Model Rocket Fin Alignment Guide & Clips for BT-20 & BT-50 Body Tubes

thingiverse

Estes's Fin Alignment tool is really cool, but costs like $20, which is a bit steep if you have a club or school class trying to build a dozen rockets at the same time. I plan to add some photos of it in use in a bit... So I printed some Fin Alignment guides for 3, 4 & 5 fin rockets using BT-20 and BT-50 tubes. They take a little while to print, but way cheaper than buying a bunch of "real" tools. Don't forget to check out L3-G0, my completely unrelated working full size Lego R2-D2 on his blog at http://L3-G0.blogspot.com or YouTube. Instructions Print one guide first to make sure it fits your tube. You may need to adjust the fudge factor in the stl. The files also have I.D. & O.D. for other tube sizes, so you can muck with the files to print more. Of course you'll also need to move the fins out a bit.... I'm expecting 1/8" ~ 3mm fin stock. If you have a different width fin stock you may need to either shim the fins when you attach them, or, if you do this a lot, adjust the STL to have a smaller offset. My first attempt was a stepped holder, but that doesn't help if the fins extend way below the tube, so don't print that one unless you really need it. Instead print the fin guide with the tall post BT-20 tube holder. If you need a BT-50, also print the "shim" to get it up to BT-50 size. What you need to print: 1) Appropriate # of fin guide(s). Make sure to use the ones with the tall center spoke unless you really want fins flush or above the bottom of the body tube. -- Check the first one at least to make sure it fits a BT-20. 2) If you need a BT-50, then print the "shim" piece. It's supposed to slip over the previous guide to make it bigger for the bigger body tube. -- If you're printing several, check the first couple to make sure it fits over the post in 1, and that the tube fits it well. I had to sand out the inside of my first one, which was a real pain. 3) Print clips. These clips are intended for about 1/8" balsa. You may need to make them skinnier, or add an extra bit of material to get enough grip. Using the fin guides: 1) Mark where you're putting the fins using your favorite method. 2) If you're using a BT-50, put the shim thing over the center post of the appropriate fin guide to make it big enough for your tube. 3) Check how the fins fit the body tube in the correct spot. Each of the guide vanes is off-center, expecting a 1/8" fin stock. Place the fins counter-clockwise to the vanes (away from the "Other Side" label). If you have the thin cardboard or balsa, you can either ignore the slight error or add a shim between the fin and the guide vane. 4) Use a clip to hold the fin in place. If you have thin stock you may need to adjust the clips or pad the gap with something. If you want to tweak the STL, there're some variables: In the Alignment Guide files: FINOFFSET is how far the vane is from the centerline of the fin, so it should be 1/2 the thickness of your fin stock. FINWIDTH and PLATE are the thickness of the parts, if they're too flimsy you could make them fatter. FUDGE modifies the "optimal" values to account for print variation, etc. You can increase this if your tubes are too tight. In the clips files the options are commented, but it wasn't really intended for tons of variation, so you may need to tweak everything else. The most interesting things are: LENGTH - how long the clips are, and INNERWIDTH - How big the gap is. Fin Alignment Shim is "just" the tube that fits over the BT-20 post. You may need to adjust the "FUDGE" or radius if it doesn't fit nicely on your post or if the BT-50 is too tight on it. You can also change the 50/20 to other tube sizes if you changed the fin guides for other tube sizes. Don't forget to check out L3-G0, my completely unrelated working full size Lego R2-D2 on his blog at http://L3-G0.blogspot.com or YouTube.