Universal Fin Alignment Fixture
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I first used a fin alignment fixture for the Dynacom Scorpion. It was simply an aluminum sleeve that fit precisely to the OD of the airframe with three slots to match the fin thickness machined 120˚apart. Very simple and effective, but a new fixture would be required for each new project. When I started the M-Python project, I wanted to construct once and for all a universal fixture that wouldn't require bothering my machinist friend for every project.
The design is the result of looking through the web and combining some of the best ideas I saw along with what I remembered of an old Estes design. I will walk through the design and process step-by-step below:
The
base plate is a 12" diameter aluminum disc that is half an inch thick. You can
see a pattern of concentric circles scribed near the center of the disc. These
circles are 54, 75, and 98mm in diameter and correspond to the motor diameters I
use. The holes near the center within these scribed circles are to locate the
simulated motors used for centering the airframe.
At each 120˚interval, three rows of holes radiate outward. The center row is used to locate pins whose diameter is equal to the fin thickness to be used. The two adjacent rows of holes are threaded and are used to position the stanchion bases shown below.
The centering cylinders mount to the base plate with three pins. Shown at left is the 54mm cylinder nested inside the 75mm cylinder.
I thought that the simplest and most versatile way to center the airframe was to make use of the airframe's motor mount, since every rocket has one and it's always centered. Loose fitting motor mounts can be accommodated with masking tape.
The centering pins are the same diameter as the fin's thickness. A set of 6 is required for each fin thickness to be used. Shown are 0.125" pins for my M-Python project. The pins can be removed once the stanchion bases are in place.
The stanchion bases are butted up against the centering pins and then kept in place by cap screws through slots into the base plate.
Once the stanchion bases are in proper alignment and secured to the base plate, the centering pins can be removed if desired.
The stanchions are placed on top of the bases, and are automatically aligned by means of pins at the top of the bases. If you look at the underside of a stanchion (the one lying on its side), three holes are visible. The outer two holes fit over the pins in the stanchion base, and the central hole accommodates a thumb screw that keeps the stanchion in place.
During machining, the stanchion was machined while assembled to its base, to ensure a perfect 90˚angle. Each stanchion shares a serial code with its base so that the pairs remain intact.
With all the stanchions in place and secured, the next step is to simply slide the airframe over the centering cylinder.
The fins fit snugly between the stanchions, so that they tend to stay where you put them, elevation-wise. This tendency lets you use them as a "third hand" to hold them while you paint the root edge with epoxy. Then they can be pushed up against the airframe. I used a large rubber band (not shown) around the tip chords of the fins just to be sure.
Once the epoxy has fully cured, just remove the stanchions by undoing their thumb screws and lift the fin can up off the centering cylinder. The result is perfect fin alignment!
The artisan behind this tool is Bob Hugo, one of the best model-maker/machinists in silicon valley. Thanks to Bob for his beautiful work and all the clever design details that took it from a sketch to reality.