Homebrew alignment fixture build thread

Suddenly everyone is talking about alignment fixtures again. A little while back JAL3 asked for a build thread on the home-made fixture I showed in the Hydra One build thread, so here goes.

Design Goals
Though I'd eventually like to make myself a Rose-style indexing fixture, that requires machine tools that I don't now have, and the designs I've seen have some limitations that would require a fair number of customized interchangeable parts if you wanted enough flexibility to build a lot of sport models in addition to 3FNC contest models.

I also like some of the ideas in the Standard Rockets magnetic fixture, but with the large flat panels it won't handle fins with embedded non-flat shapes, outboard pods, etc.

Here is what I thought was important:

• No machining required beyond a drill press
• Materials available from HD, Staples, Michaels, and/or hardware store
• Handle any number of fins/wings from 1 to 12
• Handle non-flat fins/wings with attachments like Constellation tip pods
• Materials cost under $50
• Accommodate any LPR/MPR body tube up to 3"
• Glue-resistant surface
• Handle situations with fins set considerably forward of the tube end

Design Decisions
The way to avoid having machined parts is to rely on gadgets that you can buy that are already accurately square enough. That told me pretty quickly that I would need a vertical spindle system so that I could use commercially available squares and straightedges to get the required perpendicularity to the base.

To avoid having to make fin-thickness-specific parts, I decided to just use a thick straightedge to set the fin radial angle. The straightedge can be taped down to the base with whatever offset is needed to get the radial axis of the fin pointing through the center of the spindle.

Since you only glue one fin at a time, rather than try to have N fin support plates, I decided to just have one, and rotate the model after attaching each fin.

Thus the overall design just became a flat base with a threaded rod spindle and index markings, and a straightedge and some squares to hold the fins in place.

The graphics were considerably revised after the first prototype to support optimal work positioning for both left-handers like me, and right-handers like most of the rest of the world.

Bill of Materials
The first two photos show most of the parts that are in the fixture. One picture is from the very first iteration, and the other is when I revamped the base plate graphic after building several models with the original.

Here are the materials needed:

• 8x10x5/8" MDF base plate. You could use thicker, but I would not go any thinner.
• 4 National 7/8" screw-on feet. You can use any feet that provide enough clearance for the bolt head underneath the base.
• 1 1/4-20 bolt, 6" long, threaded entire length
• 5 1/4" washers
• 1 1/4" fender washer (for bottom side of base plate)
• 5 1/4-20 nuts
• 1 sheet 3M inkjet plastic transparency media
• 1 sheet 110# cardstock, white
• 1 8" stiff (=thicker) stainless steel rule
• 1 8" Empire square (available at HD)
• 2 plywood adapter rings for each BT size you want to use, with a 1/4" hole in the middle.

These materials should cost less than $20, and most of it is for the steel rule and square. You can use additional squares etc. for special situations but the 8" one should work for almost everything.

Assembly
Drill a 1/4" hole *with a drill press* in the center of the 8x10 MDF base plate. This hole must be accurately perpendicular to the base plate.

Install the feet on the bottom corners of the base plate.

Print the baseplate graphic on a sheet of 3M inkjet transparency media. This is a plastic sheet with one smooth side and a matte side on which you print. Be sure to print the graphic backwards so it reads correctly when the smooth side is up! The Illustrator file is in the rocketry goodies section of my website at https://www.caveduck.com/rocketplans

Use spray adhesive to laminate the matte side of the transparency to a sheet of white cardstock.

Drill a 3/8" hole through the center of the graphic sheet lamination, using the center cross to be as accurate as possible.

Now use the spray adhesive again to glue the graphic sheet down to the base plate, centering the pattern on the 1/4" hole in the base plate as accurately as possible (a critical step).

Insert the 6" long bolt through the base plate from the bottom. Use a fender washer on the bottom side and a regular washer on the top. The third photo shows the base with the bolt installed.

On the top side of the base plate, secure the bolt with a 1/4-20 nut over a regular washer and tighten firmly with a wrench. The fourth photo shows the top side of the unit after this is done.

The unit is now ready for setup and use.

Tube Mounting Rings
To use this fixture, you need a pair of adapter rings to hold the tube to which you want to mount things. These rings are made to match the ID of the tube, and have a 1/4" hole in the middle to go over the fixture spindle.

I make the rings myself on a drill press by drilling a 1/4" hole in the center of a 1/4" plywood blank, inserting a 1/4" carriage bolt, clamping a nut down on the blank, chucking the thing in the drill press and turning it down to size with a rasp, checking size with calipers when it starts getting close. With practice you can make a perfect ring in any LPR size in a few minutes, and they have the required 1/4" hole in the middle with no extra work.

The rings need to be fairly tight; you don't want the model rotating or sliding up and down inadvertently as you work on it.

You could also use other systems like putting threaded inserts into engine casings, etc. or even custom turning longer mandrels with a 1/4-20 threaded center, but that gets back into things that need machine tools.

Setup
Mount each ring on the central spindle between a pair of 1/4-20 nuts, with a washer on each side. Use a wrench to tighten them up so that they won't start unscrewing when you twist the tube to rotate to the next index point.

Generally you want to put the rings as far apart as possible to provide the most stable alignment for the tube.

it's even possible to use rings of different sizes if you have a design with a transition very near the aft end.

The first photo shows the spindle set up for a BT-60 model.

Now we're ready to lay down the "slide guide" steel rule along which the fin-holder square will slide. You need to tape it down so that the surface of the square that carries the fin will move parallel to the fin radial, with an offset that puts the centerline of the fin thickness exactly on the radial guide line. Sounds complicated, but is actually pretty easy to do. There's a set of parallel lines along the primary index radial that aid you in aligning the steel rule.

The second photo shows the slide guide positioned and ready to be taped down.

Now mount the body tube on the rings. Usually you will have the rear of the tube somewhat above the base plate to account for the thickness of the square's base leg. The tube should be very snug on the rings; apply some masking tape to the rings if needed (though if they were that loose I'd probably make some new rings).

Fin Attachment
To attach each fin, begin by clipping or taping or magnet-attaching it to the arm of the square. I like to use the spring steel clips from my old Estes Fin-Kwik. Get the fin positioned so that when you slide the square inward toward the model, the fin root lands in the right spot on the tube. Make sure that everything is lying down flat and that you don't have any debris under the square. Now withdraw the fin, put a very small amount of gap-filling CA on the fin root, and slide it back in against the tube. You can either wait a minute here or use some accelerator to zap it instantly.

Now unclip the fin, remove the square and rotate the model so that the fin you just applied is on the next index radial for that number of fins. On my graphic the 45/90 radials are color-coded differently than the 30/60/120 radials. At this point you are ready to apply the next fin.

Here are a few quick photos of the fin fixture in use for different things. These photos all show the old base plate graphic of the original prototype.

The first picture shows the Semroc Hydra One inner fins being attached. This is a pretty vanilla application, though there are 6 fins.

In the second photo, the upper small fins are being applied to a Semroc Laser-X. These fins are offset about 4" up from the bottom of the tube. Here I used a masking tape guide about 1/16" below the desired position of the fins to guide the placement.

In the last picture, I'm attaching Constellation fins that already have the tip teardrop in place. This was no problem with the thin bladed square. Here again the tube is mounted up in the air a bit due to the swept fins.

Overall I'm pretty happy with how this turned out. The design goals have all been well met; it is very inexpensive and construction is easy. Some of the models built with it have been flown and all have shown ultra-straight trajectories.

Like it.

Very cool idea... thanks for sharing...

Dave- that is awesome and fills a great need for an affordable tool the rest of us can build without a machine shop. I'm going to visit my local hardware store to day and start on my very own genuine copy...would you autograph it if I bring it out to a launch? Everybody wants a Caveduck collectable!!

Thanks for the kind words...@fyrwrxz I somehow doubt that my autograph will actually add any colllecto-mojo to anything but I'm looking forward to meeting you IRL (Plaster Blaster? - I'll be there Saturday at least)

Here are a few things I've thought of that might be easily incorporated into fixtures of this type.

Magnetic Base
A layer of the thin magnetic sheet material could be put on the base plate, under the graphics. This would make steel squares and straightedges stick down to the base, but does add some cost.

Heavier Spindle
You could use a 3/8" bolt and hardware in place of the 1/4" shown here. This would add some extra rigidity and enable a longer spindle for MPR applications. The trade-off is that a 3/8" nut will not fit inside a BT-5 so you'd need to make a special adapter for BT-5, and 10mm tube would become impossible. To allow more flexibility you could try using a bushing so that you could switch between 1/4" and 3/8" spindle bolts.

Mandrels Instead of Rings
In place of the pairs of plywood centering rings, you could use longer plastic, aluminum or wood mandrels that had 1/4" central holes. This would speed up changing tube sizes on the fixture since you'd only have to install one centering adapter instead of two. I think you could still make them on a drill press up to around 3" long. Longer ones would be useful but would have to be made on a lathe.

Dave- yup- I'll be there Sat. Was gonna wear a DART t-shirt, but son's wedding precluded picking one up in time, so It'll be the old school Apogee from Ed's days back when dinosaurs roamed compuserve...good suggestions for the upgrades. I'll prolly finish mine after PB so I can use the time to get something ready to fly. Kenny (KenRico) is sposed to come over over today and maybe we'll make some ignitors. C'ya there! Up in the Air, Juniour Birdmen!-Dave-