Plywood fin material - something to practice on

I've gotten pretty good at working with balsa and basswood fins. But I'm about to step up to plywood. I'd like to find some plywood material to practice a few techniques on and see what the material is like to work with.

Specifically, I want to see what it's like to bevel the fins, fill them, and maybe even see if I can do any airfoiling stuff on them - I've got a beveling/airfoiling technique I think is pretty good for balsa, but I don't know how it will translate to larger plywood. I've been reading some of the older TRF threads on these topics, but I don't have some of the tools people like to use for them (I really like the jig/table saw method I've seen people use, but I have no experience with those!). I want to see what I can figure out on my own, and then maybe ask for help if I'm just not getting it.

So, I've seen sheets of craft plywood at craft stores (Michael's, Hobby Lobby, even Menard's). Is this the same material? I want to try a few things out before I commit to my next rocket, but I need to know if the materials behave the same way, i.e. do or to not chip when you do certain things to them, whether the plies hold together the same way, is it the same hardness (for sanding and such), etc. I don't know if rocket fins are made differently from basic craft balsa - or if it's, like, aircraft plywood or something (I'm not sure if that's a thing). So, does this stuff behave the same way? If not, what kind of plywood do I need to look for, and is it something I can get [edit]in a hardware store [end edit], or do I need to order it specially from an online rocketry vendor?

I'm interested in hearing this, too...I've inquired about this before, and a number of people wrote in to say they didn't sand plywood fins at all. I went ahead and attempted to sand my Leviathan fins, and man, even 1/8" plywood is a pain to sand by hand! I'm not sure I made much of a dent in the things...

The_Lone_Beagle said:

I'm interested in hearing this, too...I've inquired about this before, and a number of people wrote in to say they didn't sand plywood fins at all. I went ahead and attempted to sand my Leviathan fins, and man, even 1/8" plywood is a pain to sand by hand! I'm not sure I made much of a dent in the things...

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Well, I did a search and came up with a number of two- and three-year-old threads where people talk about beveling and such - some using a power sander, some by hand, and some using a table saw (let me see if I can find those again, and I'll post them here). But I'd like to get my hands on some practice material just to see what it's like to work with.

Incidentally, if you have access to power tools, John Coker's website has a jig you can build to bevel fins. I'm trying to join my local makerspace, so maybe I'll be able to go that route. I just want to understand the materials I'm using before I make any cuts or dents.

[Edit] Here's one. And another. And another.

Balsa is much softer than plywood.
Baltic Birch is what I use and it's requires different tools than working with Balsa.

I learned this many years ago when moving up.
https://www.woodworkerssource.com/b...-birch-plywood-why-its-better-when-to-use-it/

I use a router table to profile the fins.
You can use a sander but, still takes a long time.


JD

That craft birch plywood will work for more than just practice. It will work for MPR through L1 HPR stuff for sure. Start looking for better grades of birch plywood, when you are building mach-busters or more powerful HPR birds. Then you probably want real aircraft plywood. The best stuff is imported from Finland and has many more plies than the hobby shop plywood. It's way more expensive than the hobby shop or crafts store birch plywood.

If you're going to buy plywood at a hobby shop or art supply store, you want the ply that's marked "Aircraft Plywood". The other stuff is lighter and weaker. Costs less but that's not the point. I'm sure some will jump in and say they been using the light stuff for years without incident but my recommendation is go with aircraft ply. You're gonna go 500 mph so what's a few pennies saved at that speed?

You need woodworking tools to work with plywood. I recommend a scroll saw or a band saw. For sanding a belt/disc sander will work beautifully.

I use a belt sander most of the time, but if the fin has a slight warp to it, you can remove too much from the areas that are curled, so then I clamp it with the edge sticking over the table and go at it with a Porter Cable angle grinder with a sanding disc.

I've made a few sanding jigs on the past. Takes too long to set everything up. I can do it by hand just as accurately and be done with it by the time I would get everything set up.

People send fins to me to custom bevel. I don't have some special machine. I do it all by hand and a trained eye. I like the control you get by doing it that way.

Hand planes can be very effective if used properly as well.

The main difference between Baltic Birch Plywood and Aircraft Plywood is BB is all birch plys of equal thickness, were as AC ply is usually birch (can also be mahogany) outer plys over Spruce or other light but strong woods like basswood or even balsa. HomeDepot/Lowes brand Birch ply is a super thin veneer of birch (paper like) over core plys of poplar or other woods and its got issues with de-lamination and voids. The stuff sold as Luan at HD/Lowes is sometimes referred to as Light Ply IIRC, and has its uses, it is cheap and more than adequate for LPR and MPR rockets.

Taiwanluthier is correct about hand planes being useful however they must be very sharp and a backing board used on the end of the cut to prevent splintering.

Mark aka Binder also has good advice on working it on power tools, a little practice, and beveling is easy by hand.

I prefer the true Baltic Birch (its Finnish) mainly because I can get 5'x5' (I've heard its a metric thing for the odd size) sheets of 3mm (aka 1/8th inch) for about $10 from my supplier and 6mm (aka 1/4 inch) for $18 per sheet vs. Midwest Hobby brand a 6"x12" of quarter inch will set me back about $3 to $4 dollars. BB weighs a little bit more but is easier to work with IMO. For large fins a bandsaw is nice as is a belt/disc sander, a scrollsaw is really useful for cutting centering rings without as much risk as fly-cutter type circle cutter in a drill press, I use a scroll saw to cut all my CRs up to 12" diameter and up to .5 inch thick. Working with true BB is almost like working with solid wood, only its more stable (not prone to warpage/splitting/swelling/shrinking type stable) and stronger. BB is used to make lots of kids toys like puzzles and tables, its real easy to tell in that all they plys are nearly the same thickness and the finished edge is just beautiful (at least to me).

Edit: I get my Baltic Birch ply through a wholesaler to get the good price that I do

lcorinth said:

Well, I did a search and came up with a number of two- and three-year-old threads where people talk about beveling and such - some using a power sander, some by hand, and some using a table saw (let me see if I can find those again, and I'll post them here). But I'd like to get my hands on some practice material just to see what it's like to work with.

Incidentally, if you have access to power tools, John Coker's website has a jig you can build to bevel fins. I'm trying to join my local makerspace, so maybe I'll be able to go that route. I just want to understand the materials I'm using before I make any cuts or dents.

[Edit] Here's one. And another. And another.

Click to expand...

Thanks for that...I'm familiar with Coker's website (lotsa great info) and had seen that before. I guess I'm interested in what I *need* to do it effectively. I don't have much in the way of tools right now, or even space for anything big, and frankly, wonder about how much the investment would be worth it if I sand 2, 3, or 4 fins a year max (vs. going with fiberglass, or other options, etc.).

I don't mind sweat, and I don't mind taking my time...(and I'm a cheapskate)...I am curious about what it takes to do it well, and what the potential trade-offs are. From what Binder says below, all I would need would be hand tools, and (along the lines of "how to get to Carnegie Hall?"), practice, practice, practice.

A coping saw (less than $10.00), a flat surface, a block of wood and various grits of sandpaper. That is sufficient to make great fins with radius and taper. You just need to have some patience....something many do not have.

rharshberger said:

I prefer the true Baltic Birch (its Finnish) mainly because I can get 5'x5' (I've heard its a metric thing for the odd size) sheets of 3mm (aka 1/8th inch) for about $10 from my supplier and 6mm (aka 1/4 inch) for $18 per sheet ....

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That's an awesome price.

Buying retail, you have to shop around. There is "aircraft plywood" and there is Aircraft Plywood, just like there are various grades of stuff called "baltic birch plywood." True aircraft plywood is usually rated for use in aircraft. Home Depot "birch plywood" is neither baltic, nor 100% birch. That doesn't mean that you can't use it in rockets, but it seems sensible to me to use the right kind of material for the right kind of job.

Aircraft Spruce sells 1/4-sheets of REAL aircraft birch plywood, from Finland, for $33.00 - $56.00 each. The reason it costs so much, other than being imported, is that so many more plies are used, better adhesive, and no void spaces. There is also a grade less expensive Finnish baltic birch plywood sold by places like Woodcraft - quality plywood, but fewer plies.

That being said, the plywood you can buy at crafts stores or the hobby shop is good.

For beveling plywood fins, I take a sheet of 60 grit sandpaper and put it down on the table of my band saw, and use the guide to hold it in place. It is a lot of manual labor, but it works. Probably takes me 15 minutes a fin depending on the size. A belt/disc sander is on my list of things to acquire if I think I'm going to do a lot of work with plywood fins. I tried rigging something up with a sanding drum in my drill press but I just can't get a consistent bevel that way.

I found a place in MO that sells it in 20" x 30" sheets
https://woodnshop.net/baltic-birch-plywood/
Pricing isn't too bad.


JD

The_Lone_Beagle said:

I don't mind sweat, and I don't mind taking my time...(and I'm a cheapskate)...I am curious about what it takes to do it well, and what the potential trade-offs are. From what Binder says below, all I would need would be hand tools, and (along the lines of "how to get to Carnegie Hall?"), practice, practice, practice.

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Ok then, here's all you need: Sanding block, sandpaper, clamp(s). Clamp the fin to the table with the edge you need to bevel hanging off. Apply 40 or 60 grit paper to the block and holding it at a consistent angle, start sanding. Stop every so often to check your progress. You'll need to adjust it as you go because you might go deeper in one area than another. Then when you are happy, do all the other edges to match. When all of them are roughed in, hit it with some finer grits to smooth it out.

It really won't take that long if your fins are small/medium sized. If you try it, you'll probably be amazed at just how easy it is.

One thing: Make sure you use a hard sanding block such as wood or metal. Rubber won't do because rubber will deflect under pressure, causing your bevels to be off. Hand planes are actually better if you know how to use it.

Also make sure the sandpaper is glued to the block. Do not simply wrap it around the block unless you can make sure it's under considerable tension. Otherwise it will give you uneven surface. Don't be surprised by how easy it is to put a camber on the fin!

My go to sanding block is a rubber 3M.
Never had any issues....

You might be pushing too hard if you are getting some deflection.
In a worst case; use a file.
The proper tools make it easier not harder.

JD

taiwanluthiers said:

One thing: Make sure you use a hard sanding block such as wood or metal. Rubber won't do because rubber will deflect under pressure, causing your bevels to be off. Hand planes are actually better if you know how to use it.

Also make sure the sandpaper is glued to the block. Do not simply wrap it around the block unless you can make sure it's under considerable tension. Otherwise it will give you uneven surface. Don't be surprised by how easy it is to put a camber on the fin!

Click to expand...