This is a brief description and photos of the fin alignment jig shown in the Bullpup construction article. I have had several requests for information on how to build one-so here it is.
the base board is a 2' x 2' piece of 3/4" birch cabinet-grade plywood.
In the back center of the board is a 1/4-20 T-Nut. This has a piece of threaded rod screwed into it. An old spent 38mm single use motor casing is drilled out and placed over the rod and held in place with a washer & wing nut. This centers the tube assembly on the jig. I use a 29mm casing for smaller motor mount equipped rockets and a 54mm/38mm adapter for rockets with 54mm motor mounts. Use a 54mm casing with a 3/8-16 T-nut & rod if you have it and need it.
A pattern is carefully drawn out for 4-fin and 3-fin configurations.
(The lines should be visible in the photos.)
Concentric circles are laid out on the board at the various diameters of tubing I use. (do this before you drill for the T-Nut)
The aluminum brackets are sections of 1/4" x 4" aluminum angle. (steel works too but is heavier)
The brackets are drilled with oversize holes to allow some room for adjustment to various fin thickness and alignment.
Holes are drilled in the base offset from the layout lines to allow the
brackets to offset for various fin thicknesses. Place the holes to
position the front edge of the brackets at least 1"-2" from the airframe
at each diameter. You need room to work and spread epoxy--and you do
not want to glue the rocket to the jig!
Carriage bolts are pressed into the base from the back side. These bolts can be moved in or out for various airframe diameters and changed from 3-fin to 4-fin configuration as needed.
Be sure to offset the brackets from the layout line each time to 1/2 the current fin thickness . Tighten the brackets down and double check alignment & squareness before beginning to apply epoxy.
Note the way the brackets are positioned in relation to each other.
This permits you to rotate the rocket away from the jig before pulling it up and off the center motor casing.
Remember- your fins are only aligned as good as your brackets are aligned.
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