Thursday, April 20, 2017

Odd'l Rockets Goony G-16 Build, Part 3, Engine Mount - TIP

Boy, I really need to buy some 20/60 rings! This isn't the first time I've had to make my own.

TIP: The next time you are at a framing store, ask if they have any scrap mat material. It's perfect for centering rings or fiber fins. It's thicker than the centering ring material you'd find in a kit. You should be able to buy some very cheap.

I'm tracing around the BT-60 main body tube with a mechanical pencil.

Center a BT-20 on the circle you just drew. It isn't easy to get it perfectly centered and may take a few tries. Trace around the BT-20 engine mount tube.

Cut out the center first, right on the pencil line.
(Inset picture) You can cut the outside ring with scissors, cut just inside the pencil line.

You can use some 220 grit wrapped around a finger to slightly widen the inside hole until the BT-20 slides in.
(Left picture) Stack the two rings on the BT-20, sand and round the outside until it slip fits into the BT-60.

Here's the rings before and after using the rotary punch.
This rocket will have the replaceable Kevlar line, that's the small hole at the bottom of the rings.

Notice the faint pencil lines near the center of the ring. Always mark before making any punches.

Here's the finished mount with the Kevlar line running through the Q-tip tube.
The Kevlar and tied on elastic are rolled up and inside the engine tube for gluing into the main airframe tube.


  1. Just a thought -- some lightly drawn guidelines and some tickmarks could help align the tubs for tracing. The (nearly perpendicular) lines would be positioned so that the intersection would be where the center of the centering ring would be. Then you can put tick marks on those lines positioned at one radius of the tube from the intersection (the marks could be positioned a wee bit further so the marks could be visible when you put the tubing on top).
    Similarly you could probably apply some carpentry techniques too. A set of 30-60-90 and 45-45-90 triangles (you can get them pretty cheaply too as part of a "geometry set" or "artist set" found in the school supply section of a dollar store) are pretty handy.

  2. Another possibly handy tool would be a circle template (it's a plastic sheet with circular openings of various diameters). Usually there will be marks at 90-degree points, so aligning is made easier. Picked one up for $1.50 at Daiso (a chain of 100-yen shops from Japan -- the stores in the USA are mostly out west hosever).

  3. If you know the od of the inner tube and the id of the outer, a compass can be used to draw two perfectly centered concentric circles. I would set the radii a hair smaller on the inner and a hair larger on the outer, and sand to fit after cutting.

  4. I forgot to mention, scrapbookers use a circle cutting device which I presume can also cut centering rings. Similar to a compass, and very cheap.

  5. Thanks Guys!
    I appreciate the suggestions. I actually know how to center circles. I have a compass, triangles, T-Square and circle templates. I did this post for the average builder who probably doesn't have all the fancy tools. This was a quick and dirty build, I didn't feel like pulling out all my old art and drafting supplies.

  6. On the topic of potentially useful gadgets, I did find a clone of the Olfa compass cutter for only $1.50, which could be set to cut circles anywhere between 1cm to 15cm in diameter.

    For those who have not seen or heard of a compass cutter
    a link to Walmart for one