Fin alignment techniques for big rockets...

[ewomack]

Continuing on with HPR newb questions, I'm just getting to the point in the DX3 of epoxying down the fins. As I test fit them a few times, I realizied that for LPR I would usually just eyeball the alignment and consider that "good enough." Eight times out of ten it worked out fine. But with a much larger, not to mention much more expensive, kit the stakes seem higher and I would like more assurance that my eyeballs haven't deceived me. I've played around with a few possibilities but thought I would likely get better ideas and answers from the forum that has already helped me out immeasurably with all things rocket-wise.

Does the following have any validity: assume a flat surface and place a square next to the body and line the fin up with the straight line carefully as pictured?



Or do any tools exist for larger rockets to help with alignment?

 

[SkyFire]

Fin alignment template program;
https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES
Foam board from Hobby Lobby works well.

 

[_kestrel_]

I used a 3d printed fin guide for my Super DX3, printed in 3 pieces to fit smaller print beds.

 

[rharshberger]

Fin alignment template program;
https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES
Foam board from Hobby Lobby works well.

This! I use this on rockets so far up to 7.5".

 

[JackC]

Yes, Badass Rocketry fin build guide. Get one for a 4 inch motor tube and use it to glue the fins on the motor tube. Then cut slots at the rear of your slotted airframe and slide the motor tube fin assembly into the airframe.

 

[teepot]

I print a template from Payload Bay then glue that to a foam board. I cut where it is designated and slide the board on to the BT. It aligns all the fins at once.

 

[ewomack]

That is an awesome free tool. For the DX3, I used 4" diameter, 3 fins, 3.25 fin width and .25 fin thickness, all in inches.

I just need some foam board. Thank you! This will give me far more confidence in the fin alignment.

If I had a 3D printer, I would give that path a try, but alas!

 

[OverTheTop]

I used coreflute and a similar technique to teepot. Works well and you can cut the slots by hand. Foam would be easier to cut.

 

[GlueckAuf]

Setting one fin at a time with slow cure JB Weld epoxy, I fasten the rocket securely to a couple of nesting fruit boxes with the center of the fin slot carefully aligned straight up (0 degrees). I use weighted containers (stacking Crystal Light jars filled with pennies) to firmly hold basswood rods against either side of the fin to immobilize it. Then I simply align the fin visually with the vertical pattern on the door opposite my workroom (verified plumb with a bubble level). Six hours later, the fin is is set, and about as perfectly straight from center as it gets.

Simple, accurate, and effective...even if the machinist who cut the fin slots for me was off by a degree or two from their being exactly 120 degrees apart, an advantage over a jig.

Good skies,
GlueckAuf

 

[Bill Hanson]

I used a 3d printed fin guide for my Super DX3, printed in 3 pieces to fit smaller print beds.

View attachment 476263View attachment 476264

Where did you get these? Did you print them yourself?

 

[Bill Hanson]

If you have access to a 3D printer, there are several customizable fin jigs on thingiverse. Here’s the one I used for my last build.

Parametric Fin Jig

Good luck!

 

[Bruce]

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[Bruce]

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[Bruce]

Might you be able to post a link to the .STL file for this fin guide?

I used a 3d printed fin guide for my Super DX3, printed in 3 pieces to fit smaller print beds.

View attachment 476263View attachment 476264

 

[JohnCoker]

I always make a guide for the rocket and fin width.


A reusable guide is nice too, but I find myself dealing with different thickness of fins frequently.

 

[_kestrel_]

Might you be able to post a link to the .STL file for this fin guide?

Just uploaded it to Thingiverse.

https://www.thingiverse.com/thing:4927909

 

[_kestrel_]

Where did you get these? Did you print them yourself?

I designed and printed these myself. I do CAD for a living so this was a pretty easy part to model. I use the printer a fair bit for tools and jigs for rocketry.

 

[dr wogz]

Post #19 in the link below.. my fool-proof method

https://www.rocketryforum.com/threads/madcow-super-dx3-patriot-a-build-thread.140215/

 

[ewomack]

This cutout method seems to work, but I wouldn't mind combining it with dr wogz's method as well to make extra sure that the fins are as perfectly aligned as possible. The foamcore at least boosts my confidence somewhat.

 

[OverTheTop]

Here are some pics to follow my earlier post. As somebody mentioned earlier, fins are glued one at a time with the others dry-fitted for alignment.

 

[Rocketclar]

Well, I've used the level technique. First made sure my workbench was level and then used a level to hold each fin at 90deg. Very good tip to tip measurements that way. More recently (a Christmas present from my brother-in-law who also is into rocketry), I was given the guillotine type fin alignment kit that Apogee sells. Here's a pic of it in action.

 

[JimJarvis50]

I believe it's important to control both the angle of the fin (perpendicular to the body tube) and the angle along the body tube. I use a foam board template for perpendicular and a couple pieces of angle iron to get the fins straight with the body. Sort of the old Estes door jam approach. Obviously, this only works with flat fins.

Jim

 

[FlyBy01]

Look at one of Theory's posts where he used a Plumb Bob and a square. Can't get more vertical than gravity.

 

[Bruce]

It seems like many of these methods rely on a single flat piece to align the fins. I'm wondering if this could still possibly allow for a bit twisting or misalignment.

Is there something that could firmly hold the fin perpendicular to the body tube at the proper location during gluing so there would be no eyeballing needed?

 

[JohnCoker]

It seems like many of these methods rely on a single flat piece to align the fins. I'm wondering if this could still possibly allow for a bit twisting or misalignment.

Is there something that could firmly hold the fin perpendicular to the body tube at the proper location during gluing so there would be no eyeballing needed?

If there are fin slots in the tube, that axis is covered. If you have purely surface-mounted fins, then using something like Jim mentions to keep the fins aligned with the long axis is necessary as well.

 

[teepot]

With the fin jig on I can look at the fins because I left the rear centering ring unglued. I pull the rear ring and look at the fin root to make sure it' perpendicular. The JB Kwik Weld sets in 6 minutes so you don't have to wait long for it to set. I'm sure there other ways to do it besides eye balling it.

 

[Michael L]

Well, I've used the level technique. First made sure my workbench was level and then used a level to hold each fin at 90deg. Very good tip to tip measurements that way. More recently (a Christmas present from my brother-in-law who also is into rocketry), I was given the guillotine type fin alignment kit that Apogee sells. Here's a pic of it in action.

I used the jig in this post and a square. A level requires everything to be "level". A square assumes that the box is square so the vertical fin should be square to the base. I also used this 3D printed widget:

The fat part is a good fit inside the sustainer tube. The center piece fits around the motor tube. The slots are for the fins. I tacked the fins into the sustainer tube then epoxied them. I need to modify this model to let the motor tube pass through the fat part. Between this and the wood jig the fins were perfect

 

[Bill Hanson]

Just uploaded it to Thingiverse.

https://www.thingiverse.com/thing:4927909

Outstanding. Thanks.

Would it be possible to share an .f3d or .dxf file so we could modify for different sizes?

otherwise should be easy enough to reverse engineer now that I see your idea.

 

[Bill Hanson]

I used the jig in this post and a square. A level requires everything to be "level". A square assumes that the box is square so the vertical fin should be square to the base. I also used this 3D printed widget:

The fat part is a good fit inside the sustainer tube. The center piece fits around the motor tube. The slots are for the fins. I tacked the fins into the sustainer tube then epoxied them. I need to modify this model to let the motor tube pass through the fat part. Between this and the wood jig the fins were perfect

That’s a nice idea to do it from the inside … kudos!

 

[Michael L]

That’s a nice idea to do it from the inside … kudos!

Thanks. I wasn't using parameters when I made it the first time. I'm thinking about redrawing the next version. I'm about to enter the realm of fiberglass construction and I'll need something a bit bigger