Fin Fastening Jig
This is one of those common sense ideas that is anything but a brainstorm.
I've used a lot of different methods to hold through-the-wall fins in placed during the gluing/curing phase of building a rocket, but this is the one that seems to work the best.
You have to decide if it is worth the trouble to make a jig instead of just going crazy with epoxy.
I find that taking a few extra minutes up front to build the proper jig saves a lot of trouble and time later on.
The jig works well during dry-fit as you assemble the pieces prior to final assembly.
The jig also frees up your hands during assembly, like when you are applying epoxy to the fin-tenon/motor-tube joint.
I've found it is possible to fasten all the fins at once without worry about the other fins moving around during epoxy cure time.
There are some steps I describe that you may decide aren't necessary, and that's OK with me.
I chose to describe it this way because if you follow these instruction, you're sure to get it right.
Materials Needed
- Drafting Compass
- Protractor
- Carpenter's Square (or other right angle)
- Straightedge Ruler
- Sharp Knife (carpenter's retractable type)
- 1/8-inch masonite
Four-Fin Jig
Step One
This is the most fool-proof method of fabricating a four-fin jig:
Start with a piece of 1/8-inch masonite and cut it into a square. Make each side equal to the total fin span of your rocket.
Make sure the square is perfect (three right angles, four equal sides) or your fins won't end up perfectly perpendicular to your body tube.
Step Two
Draw diagonal lines from opposite corners - the point where they intersect is the center of the square.
Set your compass to the outside diameter of your rocket and scribe a circle using the intersection of the diagonal lines as your center point. (Figure 1A)
Step Three
Next, draw parallel lines on either side of each diagonal line - the distance between these lines should be one-half the thickness of your fin material.
These are the lines you will cut on in Step Four.
I tend to make the fin slots shorter than the span of each fin and wedge the jig on either the aft or leading edge -- but that's up to you.
(see Figure 1B)
Step Four
Cut out the circular center and the fin slots. (see Figure 1C)
You may have more suitable tools, but I use my retractable knife and a straight edge to cut the straight lines, and cut the circle freehand.
You can cut through the 1/8-inch masonite with only a few passes of the knife if you use a firm hand.
(Cardboard will work instead of the masonite, and is easier to cut -- but your jig won't be as durable or strong.)
Three-Fin Jig
Step One
This is the most fool-proof method of fabricating a Three-fin jig:
Start with a piece of 1/8-inch masonite and cut it into an equilateral triangle. Make each side 1.5 times the total fin span of your rocket (so the jig isn't too flimsy).
Make sure the equilateral triangle is perfect (three sixty-degree angles, three equal sides) or your fins won't end up perfectly perpendicular to your body tube. (Figure 2A)
Step Two
Draw a line from each corner to the middle of the opposite side - the point where the lines intersect is the center of the triangle/
Set your compass to the outside diameter of your rocket and scribe a circle using the intersection of the lines as your center point. (Figure 2B)
Step Three
Same as Step Three above.
Step Four
Same as Step Four above. (see Figure 2C)
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