Fin positioning: 120 degrees apart

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Kirk G

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I've assembled a dozen or so rockets now, and some have two fins and pods.... some have four fins.... and a few have just three.

I have gotten in the habit of gluing on the fin while the body tube lays flat on an up-ended egg carton. As long as the fin stays vertical (like a shark's fin) as it's drying, it seems to work well.

However, these three-fins rockets are giving me fits. I can get two of the three fins to be pretty evenly spaced, but the third fin always seems to be unequally angled. That is, it's never a perfect 120-120-120 degree separation around the body. I'm using the body tube marking guide that comes printed on the instruction sheet... but it still doesn't go right for me.

Any suggestions?
 
I use the "wrap" style fin marking guides from Payload Bay or Open Rocket. I've rarely had a problem. I also use a homemade fin alignment fixture.

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I forgot to mention, I use a piece of aluminum angle to mark the fin and rail button lines. The angle holds itself centered on the body tube.

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I use the "wrap" style fin marking guides from Payload Bay or Open Rocket. I've rarely had a problem. I also use a homemade fin alignment fixture.

I too like to preheat my fin alignment jig (needs picture 3 from post 3 to make sense). They seem to work better that way.
 
For standard Estes body tube sizes, I used the Tube Marking Guide:

https://www.estesrockets.com/rockets/accessories/302227-tube-marking-guide

With that, my fins are always perfectly spaced around the body tube. I also use the Ultimate Tube Marking Guide (or Portable Door Frame as I heard John Boren refer to it) to draw the lines on the tubes:

https://www.estesrockets.com/rockets/accessories/302228-ultimate-tube-marking-guide

These and the Tube Cutting Guides are three of the best investments I have made in building model rockets.
 
I have seen many of your builds and have always attributed the final ( and fine)results to your work bench.
 
You could print out a fin guide from payloadbay.com.

https://www.payloadbay.com/page-Tools.html

+1!

Print it (actual size, not fit to page), glue it to some foam board ($1 at Dollar Tree (+Tax if you're not in Oregon)), cut it out, use. Voila! Perfect aligned fins. A caviat, I have only used them with rockets that are TTW. Keeping surface mounted fins held in check (all at once) is like herding cats for me.

For standard Estes body tube sizes, I used the Tube Marking Guide:

https://www.estesrockets.com/rockets/accessories/302227-tube-marking-guide

I like the round bits, but the angle part let me down on my L1 build, so it has it's limitations.

With that, my fins are always perfectly spaced around the body tube. I also use the Ultimate Tube Marking Guide (or Portable Door Frame as I heard John Boren refer to it) to draw the lines on the tubes:

https://www.estesrockets.com/rockets/accessories/302228-ultimate-tube-marking-guide

These and the Tube Cutting Guides are three of the best investments I have made in building model rockets.

I use the aluminum angle from Home Depot (more solidly built than that Estes plastic).

If only the Tube Cutting Guides would come out with a 2nd edition... BT-56, BT-70, 29mm, 38mm, PSII 2", PSII 2.5", PSII 3", PSII 4" (Oh, and those round bits from the marking guide in those other sizes too).
 
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I use the "wrap" style fin marking guides from Payload Bay or Open Rocket. I've rarely had a problem. I also use a homemade fin alignment fixture.

Quake you make some gorgeous jigs man..............

Teddy
 
Estes Fin Marking Guides. I made up a sort of adapter for BT70 as it's not part of the marking steps. A BT20 tube with 2 BT20 to BT70 rings, they aren't glued in place, tape rings top and bottom of the rings keep them from moving. As to keeping 3 fins straight out from the body I just line a straight edge along the surface of a fin and check to see if it is aligned with the center line of where the motor is. Hard to describe but it works for me :)
 
For Estes tubes I use the Estes plastic fin marking guide set. For bigger diameters I use a compass and, does everyone remember their geometry? After drawing the circle and without changing the setting of the compass, place the point anywhere on the circumference and mark the circumference on both sides of the point. Move to one of the points and repeat. This will produce perfect 60 degree segments. By drawing the circle just slightly larger than the outer diameter of the tube you will be able to mark the bottom of the tube just like with the Estes guides after you get it centered.
 
For Estes tubes I use the Estes plastic fin marking guide set. For bigger diameters I use a compass and, does everyone remember their geometry? After drawing the circle and without changing the setting of the compass, place the point anywhere on the circumference and mark the circumference on both sides of the point. Move to one of the points and repeat. This will produce perfect 60 degree segments. By drawing the circle just slightly larger than the outer diameter of the tube you will be able to mark the bottom of the tube just like with the Estes guides after you get it centered.

Peter, I remember my geometry and you are exactly right. And 3/4/5 describes a right triangle. Are these things taught anymore?
 
Here's a tip when using the wrap-around fin marking guides. Sometimes (often) they aren't exactly the right width to wrap around the tube. This can cause the spacing between two of the fins to be a bit off when drawing the lines based on the guide.

The solution is to use the guide to mark the fin (and launch lug) locations. Then, remove the guide and reuse it, but upside-down (reversing its top and bottom). Align the launch lug line (or one of the fins if there is no lug line). Then use the guide to draw a new set of lines.

Some of the lines will line up just right with the earlier ones. But, in some case you'll end up with two lines side-by-side. In the second case, use the center between the two parallel lines as the line for the fin.

-- Roger
 
Here's a tip when using the wrap-around fin marking guides. Sometimes (often) they aren't exactly the right width to wrap around the tube. This can cause the spacing between two of the fins to be a bit off when drawing the lines based on the guide.

The solution is to use the guide to mark the fin (and launch lug) locations. Then, remove the guide and reuse it, but upside-down (reversing its top and bottom). Align the launch lug line (or one of the fins if there is no lug line). Then use the guide to draw a new set of lines.

Some of the lines will line up just right with the earlier ones. But, in some case you'll end up with two lines side-by-side. In the second case, use the center between the two parallel lines as the line for the fin.

-- Roger

I suspect that the wrap around fin guides that come printed upon the instruction sheets included with manufactured kits do not allow for the thickness of the paper upon which they are printed. According to my micrometer, 20 lb bond paper is about 0.004 inches thick. Wrap this around a BT60 (1.64 inches) tube and the outside diameter increases to 1.648 inches and the circumference grows by 0.025 inches. Small but not insignificant.

Perfect is the enemy of good.
 
Using a fin marking guide or wrap, I don't have much trouble spacing the lines at 120 or 90 degrees from each other.
Using the Estes fin marking guide or a piece of aluminum angle, I don't have much trouble drawing the lines straight on the body tube, either.

For me, the hard part is actually gluing the fins on straight --- straight as in aligned with the lines I've already drawn, and straight as in perpendicular to the outside wall of the body tube.

The Estes fin marking guide has helped with that, since it has that fin-holder notch in the end, but it's still very easy to glue a fin on that's perpendicular to the body tube but that has a bit of a cant to it.
 
I suspect that the wrap around fin guides that come printed upon the instruction sheets included with manufactured kits do not allow for the thickness of the paper upon which they are printed. According to my micrometer, 20 lb bond paper is about 0.004 inches thick. Wrap this around a BT60 (1.64 inches) tube and the outside diameter increases to 1.648 inches and the circumference grows by 0.025 inches. Small but not insignificant.
As it happens, increasing the diameter of *any* circle by 0.008 inches increases the circumference by just over 0.025 inches.
I think you're right; that probably is what throws off the spacing of the lines on the wrap-around fin marking guides.
 
Using a fin marking guide or wrap, I don't have much trouble spacing the lines at 120 or 90 degrees from each other.
Using the Estes fin marking guide or a piece of aluminum angle, I don't have much trouble drawing the lines straight on the body tube, either.

For me, the hard part is actually gluing the fins on straight --- straight as in aligned with the lines I've already drawn, and straight as in perpendicular to the outside wall of the body tube.

The Estes fin marking guide has helped with that, since it has that fin-holder notch in the end, but it's still very easy to glue a fin on that's perpendicular to the body tube but that has a bit of a cant to it.

Getting the fins on straight is what prompted me to develop the Guillotine Fin Jig back in 2011 and now being sold through Apogee Components. Can I interest you in a set of plans? :wink:
 
The way I align my fins is to lay the body tube on a flat table with a block of wood that is half the diameter of the body tube minus half the thickness of the fin in thickness alongside the body tube. I lay the fin on the block of wood with a weigh on top of it to hold it still while the glue dries. After the first fin is dry, I use a 30 60 degree right angle to support the first fin which rotates the body for the second fin to be glued at 120 degrees. I slide the whole system over to the edge of the table so the first two fins are off the edge of the table and use a level to set the first two fin tips at perpendicular to the table then it is in position to glue the third fin on. All 3 fins should be in perfect alignment and 120 degrees apart.
 
My off-the-beaten track vertical fin alignment fixture is shown in this thread: https://www.rocketryforum.com/showthread.php?28228-Homebrew-alignment-fixture-build-thread

It will let you make any angle accurately for 2,3,6,8,12 fins, and even allows for models with non-uniform fin angles. It's a bit fiddly to use but so far has done almost everything I've wanted. Since the thread came out I've upgraded the fixture with machined spindles instead of plywood centering rings but still use it generally as shown. I've been tempted to add a slot with a Rose-style fin plate plate and screw-driven thickness adjustment, but doing that right is going to require a mini-mill.
 
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