Method for Creating Centering Rings and Bulkheads

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By Peter Stanley

Introduction:

Using “Engine Mount Template Generator” and thin corrugated USPS Priority Mail boxes (Both are freely available.), you can quickly and easily create small to mid-sized custom rings and bulkheads. You’ll also need a printer, spray adhesive, and a hobby knife.

Software:

I found this freeware suite of rocketry tools online a few years ago. The programs are old and windows based, but I’m currently running them on Windows XP without problems. The author states that this can be freely distributed [EMRR Tools Page].

The tools in the zip file are the following: an Elliptical Fin Template Generator, an Engine Mount Template Generator, a wrap around fin guide, and a shroud designer. To use this software, extract all the files (including the .dll file) into a folder somewhere. You can create shortcuts to the files, or just run them directly from that directory. Engine Mount Template Generator (EMTG) is the program used for this tip.

Materials:

1. USPS Priority Mail cardboard size 0-1096S. This is a very thin corrugated cardboard designed for shipping VHS and related sized packages. You can order this totally free (even free shipping) from the USPS website. It is good to have this kind of stuff around anyway in case you to ever need to ship something Priority Mail. Here is the link to the actual box http://shop.usps.com/cgi-bin/vsbv/postal_store_non_ssl/display_products/productDetail.jsp?OID=1003571 In case that ever changes you can go to http://www.usps.com and browse to Priority Mail shipping supplies. There are probably other boxes that have the same thickness, but I know this one works.
2. Spray adhesive, such as Duro All-Purpose Spray Adhesive.
3. Hobby knife

Overview of Process:

1. Print centering ring or bulkhead template using EMTG, onto plain paper.
2. Rough cut around the template with scissors, and affix to a piece of O-1096S (or similarly thick cardboard) using spray adhesive.
3. Cut out with hobby knife.

Step-by-Step:

Centering ring:

In this implementation I am creating a ring that will fit between a BT-55 and BT-60 tube, which I needed for a tailcone transition.

1. Determine the size you need. The inside circle needs to be the BT-55 exterior diameter and the outside circle should correspond to the BT-60 interior diameter. The outside diameter of a BT-55 tube is 1.32 inches. The inside diameter of a BT-60 tube is 1.59 inches.

2. Open EMTG and enter the following parameters, and click PRINT

3. Your output should look like this

4. Cut out the image and affix to Priority Mail box using spray adhesive.

5. Cut out the inside circle first, then the outside circle. The best way I have found to do this is first start with an up and down sawing motion using a fresh hobby knife blade. Follow the line slowly, rotating the piece around until you have made one complete revolution. Then carefully drag the blade over the cut, rotating the piece until you have cut through completely.

I could have used this method for the other rings in the assembly, but I happened to have them already.

Bulkhead:

In this implementation I am creating a bulkhead that will fit inside a BT-60 coupler tube for a payload bay. The inside diameter of my BT-60 coupler tube is 1.524 inches.

Use the same technique for centering rings, except change how setup EMTG. See below.

GUEST's OPINION:
"" (X.X.)

1. General
   When you need to manufacture your centering rings (say you want to fly that Mean Machine on 24mm E, F and G motors), buying hole saws that nearly match is ok, but there is an easier and more cost effective way. Purchase a variable size hole cutter. The one I have as a little arm that slides out from 7/8" to 4" and can be used to get a perfect fit centering ring. I got mine from Aircraft Spruce & Specialty (part #12-10600) for $9.40 (as of 5-24-01). But you may be able to find one at a local hardware store. Just be sure when using this thing you cut the outside diameter first, then the inside. - G.S. (CA)
2. General
   For those of you that have a cat(s)....I've been wondering for a while what to do with all those empty soft cat food tins. Well as a BAR....here it is, MMT Centering Rings. The "Tins" are aluminum and can be cut out with hobby knives. The finished rings can be sanded easily, have a painted side which is good for tracing size for cutting out, feel lighter than the cardboard rings supplied with Estes kits, and will work for up to a BT80 tube. I am gluing them to the cardboard upper ring which makes a very good backing to prevent recovery wire knots from pulling thru. And you don't have to wrap or epoxy the recovery wire to the MMT which cuts down on the weight and fin interference. If you don't like this idea....the tins are great containers for brush cleaner, glue mixing, ETC. – J.R.S. (PA)
3. General
   One can make centering rings from card stock and epoxy, or super glue. First draw the ring. Any CAD program would work, I use the orthographic camera in POV-Ray (a 3D ray tracing program). 110 pound card stock will work in most printers, and one layer of card stock and epoxy is approximately 1/100th of an inch thick (10 mils). Print out as many rings as needed for desired thickness. Cut them out with a hobby knife. When using epoxy, put a pea size of resin and hardener (I use five minute epoxy). Spread the epoxy thin on one ring and apply to another. Apply epoxy to successive rings and put under a book to cure. I use wax paper to keep it from sticking. After it has cured clean up the edges with sand paper, or a Dremel. Voila, a centering ring that will work with whatever tube size will fit on a sheet of paper. And they are as stiff as Aircraft plywood! Enjoy! - E.B. (VA)