut out the fins, staying slightly outside the lines (by maybe 1/8"), using a hobby band saw. I do this freehand, since the edges do not have to be perfectly straight, or perfectly follow the line. For larger projects, I would use my full sized band saw.

Once I cut all the fins, I stack them together, and sand all fins simultaneously on a small disc/belt sander. I sand down to the line, rotate, and do the same with the next edge. Obviously this works best for fins with straight edges, but can work on curved edges as well. For balsa and most basswood, you can pin the fins together with small modeler's T-pins. For plywood, small brads hammered thru work well. This will make sure the fins do not slip in relation to each other.

The advantage is that this method is fast, all fins are identical, and you get nice straight edges, sanded nicely perpendicular to the fin surface.

Again, not for young or first time modelers, but good if you are working with an older and/or more experienced group.