Fin bevel jig specifically for LPR & smaller MPR

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Marc_G

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Hi folks,

Some of the recent discussions here have covered concepts for fin beveling techniques & jigs. These are of great interest to me because good beveling is something I'm not particularly adept at.

My default, go-to method has been using tape to mark the stop point for beveling, then go at the wood with a sanding block, or rub the fin against a sheet of sandpaper affixed to cardboard or foamboard to keep it flat.

This works OK, but can be challenging (for me) to do repeatedly and get the same results on all the fins. I'm just not good with precision performance of repetitive motion.

I've seen fin beveling concepts by several others over the years, and from my perspective they break down into two groups:

-Jigs that allow for positioning the fin so that a sanding surface can be guided over it with some precision
-Jigs that allow travel of the wood past a cutting/sanding surface in a repeatable manner

I have tried and failed in years past to reproduce the first style. The problem was not in the design, but in my poor ability to perform the repeated motions accurately... it's just not a strength of mine.

So this time I decided to try the second style. But keep in mind, I'm talking small rockets here... I've never flown anything more than an E engine. So I need a beveler that can put bevels on fins just an inch or two across made out of say 3/32" balsa. And of course a bit bigger... This lets out anything based on a table saw or bench sander.

I've come up with a prototype based on a Dremel tool with the flexible extension, and a movable sled on which the fin rests. The sled rests on some European glide drawer glides for smooth motion. Over the next weeks I'll describe it's construction and the fine tuning yet to happen. It's not finished yet, but I have been able to test the concept with some success.

Here are some pics based on the evolution of the design. I'll try to update this thread every few days as I make progress, and I invite discussion and other ideas...

Bevel 1.jpg
Bevel 2.jpg
Bevel 3.jpg
Bevel 4.jpg
Bevel 5.jpg
 
An electrified version! I like the rearrangement of the moving components.

After a close inspection, I see the force of the spinning sanding drum causes tear out on the end of the bevel. In a effort to prevent this, I would suggest soaking some thin CA into the edge of the fin that goes thru the sanding drum last and see if that does the trick.
 
That's a sharp eye, bradycros! But you are close, though no cigar. :grin:

What looks like tear-out is actually just the rough ends of the scrap pieces I've been using to demonstrate. I have some primed basswood scrap from which I've cut fins, much like the leftovers after you punch out the laser (or die crushed) fins from a fin set. I snapped it off at convenient places to create some test scraps. A few posts down you can see some more of the scrap in a zig zag shape!

So far here's how I've built it. I started with a piece of particle board. I then attached a pair of European glide drawer slides. The "European glide" ones have better bearings and glide very smoothly with little wiggle or slop perpendicular to the main direction of motion. I actually bought this set (at Lowes) thinking to use them for a fancy fin jig where the uprights would slide in and out, but it turned out to be more complicated than it needed to be.

Anyway, I attach two glides to the particle board. Two, in parallel, gives a very sturdy base of motion with almost no slop. Two is better than one.

Bevel Dremel 001.jpg

I attached a flat piece of board to the top of the glides, making sure it was firmly attached to both glides.

Bevel Dremel 002.jpg
 
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Initially I thought I would just mount the Dremel flex shaft extension somehow to the particle board.

Bevel Dremel 003.jpg

But that didn't work. I needed to be able to either angle the Dremel sander more degrees that this would allow, or angle the movable fin support platform. Much easier to adjust the angle of the Dremel.

Marc
 
So I cut down the particle board, hacked on some 2x6" legs, and used a bit of wood to hold the Dremel shaft. Notice the thumbscrews... the holes they go through are fairly big to allow adjustment of the tool at various angles to the fin on the platform.

Bevel Dremel 006.jpg

This was a viable approach (some of the examples above used it) and I may revisit it, but I decided it might be better to make the Dremel shaft more fixed, at a convenient angle that would cover routine needs, and use a calibrated surface to adjust the positioning of the fins closer or farther from the edge of the platform. Thus, factoring the angle of the sander, I could get increasing or decreasing bevels, accommodating different fin thicknesses.

Bevel Dremel 004.jpg Bevel Dremel 005.jpg

Notice the lines on the adhesive paper on the platform. They are done with different dashes for easy reference as I line fins up.
 
The jig basically works at this point, but there are two or three problems I need to tinker away:

1. When beveling basswood fins, the force of the rotating sanding drum against the wood as it sands it away is enough to cause the fin to twist in place, screwing up the uniformity of the bevel and potentially ruining the fin. This isn't an issue with balsa, because the drum chews it away as if it was smoke. However, it would be an issue if I were beveling CA-hardened balsa (let's avoid arguing over the relevance of the use case of why I might want to CA harden balsa before contouring it). Ideas I've got to fix this:
A. Put in a stop that I can push the fin against, in a way that it won't interfere with the drum (some challenges here, making it work not just for the first pass on the first side of the fin but such that it will be tall enough to help when I flip the fin and do the second side
B. Use a thin layer of latex or similar rubbery material on the wood platform to increase friction between the fin and the platform (I have some such material, sold cheap at Walmart in the guise of stretch bands for exercising [side note: if any of you see me in the exercising aisle at Walmart, rest assured I'm there for rocketry supplies])​

2. I'd like to increase the adjustability of the angle of the sander, without overly complicating the jig, and without compromising the positional rigidity of the sander when it is ready for use. I have a few ideas... ideally I'd like it to work out to some sort of thumbscrew adjustments but we'll see.

3. The particle board/glide platform flexes ever so slightly, if I push on it too hard in a downward way, causing nonuniformity of bevel. I can avoid the problem by not pushing down, but I can easily bolster the rigidity of the platform with some wood underneath the particle board.

4. Oh, one more thing: I really need a finer sanding drum, if possible, as you can see streak lines in the basswood from features on the drum.

If I get some time tonight I'll try to start working on some of these.

Marc
 
Made some progress tonight.

Using some of that stretch band material:

Bevel Dremel 009.jpg

I cut a strip and sprayed just a bit of spray adhesive onto on side. I then carefully positioned it on the wood platform (which is actually a second layer on top of the moving platform). I had to take off my nifty dashed line marker label first, of course.

I then used a mock fin made from primed basswood and made some passes, using a little dot I Sharpied onto the rubber surface as an indicator for how close to the edge to align my pieces.

Bevel Dremel 007.jpg Bevel Dremel 008.jpg

The rubber surface had significantly more friction than the label paper, so there was less of a tendency for the fin to drift as I moved it. As you can see, the results were pretty uniform. I purposefully did slightly different amounts of beveling on each of the three edges, practicing my uniformity. It came out at least as good as anything I could do manually, so the jig is already a qualified success.

Here is an extreme close up of the edges, you can see the striations from where the features of the sanding drum marked the wood:

Bevel Dremel 010.jpg

A light sanding with 400 grit would take care of it (on basswood); on un-hardened balsa I don't think it would be necessary to do much.

Still lots to do, but this is seeming promising so far.

Marc
 
Have you tried this with any other bits? Small high speed router bits by chance. Wondering what the finish difference would be.

Great jig by the way. Looks like it makes for very consistent cuts.
 
A router bit is a good idea. I believe I have a few for the Dremel collet system. I will have to check to see if I have the straight-cutter one. No time to work on it tonight, but hopefully on Friday I can take a half day off work and make more progress.

I'm considering some sort of screw-based adjustable angle for the piece that holds the Dremel. Maybe something like a barrel nut connecting two bits of allthread, such that you rotate the barrel nut to raise or lower the Dremel.

It's something to think about...

Marc
 
Thought about this after I posted but coming from an accident prone woodworker. Sanding bits are a lot safer to work with without guarding. If you put a router bit on there keep all digits away. Knick a sanding drum ouch, knick a router bit and get a taxi ride to the ER. Public Safety Announcement. I'm sure you know that but we all can use reminders sometimes.
 
Hmm. Good point on the hazards of spinning router bits. I had been considering protective shields because I like to keep my fingers, but I think sticking with sandpaper drums is probably more practical. I believe I have a fine grit one in the box that should do the trick.
 
I placed a fine grit sanding drum on the Dremel tool. After doing some test passes with scrap, I decided the tool was ready for a practical test.

I'm working on a set of Yellow Jacket clones. I've got BT 50 (original scale) and BT-60 versions built, but I decided I want a BT-56 one as well, to use up some extra BT-56 I have. So I whipped up some upscale Yellow Jacket fins out of 1/8" balsa. I decided to try skinning / CA hardening the fins, then beveling them with my new tool. The reason for papering first, THEN beveling is that I find it hard to paper pre-beveled fins without wrinkles. I figure the papered and hardened fins will machine down nicely, without the paper ripping.

I skinned them with computer paper that has just a bit of #77 spray adhesive to tack the paper to the fins.

Bevel Dremel 011.jpg

The fins will eventually be double wood glued to the body tube, and I don't want the CA to penetrate the root/bevel area of the fins. Notice I didn't paper the area that will be covered by bevels. Instead, for that area I coated it with wood glue.

Bevel Dremel 013.jpg

I then set the fins aside to dry overnight.
 
This morning, I took the fins out to the garage, and while using proper personal protective procedures (goggles, ventilation, gloves), I saturated them with thin CA. I used a Q-tip to spread the CA, but mostly it soaks right through the paper and into the wood. I made sure the edges of the fins were well-saturated and set the fins aside on some wax paper to cure for a few hours.

Bevel Dremel 014.jpg

Next, I took the fins and some similarly treated (papered, CAed) scraps to my jig. After testing on the scraps, I drew a line on the jig to allow for good alignment of the fins on the jig for even beveling. Here is the set of three fins after the leading edges were beveled.

Bevel Dremel 015.jpg

Emboldened by my success, I then did the other edges similarly. The jig even allowed me to do the interior corners, even if it wasn't quite easy to get them to meet up perfectly. When doing these inside corners, I angled the fins slightly in the jig to sort of feather out the bevel at the bottom of the fin.

Bevel Dremel 016.jpg

The fins came out pretty good in my opinion, certainly more even than I could have done manually. Nice and sharp. No tearing of the CA-hardened paper.

Bevel Dremel 017.jpg

Finally I used some fine sandpaper (320 grit I think) and polished the surfaces a bit, removed any rough edges, etc.. I then used medium thickness CA to re-seal the beveld edges. Set aside to dry. Shown here on plastic but eventually moved to wax paper.

Bevel Dremel 018.jpg
 
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